steve_giddings's picture
Theoretical Physicist; Professor, Department of Physics, University of California, Santa Barbara
New Experimental Probes Of Einstein’s Curved Spacetime—And Beyond?

One of the most profound puzzles in modern physics is to describe the quantum nature of spacetime. A real challenge here is that of finding helpful experimental guidance. Interestingly, we are just now on the verge of gaining key new experimental information about classical spacetime, in new and important regimes—and this tantalizingly also offers a possibility of learning about quantum spacetime as well.

Of course, the community has been abuzz about the possible discovery of a new particle at LHC, seen by its disintegration into pairs of photons. If this is real—and not just a fluctuation—there’s a slim chance it is even a graviton in extra dimensions, which, if true, could well be the discovery of the century. While this would indeed be a probe of quantum spacetime, I’ll put this aside until more data reveals what is happening at LHC.

But on the long-distance front, we are clearly entering a new era in several respects. First, miles long instruments built to detect gravitational waves have just reached a sensitivity where they should be able to see these spacetime ripples, emitted from collisions and mergers of distant black holes and neutron stars. In fact, there have been very recent hints of signals seen in these detectors, though the physics community eagerly awaits a verifiable signal. Once found, these will confirm a major prediction of Einstein’s general relativity, and open a new branch of astronomy, where distant objects are studied by the gravitational waves they emit. There is also the possibility that precise measurements of the microwave radiation left over from the Big Bang will reveal gravity waves, though the community has backpedaled from the premature announcement of this in 2014, and so the race may well be won by the earth-based gravity-wave detectors. Either way, these developments will be very exciting to watch.

The possibility of even more profound tests of general relativity exists with another new “instrument,” the Event Horizon Telescope, which is being brought on line to study the four million solar mass black hole at the center of our Milky Way galaxy. Instrument is in quotes because the EHT is really a network of radio telescopes, which combine to make a telescope the size of planet Earth. This will offer an unprecedentedly sharp focus on both our central black hole, and on the six billion solar mass black hole at the center of the nearby elliptical galaxy M87. In fact, with the telescopes that have been networked so far, we are beginning to see structure whose size is close to that of the event horizon of our central black hole. EHT should ultimately probe gravity in a regime where it gets extremely strong—so strong that the velocity that objects need to escape its pull is approaching the speed of light. This will give us a new view on gravity in a regime where it has so far not been well tested.

Even more tantalizing is the possibility that EHT will start to see effects that begin to reveal a more basic quantum reality underlying spacetime. For the 2014 Edge Question, I wrote that apparently our fundamental concept of spacetime is ready for retirement, and it needs to be replaced by a more basic quantum structure. There are multiple reasons for this. One very good one is the crisis arising from the attempt to explain black hole evolution within present day physics; our current foundational principles, including the idea of spacetime, come into sharp conflict with each other when describing black holes. While Stephen Hawking initially predicted that quantum mechanics must break down when we account for emission of particles of “Hawking radiation” from a black hole, there are now good indications that it should not be abandoned. And if quantum mechanics is to be saved, this tells us that quantum information must be able to escape a black hole as it radiates particles—and this confronts our understanding of spacetime.

The need for information to escape an evaporating black hole conflicts with our current notions of how fields and particles move in spacetime; here, escape is forbidden by the prohibition of faster-than-light travel. A key question is how the familiar spacetime picture of a black hole must be modified to allow such escape. The modifications must apparently extend out at least to the event horizon of a black hole. Some have postulated that the new effects abruptly stop right there, at the horizon, but this abruptness is rather unnatural, and leads to other apparently crazy conclusions associated with what has been recently renamed the “firewall” scenario. A more natural scenario, though, is that the usual spacetime description is also modified in a region that extends outward beyond the black hole horizon, at least through the region where gravity is very strong; the size of this region is perhaps a few times the horizon radius. So, in short, the need to save quantum mechanics indicates quantum modifications to our current spacetime description that extend into the region that EHT observations will be probing! Important problems are to improve understanding of the nature of these alterations to the familiar spacetime picture, and to determine more carefully their possible observability via EHT’s measurements.