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Clever Physics Experiment That Produces “Something From Nothing”

Black holes are regions of space-time that have an extremely high gravitational field. Initially, scientists believed that nothing, including light, could escape the boundaries of these massive objects.

Since Albert Einstein's general theory of relativity introduced the possibility of black holes, their precise nature has been contested. Among the most famous discoveries was English physicist Stephen Hawking's prediction that certain particles are actually emitted at the black hole's edge.

Additionally, physicists have studied the operation of vacuums. While Hawking was describing how light can escape a black hole's gravitational pull in the early 1970s, Canadian physicist William Unruh proposed that a photodetector accelerated quickly enough could "see" light in a vacuum.

Dartmouth University research advances these theories by describing a method for producing and detecting previously unobservable light.

"From a practical standpoint, the findings appear to imply the ability to generate light from an empty vacuum," said Miles Blencowe, the Eleanor and A. Kelvin Smith Distinguished Professor of Physics at Dartmouth and the study's senior researcher. "In essence, we have created something out of nothing; the thought of that is quite cool."

The vacuum is defined in classical physics as the absence of matter, light, and energy. In quantum physics, the vacuum is not so much empty as it is filled with photons that exist and vanish. However, this type of light is nearly impossible to quantify.

The "equivalence principle," a component of Einstein's general theory of relativity, establishes a link between Hawking's prediction of radiating black holes and Unruh's prediction of accelerating photodetectors seeing light. Equivalence states that gravity and acceleration are fundamentally indistinguishable: a person trapped in a windowless, accelerating elevator would have no way of knowing whether they are being acted on by gravity, an inertial force, or both.

Thus, if black hole gravity can generate photons in a vacuum, acceleration can as well.

With science demonstrating that it is possible to observe light in a vacuum, the Dartmouth team set out to develop a practical method for detecting the photons.

According to the Dartmouth research theory, which was published in Nature Research's Communications Physics, nitrogen-based imperfections in a rapidly accelerating diamond membrane can be used to detect it.

A postage stamp-sized synthetic diamond containing nitrogen-based light detectors is suspended in a super-cooled metal box that creates a vacuum in the proposed experiment. The membrane, which functions similarly to a tethered trampoline, is accelerated at tremendous speeds.

The research paper explains that when the detector number exceeds a critical value, the resulting photon production from the cavity vacuum is collectively enhanced and measurable, with the vacuum photon production transitioning from a normal phase to "an enhanced superradiant-like, inverted lasing phase."

"The diamond's motion generates photons," explained Hui Wang, a postdoctoral researcher who wrote the theoretical paper while a Dartmouth graduate student. "Essentially, all that is required is to shake something violently enough to generate entangled photons."

The Dartmouth paper examines the use of multiple photon detectors—diamond defects—to amplify the membrane's acceleration and thus increase detection sensitivity. Additionally, oscillating the diamond enables the experiment to take place in a controlled environment at high rates of acceleration.

"Our work is the first to investigate what happens when multiple accelerating photodetectors are used instead of one," Blencowe explained. "We discovered a quantum-enhanced amplification effect for the creation of light from vacuum, in which the aggregate effect of the many accelerating detectors is greater than the sum of their individual effects."

To demonstrate that the detected photons originate in the vacuum rather than the surrounding environment, the team shows that the theory observes "entangled light," a unique property of quantum mechanics that cannot be explained by external radiation.

"The diamond detects photons in pairs," Hui explained. "This production of paired, entangled photons demonstrates that photons are generated in vacuum and not from an external source."

Although the proposal to observe light in a vacuum has no immediate application, the research team hopes that it contributes to our understanding of physical forces in the same way that other theoretical research has. The work may, in particular, shed experimental light on Hawking's prediction for radiating black holes when viewed through Einstein's equivalence principle.

"Part of our responsibility and joy as theorists is to put ideas out there," Blencowe explained. "We're attempting to demonstrate that it is possible to conduct this experiment, to test something that has previously been exceedingly difficult."

The experiment generates photons, as depicted in a technical animation created by the team. Because the detected light is microwave in frequency, it is invisible to the naked eye.

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