Probing The Unruh Effect with Quantum Optics

^{Lando Calrissian Heads to Hyperspace. An accelerating object, such as a spaceship traveling at relativistic speeds (close to the speed of light), should generate showers of faintly glowing particles, according to the predicted phenomenon known as the Unruh effect.}

Einstein’s principle of equivalence states that gravity and acceleration are indistinguishable from each other. The best example to see this, is to place oneself inside an elevator. Initially it is static in a floor, and when it starts to move upwards (i.e., it accelerates or changes its speed) one feels pushed towards the floor, as if one is pulled by gravity, even though the effect comes from an acceleration in the opposite direction.

Now, let us imagine that an object is in space, in a perfect vacuum; a supposedly friction-less environment, and it accelerates suddenly. The first question that arises is … how can it even move, if there is nothing to push against it? It needs a source of acceleration. In the case of a rocket, the propellant is playing that role: carrying away linear momentum in one direction so that the rocket can gain momentum and move in the opposite direction. The speeds obtained by this means, are very far from relativistic speeds (they are very far from light speed).

In case of an interstellar spaceship, we recall the typical image from a sci-fi movie, showing a spaceship that when reaching extremely high speeds, it suddenly is immersed in a bath of radiation field, which in principle would be impossible in complete emptiness. Therefore, where is this glow coming from? It would have to come from friction against vacuum. This is where the infinitely energetic quantum vacuums electromagnetic fluctuations come into play. The only plausible explanation would be that the spaceship is interacting with the vacuum fluctuations in an accelerated frame. And when this interaction results in thermal radiation (a measure of kinetic energy, associated to particles), this is called the Unruh effect.

^{@ https://www.scholarpedia.org/article/Unruh_effect}

Quantum field theory predicts that an observer under acceleration will observe a thermal bath, like blackbody radiation; the background appears to be warm from an accelerating reference frame. And the thermal radiation, known as Unruh radiation, would be a measure of the kinetic energy emitted by the Unruh particles that result from the interaction of the accelerating object with the vacuum fluctuations.

Because of the principle of equivalence, if Unruh effect comes from acceleration, the radiated gravitational counterpart would be the Hawking radiation, an almost imperceptible halo of light that Hawking predicted should leak from black holes as they slowly evaporate. Therefore, the analogy holds, Hawking radiation would be the thermal radiation coming from the particles created at the Event horizon of a black hole.

From the perspective of relativity, Unruh effect would be an experimental evidence of a sort of “friction against vacuum”, or frame dragging, that results in generation of heat, or thermal radiation. The problem is that to test this effect in laboratory requires accelerations way beyond what we can create, of the order of billions of times the gravitational field, as the video below explains.

Nevertheless, the effect can be tested from the quantum mechanics perspective. A work published in Physics Review Letters, shows a clever way to test the effect using atoms in a laboratory, by manipulating the quantum properties of the system, namely, by finding the experimental conditions in which light matter interactions at atomic scale enhance the stimulated emission, with respect to other atomic responses to the field, such as spontaneous emission and light absorption. The enhancement of the stimulated emission is such, that it could be detected in laboratory, compensating the requirement of an extreme, unachievable acceleration. The authors find the theoretical conditions for this effect to be measured realistically through the intensity of the stimulated emission in an accelerated frame, proposing an experimental set up that is so simple, that many labs in universities could make it.

Additionally, since the equivalence principle relates the Unruh effect to the Hawking effect, this suggests the existence of the gravity-induced analogs of the new phenomena that the authors find in this study, such as the stimulation of Hawking radiation.

Highlights:

In a recent RSF article by Biophysicist William Brown, another quantum mechanical setup was discussed, where entanglement would be the mechanism enabling the detection of Hawking radiation in a Bose Einstein condensate that serves as an optical analog for a white-black hole in a co-moving frame.

Such laboratory analogs are extremely important to probe and explore the properties of black holes, which are foundational to the Unified field theory developed by Nassim Haramein, and wherein cosmological black holes have an analog particle like behavior, while subatomic particles such as protons, are analogous to quantum scale black holes.

Cosmological black holes are extremely relevant in the frame of a unified theory, since they are macroscopical quantum objects. They are a natural bridge between quantum field theory and general relativity, since black holes have huge gravitational effects. Quantum gravity is therefore, unambiguously related to black holes, as Haramein’s generalized holographic theory has proven [1,2].