Fiber-Optical Analog of the Event Horizon
Thomas G. Philbin · Chris Kuklewicz · Scott Robertson · Stephen Hill · Friedrich König · Ulf Leonhardt
Abstract and summary · read the original at the source · none found
In one page
A black hole horizon behaves like a waterfall: waves that swim upstream at a fixed speed can no longer make headway once the current outruns them. Ulf Leonhardt’s group at St Andrews built that waterfall out of light. They fired 70-femtosecond pulses through a metre and a half of microstructured optical fibre; each pulse raises the fibre’s refractive index as it passes, so a slab of slow glass travels along with the pulse even though no glass moves. A second, slower-tuned beam chasing the pulse cannot get past its trailing edge — a white-hole horizon — and the team measured the signature that predicts: the chasing light piles up and shifts to the blue. Their calculations then show the same arrangement should create photon pairs out of the vacuum, the optical version of Hawking radiation, at a temperature far above anything the cold-atom analogues can reach, and with both partners of each pair available to measure.
Why it matters hereChapter 5 treats the vacuum as a medium with fluid behaviour, and analogue horizons are how that idea gets tested on a bench rather than at a black hole. This experiment makes the point sharply: an effective medium can move at the speed of light without anything material moving, because what travels is a change in the fibre’s optical properties. It also removes the objection that dogs the astrophysical case — the runaway blue-shift past the Planck scale — because here the physics that stops it is known and measured, the fibre’s own dispersion. For chapter 13’s unified picture that is the useful lesson: the same wave equations describe the horizon, the fluid and the vacuum, so a laboratory can interrogate the geometry directly.
What it claims
01The physics at an event horizon resembles the behaviour of waves in a moving medium, and horizons form wherever the local speed of the medium exceeds the wave velocity — a black-hole horizon where the flow accelerates past the wave speed, a white-hole horizon where a fast flow slows down and waves can no longer enter.Abstract; Introduction, paragraphs 1 to 2
Settled physics02The medium does not have to move physically. An ultrashort pulse raises the refractive index of the fibre through the Kerr effect by an amount proportional to its instantaneous intensity, and that index change travels with the pulse — so the pulse establishes a moving medium, at the speed of light in the fibre, although nothing material is moving.Paragraph 5 and Eq. (1)
Published and peer-reviewed03In the experiment, 70-femtosecond solitons at 803 nanometres carrier wavelength and 80 megahertz repetition rate were launched into 1.5 metres of microstructured fibre, with a continuous probe tunable from 1460 to 1540 nanometres chasing them; the fibre was chosen so the two group velocities match. The measured probe spectrum shows a clear blue-shifted peak, in very good agreement with the theory of light propagation in the presence of horizons — the classical optical effect of a white-hole horizon.Paragraphs 6 to 10; Fig. 2 and Fig. 3; Doppler formula Eq. (2)
Published and peer-reviewed04The analogue supplies its own answer to the trans-Planckian problem. At an astrophysical horizon light freezes at wavelengths shorter than the Planck scale, where the physics is unknown; in the fibre the corresponding physics is known and simple — the frequency dependence of the refractive index blue-shifts the incoming modes and then limits the shift by tuning the probe out of the horizon.Paragraph 8
Published and peer-reviewed05Near 300 nanometres the glass dispersion makes the effective medium superluminal for those modes, so each subluminal mode has a superluminal partner at the same co-moving frequency, and the pulse converts them partially into one another. Even with every mode starting in its vacuum state the horizon spontaneously creates photon pairs — the optical analogue of Hawking radiation, at both the black-hole and the white-hole horizon. The pairs are separable from the pump by polarisation and by an octave in frequency, they escape the phase-matching conditions of four-wave mixing, and unlike in astrophysics both partners can be detected, so their correlations can be measured.Paragraphs 11 to 13
Published and peer-reviewed06The Hawking temperature in this system does not depend on the size of the refractive-index change, only on how steeply it varies with retarded time — so even the tiny index changes of nonlinear fibre optics give a substantial temperature. Taking the steepness of the self-steepened optical shock front as comparable to twice the carrier frequency, 8 times ten to the fourteen hertz, the calculated temperature reaches about a thousand kelvin, many orders of magnitude above the condensed-matter analogues.Eq. (8) and the two closing paragraphs
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Read it · abstract
Abstract
The physics at the event horizon resembles the behavior of waves in moving media. Horizons are formed where the local speed of the medium exceeds the wave velocity. We used ultrashort pulses in microstructured optical fibers to demonstrate the formation of an artificial event horizon in optics. We observed a classical optical effect: the blue-shifting of light at a white-hole horizon. We also showed by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation.
The way in
https://doi.org/10.1126/science.1153625LICENCE CHECK. The version of record is Science volume 319, pages 1367 to 1370 (2008), copyright AAAS, and the author copy arXiv:0711.4796v2, dated 13 February 2008, carries the arXiv assumed licence rather than a Creative Commons grant. The sheet therefore stays abstract-only. The abstract below is the published one; the arXiv preprint carries the same text in the present tense. The work is from the School of Physics and Astronomy, University of St Andrews, with Philbin also at the Max Planck Research Group of Optics, Information and Photonics in Erlangen.
How to cite it
Thomas G. Philbin, Chris Kuklewicz, Scott Robertson, Stephen Hill, Friedrich König, Ulf Leonhardt (2008) Fiber-Optical Analog of the Event Horizon. doi:10.1126/science.1153625
Where it sits in the curriculum
The vacuum as a quantum fluidWhat the vacuum isThe unified picture