The essential fuzziness of time may be the limiting factor for a gravitational-wave detector in Germany.
Poets have long believed the passage of time to be unavoidable, inexorable and generally melancholic. Quantum mechanics says it is fuzzy, ticking along at minimum intervals within which the notion of time is meaningless. And Craig Hogan claims he can 'see' it — in the thus far unexplained noise of a gravitational-wave detector. "It's potentially the most transformative thing I've ever worked on," says Hogan, director of the Center for Particle Astrophysics at the Fermi National Accelerator Laboratory in Batavia, Illinois. "It's actually a possibility that we can access experimentally the minimum interval of time, which we thought was out of reach."
In a classical view of the world, space and time are smooth. The minimum scales at which, according to quantum mechanics, the smoothness breaks down — the Planck length and time — can be derived from other quantities, but they have not been tested experimentally, nor would they be, given their impossibly small size.
Yet if Hogan's ideas are right, noise associated with this fundamental fuzziness should be prominent at GEO600, a joint British and German machine operating near Hannover, Germany, that is searching for gravitational waves. These waves are thought to arise during events such as the massive cosmic collisions of black holes and neutron stars. Confirmation of the idea — which could come as experimental upgrades to GEO600 are put in place over the coming year — would be a big step towards a verifiable quantum theory of gravity, a long-sought unification of quantum mechanics (the physics of the very small) with general relativity (the physics of the very big). Hogan outlines his predictions in a paper published on 30 October in Physical Review D1.
If it's true, it's Nobel-prize-winning stuff Karsten Danzmann , Max Planck Institute for Gravitational Physics
Of course, theorists are full of extraordinary ideas that never pan out, so physicists at GEO600 are treating Hogan's ideas with a healthy dose of scepticism. "To me as an experimentalist, this all seems a bit like black magic," says Karsten Danzmann, principal investigator for GEO600, and director of the Max Planck Institute for Gravitational Physics. "It seems a bit far-fetched and artificial. But if it's true, it's Nobel-prize-winning stuff."
Hogan says that the noise could be responsible for about 70% of some unaccounted for noise that GEO600 is recording. Danzmann says it's "intriguing" that this noise just happens to be the right magnitude and shape to account for most of the 'mystery' noise that his team has been unable identify for a year now.
The predictions are based on a lower-dimensional view of spacetime: two spatial dimensions, plus time. Spacetime would be a plane of waves, travelling at the speed of light. The fundamental fuzziness of the waves, on the order of the Planck length and time, could be amplified in large systems such as gravitational-wave detectors. The third spatial dimension of the macroscopic world would be encoded in information contained in the two-dimensional waves. "It's as if, in the real world, we are living inside a hologram," says Hogan. "The illusion is almost perfect. You really need a machine like GEO600 to see it."
According to Hogan, the 'holographic' noise is more likely to be seen in certain detectors, because the fuzziness gets translated into noise only in the plane of the underlying wavy two-dimensional fabric of spacetime. GEO600 is less sensitive to gravity waves than are detectors such as those in LIGO (Laser Interferometer Gravitational-Wave Observatory), two similar, large L-shaped detectors in Washington and Louisiana. But Hogan says GEO600 is more sensitive to holographic noise, because its power is locked in a beamsplitter that amplifies the peculiar transverse quality of the fuzziness.
The idea for an essentially holographic Universe has gained traction in recent years, as string theorists have found ways to trim the 10 dimensions that their theories call for. A decade ago, Juan Maldacena, now of the Institute for Advanced Study in Princeton, New Jersey, put forward the idea that most of the 10 dimensions can be reduced when the information is encoded, like a hologram, in three or four basic dimensions. "The ideas of holography in string theory are extremely well accepted," says Gary Horowitz of the University of California, Santa Barbara. He adds, however, that Hogan's ideas about holography don't use conventional starting points. "There is reason to be somewhat sceptical. I don't find the theoretical motivation totally convincing."
But Hogan's predictions are striking and specific enough to get the attention of the GEO600 staff. Hogan will travel to Hannover to work with GEO600 scientists such as Harald Lück, who is leading an effort to double the sensitivity of the machine by the end of 2009. That should mean that the instrumental noise also drops. But if most of the noise remains, then it could be a sign that it is due to holographic noise, which would be fundamental, and pervasive throughout the Universe. "If the noise is still there, we have to be serious" about the observations, says Lück.
Hogan, C. J. Phys. Rev. D 78, 087501 (2008).
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Hand, E. Time to test time. Nature (2008). https://doi.org/10.1038/news.2008.1217