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Time Travel without Regrets

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Time travel is not ruled out by general relativity, but it might well create problems for the laws of common sense. In the 28 January Physical Review Letters , a team proposes a new way of deciding the possibility or impossibility of quantum states that travel forward and backward in time. The new criterion automatically disallows quantum versions of the “grandfather paradox,” in which a person travels back in time and kills her ancestor, thereby ensuring her own demise. The team also performed an experiment that illustrates the paradox-nullifying mechanism.

General relativity, Einstein’s theory of space and time, allows the existence of closed timelike curves (CTCs)–paths that go forward in time, then back again to reconnect and form closed loops. Although it’s unclear whether CTCs can be created, physicists have nevertheless explored their possible consequences, including their influence on quantum mechanics.

An ordinary quantum event might involve two particles moving forward in time, changing each other by interacting at some time, then going their separate ways into the future. However, if one outgoing particle enters a CTC, it can double back and become one of the ingoing particles–thus influencing its own transformation. In 1991, Oxford University physicist David Deutsch proposed a consistency condition to avoid time-travel paradoxes: a particle that loops back in time in this way should be in the same quantum state when in reappears in the immediate past of the interaction as it was when it departed the interaction for the immediate future [1] .

To see how this condition works, imagine a quantum particle having states labeled 0 and 1. It travels around a CTC and, on its return, interacts with an “external” particle in such a way that 0 becomes 1 and 1 becomes 0. Such a particle presents a quantum grandfather paradox: when it comes back around the loop, it flips its former self to the opposite state. However, Deutsch showed that consistency is possible if the particle is in a superposition–a state that is equal parts 0 and 1. The interaction exchanges the 0 and the 1, but the state overall remains unchanged. For this to work, the external particle must also be in a superposition that flips back and forth.

The paradox is avoided, but a difficulty arises if the external particle is measured. Then it cannot remain in a superposition but must become definitely either 0 or 1–which means that the CTC particle cannot remain in a superposition, either. To preserve consistency, Deutsch argued that the CTC particle must exist in two parallel universes–the “1-universe” and the “0-universe”–and continually switch between them, so that no contradiction occurs in either one.

Lorenzo Maccone, of the Massachusetts Institute of Technology and the University of Pavia, Italy, and his colleagues propose a more stringent condition that avoids these difficulties. They require that any measurement of the particle going into the future should yield the same result as measuring it when it returns from the past. So any state that would alter the past when it came around again is disallowed, and no grandfather-type paradoxes can arise.

Perhaps surprisingly, “we can still have CTCs even with this strong condition,” Maccone says. Only states that avoid paradoxes after the interaction are able to exist beforehand, so the team calls their condition “post-selection.”

To demonstrate these ideas, the team performed an experiment with photons showing that the consistency condition indeed picks out specific states and destroys all the rest. Lacking an actual CTC to perform the post-selection, the team created photons in a specific quantum state for the input, a state where the polarization was not known or measured but had a correlation with another property, associated with the photon’s path. As the photon went through the experiment, it experienced changes that mimicked the 0-to-1 flipping that occurs in the imagined time-travel arrangement. The team found that only those photons that wouldn’t lead to paradoxes made it through unscathed. Although the result is in line with expectation, no one has simulated time travel in this way before.

An odd consequence of post-selection is that because the presence of a CTC annuls paradoxical states completely, it can disallow some states that seem innocuous today but have unacceptable consequences later. “In principle, one could detect the future existence of time machines by … looking for deviations now from the predictions of quantum mechanics,” says Todd Brun of the University of Southern California in Los Angeles. Although, he adds, it’s hard to know in advance what to measure.

–David Lindley

David Lindley is a freelance science writer in Alexandria, Virginia.

  • D. Deutsch, “Quantum Mechanics near Closed Timelike Lines,” Phys. Rev. D 44, 3197 (1991)

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Time Travel Beats Quantum Mechanics ( Focus story from 2009)

Closed Timelike Curves via Postselection: Theory and Experimental Test of Consistency

Seth Lloyd, Lorenzo Maccone, Raul Garcia-Patron, Vittorio Giovannetti, Yutaka Shikano, Stefano Pirandola, Lee A. Rozema, Ardavan Darabi, Yasaman Soudagar, Lynden K. Shalm, and Aephraim M. Steinberg

Phys. Rev. Lett. 106 , 040403 (2011)

Published January 27, 2011

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General Relativity and Quantum Cosmology

Title: emergent time and time travel in quantum physics.

Abstract: Entertaining the possibility of time travel will invariably challenge dearly held concepts of fundamental physics. It becomes relatively easy to construct multiple logical contradictions using differing starting points from various well-established fields of physics. Sometimes, the interpretation is that only a full theory of quantum gravity will be able to settle these logical contradictions. Even then, it remains unclear if the multitude of problems could be overcome. Yet as definitive as this seems to the notion of time travel in physics, such a recourse to quantum gravity comes with its own, long-standing challenge to most of these counter-arguments to time travel: These arguments rely on time, while quantum gravity is (in)famously stuck with and dealing with the problem of time. One attempt to answer this problem within the canonical framework resulted in the Page-Wootters formalism, and its recent gauge-theoretic re-interpretation - as an emergent notion of time. Herein, we will begin a programme to study toy models implementing the Hamiltonian constraint in quantum theory, with an aim towards understanding what an emergent notion of time can tell us about the (im)possibility of time travel.

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Physical Review A

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Treating time travel quantum mechanically

John-mark a. allen, phys. rev. a 90 , 042107 – published 9 october 2014.

  • Citing Articles (19)
  • INTRODUCTION
  • MODELING TIME TRAVEL WITH QUANTUM…
  • D-CTCs AND P-CTCs
  • ALTERNATE THEORIES
  • ACKNOWLEDGMENTS

The fact that closed timelike curves (CTCs) are permitted by general relativity raises the question as to how quantum systems behave when time travel to the past occurs. Research into answering this question by utilizing the quantum circuit formalism has given rise to two theories: Deutschian-CTCs (D-CTCs) and “postselected” CTCs (P-CTCs). In this paper the quantum circuit approach is thoroughly reviewed, and the strengths and shortcomings of D-CTCs and P-CTCs are presented in view of their nonlinearity and time-travel paradoxes. In particular, the “equivalent circuit model”—which aims to make equivalent predictions to D-CTCs, while avoiding some of the difficulties of the original theory—is shown to contain errors. The discussion of D-CTCs and P-CTCs is used to motivate an analysis of the features one might require of a theory of quantum time travel, following which two overlapping classes of alternate theories are identified. One such theory, the theory of “transition probability” CTCs (T-CTCs), is fully developed. The theory of T-CTCs is shown not to have certain undesirable features—such as time-travel paradoxes, the ability to distinguish nonorthogonal states with certainty, and the ability to clone or delete arbitrary pure states—that are present with D-CTCs and P-CTCs. The problems with nonlinear extensions to quantum mechanics are discussed in relation to the interpretation of these theories, and the physical motivations of all three theories are discussed and compared.

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  • Received 22 January 2014

DOI: https://doi.org/10.1103/PhysRevA.90.042107

©2014 American Physical Society

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  • Department of Computer Science, University of Oxford, Wolfson Building, Parks Road, Oxford OX1 3QD, United Kingdom
  • * [email protected]

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Vol. 90, Iss. 4 — October 2014

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Schematic diagram for the standard form circuit described in the text. The n CR and m CV qubits are shown entering the gate labeled by unitary U . The double bars represent the time-travel event and may be thought of as two depictions of the same spacelike hypersurface, thus forming a CTC. The dashed lines represent the spacelike boundaries of the region in which time travel takes place; CV qubits are restricted to that region.

Circuit diagram for the unproven theorem circuit of Ref. [ 16 ] in the standard form. The book qubit is labeled B , mathematician labeled M , and time traveler labeled T . The input to the circuit is taken to be the pure state | 0 〉 B | 0 〉 M . The notation used for the gates is the standard notation as defined in Ref. [ 14 ] and describes a unitary U = s w a p M T c n o t B M c n o t T B .

The equivalent circuit model. (a) A circuit equivalent to the standard form of Fig.  1 with V = s w a p U . (b) The same circuit “unwrapped” in the equivalent circuit model. There is an infinite ladder of unwrapped circuits, each with the same input state ρ i . To start the ladder, an initial CV state σ 0 is guessed. The CV state of the N th rung of the ladder is σ N . The output state ρ f is taken after a number N → ∞ of rungs of the ladder have been iterated.

Schematic diagram illustrating the protocol defining the action of P-CTCs as described in the text. When Alice measures the final state of the B and A qubits to be | Φ 〉 , the standard teleportation protocol would have Bob (who holds the other half of the initially entangled state) then holding the state with which Alice measured the A system. There are two unusual things about this situation: first, that Bob's system is the same one that Alice wants to teleport, just at an earlier time, and, second, that Alice can get the outcome | Φ 〉 with certainty and so no classical communication to Bob is required to complete the teleportation.

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Quantum Mechanics Proves 'Back to the Future' Is B.S.

But the Avengers' method of time travel totally checks out.

zurueck in die zukunft, back to the future

  • Like in certain mathematical conditions , the timeline can overcome some disruptions.
  • No one can take Back to the Future away from you.

In trippy new research, scientists say they’ve confirmed what they call the Avengers: Endgame model of time travel.

They did this by running a quantum time travel simulation that runs backward and forward, letting them “damage” the past and see what resulted. And, as they say, the devil is in the details—the experiment involves an extremely simplified idea of a “world,” and is only the very first step toward demonstrating any big ideas about causality.

🤯 The universe is a mindf#!@. Let's explore it together.

In a statement, sponsoring Los Alamos National Laboratory likens the movie Back to the Future , where Marty McFly must carefully not disrupt the timeline of his own inception, to the idea of the “butterfly effect.” The idea is simple: Because of the complex way time moves and how causality “ripples out” in unexpected or just unfathomable ways, stepping on a butterfly in the past could change the entire world you try to return to.

Games with procedurally generated worlds use a reverse butterfly effect to create those worlds—a randomized “seed,” which is a string of characters that determines different variables. In Songbringer , for example, the player makes up their own six-letter seed and then plays through the world they created. Since the randomness is an illusion based on high numbers, entering the same seed makes the same world over and over.

christopher lloyd in 'back to the future'

But what if the world is, to some extent, self healing? This is where the Endgame version of events comes into play, and where the quantum experiment begins. In this version of events, no matter what “seed” shifted the world—what information was included, left out, or examined—the timelines would all eventually converge. The Avengers can go back, gently meddle, and return to the present without making a ripple.

This may sound like the same narrative handwaving that enables all time travel stories, but the Los Alamos experiment came to the same conclusion.

Here’s the scenario: Alice and Bob, two agents in a system, each have a piece of data—a qubit , or quantum bit. Alice sends her data backward in time, where Bob measures it, altering it in accordance with the Heisenberg uncertainty principle. What happens when the qubit returns?

In their simulations, the researchers found the qubit didn't change—or even affect—the original timeline. They used a series of quantum logic gates and an operator called a Hamiltonian, which in quantum mechanics is a measurement of potential and kinetic energy. Here’s where the idea stops being the stuff of sci-fi watercooler talk and starts to sound real. From the paper :

“The evolution with a complex Hamiltonian generally leads to information scrambling. A time-reversed dynamics unwinds this scrambling and thus leads to the original information recovery. We show that if the scrambled information is, in addition, partially damaged by a local measurement, then such a damage can still be treated by application of the time-reversed protocol.”

In other words, the nature of the time-affected operator they studied is such that it ends up affecting information that travels through it, and for some far-off application like quantum internet, that fact could end up making a big difference.

If researchers know they can “rewind” the information to see how it looked originally without changing the outcome, they may be able to guarantee fidelity of data in a situation previously thought to result in, well, some digital broken eggs.

preview for Pop News: Airports, Lava Floors and Movie Stunts

Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all. 

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Can we time travel? A theoretical physicist provides some answers

time travel theory quantum physics

Emeritus professor, Physics, Carleton University

Disclosure statement

Peter Watson received funding from NSERC. He is affiliated with Carleton University and a member of the Canadian Association of Physicists.

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

a woman stands among a crowd of people moving around her

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

Image of galaxies, taken by the Hubble Space Telescope.

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

Illustration of GPS satellites orbiting around Earth

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

Illustration of a hand holding a phone with a maps application active.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

If you liked this, you may like:

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Simulations of ‘backwards time travel’ can improve scientific experiments

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Physicists have shown that simulating models of hypothetical time travel can solve experimental problems that appear impossible to solve using standard physics.

We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics David Arvidsson-Shukur

If gamblers, investors and quantum experimentalists could bend the arrow of time, their advantage would be significantly higher, leading to significantly better outcomes. 

Researchers at the University of Cambridge have shown that by manipulating entanglement – a feature of quantum theory that causes particles to be intrinsically linked – they can simulate what could happen if one could travel backwards in time. So that gamblers, investors and quantum experimentalists could, in some cases, retroactively change their past actions and improve their outcomes in the present.

Whether particles can travel backwards in time is a controversial topic among physicists, even though scientists have previously simulated models of how such spacetime loops could behave if they did exist. By connecting their new theory to quantum metrology, which uses quantum theory to make highly sensitive measurements, the Cambridge team has shown that entanglement can solve problems that otherwise seem impossible. The study appears in the journal  Physical Review Letters .

“Imagine that you want to send a gift to someone: you need to send it on day one to make sure it arrives on day three,” said lead author David Arvidsson-Shukur, from the Hitachi Cambridge Laboratory. “However, you only receive that person’s wish list on day two. So, in this chronology-respecting scenario, it’s impossible for you to know in advance what they will want as a gift and to make sure you send the right one.

“Now imagine you can change what you send on day one with the information from the wish list received on day two. Our simulation uses quantum entanglement manipulation to show how you could retroactively change your previous actions to ensure the final outcome is the one you want.”

The simulation is based on quantum entanglement, which consists of strong correlations that quantum particles can share and classical particles—those governed by everyday physics—cannot.

The particularity of quantum physics is that if two particles are close enough to each other to interact, they can stay connected even when separated. This is the basis of quantum computing – the harnessing of connected particles to perform computations too complex for classical computers.

“In our proposal, an experimentalist entangles two particles,” said co-author Nicole Yunger Halpern, researcher at the National Institute of Standards and Technology (NIST) and the University of Maryland. “The first particle is then sent to be used in an experiment. Upon gaining new information, the experimentalist manipulates the second particle to effectively alter the first particle’s past state, changing the outcome of the experiment.”

“The effect is remarkable, but it happens only one time out of four!” said Arvidsson-Shukur. “In other words, the simulation has a 75% chance of failure. But the good news is that you know if you have failed. If we stay with our gift analogy, one out of four times, the gift will be the desired one (for example a pair of trousers), another time it will be a pair of trousers but in the wrong size, or the wrong colour, or it will be a jacket.”

To give their model relevance to technologies, the theorists connected it to quantum metrology. In a common quantum metrology experiment, photons—small particles of light—are shone onto a sample of interest and then registered with a special type of camera. If this experiment is to be efficient, the photons must be prepared in a certain way before they reach the sample. The researchers have shown that even if they learn how to best prepare the photons only after the photons have reached the sample, they can use simulations of time travel to retroactively change the original photons.

To counteract the high chance of failure, the theorists propose to send a huge number of entangled photons, knowing that some will eventually carry the correct, updated information. Then they would use a filter to ensure that the right photons pass to the camera, while the filter rejects the rest of the ‘bad’ photons.

“Consider our earlier analogy about gifts,” said co-author Aidan McConnell, who carried out this research during his master’s degree at the Cavendish Laboratory in Cambridge, and is now a PhD student at ETH, Zürich. “Let’s say sending gifts is inexpensive and we can send numerous parcels on day one. On day two we know which gift we should have sent. By the time the parcels arrive on day three, one out of every four gifts will be correct, and we select these by telling the recipient which deliveries to throw away.”

“That we need to use a filter to make our experiment work is actually pretty reassuring,” said Arvidsson-Shukur. “The world would be very strange if our time-travel simulation worked every time. Relativity and all the theories that we are building our understanding of our universe on would be out of the window.

“We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics. These simulations do not allow you to go back and alter your past, but they do allow you to create a better tomorrow by fixing yesterday’s problems today.”

This work was supported by the Sweden-America Foundation, the Lars Hierta Memorial Foundation, Girton College, and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).

Reference: David R M Arvidsson-Shukur, Aidan G McConnell, and Nicole Yunger Halpern, ‘ Nonclassical advantage in metrology established via quantum simulations of hypothetical closed timelike curves ’, Phys. Rev. Lett. 2023. DOI: 10.1103/PhysRevLett.131.150202

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Physicists Say Time Travel Can Be Simulated Using Quantum Entanglement

“whether closed timelike curves exist in reality, we don’t know.".

Image for article titled Physicists Say Time Travel Can Be Simulated Using Quantum Entanglement

The quantum world operates by different rules than the classical one we buzz around in, allowing the fantastical to the bizarrely normal. Physicists have described using quantum entanglement to simulate a closed timelike curve—in layman’s terms, time travel.

Before we proceed, I’ll stress that no quantum particles went back in time. The recent research was a Gedankenexperiment , a term popularized by Einstein to describe conceptual studies conducted in lieu of real tests—a useful thing when one is testing physics at its limits, like particles moving at the speed of light. But a proposed simulation involves “effective time travel,” according to the team’s recent paper in Physical Review Letters, thanks to a famously strange way that quantum particles can interact.

That interaction is called quantum entanglement , and it describes when the characteristics of two or more quantum particles are defined by each other. This means that knowing the properties of one entangled particle gives you information about the other, regardless of the distance between the two particles; their entanglement is on a quantum level, so a little thing like their physical distance has no bearing on the relationship. Space is big and time is relative, so a change to a quantum particle on Earth that’s entangled with a particle near a black hole 10 billion light-years away would mean changing the behavior of something in the distant past.

The recent research explores the possibility of closed-timelike curves, or CTCs—a hypothetical pathway back in time. The curve is a worldline—the arc of a particle in spacetime over the course of its existence—that runs backwards. Steven Hawking posited in his 1992 “Chronology protection conjecture” paper that the laws of physics don’t allow for closed timelike curves to exist—thus, that time travel is impossible. “Nevertheless,” the recent study authors wrote, “they can be simulated probabilistically by quantum-teleportation circuits.”

The team’s Gedankenexperiment goes like this: Physicists put photonic probes through a quantum interaction, yielding a certain measurable result. Based on that result, they can determine what input would have yielded an optimal result—hindsight is 20/20, just like when you can look over a graded exam. But because the result was yielded from a quantum operation, instead of being stuck with a less-than-optimal result, the researchers can tweak the values of the quantum probe via entanglement, producing a better result even though the operation already happened. Capiche?

The team demonstrated that one could “probabilistically improve one’s past choice,” explained study co-author Nicole Yunger Halpern, a physicist at the National Institute of Standards and Technology and the University of Maryland at College Park, in an email to Gizmodo, though she noted that the proposed time travel simulation has not yet taken place. 

In their study, the apparent time travel effect would occur one time in four—a failure rate of 75%. To address the high failure rate, the team suggests sending a large number of entangled photons, using a filter to ensure the photons with the corrected information got through while sifting out the outdated particles.

“The experiment that we describe seems impossible to solve with standard (not quantum) physics, which obeys the normal arrow of time,” said David Arvidsson-Shukur, a quantum physicist at the University of Cambridge and the study’s lead author, in an email to Gizmodo. “Thus, it appears as if quantum entanglement can generate instances which effectively look like time travel.”

The behavior of quantum particles—specifically, the ways in which those behaviors differ from macroscopic phenomena—are a useful means for physicists to probe the nature of our reality. Entanglement is one aspect of how quantum things operate by different laws.

Last year, another group of physicists claimed that they managed to create a quantum wormhole—basically, a portal through which quantum information could instantaneously travel. The year before, a team synchronized drums as wide as human hairs using entanglement. And the 2022 Nobel Prize in Physics went to three physicists for their interrogation of quantum entanglement, which is clearly an important subject to study if we are to understand how things work.

A simulation offers a means of probing time travel without worrying about whether it’s actually permitted by the rules of the universe.

“Whether closed timelike curves exist in reality, we don’t know. The laws of physics that we know of allow for the existence of CTCs, but those laws are incomplete; most glaringly, we don’t have a theory of quantum gravity,” said Yunger Halpern. “Regardless of whether true CTCs exist, though, one can use entanglement to simulate CTCs, as others showed before we wrote our paper.”

In 1992, just a couple weeks before Hawking’s paper was published, the physicist Kip Thorne presented a paper at the 13th International Conference on General Relativity and Gravitation. Thorne concluded that, “It may turn out that on macroscopic lengthscales chronology is not always protected, and even if chronology is protected macroscopically, quantum gravity may well give finite probability amplitudes for microscopic spacetime histories with CTCs.” In other words, whether time travel is possible or not is a quandary beyond the remit of classical physics. And since quantum gravity remains an elusive thing , the jury’s out on time travel.

But in a way, whether closed-timelike curves exist in reality or not isn’t that important, at least in the context of the new research. What’s important is that the researchers think their Gedankenexperiment provides a new way of interrogating quantum mechanics. It allows them to take advantage of the quantum realm’s apparent disregard for time’s continuity in order to achieve some fascinating results.

The headline and text of this article have been updated to clarify that the team describe a way that time travel can be simulated; they did not simulate time travel in this experiment.

More: Scientists Tried to Quantum Entangle a Tardigrade

The Debrief

Time Travel May be Possible Inside the Quantum Realm

Time travel may be possible after all, particularly in the quantum realm. And based on recently published research, this may include moving both backward and forward in time.

BACKGROUND: BUT ISN’T TIME TRAVEL IMPOSSIBLE?

In classical physics, the movement of time is more or less described as a movement from a more organized state to a less organized state. Physicists called this entropy. Such movement can be seen in everyday systems like the rotting of food or the growing of a tree, or the simple process of a meal cooking on the stove. 

Due to this seeming unidirectional aspect to the movement of time, often called time’s arrow, most physicists agree that traveling backwards in time would violate a number of known processes and properties of physics, likely making it all but impossible outside of science fiction .

By comparison, moving forward in time is relatively straightforward. Simply speed up to as close to the speed of light as possible, thereby taking advantage of the relativistic effects that will cause time on the outside world to travel significantly faster than it will for you. In short, if you travel close enough to the speed of light, you will age significantly slower than the world around you, meaning that for all intents and purposes, you will have traveled into the future.

Now, based on new research published in the journal Communications Physics , traveling to the past may be back on the proverbial time travel table.

ANALYSIS: TO TRAVEL BACKWARDS YOU FIRST MUST GO QUANTUM

According to a recent press release ; “a team of physicists from the Universities of Bristol, Vienna, the Balearic Islands and the Institute for Quantum Optics and Quantum Information (IQOQI-Vienna), has shown how quantum systems can simultaneously evolve along two opposite time arrows – both forward and backward in time.”

This unique ability is governed by the quantum principle of superposition, where a single particle of matter can exist in two different states at the same time. According to the researchers behind the latest study, this unique state of matter also allows for the travel of time in both directions, forward and backward.

“We can take the sequence of things we do in our morning routine as an example,” said the study’s lead-author Dr. Giulia Rubino from the University of Bristol’s Quantum Engineering Technology Labs (QET labs). “If we were shown our toothpaste moving from the toothbrush back into its tube, we would be in no doubt it was a re-winded recording of our day. However, if we squeezed the tube gently so only a small part of the toothpaste came out, it would not be so unlikely to observe it re-entering the tube, sucked in by the tube’s decompression.”

“Extending this principle to time’s arrows,” added Rubino, “it results that quantum systems evolving in one or the other temporal direction (the toothpaste coming out of or going back into the tube), can also find themselves evolving simultaneously along both temporal directions.” 

Basically, if this quantum sized hypothetical “toothpaste” can evolve along two different paths as allowed by superposition, then it is possible that one of those paths results in the toothpaste moving back into the tube, essentially going back in time to a previous, less entropic state. According to Rubino, it is precisely this process that his team’s research shows.

“In our work, we quantified the entropy produced by a system evolving in quantum superposition of processes with opposite time arrows,” explained Rubino. “We found this most often results in projecting the system onto a well-defined time’s direction, corresponding to the most likely process of the two.”

In short, most of the time things moved forward in time, just as researchers and classical physics would predict. However, sometimes the opposite happened.

“And yet,” he added, “when small amounts of entropy are involved (for instance, when there is so little toothpaste spilled that one could see it being reabsorbed into the tube), then one can physically observe the consequences of the system having evolved along the forward and backward temporal directions at the same time.”

In conclusion, based on the size and timing of the event, the quantum realm may possess its own equivalent of a five second rule , allowing the movement back in time to a less entropic state in certain circumstances. But what does this mean in the macro world?

time travel

OUTLOOK: SO FLUX CAPACITOR OR NO FLUX CAPACITOR?

Like many things that occur in the quantum realm, the findings of this latest research may be counter-intuitive. However, the researchers behind the study say it is a real, actual principle operating inside the quantum world that may have real, macro-world level impacts.

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“Although this idea seems rather nonsensical when applied to our day-to-day experience, at its most fundamental level, the laws of the universe are based on quantum-mechanical principles.”

This revelation also likely means that macro level systems that are affected by processes in the quantum realm, like the ability of birds to sense the Earth’s magnetic field using a quantum mechanical process , may be able to take advantage of these uniquely quantum effects. Apparently, this now includes time travel.

“Although time is often treated as a continuously increasing parameter, our study shows the laws governing its flow in quantum mechanical contexts are much more complex,” said Rubino. ”This may suggest that we need to rethink the way we represent this quantity in all those contexts where quantum laws play a crucial role.”

Okay, somebody call Doc Brown. Anyone seen the keys to my DeLorean?

Follow and connect with author Christopher Plain on Twitter: @plain_fiction  

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  • Published: 26 March 2024

Search for decoherence from quantum gravity with atmospheric neutrinos

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Neutrino oscillations at the highest energies and longest baselines can be used to study the structure of spacetime and test the fundamental principles of quantum mechanics. If the metric of spacetime has a quantum mechanical description, its fluctuations at the Planck scale are expected to introduce non-unitary effects that are inconsistent with the standard unitary time evolution of quantum mechanics. Neutrinos interacting with such fluctuations would lose their quantum coherence, deviating from the expected oscillatory flavour composition at long distances and high energies. Here we use atmospheric neutrinos detected by the IceCube South Pole Neutrino Observatory in the energy range of 0.5–10.0 TeV to search for coherence loss in neutrino propagation. We find no evidence of anomalous neutrino decoherence and determine limits on neutrino–quantum gravity interactions. The constraint on the effective decoherence strength parameter within an energy-independent decoherence model improves on previous limits by a factor of 30. For decoherence effects scaling as E 2 , our limits are advanced by more than six orders of magnitude beyond past measurements compared with the state of the art.

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Data availability

A list of selected event energies and zenith angles, a Monte Carlo simulation set and information on systematic uncertainty effects, along with other public IceCube data releases, are available at https://icecube.wisc.edu/science/data-releases/ .

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Acknowledgements

We acknowledge support from the following sources, grouped by country. United States: US National Science Foundation, Office of Polar Programs; US National Science Foundation, Physics Division; US National Science Foundation, EPSCoR; US National Science Foundation, Office of Advanced Cyberinfrastructure, Wisconsin Alumni Research Foundation, Center for High Throughput Computing (CHTC) at the University of Wisconsin–Madison; Open Science Grid (OSG), Partnership to Advance Throughput Computing (PATh), Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS), Frontera computing project at the Texas Advanced Computing Center; US Department of Energy, National Energy Research Scientific Computing Center, Particle Astrophysics Research Computing Center at the University of Maryland; Institute for Cyber-Enabled Research at Michigan State University; Astroparticle Physics Computational Facility at Marquette University; NVIDIA Corporation; and Google Cloud Platform. Belgium: Funds for Scientific Research (FRS-FNRS and FWO), FWO Odysseus and Big Science programmes and Belgian Federal Science Policy Office (Belspo). Germany: Bundesministerium für Bildung und Forschung (BMBF), Deutsche Forschungsgemeinschaft (DFG), Helmholtz Alliance for Astroparticle Physics (HAP), Initiative and Networking Fund of the Helmholtz Association, Deutsches Elektronen Synchrotron (DESY), and High Performance Computing cluster of the RWTH Aachen. Sweden: Swedish Research Council, Swedish Polar Research Secretariat, Swedish National Infrastructure for Computing (SNIC) and Knut and Alice Wallenberg Foundation. European Union: EGI Advanced Computing for Research. Australia: Australian Research Council. Canada: Natural Sciences and Engineering Research Council of Canada, Calcul Québec, Compute Ontario, Canada Foundation for Innovation, WestGrid and Digital Research Alliance of Canada. Denmark: Villum Fonden, Carlsberg Foundation and European Commission. New Zealand: Marsden Fund. Japan: Japan Society for Promotion of Science (JSPS) and Institute for Global Prominent Research (IGPR) of Chiba University. Korea: National Research Foundation of Korea (NRF). Switzerland: Swiss National Science Foundation (SNSF).

Author information

S. K. Agarwalla, S. Chattopadhyay, J. Krishnamoorthi & A. K. Upadhyay

Present address: Institute of Physics, Sachivalaya Marg, Sainik School Post, Bhubaneswar, India

J. Becker Tjus

Present address: Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden

Present address: Earthquake Research Institute, University of Tokyo, Bunkyo, Japan

Authors and Affiliations

Department of Physics, Loyola University Chicago, Chicago, IL, USA

Deutsches Elektronen-Synchrotron DESY, Zeuthen, Germany

M. Ackermann, S. Athanasiadou, S. Blot, J. Brostean-Kaiser, L. Fischer, T. Karg, M. Kowalski, A. Kumar, N. Lad, C. Lagunas Gualda, S. Mechbal, R. Naab, J. Necker, T. Pernice, S. Reusch, C. Spiering, A. Trettin & J. van Santen

Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand

Department of Physics and Wisconsin IceCube Particle Astrophysics Center, University of Wisconsin–Madison, Madison, WI, USA

S. K. Agarwalla, A. Balagopal V, M. Baricevic, V. Basu, J. Braun, D. Butterfield, S. Chattopadhyay, D. Chirkin, A. Desai, P. Desiati, J. C. Díaz-Vélez, H. Dujmovic, M. A. DuVernois, H. Erpenbeck, K. Fang, S. Griffin, F. Halzen, K. Hanson, S. Hori, K. Hoshina, R. Hussain, M. Jacquart, A. Karle, M. Kauer, J. L. Kelley, A. Khatee Zathul, J. Krishnamoorthi, J. P. Lazar, L. Lu, J. Madsen, Y. Makino, S. Mancina, W. Marie Sainte, K. Meagher, R. Morse, M. Moulai, M. Nakos, V. O’Dell, J. Osborn, J. Peterson, A. Pizzuto, M. Prado Rodriguez, Z. Rechav, B. Riedel, I. Safa, P. Savina, M. Silva, R. Snihur, J. Thwaites, D. Tosi, A. K. Upadhyay, J. Vandenbroucke, J. Veitch-Michaelis, C. Wendt, E. Yildizci, T. Yuan, P. Zilberman & M. Zimmerman

Université Libre de Bruxelles, Science Faculty, Brussels, Belgium

J. A. Aguilar, N. Chau, I. C. Mariş, F. Schlüter & S. Toscano

Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

M. Ahlers, K. M. Groth, D. J. Koskinen, T. Kozynets, J. V. Mead, A. Søgaard & T. Stuttard

Department of Physics, TU Dortmund University, Dortmund, Germany

J. M. Alameddine, D. Elsässer, P. Gutjahr, M. Hünnefeld, K. Hymon, L. Kardum, W. Rhode, T. Ruhe, J. Soedingrekso, J. Werthebach & L. Witthaus

Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, DE, USA

N. M. Amin, S. N. Axani, P. A. Evenson, J. G. Gonzalez, R. Koirala, A. Leszczyńska, A. Novikov, H. Pandya, E. N. Paudel, A. Rehman, F. G. Schröder, D. Seckel, T. Stanev, S. Tilav & S. Verpoest

Department of Physics, Marquette University, Milwaukee, WI, USA

K. Andeen & A. Vaidyanathan

Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany

G. Anton, A. Domi, A. Eimer, S. Fiedlschuster, T. Glüsenkamp, C. Haack, U. Katz, C. Kopper, M. Rongen, S. Schindler, J. Schneider, L. Schumacher & G. Wrede

Department of Physics and Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, MA, USA

C. Argüelles, J. Y. Book, K. Carloni, D. Delgado, A. Garcia, M. Jin, N. Kamp, J. P. Lazar, I. Martinez-Soler, I. Safa, B. Skrzypek, W. G. Thompson, A. Y. Wen & P. Zhelnin

Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA

Y. Ashida, M. Jeong & C. Rott

III. Physikalisches Institut, RWTH Aachen University, Aachen, Germany

L. Ausborm, C. Benning, J. Böttcher, L. Brusa, S. Deng, S. El Mentawi, P. Fürst, E. Ganster, O. Gries, C. Günther, L. Halve, M. Handt, J. Häußler, J. Hermannsgabner, L. Heuermann, O. Janik, S. Latseva, A. Noell, S. Philippen, A. Rifaie, J. Savelberg, M. Schaufel, L. Schlickmann, P. Soldin, M. Thiesmeyer, C. H. Wiebusch & A. Wolf

Physics Department, South Dakota School of Mines and Technology, Rapid City, SD, USA

X. Bai, L. Paul & M. Plum

Department of Physics and Astronomy, University of California, Irvine, CA, USA

S. W. Barwick

Department of Physics, University of California, Berkeley, CA, USA

R. Bay, S. R. Klein, Y. Lyu & S. Robertson

Department of Astronomy, Ohio State University, Columbus, OH, USA

J. J. Beatty, A. Connolly & W. Luszczak

Department of Physics and Center for Cosmology and Astro-Particle Physics, Ohio State University, Columbus, OH, USA

J. J. Beatty, A. Connolly, W. Luszczak, A. Medina & M. Stamatikos

Fakultät für Physik & Astronomie, Ruhr-Universität Bochum, Bochum, Germany

J. Becker Tjus, A. Franckowiak, J. Hellrung, E. Kun, M. Lincetto, L. Merten, P. Reichherzer & G. Sommani

Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden

J. Beise, O. Botner, A. Coleman, C. Glaser, T. Glüsenkamp, A. Hallgren, N. Heyer, E. O’Sullivan, C. Pérez de los Heros, A. Pontén & N. Valtonen-Mattila

Physik-department, Technische Universität München, Garching, Germany

C. Bellenghi, P. Eller, M. Ha Minh, M. Karl, T. Kontrimas, E. Manao, R. Orsoe, E. Resconi, L. Ruohan, C. Spannfellner, A. Terliuk & M. Wolf

Department of Physics and Astronomy, University of Rochester, Rochester, NY, USA

S. BenZvi & S. Griswold

Department of Physics, University of Maryland, College Park, MD, USA

D. Berley, E. Blaufuss, B. A. Clark, J. Evans, K. L. Fan, S. J. Gray, K. D. Hoffman, M. J. Larson, A. Olivas, R. Procter-Murphy, T. Schmidt, S. Sclafani, G. W. Sullivan & A. Vijai

Dipartimento di Fisica e Astronomia Galileo Galilei, Università Degli Studi di Padova, Padova, Italy

E. Bernardini, C. Boscolo Meneguolo & S. Mancina

Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA

D. Z. Besson, M. Seikh & R. Young

Karlsruhe Institute of Technology, Institute for Astroparticle Physics, Karlsruhe, Germany

F. Bontempo, R. Engel, A. Haungs, W. Hou, T. Huber, D. Kang, P. Koundal, T. Mukherjee, P. Sampathkumar, H. Schieler, F. G. Schröder, R. Turcotte, M. Venugopal, A. Weindl & M. Weyrauch

Institute of Physics, University of Mainz, Mainz, Germany

S. Böser, T. Ehrhardt, D. Kappesser, L. Köpke, E. Lohfink, Y. Popovych & J. Rack-Helleis

School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, Atlanta, GA, USA

B. Brinson, C. Chen, P. Dave, I. Taboada & C. F. Tung

Department of Physics, University of Adelaide, Adelaide, South Australia, Australia

R. T. Burley, E. G. Carnie-Bronca, G. C. Hill & E. J. Roberts

Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany

R. S. Busse, M. Dittmer, A. Kappes, C. J. Lozano Mariscal, M. Neumann, B. Schlüter, M. A. Unland Elorrieta & J. Vara

Department of Physics, Drexel University, Philadelphia, PA, USA

M. A. Campana, X. Kang, M. Kovacevich, N. Kurahashi, C. Love & R. Shah

Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA

Z. Chen, H. Hamdaoui, J. Kiryluk & Z. Zhang

Department of Physics, Sungkyunkwan University, Suwon, Republic of Korea

S. Choi, S. In, W. Kang, J. W. Lee, S. Rodan, G. Roellinghoff, C. Rott & C. Tönnis

Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA

G. H. Collin, J. M. Conrad, A. Diaz, J. Hardin, D. Vannerom & P. Weigel

Vrije Universiteit Brussel (VUB), Dienst ELEM, Brussels, Belgium

P. Coppin, P. Correa, C. De Clercq, K. D. de Vries, E. Magnus, Y. Merckx & N. van Eijndhoven

Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA, USA

D. F. Cowen, D. Fox, Y. Liu & Y. Wang

Department of Physics, Pennsylvania State University, University Park, PA, USA

D. F. Cowen, K. Leonard DeHolton, Y. Liu, Y. Wang & J. Weldert

Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL, USA

J. J. DeLaunay, A. Ghadimi, M. Marsee, M. Santander & D. R. Williams

Oskar Klein Centre and Department of Physics, Stockholm University, Stockholm, Sweden

K. Deoskar, C. Finley, A. Hidvegi, K. Hultqvist, M. Jansson & C. Walck

Centre for Cosmology, Particle Physics and Phenomenology—CP3, Université catholique de Louvain, Louvain-la-Neuve, Belgium

G. de Wasseige, K. Kruiswijk, M. Lamoureux, C. Raab & M. Vereecken

Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA

T. DeYoung, D. Grant, R. Halliday, A. A. Harnisch, A. Kochocki, E. Krupczak, K. B. M. Mahn, F. Mayhew, J. Micallef, H. Niederhausen, M. U. Nisa, S. C. Nowicki, B. Pries, D. Salazar-Gallegos, S. E. Sanchez Herrera, K. Tollefson, J. P. Twagirayezu, C. Weaver, N. Whitehorn & S. Yu

Department of Physics, University of Wuppertal, Wuppertal, Germany

E. Ellinger, K. Helbing, S. Hickford, F. Lauber, U. Naumann, A. Sandrock, N. Schmeisser & T. Stürwald

Karlsruhe Institute of Technology, Institute of Experimental Particle Physics, Karlsruhe, Germany

R. Engel, J. Saffer, S. Shefali & D. Soldin

Department of Physics and The International Center for Hadron Astrophysics, Chiba University, Chiba, Japan

K. Farrag, C. Hill, A. Ishihara, M. Meier, Y. Morii, R. Nagai, A. Obertacke Pollmann, A. Rosted, N. Shimizu & S. Yoshida

Department of Physics, Southern University, Baton Rouge, LA, USA

A. R. Fazely, J. Mitchell, S. Ter-Antonyan, K. Upshaw & X. W. Xu

Institute of Physics, Academia Sinica, Taipei, Taiwan

A. Fedynitch

Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany

N. Feigl, H. Kolanoski & M. Kowalski

Department of Astronomy, University of Wisconsin–Madison, Madison, WI, USA

J. Gallagher

Lawrence Berkeley National Laboratory, Berkeley, CA, USA

L. Gerhardt, S. R. Klein, Y. Lyu, G. T. Przybylski, S. Robertson & T. Stezelberger

Department of Physics, Chung-Ang University, Seoul, Korea

Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, Ontario, Canada

P. Hatch & N. Park

CTSPS, Clark-Atlanta University, Atlanta, GA, USA

G. S. Japaridze

Department of Physics, University of Texas at Arlington, Arlington, TX, USA

B. J. P. Jones, A. Negi, G. K. Parker & B. Smithers

Department of Physics, University of Alberta, Edmonton, Alberta, Canada

A. Katil, M. Liubarska, T. McElroy, R. W. Moore, S. Sarkar & J. P. Yanez

Department of Physics & Astronomy, University of Nevada, Las Vegas, NV, USA

A. Kheirandish

Nevada Center for Astrophysics, University of Nevada, Las Vegas, NV, USA

Département de physique nucléaire et corpusculaire, Université de Genève, Genève, Switzerland

F. Lucarelli & T. Montaruli

Columbia Astrophysics and Nevis Laboratories, Columbia University, New York, NY, USA

S. Marka, Z. Marka & D. Veske

Department of Physics, Yale University, New Haven, CT, USA

R. Maruyama

Department of Physics, Mercer University, Macon, GA, USA

Department of Physics and Astronomy, University of Gent, Gent, Belgium

B. Oeyen & D. Ryckbosch

Department of Physics and Astronomy, University of Alaska Anchorage, Anchorage, AK, USA

Department of Physics, University of Oxford, Oxford, UK

Department of Physics, University of Wisconsin, River Falls, WI, USA

S. Seunarine & G. M. Spiczak

  • , M. Ackermann
  • , S. K. Agarwalla
  • , J. A. Aguilar
  • , M. Ahlers
  • , J. M. Alameddine
  • , N. M. Amin
  • , K. Andeen
  • , C. Argüelles
  • , Y. Ashida
  • , S. Athanasiadou
  • , L. Ausborm
  • , S. N. Axani
  • , A. Balagopal V
  • , M. Baricevic
  • , S. W. Barwick
  • , J. J. Beatty
  • , J. Becker Tjus
  • , C. Bellenghi
  • , C. Benning
  • , S. BenZvi
  • , D. Berley
  • , E. Bernardini
  • , D. Z. Besson
  • , E. Blaufuss
  • , F. Bontempo
  • , J. Y. Book
  • , C. Boscolo Meneguolo
  • , O. Botner
  • , J. Böttcher
  • , B. Brinson
  • , J. Brostean-Kaiser
  • , R. T. Burley
  • , R. S. Busse
  • , D. Butterfield
  • , M. A. Campana
  • , K. Carloni
  • , E. G. Carnie-Bronca
  • , S. Chattopadhyay
  • , D. Chirkin
  • , B. A. Clark
  • , A. Coleman
  • , G. H. Collin
  • , A. Connolly
  • , J. M. Conrad
  • , P. Coppin
  • , P. Correa
  • , D. F. Cowen
  • , C. De Clercq
  • , J. J. DeLaunay
  • , D. Delgado
  • , K. Deoskar
  • , P. Desiati
  • , K. D. de Vries
  • , G. de Wasseige
  • , T. DeYoung
  • , J. C. Díaz-Vélez
  • , M. Dittmer
  • , H. Dujmovic
  • , M. A. DuVernois
  • , T. Ehrhardt
  • , E. Ellinger
  • , S. El Mentawi
  • , D. Elsässer
  • , H. Erpenbeck
  • , P. A. Evenson
  • , K. L. Fan
  • , K. Farrag
  • , A. R. Fazely
  • , A. Fedynitch
  • , S. Fiedlschuster
  • , C. Finley
  • , L. Fischer
  • , A. Franckowiak
  • , J. Gallagher
  • , E. Ganster
  • , A. Garcia
  • , L. Gerhardt
  • , A. Ghadimi
  • , C. Glaser
  • , T. Glüsenkamp
  • , J. G. Gonzalez
  • , S. J. Gray
  • , S. Griffin
  • , S. Griswold
  • , K. M. Groth
  • , C. Günther
  • , P. Gutjahr
  • , A. Hallgren
  • , R. Halliday
  • , F. Halzen
  • , H. Hamdaoui
  • , M. Ha Minh
  • , K. Hanson
  • , J. Hardin
  • , A. A. Harnisch
  • , A. Haungs
  • , J. Häußler
  • , K. Helbing
  • , J. Hellrung
  • , J. Hermannsgabner
  • , L. Heuermann
  • , S. Hickford
  • , A. Hidvegi
  • , G. C. Hill
  • , K. D. Hoffman
  • , K. Hoshina
  • , K. Hultqvist
  • , M. Hünnefeld
  • , R. Hussain
  • , A. Ishihara
  • , M. Jacquart
  • , M. Jansson
  • , G. S. Japaridze
  • , B. J. P. Jones
  • , A. Kappes
  • , D. Kappesser
  • , L. Kardum
  • , J. L. Kelley
  • , A. Khatee Zathul
  • , A. Kheirandish
  • , J. Kiryluk
  • , S. R. Klein
  • , A. Kochocki
  • , R. Koirala
  • , H. Kolanoski
  • , T. Kontrimas
  • , C. Kopper
  • , D. J. Koskinen
  • , P. Koundal
  • , M. Kovacevich
  • , M. Kowalski
  • , T. Kozynets
  • , J. Krishnamoorthi
  • , K. Kruiswijk
  • , E. Krupczak
  • , N. Kurahashi
  • , C. Lagunas Gualda
  • , M. Lamoureux
  • , M. J. Larson
  • , S. Latseva
  • , F. Lauber
  • , J. P. Lazar
  • , J. W. Lee
  • , K. Leonard DeHolton
  • , A. Leszczyńska
  • , M. Lincetto
  • , M. Liubarska
  • , E. Lohfink
  • , C. J. Lozano Mariscal
  • , F. Lucarelli
  • , W. Luszczak
  • , J. Madsen
  • , E. Magnus
  • , K. B. M. Mahn
  • , Y. Makino
  • , S. Mancina
  • , W. Marie Sainte
  • , I. C. Mariş
  • , M. Marsee
  • , I. Martinez-Soler
  • , R. Maruyama
  • , F. Mayhew
  • , T. McElroy
  • , F. McNally
  • , J. V. Mead
  • , K. Meagher
  • , S. Mechbal
  • , A. Medina
  • , Y. Merckx
  • , L. Merten
  • , J. Micallef
  • , J. Mitchell
  • , T. Montaruli
  • , R. W. Moore
  • , M. Moulai
  • , T. Mukherjee
  • , U. Naumann
  • , J. Necker
  • , M. Neumann
  • , H. Niederhausen
  • , M. U. Nisa
  • , A. Novikov
  • , S. C. Nowicki
  • , A. Obertacke Pollmann
  • , V. O’Dell
  • , A. Olivas
  • , J. Osborn
  • , E. O’Sullivan
  • , H. Pandya
  • , G. K. Parker
  • , E. N. Paudel
  • , C. Pérez de los Heros
  • , T. Pernice
  • , J. Peterson
  • , S. Philippen
  • , A. Pizzuto
  • , A. Pontén
  • , Y. Popovych
  • , M. Prado Rodriguez
  • , R. Procter-Murphy
  • , G. T. Przybylski
  • , J. Rack-Helleis
  • , K. Rawlins
  • , Z. Rechav
  • , A. Rehman
  • , P. Reichherzer
  • , E. Resconi
  • , S. Reusch
  • , B. Riedel
  • , A. Rifaie
  • , E. J. Roberts
  • , S. Robertson
  • , G. Roellinghoff
  • , M. Rongen
  • , A. Rosted
  • , L. Ruohan
  • , D. Ryckbosch
  • , J. Saffer
  • , D. Salazar-Gallegos
  • , P. Sampathkumar
  • , S. E. Sanchez Herrera
  • , A. Sandrock
  • , M. Santander
  • , S. Sarkar
  • , J. Savelberg
  • , P. Savina
  • , M. Schaufel
  • , H. Schieler
  • , S. Schindler
  • , L. Schlickmann
  • , B. Schlüter
  • , F. Schlüter
  • , N. Schmeisser
  • , T. Schmidt
  • , J. Schneider
  • , F. G. Schröder
  • , L. Schumacher
  • , S. Sclafani
  • , D. Seckel
  • , S. Seunarine
  • , S. Shefali
  • , N. Shimizu
  • , B. Skrzypek
  • , B. Smithers
  • , R. Snihur
  • , J. Soedingrekso
  • , A. Søgaard
  • , D. Soldin
  • , P. Soldin
  • , G. Sommani
  • , C. Spannfellner
  • , G. M. Spiczak
  • , C. Spiering
  • , M. Stamatikos
  • , T. Stanev
  • , T. Stezelberger
  • , T. Stürwald
  • , T. Stuttard
  • , G. W. Sullivan
  • , I. Taboada
  • , S. Ter-Antonyan
  • , A. Terliuk
  • , M. Thiesmeyer
  • , W. G. Thompson
  • , J. Thwaites
  • , K. Tollefson
  • , C. Tönnis
  • , S. Toscano
  • , A. Trettin
  • , C. F. Tung
  • , R. Turcotte
  • , J. P. Twagirayezu
  • , M. A. Unland Elorrieta
  • , A. K. Upadhyay
  • , K. Upshaw
  • , A. Vaidyanathan
  • , N. Valtonen-Mattila
  • , J. Vandenbroucke
  • , N. van Eijndhoven
  • , D. Vannerom
  • , J. van Santen
  • , J. Veitch-Michaelis
  • , M. Venugopal
  • , M. Vereecken
  • , S. Verpoest
  • , C. Weaver
  • , P. Weigel
  • , A. Weindl
  • , J. Weldert
  • , A. Y. Wen
  • , J. Werthebach
  • , M. Weyrauch
  • , N. Whitehorn
  • , C. H. Wiebusch
  • , D. R. Williams
  • , L. Witthaus
  • , J. P. Yanez
  • , E. Yildizci
  • , S. Yoshida
  • , P. Zhelnin
  • , P. Zilberman
  •  & M. Zimmerman

Contributions

The IceCube Collaboration acknowledges significant contributions to this manuscript by the IceCube groups from the University of Texas at Arlington, the Niels Bohr Institute and Harvard University. The IceCube Collaboration designed, constructed and now operates the IceCube Neutrino Observatory. Data processing and calibration, Monte Carlo simulations of the detector and of the theoretical models, and data analyses were performed by a large number of collaboration members, who also discussed and approved the scientific results presented here.

Ethics declarations

Competing interests.

The authors declare no competing interests.

Peer review

Peer review information.

Nature Physics thanks Ricardo A. Gomes, Atsuko Ichikawa and Giuseppe Gaetano Luciano for their contribution to the peer review of this work.

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Extended data

Extended data fig. 1 systematic pulls for phase perturbation (top) and state selection (bottom)..

The pull is defined as the value of the nuisance parameter minus its central value, divided by the Gaussian prior width. Each of the nuisance parameters (outlined in the Methods section) is represented by four color bars, one corresponding to the best fit point under each power law n . Since the best fit point is very close to no decoherence in all power law models, the distributionsof pulls are similar in all cases.

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We've glimpsed something that behaves like a particle of gravity

Gravitons, the particles thought to carry gravity, have never been seen in space – but something very similar has been detected in a semiconductor

By Karmela Padavic-Callaghan

27 March 2024

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Have we spotted hints of gravitons?

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Physicists have been searching for gravitons, the hypothetical particles thought to carry gravity, for decades. These have never been detected in space, but graviton-like particles have now been seen in a semiconductor. Using these to understand gravitons’ behaviour could help unite the general theory of relativity and quantum mechanics, which have long been at odds.

“This is a needle in a haystack [finding]. And the paper that started this whole thing is from way back in 1993,” says Loren Pfeiffer at Princeton University. He wrote that paper with several colleagues including Aron Pinczuk , who passed away in 2022 before they could find hints of the elusive particles.

Rethinking reality: Is the entire universe a single quantum object?

In the face of new evidence, physicists are starting to view the cosmos not as made up of disparate layers, but as a quantum whole linked by entanglement

Pinczuk’s students and collaborators, including Pfeiffer, have now completed the experiment the two began discussing 30 years ago. They focused on electrons within a flat piece of the semiconductor gallium arsenide, which they placed in a powerful refrigerator and exposed to a strong magnetic field. Under these conditions, quantum effects make the electrons behave strangely – they strongly interact with each other and form an unusual incompressible liquid.

This liquid is not calm but features collective motions where all the electrons move in concert, which can give rise to particle-like excitations. To examine those excitations, the team shined a carefully tuned laser on the semiconductor and analysed the light that scattered off it.

This revealed that the excitation had a kind of quantum spin that has only ever been theorised to exist in gravitons. Though this isn’t a graviton per se, it is the closest thing we have seen.

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Ziyu Liu at Columbia University in New York who worked on the experiment says he and his colleagues knew that graviton-like excitations could exist in their semiconductor, but it took years to make the experiment precise enough to detect them. “From the theoretical side, the story was kind of complete, but in experiments, we were really not sure,” he says.

The experiment isn’t a true analogue to space-time – electrons are confined to a flat, two-dimensional space and move more slowly than objects governed by the theory of relativity.

But it is “extremely important” and bridges different branches of physics, like the physics of materials and theories of gravity, in a previously underappreciated way, says Kun Yang at Florida State University, who was not involved in the work.

The physicist searching for quantum gravity in gravitational rainbows

Claudia de Rham thinks that gravitons, hypothetical particles thought to carry gravity, have mass. If she’s right, we can expect to see “rainbows” in ripples in space-time

However, Zlatko Papic at the University of Leeds in the UK cautions against equating the new finding with detection of gravitons in space. He says the two are sufficiently equivalent for electron systems like those in the new experiment to become testing grounds for some theories of quantum gravity, but not for every single quantum phenomenon that happens to space-time at cosmic scales .

Connections between this particle-like excitation and theoretical gravitons also raise new ideas about exotic electron states, says team member Lingjie Du at Nanjing University in China.

Journal reference:

Nature DOI: 10.1038/s41586-024-07201-w

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Supplement to Quantum Field Theory

The history of qft, the early development, the emergence of infinities, the taming of infinities, the standard model of particle physics.

The historical development of QFT is very instructive until the present day. Its first achievement, namely the quantization of the electromagnetic field is “still the paradigmatic example of a successful quantum field theory” (Weinberg 1995). Ordinary QM cannot give an account of photons which constitute the prime case of relativistic ‘particles’. Since photons have the rest mass zero, and correspondingly travel in the vacuum at the velocity, naturally, of light \(c\) it is ruled out that a non-relativistic theory such as ordinary QM could give even an approximate description. Photons are implicitly contained in the emission and absorption processes which have to be postulated, for instance, when one of an atom’s electrons makes a transition from a higher to a lower energy level or vice versa. However, only the formalism of QFT contains an explicit description of photons. In fact most topics in the early development of quantum theory (1900–1927) were related to the interaction of radiation and matter and should be treated by quantum field theoretical methods. However, the approach to quantum mechanics formulated by Dirac, Heisenberg and Schrödinger (1926/27) started from atomic spectra and did not rely very much on problems of radiation.

As soon as the conceptual framework of quantum mechanics was developed, a small group of theoreticians immediately tried to extend the methods to electromagnetic fields. A good example is the famous three-man paper by M. Born, W. Heisenberg, and P. Jordan (1926). Especially P. Jordan was acquainted with the literature on light quanta and made important contributions to QFT. The basic analogy was that in QFT field quantities, i.e., the electric and magnetic field, should be represented by matrices in the same way as in QM position and momentum are represented by matrices. The ideas of QM were extended to systems having an infinite number of degrees of freedom.

The inception of QFT is usually dated 1927 with Dirac’s famous paper on “The quantum theory of the emission and absorption of radiation” (Dirac 1927). Here Dirac coined the name quantum electrodynamics (QED) which is the part of QFT that has been developed first. Dirac supplied a systematic procedure for transferring the characteristic quantum phenomenon of discreteness of physical quantities from the quantum mechanical treatment of particles to a corresponding treatment of fields. Employing the quantum mechanical theory of the harmonic oscillator, Dirac gave a theoretical description of how photons appear in the quantization of the electromagnetic radiation field. Later, Dirac’s procedure became a model for the quantization of other fields as well. During the following three years the first approaches to QFT were further developed. P. Jordan introduced creation operators for fields obeying Fermi statistics. For an elementary discussion of quantum statistics (Fermi and Bose), see the entry on quantum theory: identity and individuality .

So the methods of QFT could be applied to equations resulting from the quantum mechanical (field like) treatment of particles like the electron (e.g., Dirac equation). Schweber points out (Schweber 1994, p. 28) that the idea and procedure of that “second quantization” goes back to Jordan, in a number of papers from 1927 (see references in Schweber 1994, pp. 695f), while the expression itself was coined by Dirac. Some difficult problems concerning commutation relations, statistics and Lorentz invariance could be solved. The first comprehensive account of a general theory of quantum fields, in particular the method of canonical quantization, was presented in Heisenberg & Pauli 1929. Whereas Jordan’s second quantization procedure applies to the coefficients of the normal modes of the field, Heisenberg & Pauli 1929 started with the fields themselves and subjected them to the canonical procedure. Heisenberg and Pauli thus established the basic structure of QFT which can be found in any introduction to QFT up to the present day. Fermi and Dirac, Fock and Podolski presented different formulations which played a heuristic role in the following years.

Quantum electrodynamics, the historical as well as systematical entrée to QFT, rests on two pillars (see, e.g., the short and lucid “Historical Introduction” of Scharf’s (1995) original book). The first pillar results from the quantization of the electromagmetic field, i.e., it is about photons as the quantized excitations or ‘quanta’ of the electromagnetic field. This procedure will be described in some more detail in the section on the particle interpretation. As Weinberg points out the “photon is the only particle that was known as a field before it was detected as a particle” so that it is natural that QED began with the analysis of the radiation field (Weinberg 1995, p. 15). The second pillar of QED consists in the relativistic theory of the electron, with the Dirac equation in its centre.

Quantum field theory started with a theoretical framework that was built in analogy to quantum mechanics. Although there was no unique and fully developed theory, quantum field theoretical tools could be applied to concrete processes. Examples are the scattering of radiation by free electrons (“Compton scattering”), the collision between relativistic electrons or the production of electron-positron pairs by photons. Calculations to the first order of approximation were quite successful, but most people working in the field thought that QFT still had to undergo a major change. On the one side some calculations of effects for cosmic rays clearly differed from measurements. On the other side and, from a theoretical point of view more threatening, calculations of higher orders of the perturbation series led to infinite results. The self-energy of the electron as well as vacuum fluctuations of the electromagnetic field seemed to be infinite. The perturbation expansions did not converge to a finite sum and even most individual terms were divergent.

The various forms of infinities suggested that the divergences were more than failures of specific calculations. Many physicists tried to avoid the divergences by formal tricks (truncating the integrals at some value of momentum, or even ignoring infinite terms) but such rules were not reliable, violated the requirements of relativity and were not considered as satisfactory. Others came up with first ideas of coping with infinities by a redefinition of the parameters of the theory and using a measured finite value (for example of the charge of the electron) instead of the infinite ‘bare’ value (“renormalization”).

From the point of view of philosophy of science it is remarkable that these divergences did not give enough reason to discard the theory. The years from 1930 to the beginning of World War II were characterized by a variety of attitudes towards QFT. Some physicists tried to circumvent the infinities by more-or-less arbitrary prescriptions, others worked on transformations and improvements of the theoretical framework. Most of the theoreticians believed that QED would break down at high energies. There was also a considerable number of proposals in favour of alternative approaches. These proposals included changes in the basic concepts (e.g., negative probabilities), interactions at a distance instead of a field theoretical approach, and a methodological change to phenomenological methods that focusses on relations between observable quantities without an analysis of the microphysical details of the interaction (the so-called S-matrix theory where the basic elements are amplitudes for various scattering processes).

Despite the feeling that QFT was imperfect and lacking rigour, its methods were extended to new areas of applications. In 1933 Fermi’s theory of the beta decay started with conceptions describing the emission and absorption of photons, transferred them to beta radiation and analyzed the creation and annihilation of electrons and neutrinos (weak interaction). Further applications of QFT outside of quantum electrodynamics succeeded in nuclear physics (strong interaction). In 1934 a new type of fields (scalar fields), described by the Klein-Gordon equation, could be quantized (another example of “second quantization”). This new theory for matter fields could be applied a decade later when new particles, pions, were detected.

After the end of World War II more reliable and effective methods for dealing with infinities in QFT were developed, namely coherent and systematic rules for performing relativistic field theoretical calculations, and a general renormalization theory. On three famous conferences between 1947 and 1949 developments in theoretical physics were confronted with relevant new experimental results. In the late forties there were two different ways to address the problem of divergences. One of these was discovered by Feynman, the other one (based on an operator formalism) by Schwinger and independently by Tomonaga. In 1949 Dyson showed that the two approaches are in fact equivalent. Thus, Freeman Dyson, Richard P. Feynman, Julian Schwinger and Sin-itiro Tomonaga became the inventors of renormalization theory. The most spectacular experimental successes of renormalization theory were the calculations of the anomalous magnetic moment of electron and the Lamb shift in the spectrum of hydrogen. These successes were so outstanding because the theoretical results were in better agreement with high precision experiments than anything in physics before. Nevertheless, mathematical problems lingered on and prompted a search for rigorous formulations (to be discussed in the main article).

The basic idea of renormalization is to avoid divergences that appear in physical predictions by shifting them into a part of the theory where they do not influence empirical propositions. Dyson could show that a rescaling of charge and mass (‘renormalization’) is sufficient to remove all divergences in QED to all orders of perturbation theory. In general, a QFT is called renormalizable, if all infinities can be absorbed into a redefinition of a finite number of coupling constants and masses. A consequence is that the physical charge and mass of the electron must be measured and cannot be computed from first principles. Perturbation theory gives well defined predictions only in renormalizable quantum field theories, and luckily QED, the first fully developed QFT, belonged to this class of renormalizable theories. There are various technical procedures to renormalize a theory. One way is to cut off the integrals in the calculations at a certain value \(\Lambda\) of the momentum which is large but finite. This cut-off procedure is successful if, after taking the limit \(\Lambda \rightarrow \infty\), the resulting quantities are independent of \(\Lambda\) (Part II of Peskin & Schroeder 1995 gives an extensive description of renormalization).

Feynman’s formulation of QED is of special interest from a philosophical point of view. His so-called space-time approach is visualized by the famous Feynman diagrams that look like depicting paths of particles. Feynman’s method of calculating scattering amplitudes is based on the functional integral formulation of field theory (for an introduction to the theory and practice of Feynman diagrams see, e.g., chapter 4 in Peskin & Schroeder 1995). A set of graphical rules can be derived so that the probability of a specific scattering process can be calculated by drawing a diagram of that process and then using the diagram to write down the mathematical expressions for calculating its amplitude. The diagrams provide an effective way to organize and visualize the various terms in the perturbation series, and they seem to display the flow of electrons and photons during the scattering process. External lines in the diagrams represent incoming and outgoing particles, internal lines are connected with ‘virtual particles’ and vertices with interactions. Each of these graphical elements is associated with mathematical expressions that contribute to the amplitude of the respective process. The diagrams are part of Feynman’s very efficient and elegant algorithm for computing the probability of scattering processes. The idea of particles travelling from one point to another was heuristically useful in constructing the theory, and moreover, this intuition is useful for concrete calculations. Nevertheless, an analysis of the theoretical justification of the space-time approach shows that its success does not imply that particle paths have to be taken seriously. General arguments against a particle interpretation of QFT clearly exclude that the diagrams represent paths of particles in the interaction area. Feynman himself was not particularly interested in ontological questions.

In the beginning of the 1950s QED had become a reliable theory which no longer counted as preliminary. It took two decades from writing down the first equations until QFT could be applied to interesting physical problems in a systematic way. The new developments made it possible to apply QFT to new particles and new interactions. In the following decades QFT was extended to describe not only the electromagnetic force but also weak and strong interaction so that new Lagrangians had to be found which contain new classes of ‘particles’ or quantum fields. The research aimed at a more comprehensive theory of matter and in the end at a unified theory of all interactions. New theoretical concepts had to be introduced, mainly connected with non-Abelian gauge theories and spontaneous symmetry breaking. See also the entry on symmetry and symmetry breaking . Today there are trustworthy theories of the strong, weak, and electromagnetic interactions of elementary particles which have a similar structure as QED. A combined theory associated with the gauge group \(\text{SU}(3) \otimes \text{SU}(2) \otimes \text{U}(1)\) is considered as ‘the standard model’ of elementary particle physics which was achieved by Glashow, Weinberg and Salam in 1962. According to the standard model there are, one the one side, six types of leptons (e.g. the electron and its neutrino) and six types of quarks, where the members of both group are all fermions with spin 1/2. On the other side, there are spin 1 particles (thus bosons) that mediate the interaction between elementary particles and the fundamental forces, namely the photon for electromagnetic interaction, two \(W\) and one \(Z\) boson for weak interaction, and the gluon for strong interaction. Altogether there is good agreement with experimental data, for example the masses of \(W^+\) and \(W^-\) bosons (detected in 1983) confirmed the theoretical prediction within one per cent deviation.

Further Reading . The first chapter in Weinberg 1995 is a very good short description of the earlier history of QFT. Detailed accounts of the historical development of QFT can be found, e.g., in Darrigol 1986, Schweber 1994 and Cao 1997a. Various historical and conceptual studies of the standard model are gathered in Hoddeson et al . 1997 and of renormalization theory in Brown 1993.

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Scientists on the hunt for evidence of quantum gravity's existence at the South Pole

by University of Copenhagen

south pole

Several thousand sensors distributed over a square kilometer near the South Pole are tasked with answering one of the large outstanding questions in physics: does quantum gravity exist? The sensors monitor neutrinos—particles with no electrical charge and almost without mass—arriving at the Earth from outer space. A team from the Niels Bohr Institute (NBI), University of Copenhagen, has contributed to developing the method that exploits neutrino data to reveal if quantum gravity exists.

"If as we believe, quantum gravity does indeed exist, this will contribute to unite the current two worlds in physics. Today, classical physics describes the phenomena in our normal surroundings such as gravity, while the atomic world can only be described using quantum mechanics .

"The unification of quantum theory and gravitation remains one of the most outstanding challenges in fundamental physics. It would be very satisfying if we could contribute to that end," says Tom Stuttard, Assistant Professor at NBI.

Stuttard is co-author of a article published by the journal Nature Physics . The article presents results from a large study by the NBI team and American colleagues. More than 300,000 neutrinos have been studied.

However, these are not neutrinos of the most interesting type originating from sources in deep space. The neutrinos in this study were created in Earth's atmosphere, as high-energy particles from space collided with nitrogen or other molecules.

"Looking at neutrinos originating from the Earth's atmosphere has the practical advantage that they are by far more common than their siblings from outer space. We needed data from many neutrinos to validate our methodology. This has been accomplished now. Thus, we are ready to enter the next phase in which we will study neutrinos from deep space," says Stuttard.

Traveling undisturbed through Earth

The IceCube Neutrino Observatory is situated next to the Amundsen-Scott South Pole Station in Antarctica. In contrast to most other astronomy and astrophysics facilities, IceCube works the best for observing space at the opposite side of the Earth, meaning the Northern hemisphere. This is because while the neutrino is perfectly capable of penetrating our planet—and even its hot, dense core—other particles will be stopped, and the signal is thus much cleaner for neutrinos coming from the Northern hemisphere.

The IceCube facility is operated by the University of Wisconsin-Madison, U.S. More than 300 scientists from countries around the world were engaged in the IceCube collaboration. University of Copenhagen is one of more than 50 universities with an IceCube center for neutrino studies.

Since the neutrino has no electrical charge and is nearly massless, it is undisturbed by electromagnetic and strong nuclear forces, allowing it to travel billions of lightyears through the universe in its original state.

The key question is whether the properties of the neutrino are in fact completely unchanged as it travels over large distances or if tiny changes are notable after all.

"If the neutrino undergoes the subtle changes that we suspect, this would be the first strong evidence of quantum gravity," says Stuttard.

The neutrino comes in three flavors

To understand which changes in neutrino properties the team is looking for, some background information is called for. While we refer to it as a particle, what we observe as a neutrino is really three particles produced together, known in quantum mechanics as superposition.

The neutrino can have three fundamental configurations—flavors as they are termed by the physicists—which are electron, muon, and tau. Which of these configurations we observe changes as the neutrino travels, a truly strange phenomenon known as neutrino oscillations. This quantum behavior is maintained over thousands of kilometers or more, which is referred to as quantum coherence.

"In most experiments, the coherence is soon broken. But this is not believed to be caused by quantum gravity. It is just very difficult to create perfect conditions in a lab. You want perfect vacuum, but somehow a few molecules manage to sneak in etc.

"In contrast, neutrinos are special in that they are simply not affected by matter around them, so we know that if coherence is broken it will not be due to shortcomings in the man-made experimental setup," Stuttard explains.

Many colleagues were skeptical

Asked whether the results of the study published in Nature Physics were as expected, the researcher replies, "We find ourselves in a rare category of science projects, namely experiments for which no established theoretical framework exists. Thus, we just did not know what to expect. However, we knew that we could search for some of the general properties we might expect a quantum theory of gravity to have."

"While we did have hopes of seeing changes related to quantum gravity, the fact that we didn't see them does not exclude at all that they are real. When an atmospheric neutrino is detected at the Antarctic facility, it will typically have traveled through the Earth. Meaning approximately 12,700 km—a very short distance compared to neutrinos originating in the distant universe. Apparently, a much longer distance is needed for quantum gravity to make an impact, if it exists," says Stuttard, noting that the top goal of the study was to establish the methodology.

"For years, many physicists doubted whether experiments could ever hope to test quantum gravity . Our analysis shows that it is indeed possible, and with future measurements with astrophysical neutrinos , as well as more precise detectors being built in the coming decade, we hope to finally answer this fundamental question."

Journal information: Nature Physics

Provided by University of Copenhagen

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    What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true. For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going ...

  15. Simulations of 'backwards time travel' can improve scientific

    Researchers at the University of Cambridge have shown that by manipulating entanglement - a feature of quantum theory that causes particles to be intrinsically linked - they can simulate what could happen if one could travel backwards in time. So that gamblers, investors and quantum experimentalists could, in some cases, retroactively ...

  16. Physicists Say Time Travel Can Be Simulated Using Quantum ...

    Physicists have described using quantum entanglement to simulate a closed timelike curve—in layman's terms, time travel. Before we proceed, I'll stress that no quantum particles went back in ...

  17. The quantum physics of time travel

    The present paper defines and analyzes a new model of time travel paradoxes, which consists of a traversable Morris-Thorne wormhole time machine in 3+1 spacetime dimensions, and proves that this model inevitably leads to paradoxes which cannot be resolved using Novikov's conjecture, but can be resolved use multiple histories. 2.

  18. Time Travel May be Possible Inside the Quantum Realm

    In short, if you travel close enough to the speed of light, you will age significantly slower than the world around you, meaning that for all intents and purposes, you will have traveled into the future. Now, based on new research published in the journal Communications Physics, traveling to the past may be back on the proverbial time travel table.

  19. Understanding Time Travel and Quantum Physics for Anyone: A ...

    Artistic Rendition of a black hole. [credit: Ashley Mackenzie for Quanta Magazine] T ime travel and quantum physics are two fascinating topics that have captured the imagination of people for years. These concepts have been used in various movies, TV shows, and books to tell exciting stories about characters traveling through time, exploring parallel universes, and experiencing strange phenomena.

  20. Rewinding Reality: Cambridge Uses Time-Travel ...

    By connecting their new theory to quantum metrology, which uses quantum theory to make highly sensitive measurements, the Cambridge team has shown that entanglement can solve problems that otherwise seem impossible. The study was published on October 12 in the journal Physical Review Letters. "Imagine that you want to send a gift to someone ...

  21. Time Twisted in Quantum Physics: How the Future Might ...

    The 2022 Nobel Prize in physics highlighted the challenges quantum experiments pose to "local realism.". However, a growing body of experts propose "retrocausality" as a solution, suggesting that present actions can influence past events, thus preserving both locality and realism. This concept offers a novel approach to understanding ...

  22. Time travel

    The first page of The Time Machine published by Heinemann. Time travel is the hypothetical activity of traveling into the past or future.Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction. In fiction, time travel is typically achieved through the use of a hypothetical device known as a time machine.The idea of a time machine was popularized by H ...

  23. Search for decoherence from quantum gravity with atmospheric ...

    The construction of a consistent and predictive quantum theory of gravity is an outstanding challenge in fundamental physics. A central experimental and theoretical question is whether the metric ...

  24. Scientists on the hunt for evidence of quantum gravity's existence at

    An Antarctic large-scale experiment is striving to find out if gravity also exists at the quantum level. An extraordinary particle able to travel undisturbed through space seems to hold the answer.

  25. We've glimpsed something that behaves like a particle of gravity

    Using these to understand gravitons' behaviour could help unite the general theory of relativity and quantum mechanics, which have long been at odds. "This is a needle in a haystack [finding].

  26. Quantum Field Theory > The History of QFT (Stanford Encyclopedia of

    For an elementary discussion of quantum statistics (Fermi and Bose), see the entry on quantum theory: identity and individuality. So the methods of QFT could be applied to equations resulting from the quantum mechanical (field like) treatment of particles like the electron (e.g., Dirac equation). Schweber points out (Schweber 1994, p.

  27. Scientists on the hunt for evidence of quantum gravity's existence at

    "The unification of quantum theory and gravitation remains one of the most outstanding challenges in fundamental physics. It would be very satisfying if we could contribute to that end," says Tom ...

  28. Applicability of mean-field theory for time-dependent open quantum

    Understanding quantum many-body systems with long-range or infinite-range interactions is of relevance across a broad set of physical disciplines, including quantum optics, nuclear magnetic resonance and nuclear physics. From a theoretical viewpoint, these systems are appealing since they can be efficiently studied with numerics, and in the thermodynamic limit are expected to be governed by ...

  29. China-led team seeking elusive quantum of gravity finds first evidence

    "The graviton is a bridge connecting quantum mechanics and general relativity theory. If confirmed, it will have huge implications for modern physics research," he said. 02:01

  30. Opinion: 25 years later, 'The Matrix' is less sci-fi than tech reality

    Editor's Note: Rizwan Virk, who founded Play Labs @ MIT, is the author of "The Simulation Hypothesis: An MIT Computer Scientist Shows Why AI, Quantum Physics and Eastern Mystics Agree We Are ...