The Spacetime Metric

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Ulf Leonhardt

Professor of physics, Weizmann Institute of Science

Leonhardt studies quantum optics, electromagnetic media and laboratory analogues of gravity. With collaborators, he developed fibre-optical horizon experiments and investigated how stimulated analogue Hawking radiation acts back on the optical field. His theoretical work with Ziv Landau models quantum fluctuations in media that change with time and explores their connection to cosmic expansion.

Affiliations

  • Weizmann Institute of Science, Department of Physics of Complex Systems

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Bibliography

A bibliography listing does not establish authorship. Credits appear under attribution.

Published bibliography entries: 3. Showing 1–3 of 3.

  1. Listed work

    Paper · 2026

    Backreaction of stimulated Hawking radiation in an optical analogue

    1. Lorenzo M. Procopio(Author)
    2. Raul Aguero-Santacruz(Author)
    3. David Bermudez(Author)
    4. Ulf Leonhardt(Author)

    Procopio, Aguero-Santacruz, Bermudez and Leonhardt address an open question: Hawking radiation draws its energy from the field around a black hole, but how field quanta create Hawking quanta was unknown. In a fibre-optical analogue of an event horizon, they present experimental and theoretical evidence for the generating process. Where a complicated cascade had been assumed, they identify a simple direct process and observe how it reacts back on the field. Such analogues rest on waves in a medium whose flow comes to exceed the wave speed. The findings point to an equally direct process in other analogues and perhaps in gravity, shedding light on how black holes might radiate.

  2. Listed work

    Paper · 2024

    Quantum noise in time-dependent media and cosmic expansion

    1. Ziv Landau(Author)
    2. Ulf Leonhardt(Author)

    Ziv Landau and Ulf Leonhardt model electromagnetic quantum noise in spatially uniform media with equal electric and magnetic responses that vary over time. Light and its fluctuations propagate using conformal time, while their local, causal renormalization predicts nonzero vacuum energy in the changing medium. They apply this model to spatially flat cosmic expansion, assuming negligible dispersion until near the Planck scale. Their proposed quantum buoyancy reduces the effective gravitational contribution of radiation and matter and can match the Hubble discrepancy for a chosen cutoff. The authors explicitly state that this does not definitively resolve the Hubble tension: the coupling depends on unknown microscopic physics and must be fitted to astronomical data.

  3. Listed work

    Paper · 2008

    Fiber-Optical Analog of the Event Horizon

    1. Thomas G. Philbin(Author)
    2. Chris Kuklewicz(Author)
    3. Scott Robertson(Author)
    4. Stephen Hill(Author)
    5. Friedrich König(Author)
    6. Ulf Leonhardt(Author)

    Philbin, Leonhardt and colleagues build on the fact that an event horizon behaves like waves in a moving medium: a horizon forms wherever the medium moves faster than the waves. Their picture is a river flowing towards a waterfall, where beyond some point waves can no longer travel upstream. Using ultrashort pulses in microstructured optical fibres, they demonstrate an artificial event horizon made of light. They observe a classical optical effect, light blue-shifting at a white-hole horizon. Theoretical calculations show the same system can probe the quantum effects of horizons, in particular Hawking radiation.