unruh effect experiment

Warp drive experiment to turn atoms invisible could finally test Stephen Hawking's most famous prediction

The Unruh effect has been predicted half a century

A new warp speed experiment could finally offer an indirect test of famed physicist Stephen Hawking's most famous prediction about black holes.

The new proposal suggests that, by nudging an atom to become invisible, scientists could catch a glimpse of the ethereal quantum glow that envelops objects traveling at close to the speed of light. 

The glow effect, called the Unruh (or Fulling-Davies-Unruh) effect, causes the space around rapidly accelerating objects to seemingly be filled by a swarm of virtual particles, bathing those objects in a warm glow. As the effect is closely related to the Hawking effect — in which virtual particles known as Hawking radiation spontaneously pop up at the edges of black holes — scientists have long been eager to spot one as a hint of the other’s existence.  

Related: 'X particle' from the dawn of time detected inside the Large Hadron Collider

But spotting either effect is incredibly hard. Hawking radiation only occurs around the terrifying precipice of a black hole, and achieving the acceleration needed for the Unruh effect would probably need a warp drive. Now, a groundbreaking new proposal, published in an April 26 study in the journal Physical Review Letters , could change that. Its authors say they have uncovered a mechanism to dramatically boost the strength of the Unruh effect through a technique that can effectively turn matter invisible. 

"Now at least we know there is a chance in our lifetimes where we might actually see this effect," co-author Vivishek Sudhir, an assistant professor of mechanical engineering at MIT and a designer of the new experiment, said in a statement . "It’s a hard experiment, and there’s no guarantee that we’d be able to do it, but this idea is our nearest hope."

First proposed by scientists in the 1970s, the Unruh effect is one of many predictions to come out of quantum field theory. According to this theory, there is no such thing as an empty vacuum. In fact, any pocket of space is crammed with endless quantum-scale vibrations that, if given sufficient energy, can spontaneously erupt into particle-antiparticle pairs that almost immediately annihilate each other. And any particle — be it matter or light — is simply a localized excitation of this quantum field.

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In 1974, Stephen Hawking predicted that the extreme gravitational force felt at the edges of black holes — their event horizons — would also create virtual particles. 

Gravity, according to Einstein’s theory of general relativity , distorts space-time , so that quantum fields get more warped the closer they get to the immense gravitational tug of a black hole’s singularity . Because of the uncertainty and weirdness of quantum mechanics, this warps the quantum field, creating uneven pockets of differently moving time and subsequent spikes of energy across the field. It is these energy mismatches that make virtual particles emerge from what appears to be nothing at the fringes of black holes. 

"Black holes are believed to be not entirely black," lead author Barbara Šoda, a doctoral student in physics at the University of Waterloo in Canada, said in a statement . "Instead, as Stephen Hawking discovered, black holes should emit radiation."

Much like the Hawking effect, the Unruh effect also creates virtual particles through the weird melding of quantum mechanics and the relativistic effects predicted by Einstein. But this time, instead of the distortions being caused by black holes and the theory of general relativity, they come from near light-speeds and special relativity, which dictates that time runs slower the closer an object gets to the speed of light. 

According to quantum theory, a stationary atom can only increase its energy by waiting for a real photon to excite one of its electrons . To an accelerating atom, however, fluctuations in the quantum field  can add up to look like real photons. From an accelerating atom’s perspective, it will be moving through a crowd of warm light particles, all of which heat it up. This heat would be a telltale sign of the Unruh effect.

But the accelerations required to produce the effect are far beyond the power of any existing particle accelerator.  An atom would need to accelerate to the speed of light in less than a millionth of a second — experiencing a g force of a quadrillion meters per second squared — to produce a glow hot enough for current detectors to spot.

"To see this effect in a short amount of time, you’d have to have some incredible acceleration," Sudhir said. "If you instead had some reasonable acceleration, you’d have to wait a ginormous amount of time — longer than the age of the universe — to see a measurable effect."

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To make the effect realizable, the researchers proposed an ingenious alternative. Quantum fluctuations are made denser by photons, which means that an atom made to move through a vacuum while being hit by light from a high-intensity laser could, in theory, produce the Unruh effect, even at fairly small accelerations. The problem, however, is that the atom could also interact with the laser light, absorbing it to raise the atom's energy level, producing heat that would drown out the heat generated by the Unruh effect.

But the researchers found yet another workaround: a technique they call acceleration-induced transparency. If the atom is forced to follow a very specific path through a field of photons, the atom will not be able to "see" the photons of a certain frequency, making them essentially invisible to the atom. So by daisy-chaining all these workarounds, the team would then be able to test for the Unruh effect at this specific frequency of light.

Making that plan a reality will be a tough task. The scientists plan to build a lab-size particle accelerator that will accelerate an electron to light speeds while hitting it with a microwave beam. If they’re able to detect the effect, they plan to conduct experiments with it, especially those that will enable them to explore the possible connections between Einstein's theory of relativity and quantum mechanics.

"The theory of general relativity and the theory of quantum mechanics are currently still somewhat at odds, but there has to be a unifying theory that describes how things function in the universe," co-author Achim Kempf, a professor of applied mathematics at the University of Waterloo, said in a statement . "We've been looking for a way to unite these two big theories, and this work is helping to move us closer by opening up opportunities for testing new theories against experiments."

Originally published on Live Science.

Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.

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The Unruh Effect: Newly Proposed Experiment Aims to Detect Vacuum Radiation Due to Acceleration

Sai Rayasam, Physical Sciences, Fall 2020

Figure 1: The Unruh effect states that for an accelerating observer, the temperature of empty space rises, causing the appearance of a swarm of particles and radiation. Current technology does not allow for the direct confirmation of this effect, for the phenomenon only becomes apparent at speeds close to light. However, a team of researchers believe they can confirm its existence utilizing a quantum simulation involving a Bose-Einstein condensate and sound particles. (Source: Flickr)

Albert Einstein’s papers on the theory of relativity are some of the most influential scientific papers to date. One of the overarching principles he proposes is that many forms of measurement are relative, dependent on an observer’s motion. The ticking of a clock, the length of an object, the wavelength of a ray of light—there are no explicit universal measures of these properties since they change depending on how fast an observer is moving (Norton, 2013). In 1976, W.G. Unruh described a phenomenon that would expand upon Einstein’s theories of relativity. The Unruh effect states that from the point of view of an accelerating observer, empty space, which would otherwise be totally blank for a non-accelerating observer, becomes a swarm of particles and radiation, taking on a temperature proportional to the observer’s acceleration. This phenomenon was only an abstract theory; it would be impossible to confirm this theory via direct observation because the observer would need to accelerate to speeds comparable to light, which is not possible with current technology (Fulling and Matsas, 2014). However, a research team led by the University of Nottingham in collaboration with the University of British Columbia and Vienna University of Technology has proposed an experiment that could detect this phenomenon using sound particles (“Scientists make sound-waves,” 2020).

The researchers proposed that instead of studying empty space, they could create a two-dimensional cloud of super cold atoms, known as a Bose-Einstein condensate, and study the sound particles, or phonons, that become audible to an accelerated observer. Just as particles can be viewed as “disturbances” in empty space, the Bose-Einstein condensate also experiences disturbances in the form of irregularities (the phonons) that spread out in waves. By focusing a laser beam on the condensate and guiding it in accelerated motion, it is possible that the observer could detect sound disturbances that would not exist for a stationary observer. Because the laws of physics are consistent in different quantum systems, such an experiment would be a perfect way to simulate the effect in empty space. In fact, such so-called quantum simulations were also used in the study of black holes (“Scientists make sound-waves,” 2020).

Preliminary planning at the university is already underway to conduct a version of this experiment utilizing superfluid helium. However, it seems that this will be a very complex and arduous feat. Jorg Schmiedmayer from Vienna University of Technology explains that although “it would be a wonderful way to learn about an important effect that was previously thought to be practically unobservable,” it is a time-consuming project with many “technical hurdles for [them] to overcome.” Along with the victory of detecting the undetectable, the experiment also has implications in the field of astrophysics. Because the Unruh effect is closely related to Hawking radiation, the emission of particles by a black hole due to extreme gravitational effects, the experiment would offer insights into the behavior of black holes. Furthermore, the project would also advance the studies of virtual particles, particles that randomly appear out of energy due to the equivalence principle. From multiple perspectives, this experiment would be a great stride in theoretical physics (“Scientists make sound-waves,” 2020; Gooding et al., 2020).

• Norton, J. D. (2013, February 10). “Special Theory of Relativity: The Principles.” Einstein for Everyone. Retrieved November 25, 2020 from http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_principles/
• Fulling, S. A. and Matsas, G. E. A. (2014, October 10). “Unruh effect.” Scholarpedia. Retrieved November 25, 2020 from http://www.scholarpedia.org/article/Unruh_effect#Wikipedia_references .
• ScienceDaily. (2020, November 23). “Scientists make sound-waves from a quantum vacuum at the Black Hole laboratory.” ScienceDaily. Retrieved November 25, 2020 from https://www.sciencedaily.com/releases/2020/11/201123100943.htm .
• Sutter, P. (2016, August 22). “‘Virtual’ Particles Are Just ‘Wiggles’ in the Electromagnetic Field.” LiveScience. Retrieved November 25, 2020 from https://www.livescience.com/55833-what-are-virtual-particles.html .
• Gooding C., Biermann, S., Erne, S., Louko, J., Unruh, W.G., Schmiedmayer, J. and Weinfurtner, S. (2020, November 20). “Interferometric Unruh Detectors for Bose-Einstein Condensates.” Physical Review Letters. Retrieved November 25, 2020 from https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.213603 .
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Physicists embark on a hunt for a long-sought quantum glow

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Editor's note: This article has been updated to clarify that the new study in  Physical Review Letters  focuses on the detection of Unruh effect on a single accelerated particle, and to note that other groups have also accumulated evidence of the Unruh effect.

For “Star Wars” fans, the streaking stars seen from the cockpit of the Millennium Falcon as it jumps to hyperspace is a canonical image. But what would a pilot actually see if she could accelerate in an instant through the vacuum of space? According to a prediction known as the Unruh effect, she would more likely see a warm glow.

Since the 1970s when it was first proposed, scientists have accumulated evidence for the Unruh effect, most recently by a group that reported evidence for the effect in a particle-accelerator experiment involving a large number of relativistic particles. The direct detection of the fundamental Unruh effect on a single accelerated particle has still proven elusive. This is because to study the effect as it occurs to an individual particle, such as an accelerated atom, has been thought to require either unachievably large accelerations or astronomical amounts of observation time.   Now, researchers at MIT and the University of Waterloo believe they have found a way to significantly increase the probability of observing the Unruh effect in single particles, which they detail in a study appearing today in Physical Review Letters .

Rather than observe the effect spontaneously as others have attempted in the past, the team proposes stimulating the phenomenon, in a very particular way that enhances the Unruh effect while suppressing other competing effects. The researchers liken their idea to throwing an invisibility cloak over other conventional phenomena, which should then reveal the much less obvious Unruh effect.

If it can be realized in a practical experiment, this new stimulated approach, with an added layer of invisibility (or “acceleration-induced transparency,” as described in the paper) could vastly increase the probability of observing the Unruh effect. Instead of waiting longer than the age of the universe for an accelerating particle to produce a warm glow as the Unruh effect predicts, the team’s approach would shave that wait time down to a few hours.

“Now at least we know there is a chance in our lifetimes where we might actually see this effect on a single particle,” says study co-author Vivishek Sudhir, assistant professor of mechanical engineering at MIT, who is designing an experiment to catch the effect based on the group’s theory. “It’s a hard experiment, and there’s no guarantee that we’d be able to do it, but this idea is our nearest hope.”

The study’s co-authors also include Barbara Šoda and Achim Kempf of the University of Waterloo.

Close connection

The Unruh effect is also known as the Fulling-Davies-Unruh effect, after the three physicists who initially proposed it. The prediction states that a body that is accelerating through a vacuum should in fact feel the presence of warm radiation purely as an effect of the body’s acceleration. This effect has to do with quantum interactions between accelerated matter and quantum fluctuations within the vacuum of empty space.

To produce a glow warm enough for detectors to measure, a body such as an atom would have to accelerate to the speed of light in less than a millionth of a second. Such an acceleration would be equivalent to a g-force of a quadrillion meters per second squared (a fighter pilot typically experiences a g-force of 10 meters per second squared).

“To see this effect in a short amount of time, you’d have to have some incredible acceleration,” Sudhir says. “If you instead had some reasonable acceleration, you’d have to wait a ginormous amount of time — longer than the age of the universe — to see a measurable effect.”

What, then, would be the point? For one, he says that observing the Unruh effect would be a validation of fundamental quantum interactions between matter and light. And for another, the detection could represent a mirror of the Hawking effect — a proposal by the physicist Stephen Hawking that predicts a similar thermal glow, or “Hawking radiation,” from light and matter interactions in an extreme gravitational field, such as around a black hole.

“There’s a close connection between the Hawking effect and the Unruh effect — they’re exactly the complementary effect of each other,” says Sudhir, who adds that if one were to observe the Unruh effect, “one would have observed a mechanism that is common to both effects.”

A transparent trajectory

The Unruh effect is predicted to occur spontaneously in a vacuum. According to quantum field theory, a vacuum is not simply empty space, but rather a field of restless quantum fluctuations, with each frequency band measuring about the size of half a photon. Unruh predicted that a body accelerating through a vacuum should amplify these fluctuations, in a way that produces a warm, thermal glow of particles.

In their study, the researchers introduced a new approach to increase the probability of the Unruh effect, by adding light to the entire scenario — an approach known as stimulation.

“When you add photons into the field, you’re adding ‘n’ times more of those fluctuations than this half a photon that’s in the vacuum,” Sudhir explains. “So, if you accelerate through this new state of the field, you’d expect to see effects that also scale ‘n’ times what you would see from just the vacuum alone.”

However, in addition to the quantum Unruh effect, the additional photons would also amplify other effects in the vacuum — a major drawback that has kept other hunters of the Unruh effect from taking the stimulation approach.

Šoda, Sudhir, and Kempf, however, found a work-around, through “acceleration-induced transparency,” a concept they introduce in the paper. They showed theoretically that if a body such as an atom could be made to accelerate with a very specific trajectory through a field of photons, the atom would interact with the field in such a way that photons of a certain frequency would essentially appear invisible to the atom.

“When we stimulate the Unruh effect, at the same time we also stimulate the conventional, or resonant, effects, but we show that by engineering the trajectory of the particle, we can essentially turn off those effects,” Šoda says.

By making all other effects transparent, the researchers could then have a better chance of measuring the photons, or the thermal radiation coming from only the Unruh effect, as the physicists predicted.

The researchers already have some ideas for how to design an experiment based on their hypothesis. They plan to build a laboratory-sized particle accelerator capable of accelerating an electron to close to the speed of light, which they would then stimulate using a laser beam at microwave wavelengths. They are looking for ways to engineer the electron’s path to suppress classical effects, while amplifying the elusive Unruh effect.

“Now we have this mechanism that seems to statistically amplify this effect via stimulation,” Sudhir says. “Given the 40-year history of this problem, we’ve now in theory fixed the biggest bottleneck.”

This research was supported, in part, by the National Science and Engineering Research Council of Canada, the Australian Research Council, and a Google Faculty Research Award.

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Press mentions.

Researchers at MIT and the University of Waterloo have discovered two properties of accelerating matter that they believe could lead to the direct observation of the Unruh effect, reports Isaac Schultz for Gizmodo . “We’d like to build a dedicated experiment that can unambiguously detect the Unruh effect, and later provide a platform for studying various associated aspects,” says Prof. Vivishek Sudhir.

Scientific American

Researchers from MIT and the University of Waterloo have found a way to test the Unruh effect, a phenomenon predicted to arise from objects moving through empty space, reports Joanna Thompson for Scientific American . “If scientists are able to observe the effect, the feat could confirm some long-held assumptions about the physics of black holes,” writes Thompson.

Popular Mechanics

Researchers from MIT and the University of Waterloo have developed a new approach aimed at significantly increasing the probability of observing the Unruh effect, reports Juandre for Popular Mechanics . “Now at least we know there is a chance in our lifetimes where we might actually see this effect,” says Prof. Vivishek Sudhir. “It’s a hard experiment, and there’s no guarantee that we’d be able to do it, but this idea is our nearest hope.”

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The Unruh effect: Probing the structure of a vacuum

by Andrey Feldman | May 6, 2022

Is the vacuum of space truly empty? If quantum field theory is correct, then the answer is no. According to this theory — used to describe the physics of elementary particles or field quanta — vacuums are filled with continuously forming and annihilating particles with observable manifestations; for example, they force structureless quarks to be confined within composite particles, like protons and neutrons.

Moreover, the mathematical machinery of quantum field theory shows that even the number of stable particles that we see in the vacuum is never fixed but dependent on the motion of the observer — moving at a constant speed, then there are no observable particles, but at an accelerated rate, electromagnetic radiation will be observed, similar to the radiation of a heated body. The more rapid the acceleration, the higher the radiation temperature. This prediction of quantum field theory is called the Unruh effect.

The acceleration needed to observe Unruh radiation with modern photon detectors is about one quadrillion times bigger than the free fall acceleration at the Earth’s surface, making it very difficult to confirm experimentally.

In a new study, a team of physicists at MIT and the University of Waterloo have perfected a classical thought experiment, paving the way for a possible future observation of Unruh radiation. The general idea is to rapidly accelerate an atom which, if the Unruh hypothesis is correct, would then “observe” the flow of electromagnetic field quanta — photons. Interaction with electromagnetic radiation can put an atom into an excited state in which its electrons occupy higher energy orbitals. If such an excitation of an accelerated atom could be found experimentally, this will confirm the existence of the Unruh effect.

As mentioned, the necessary acceleration is unattainable, and to avoid this complication, the physicists suggested studying a stimulated Unruh effect by placing the atom into an electromagnetic field before accelerating it.

This idea is based on a famous effect put forth by Albert Einstein about a century ago where the more intense the electromagnetic field around an atom, the higher the probability that the atom will be excited. Due to this phenomenon, the acceleration required to excite the atom is much smaller than the acceleration required for the atom in a vacuum and could maybe be achieved experimentally in the near future.

The problem here is that if an atom is placed in electromagnetic field, then there is a chance that it will be excited even without the Unruh effect, which makes it difficult to disentangle the contribution of the latter from that of the atom’s interaction with electromagnetic field.

However, if an atom’s acceleration is non-uniform, it is possible to completely suppress this competing effect, reducing the corresponding excitation probability to zero. What is even more important, by analyzing the interaction of the atom with electromagnetic field using quantum electrodynamics, the scientists found the exact law of atom’s motion, which guarantees this suppression.

In their paper, the researchers did not propose a specific experimental set up for testing the theoretical results they obtained, but the work on this problem is ongoing.

There is a very important application of the study, which relies on a mathematical similarity between the Unruh effect and the Hawking radiation of black holes . If Hawking’s theory is correct, then the temperature and radiation of a black hole are extremely small, and as a result, there is almost no chance of detecting them, even with space-based telescopes. A study of the Unruh effect could be the only chance to understand this aspect of their bizarre physics.

Reference: Barbara Šoda, Vivishek Sudhir, and Achim Kempf, Acceleration-Induced Effects in Stimulated Light-Matter Interactions , Phys. Rev. Lett. (2022). arXiv:2103.15838

Feature image: Fred Moon on Unsplash

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By Media Relations

A major hurdle for work at the forefront of fundamental physics is the inability to test cutting-edge theories in a laboratory setting. But a recent discovery opens the door for scientists to see ideas in action that were previously only understood in theory or represented in science fiction.

One such theory is on the Unruh effect. When astronauts in a spacecraft undergo super strong acceleration and see the light of stars stream by, then the Unruh effect is an additional warm glow on top of the streaming light. First predicted by Canadian physicist Bill Unruh, this effect is closely related to the glow from black holes predicted by Stephen Hawking. This is because black holes strongly accelerate everything towards them. 

“Black holes are believed to be not entirely black,” says Barbara Šoda, a PhD student in physics at the University of Waterloo. “Instead, as Stephen Hawking discovered, black holes should emit radiation. This is because, while nothing else can escape a black hole, quantum fluctuations of radiation can.”

Similar to how the Hawking effect needs a black hole, the Unruh effect requires enormous accelerations to produce a significant glow. The Unruh effect was therefore thought to be so weak that it would be impossible to measure with the accelerations that can be achieved in experiments with current technology.

The research team found an innovative way to experiment on the Unruh effect through a novel use of high-intensity lasers. They discovered that shining a high-intensity laser on an accelerated particle can amplify the Unruh effect so much that it actually becomes measurable.

In an unexpected twist, the team also discovered that by delicately balancing acceleration and deceleration, one should even be able to make accelerated matter transparent.

The ability to experiment on the Unruh effect as well as on the new phenomenon of acceleration-induced transparency provide a big boost for physicists, who have long been searching for ways to unify Einstein’s theory of general relativity with quantum mechanics.

“The theory of general relativity and the theory of quantum mechanics are currently still somewhat at odds, but there has to be a unifying theory that describes how things function in the universe,” says co-author Achim Kempf, a professor of applied mathematics and member of the Institute for Quantum Computing at Waterloo. “We’ve been looking for a way to unite these two big theories, and this work is helping to move us closer by opening up opportunities for testing new theories against experiments.”

The team is now setting out to conduct further laboratory experiments. They are also excited by the impacts of the research on some of the fundamental questions about physics and the nature of the universe.

“For over 40 years, experiments have been hindered by an inability to explore the interface of quantum mechanics and gravity,” says co-author Vivishek Sudhir, an assistant professor of mechanical engineering at the Massachusetts Institute of Technology and an affiliate of the Laser Interferometer Gravitational-Wave Observatory (LIGO). “We have here a viable option to explore this interface in a laboratory setting. If we can figure out some of these big questions, it could change everything.”

The new paper by Šoda, Sudhir and Kempf, “ Acceleration-induced effects in stimulated light-matter interactions ,” is published in the latest edition of the journal Physical Review Letters.

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General Relativity and Quantum Cosmology

Title: observable unruh effect and unmasked unruh radiation.

Abstract: The Unruh effect, thereby an ideally accelerated quantum detector is predicted to absorb thermalized virtual photons and re-emit real photons, is significantly extended for laboratory accessible configurations. Using modern influence functional techniques, we obtain explicit expressions describing the excitation and relaxation of the quantum levels of an Unruh detector as a general noninertial open quantum system. Remarkably, for controllable periodical motions, an exact master equation is found for the Unruh detector within the prevailing framework of quantum optics with a well-defined Unruh temperature for given acceleration ($\alpha$), acceleration frequency ($\omega_\alpha$), and transition frequency ($\omega_0$) of the detector. We further show that the measurable Unruh temperatures and corresponding transition rates are comparable or higher than their values for the ideally accelerated cases if $c\omega_0$ and $c\omega_\alpha$ have similar orders of magnitude as $\alpha$. This allows us to select the transition rates of the detector to unmask Unruh radiation against Larmor radiation which has been a major competing noise. Our work suggests experiments with such settings may directly confirm the Unruh effect within the current technology, based on which a laboratory test of black hole thermodynamics will become possible.

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