Gravitational waves have opened up new ways to test the properties of black holes — and Einstein’s theory of gravity along with them.
Quanta Magazine said:We all dream the same dream, here in theoretical physics. We dream of the day when one of our equations will be plotted against data and fit spot on. It’s rare for this dream to come true. Even if it does, some don’t live to see it.
Take, for example, Albert Einstein, who passed away in 1955, 60 years before his equations’ most stunning consequence was confirmed: Space-time has periodic ripples — gravitational waves — that can carry energy across billions of light-years.
Since that September 2015 black hole collision, the Laser Interferometer Gravitational-Wave Observatory (LIGO) team has reported five more events (a sixth fell just short of the standard of significance). But the LIGO data is still virgin territory. It is an entirely new way of decoding the universe, and physicists must develop methods of data analysis along with the measurements.
It’s not a simple task. Measuring gravitational waves is not the kind of discovery you make by accident. But now that they have the data, physicists have been able to extract insights about the astrophysics of black holes and neutron stars, including their location, composition and masses. They’ve measured the expansion of the universe and made new precision tests of Einstein’s general theory of relativity. The theory has passed all tests — so far.
But the same measurements that have so spectacularly confirmed Einstein’s theory could also, perhaps, reveal where it goes wrong.
Quantized Columns
A regular column in which top researchers explore the process of discovery. This month’s columnist, Sabine Hossenfelder, is a theoretical physicist based at the Frankfurt Institute for Advanced Studies in Frankfurt, Germany.
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Physicists know that general relativity breaks down close to a black hole’s center. Yet the center of a black hole is, famously, a place where we can never look. It’s protected by the black hole’s horizon — the surface surrounding the black hole from which light can never escape. In general relativity, the black hole horizon has no substance; it poses no obstacle. The black hole simply swallows whatever dares to pass the horizon.
Most physicists believe that general relativity correctly describes the horizons of black holes. Yet some have argued that contradictions between general relativity and quantum theory mean that something else could be going on. In particular, the claim that black holes are surrounded by a “firewall,” though controversial, has spurred work on alternative descriptions of the horizon.
If the horizon of a black hole is obstructed by something like a firewall, then the horizon could potentially reflect gravitational waves. If that was so, then LIGO should see evidence for these modifications. In particular, a collision between two black holes should produce an echo.
That’s the basic idea put forward by two independent groups of physicists, one led by Vítor Cardoso and the other by Niayesh Afshordi. Using simple models for a horizon with substance, the researchers showed that some of the gravitational waves emitted by a black hole collision should reflect back toward the black hole’s center. The waves would then reflect outwards again, where some would then once again be reflected at the horizon. The black hole would act like a resonant cavity with a semitransparent mirror at one end. It would emit periodic signals with decreasing amplitude. It would echo.
So much for the theory. What about the data?
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