Hawking(ish) Radiation Observed

In 1974, Steven Hawking proposed a seemingly ridiculous hypothesis. Black holes, the gravitational monsters from which nothing escapes, evaporate. To justify this, he proposed that pairs of virtual particles in which one strayed too close to the event horizon, could be split, causing one particle to escape and become an actual particle that could escape. This carrying off of mass would take energy and mass away from the black hole and deplete it. Due to the difficulty of observing astronomical black holes, this emission has gone undetected. But recently, a team of Italian physicists, led by Francesco Belgiorno, claims to have observed Hawking radiation in the lab. Well, sort of. It depends on your definition.

The experiment worked by sending powerful laser pulses through a block of ultra-pure glass. The intensity of the laser would change the optical properties of the glass increasing the refractive index to the point that light could not pass. In essence, this created an artificial event horizon. But instead of being a black hole which particles could pass but never return, this created a “white hole” in which particles could never pass in the first place. If a virtual pair were created near this barrier, one member could be trapped on one side while the other member could escape and be detected creating a situation analogous to that predicted by Hawking radiation.

Readers with some background in quantum physics may be scratching their heads at this point. The experiment uses a barrier to impede the photons, but quantum tunneling has demonstrated that there’s no such thing as a perfect barrier. Some photons should tunnel through. To avoid detecting these photons, the team simply moved the detector. While some photons will undoubtedly tunnel through, they would continue on the same path with which they were set. The detector was moved 90º to avoid detecting such photons.

The change in position also helped to minimize other sources of false detections such as scattering. At 90º, scattering only occurs for vertically polarized light and the experiment used horizontally polarized light. As a check to make sure none of the light became mispolarized, the team checked to ensure no photons of the emitted wavelength were observed. The team also had to guard against false detections from absorption and re-emission from the molecules in the glass (fluorescence). This was achieved through experimentation to gain an understanding of how much of this to expect so the effects could be subtracted out. Additionally, the group chose a wavelength in which fluorescence was minimized.

After all the removal of sources of error for which the team could account, they still reported a strong signal which they interpreted as coming from separated virtual particles and call a detection of Hawking radiation. Other scientists disagree in the definition. While they do not question the interpretation, others note that Hawking radiation, by definition, only occurs at gravitational event horizons. While this detection is interesting, it does not help to shed light on the more interesting effects that come with such gravitational event horizons such as quantum gravity or the paradox provided by the Trans-Planckian problem. In other words, while this may help to establish that virtual particles like this exist, it says nothing of whether or not they could truly escape from near a black hole, which is a requirement for “true” Hawking radiation.

Meanwhile, other teams continue to explore similar effects with other artificial barriers and event horizons to explore the effects of these virtual particles. Similar effects have been reported in other such systems including ones with water waves to form the barrier.

8 Replies to “Hawking(ish) Radiation Observed”

  1. The research paper may be found here


    There do still need to be some sanity checks I think to make sure that some nonlinear optical effect is not fooling us.

    The experiment sort of simulates a white hole, or a time reversed black hole, as the post here indicates. There is a stationary beam through a medium which sets up an index of refraction by nonlinear means. The index of refraction slows down the speed of another laser pulse. The nonlinear effect has a length dependency, so there is a point where the other incoming light pulse is stopped in its tracks. This is a sort of particle horizon, which has its analogue in general relativity. To a good degree of approximation a gravity field can be modeled as a spatial dependency on the index of refraction of light due to the vacuum. The experimentalists did find that the secondary pulse sent on the medium did tunnel through. From a quantum mechanical perspective this is no different from Hawking radiation.


  2. In essence, this created an artificial event horizon. But instead of being a black hole which particles could pass but never return, this created a “white hole” in which particles could never pass in the first place.

    Actually since the refractive index perturbation was traveling, the “white hole” trapped light escapes from two event horizons as described in the paper, one black and one white.

    I would be surprised if these phase velocity horizons would have anything to do with Hawking radiation, since a horizon has to capture and then re-emit group velocity information in this analogy instead of the missing quantum entanglement corrections. Even Hawking thinks BHs re-emit entanglement information now, AFAIU.

    So I think this perfect black body emission from the traveling photon trap cavity is too perfect to make the analogy go through. OTOH there will be things learned, so not too much rain on the parade I guess.

  3. You are right, this “horizon” is moving. In theory this could be done where the horizon is stopped. I should have been clearer on that. There are optical materials proposed where the pulse would indeed stop.

    The phase velocity horizon can act as a horizon, where there are some materials that bring the effective phase velocity to zero. The group velocity of the other photon pulse can scatter off of this in a manner analogous to in and out modes scattering off a black hole event horizon.

    The spectrum should be blackbody, as the process invovles raising and lowering operators that are Bogoliubov coefficients. It is the case that information is preserved, but it is so utterly scrambled or “encrypted” that you can access it.


  4. i would like to ask…where can i read Hawking’s thesis or the paper anywhere on the internet about the great hypothesis 🙂

  5. Hawking’s orginal paper was published in Commumications Math. Phys vol-33, #323 (1973) and the follow up is in the same journal vol-43. The first paper is tricky to read, and frankly the best source in my opinion on this physics is Robert Wald’s book “Quantum Fields In Curved Spacetime and Black Hole Thermodynamics,” U Chicago press.


  6. The direction of emission is a MAJOR point of ongoing discussion. Photons emerge from the glass at a 90 degree angle from the direction of the laser pulse. That is the wrong direction, and it is really hard to see how that could happen, says Unruh. It is impossible to verify detection of correlated pairs at the event horizon, unless they use optical glass fibers instead of glass blocks. It seems unlikely to me that this is a natural occuring white hole that emits genuine predicted hawking radiation

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