A quantum simulation of Unruh radiation

A quantum simulation of Unruh radiation
(a) illustrates how Unruh radiation is expected to emerge in an accelerating frame. (b) shows the image of our experiment that simulates Unruh radiation. Credit: Hu et a.

Researchers at the University of Chicago (UChicago) have recently reported an experimental observation of a matter field with thermal fluctuations that is in accordance with Unruh's radiation predictions. Their paper, published in Nature Physics, could open up new possibilities for research exploring the dynamics of quantum systems in a curved spacetime.

"Our team at UChicago has been investigating a new quantum phenomena called Bose fireworks that we discovered two years ago," Cheng Chin, one of the researchers who carried out the study, told Phys.org. "Our paper reports its hidden connection to a gravitational phenomenon called Unruh radiation."

The Unruh effect, or Unruh radiation, is closely connected to Hawking radiation. In 1974, theoretical physicist Stephen Hawking predicted that the strong gravitational force near black holes leads to the emission of a thermal radiation of particles, which resembles the emitted by an oven. This phenomenon remains speculative with no direct experimental confirmation.

A few years later, in 1976, physicist William Unruh hypothesized that a person could observe the same radiation when she is moving with a high acceleration. The equivalence between Hawking and Unruh radiation is based on Einstein's equivalence principle, which has now been confirmed by many experiments.

Despite Unruh's predictions, no one has yet observed Unruh radiation, which is not surprising, as this phenomenon is particularly difficult to capture. In fact, a person would need to endure a G-force of 25 billion billion (25*1018) to see a weak radiation of 1 Kelvin. This is an astounding number when considering that, for instance, the G-force experienced by a fighter jet pilot is no more than 10.

"In our lab, we simulate Unruh physics by precisely modulating a Bose-Einstein condensate with the magnetic field," Chin said. "Even through our sample is not moving, the modulation has the same effect as boosting the sample to an accelerating reference frame. We observe radiation at 2 micro-Kelvin, and the measurement excellently agrees with the Unruh's prediction and confirms the quantum nature of the radiation field."

In their experiment, Chin and his colleagues prepared 60,000 and cooled them to about 10 nano-Kelvin, then started the modulation of the magnetic field. A few milliseconds after the modulation, they observed a thermal emission of atoms in all directions. To confirm the thermal distribution of atoms, the researchers collected a larger number of samples and showed that the atom number fluctuates precisely according to the thermal Boltzmann distribution.

"The temperatures we extracted from the images agree excellently with Unruh's prediction," Chin said. "In addition to the thermal distribution, we also observe the spatial and temporal coherence of the matter wave emission. The coherence is the hallmark of quantum mechanics and reveals that Unruh radiation originates from quantum mechanics. This is in sharp contrast to classical thermal sources, like an oven or sunlight, which come from thermal equilibrium."

Essentially, Chin and his colleagues observed a matter wave field using a framework for quantum physics simulations in non-inertial frames. They observed that this matter wave's fluctuations, as well as long-range phase coherence and its temporal coherence are aligned with Unruh's predictions.

The study carried out by the team at UChicago was funded by the National Science Foundation, Army Research Office and Chicago MRSEC. In the future, their observations could have important implications for the study of quantum phenomena in a curved spacetime.

"Our method applies to generic quantum states in non-inertial reference frames. In our future work, we wish to identify novel quantum phenomena in curved spacetimes," Chin said. "There has been much discussion whether Einstein's general relativity is compatible with quantum mechanics. There are proposals, speculations and even paradox, and we wish to carry out experiments that can help to better understand how mechanics works in curved spacetimes."


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Chinese scientists realize quantum simulation of the Unruh effect

More information: Jiazhong Hu et al. Quantum simulation of Unruh radiation, Nature Physics (2019). DOI: 10.1038/s41567-019-0537-1

ultracold.uchicago.edu/

Journal information: Nature Physics

© 2019 Science X Network

Citation: A quantum simulation of Unruh radiation (2019, June 7) retrieved 21 August 2019 from https://phys.org/news/2019-06-quantum-simulation-unruh.html
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Jun 07, 2019
I remember Prof. Unruh. A jovial guy as much at home with the most abstruse theoretical physics as explaining standing waves in organ pipes and strings to students in music concentration.

Jun 07, 2019
Unruh's discovery that Hawking radiation plus the equivalence principle equals the radiation named for him appears to be seminal now that it's been confirmed. It's also yet another confirmation of the Einstein equivalence principle.

Jun 08, 2019
This is all the more remarkable as Unruh derived the effect purely classical.

Had the pleasure to talk to him a while back after a public lecture. Amazing mind deeply steeped in classical GR, I figure this experiment and quantum analog will be very exciting to him.

Jun 09, 2019
Considering how this is proof of GRT, I'm surprised the cranks aren't here asserting 1 = 0.

Jun 09, 2019
The radiation relies on a local reference inertial frame. General Relativity, as opposed to Special Relativity, defines preferred local reference inertial frames. Any decent realistic quantum gravity theory will also define preferred local reference inertial frames. Suggesting anything more than that about General Relativity versus any other gravity theories in response to this topic is kind of dim if you ask me.

Jun 09, 2019
I guess some people fixate on criticisms of GR having nothing to do with perceiving a glossiness or lack of fine detail, or else maybe a cosmological-scale overextension of the theory, neither of which situations seems particularly relevant here.

Jun 09, 2019
Relativity defines preferred local reference inertial frames
Nope. Neither SRT nor GRT defines any "preferred local reference inertial frames" (whatever that means). Both of them define transforms between frames; that's their purpose.

Jun 09, 2019
Without gravity an inertial frame always follows a straight path, with gravity an inertial frame will follow a curved or circular path. It's possible to transform a circle to a straight line, but not always realistic.

Jun 09, 2019
This also doesn't mention the concept of relativistic mass being identical to gravitational mass because that's not tested here.

Jun 09, 2019
Without gravity an inertial frame always follows a straight path, with gravity an inertial frame will follow a curved or circular path. It's possible to transform a circle to a straight line, but not always realistic.
An inertial frame presupposes mass, which entails gravity.
It is topologically meaningless to transform a circle to a straight line. In a circle, there exists two paths that return to any given point. In a straight line, there exists no path that returns to a given point. I think you must mean a curved line segment to a straight line segment.

Jun 09, 2019
"Our method applies to generic quantum states in non-inertical reference frames. In our future work, we wish to identify novel quantum phenomena in curved spacetimes"

Aside from the spelling, and the simplistic equating of gravity fields to "curved spacetimes" there is not much to say about this prediction.

Jun 09, 2019
Curved vs straight it makes a difference, maybe only appreciably in the case of the black hole, but anyway I do not see where there is any meaningful affirmation of general relativity vs something else that may be very similar yet still different.

Jun 09, 2019
"An inertial frame presupposes mass"

Special relativity does not require non-negligible mass. Anyway, this pre-supposed mass does not bend the path of an inertial frame for the object with mass, of course external masses may cause the path of an object with mass to bend, but that has nothing to do with special relativity.

Jun 09, 2019
So anyway, the article clearly says they haven't tested any "curved spacetime" notions with this, so even at that point it seems clearly foolish to say general relativity was validated here. On the other hand, the point that certain disagreements have unpredictable consequences of varying intensity is developed very well by the contributors here.

Jun 09, 2019
"An inertial frame presupposes mass"

Special relativity does not require non-negligible mass. Anyway, this pre-supposed mass does not bend the path of an inertial frame for the object with mass, of course external masses may cause the path of an object with mass to bend, but that has nothing to do with special relativity.
SR speaks to the question of mass beyond gravity in differences of mass in bodies in two inertial reference-frames differing in their velocities; say two hydrogen atoms. It's rather academic to consider a single 'reference' frame with one arbitrarily near-zero mass. A reference should entail a reference to something else. Hence we get the classic trains at 'rest' vs the 'moving' trains, and the whole (contracted) nine-yards...

Jun 09, 2019
Not to put too fine a point on it but after explaining how the authors do not claim to validate any particular theory of gravity including general relativity and after noting that special relativity's notion that relativistic mass is equivalent to gravitational mass is also not validated, these topics seem adequately, and uniquely constructively, addressed by me.

Jun 09, 2019
SR speaks to the question of mass beyond gravity in differences of mass in bodies in two inertial reference-frames differing in their velocities; say two hydrogen atoms. It's rather academic to consider a single 'reference' frame with one arbitrarily near-zero mass.


I can but read this side of the conversation, but the Unruh as well as Hawking radiation are quantum field effects under general relativity. The mapping from special relativity of non-accelerating inertial reference frames to general relativity of accelerating inertial reference frames is due to the equivalence principle, and gravity is just coincidental to that as an interaction amomg others. We can approximate gravity as a field in flat space, it is a good approximation outside of black holes, so dragging gravity into specifically this is - aside from the Hawking/Unruh equivalence - in my opinion too "rather academic".

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