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| Trans-Planckian problem | |
|---|---|
| Name | Trans-Planckian problem |
| Field | Theoretical physics |
| Introduced | 1970s–1990s |
| Notable people | Stephen Hawking, Jacob Bekenstein, Gabriele Veneziano, Ted Jacobson, Martin Rees |
| Related | Quantum field theory, General relativity, Planck scale, Cosmic microwave background |
Trans-Planckian problem The Trans-Planckian problem is a conceptual and technical tension arising when modes or degrees of freedom in Quantum field theory are traced back to scales shorter than the Planck length or higher than the Planck energy, where known physics like General relativity and Standard Model are expected to fail. It appears prominently in discussions of Cosmology, Black hole thermodynamics, and semiclassical gravity, challenging extrapolations used in derivations such as Hawking radiation and predictions for the Cosmic microwave background. The problem motivates research across approaches including String theory, Loop quantum gravity, and effective field theory programs associated with figures like Stephen Hawking and Jacob Bekenstein.
The problem was emphasized in analyses that combine Quantum field theory on curved spacetimes with classical backgrounds from General relativity, notably in derivations by Stephen Hawking of black hole evaporation and in inflationary calculations influenced by authors connected to Alan Guth and Andrei Linde. When one traces a low-energy mode backward in time in an expanding Friedmann–Lemaître–Robertson–Walker spacetime or backward along the near-horizon region of a Schwarzschild metric or Kerr metric, its comoving wavelength can be blue-shifted below the Planck length or above the Planck energy, invoking degrees of freedom beyond those of the Standard Model or semiclassical frameworks. Early discussions linking short-distance sensitivity to observable predictions involved researchers associated with Gerard 't Hooft and Gabriele Veneziano, and later formalizations used tools from Effective field theory and the philosophy of cutoff dependence explored by scholars in Princeton University and Cambridge University circles.
In inflationary Cosmology the origin of primordial perturbations relies on vacuum fluctuations of quantum fields in an approximately de Sitter background introduced by Alan Guth and developed by Andrei Linde and Alexei Starobinsky. Modes observed in the Cosmic microwave background anisotropy analyses by teams at WMAP and Planck (spacecraft) were, during early inflation, compressed to trans-Planckian wavelengths according to calculations using the Bunch–Davies vacuum assumption and techniques refined by researchers at Harvard University and University of Cambridge. This raises questions examined by groups including those around Ted Jacobson and Jerome Martin: whether unknown ultraviolet physics from String theory or alternative frameworks such as Loop quantum gravity could imprint measurable signatures in the power spectrum or in non-Gaussian statistics targeted by projects like BICEP2 and South Pole Telescope.
In black hole contexts the original derivation of Hawking radiation by Stephen Hawking similarly traces low-frequency outgoing quanta back to ultra-high-frequency near-horizon precursors in the background of metrics studied by Karl Schwarzschild and generalized in rotating solutions by Roy Kerr. Critics and proponents including Gerard 't Hooft, Leonard Susskind, and Samir Mathur have argued that trans-Planckian precursors implicate unknown microscopic structure such as proposed by String theory dualities, the AdS/CFT correspondence promoted by Juan Maldacena, or by quantum gravitational scenarios advocated in Loop quantum gravity. Debates intersect with proposals like the Black hole information paradox as formulated by participants at conferences at institutions such as Institute for Advanced Study and Perimeter Institute.
Several strategies have been advanced: adopting modified dispersion relations in the spirit of the Unruh effect analogues and work by W. G. Unruh, employing specific ultraviolet completions like String theory or M-theory championed by figures such as Edward Witten, invoking background-independent quantizations from Loop quantum gravity proponents like Carlo Rovelli, or treating trans-Planckian modes via Effective field theory cutoffs following methods used by researchers at Stanford University and Caltech. Other approaches include the trans-Planckian censorship conjecture proposed in the landscape/swampland discourse influenced by Cumrun Vafa and collaborators, and analogue gravity experiments developed by groups at University of Glasgow and CNRS that test modified Hawking-like emission using condensed-matter setups first suggested by Bill Unruh. Each model yields different predictions for deviations from standard spectra and for robustness of semiclassical results.
Potential observational signatures include small oscillatory features, running, or modifications to the amplitude and non-Gaussianity of primordial perturbations targeted by Planck (spacecraft), WMAP, BICEP2, and large-scale structure surveys coordinated with teams at Sloan Digital Sky Survey and Dark Energy Survey. In black hole analogues, laboratory measurements in systems pursued by researchers at Universität Wien and Imperial College London aim to detect analog Hawking radiation or stimulated emission consistent with trans-Planckian-modified dispersion relations. Constraints from data analyses performed by collaborations such as those at European Southern Observatory and NASA place limits on the scale and form of possible trans-Planckian effects, informing theoretical programs at centers like Perimeter Institute and CERN.
The Trans-Planckian problem raises conceptual questions debated by philosophers and physicists associated with Princeton University, University of Oxford, and Rutgers University about the nature of effective descriptions, emergence, and the limits of extrapolation in physics. It engages themes from the Philosophy of science regarding theory-ladenness of inference and underdetermination as discussed in seminars at institutes like London School of Economics and Humboldt University of Berlin. The problem also interacts with broader research agendas concerning the Black hole information paradox, the role of holography advanced by Gerard 't Hooft and Leonard Susskind, and conjectures on the viability of ultraviolet completions advocated by Edward Witten and Cumrun Vafa.