What happened?
Researchers from Paderborn University, the Weizmann Institute of Science and Cinvestav have studied a laboratory analogue of an event horizon inside a photonic-crystal optical fibre. They identified a direct process that generates stimulated Hawking-like radiation and measured its backreaction on the light field driving the experiment.
The team sent an intense pump pulse and a weaker probe pulse through a 7-millimetre piece of specially structured fibre. Each pulse came from an 8-femtosecond laser system centred near 800 nanometres. Through the nonlinear Kerr effect, the pump pulse temporarily changed the fibre's refractive index as it travelled.
That moving change in refractive index altered the speed of the probe waves. At the point where the probe group velocity matched the moving optical disturbance, the setup behaved mathematically like an event horizon: waves on one side could no longer overtake the moving boundary.
The experiment detected stimulated Hawking partner light in the ultraviolet and a second, smaller wavelength shift associated with backreaction. The result was published in Nature. It is evidence from an optical analogue, not a detection of Hawking radiation from an astronomical black hole.
The simple version
An analogue experiment copies the mathematical behaviour of one system using a different physical system that is easier to control. The fibre did not contain a real black hole. Instead, a travelling laser pulse created a boundary for other light waves that behaved like a horizon.
Imagine waves trying to move upstream in flowing water. If the water moves faster than the waves can travel upstream, they cannot cross the boundary. In the fibre, the changing optical properties played the role of the flowing medium and the probe light played the role of the waves.
The probe stimulated a paired light component with the frequency pattern expected for analogue Hawking radiation. The researchers also saw that producing this radiation altered the pump spectrum. That response is called backreaction: the emitted radiation is not treated as having appeared without affecting its energy source.
The measured radiation was classical and stimulated by a probe. The experiment did not yet observe the fully spontaneous quantum process or the entanglement expected between a pair of genuine quantum Hawking partners.
Worked equations
Light speed inside the fibre
The refractive index n compares the speed of light in vacuum with its wave speed v in a material.
- A larger refractive index means a smaller wave speed in the material.
- The experiment depends on group velocity, which describes the speed of a pulse envelope rather than one single wave crest.
Photon energy at 800 nanometres
Shorter-wavelength ultraviolet photons have more energy than the original near-infrared photons.
- In electronvolts: E = 2.48 x 10^-19 J / 1.602 x 10^-19 J/eV = 1.55 eV
Duration of each laser pulse
Such an ultrashort pulse contains a very broad range of frequencies, which is important for the direct optical interaction reported in the paper.
- Prefix: femto = 10^-15
- Pulses each second: 80 MHz = 8.0 x 10^7 Hz
Why it matters
Hawking radiation sits where quantum physics, gravity and thermodynamics meet. For a real black hole, the radiation would be extremely faint, so laboratory analogues provide a controlled way to test pieces of the underlying mathematics.
Earlier explanations of this optical setup used a complicated cascade of processes. The new theoretical analysis identifies a more direct interaction, and the experiment found the predicted radiation and backreaction signals. A simpler mechanism is easier to calculate and test in other analogue systems.
Backreaction matters because energy conservation requires emitted radiation to affect the system supplying the energy. In a real black-hole description, Hawking radiation would gradually reduce the black hole's mass. In this fibre experiment, the analogue appeared as a redistribution of pump light into a slightly different frequency.
The work does not prove that astronomical black holes radiate through exactly the same microscopic mechanism. It demonstrates that a controlled optical system with horizon-like mathematics can produce and respond to Hawking-like radiation in the predicted way.
Physics you already know
This experiment begins with waves and refraction. The wave speed in a medium depends on refractive index, and a very intense pulse can change that index briefly through a nonlinear optical effect.
It also uses photons. The energy of each photon is E = hf, so moving light towards a higher ultraviolet frequency means increasing the energy per photon even when the total number of photons in a field is redistributed rather than simply increased.
The pump and probe travelled as wave packets. Their group velocities determine whether the probe can pass the moving index change, which is why the horizon analogy is about pulse propagation rather than ordinary reflection from a surface.
The ultraviolet signal sits in the electromagnetic spectrum beyond visible violet light. Filters, a prism and a photomultiplier allowed the researchers to separate weak wavelength components and count photons.
The experiment also illustrates proportional reasoning. The stimulated Hawking signal was expected to vary linearly with probe intensity, while the backreaction signal varied with the square of probe intensity. Those different trends helped distinguish the two effects.
Science ideas to understand
What was directly measured?
The researchers counted wavelength-resolved ultraviolet photons and measured how signal strength changed with probe power. They identified the stimulated partner signal and the smaller spectral component predicted for backreaction.
What is the black-hole connection?
The moving optical disturbance and probe waves obey horizon-like mathematics. This lets physicists investigate an analogue of Hawking processes, but the fibre has no strong gravitational field and is not a miniature black hole.
Common misconception
The experiment did not detect Hawking radiation arriving from space, prove that black holes evaporate, or observe spontaneous quantum Hawking entanglement. It tested a stimulated optical analogue under controlled laboratory conditions.
A Level stretch
The Kerr effect makes the refractive-index change depend on light intensity. In the experiment the change was around one part in a thousand, but dispersion means different optical frequencies already travel with different phase and group velocities inside the photonic-crystal fibre.
The relevant horizon condition occurs when the group velocity of the probe matches the speed of the moving pump-induced disturbance. Group velocity is the speed at which the envelope of a pulse and its information normally travel.
The direct interaction creates positive- and negative-frequency partners in the reference frame moving with the pump. Negative co-moving frequency is a mathematical property of the mode relative to that moving frame; it does not mean a detector measures a physically negative frequency in the laboratory.
Spontaneous quantum Hawking radiation should produce entangled partner modes from quantum fluctuations. This experiment used stimulated radiation, so it could measure the spectral mechanism and backreaction with a stronger signal but could not demonstrate that entanglement.
The reported backreaction shifted part of the pump spectrum about 0.5 to 1 nanometre farther into the ultraviolet. Detecting such a small secondary signal required improved stability, calibration and signal-to-noise ratio.
Key words
Quick pupil questions
Did scientists observe Hawking radiation from a real black hole?
No. They observed stimulated Hawking-like radiation and its backreaction in a fibre-optic laboratory analogue of an event horizon.
How can an optical fibre imitate an event horizon?
An intense moving light pulse changes the fibre's refractive index. Where a probe pulse can no longer overtake that moving change, its wave behaviour is mathematically similar to a horizon.
What is backreaction in the Hawking radiation experiment?
It is the measurable change in the pump-light spectrum caused by the process producing the stimulated Hawking-like radiation.
How does the optical Hawking experiment link to A Level Physics?
It applies refractive index, group velocity, photons, E = hf, the electromagnetic spectrum, proportional relationships and energy conservation.