Black Hole Echoes Would Reveal Break With Einstein’s Theory

Quanta Magazine ^ | 3/22/18 | Sabine Hossenfelder

[LibWhacker]

Gravitational waves have opened up new ways to test the properties of black holes — and Einstein’s theory of gravity along with them.

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.

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? In late 2016, Afshordi and collaborators applied a custom-designed analysis to the publicly available LIGO data and looked for evidence of echoes. Amazingly enough, they found echoes just where they sought them. The signal was not highly significant — the researchers estimated a 1 in 100 chance that the signal was just noise — but a signal nevertheless.

Theirs was a big claim, a daring claim. If correct, it would be evidence for the failure of Einstein’s theory.

A few weeks after Afshordi’s group published their paper, members of the LIGO collaboration put out a reply, raising concerns about Afshordi’s analysis. The LIGO team then set out to do their own study. But large collaborations work slowly, and so it took more than a year until they were able to finish the analysis and get the collaboration’s approval to publish the work.

The LIGO analysis is now available. The researchers found the echo, but at a lower statistical significance than before. They concluded that there’s a 1 in 50 chance that the echo is merely noise. Moreover, the study found the strongest evidence for an echo in one particular event — the event that itself has the lowest significance. When they remove this event from the sample, the probability that the echo isn’t real goes up to almost 20 percent.

For this data analysis to be doable at all, the physicists must make assumptions about the signal that they search for. Inspired by Afshordi’s model, researchers assumed that the echoes should come at regular intervals, that they decay exponentially, and that they remain unmodified (aside from the decrease of amplitude). In many regards, they’re searching for the simplest possible echo.

The theorists could now review the data analysis and develop hypotheses that fit the data better. But reanalyzing the same data over and over again carries a big risk: Instead of developing a better theory, they could merely find a way to better amplify noise.

The more types of echoes they look for, the more likely they are to find something. But these repeated attempts will render measures of statistical significance unreliable.

The only way to overcome this impasse is fresh data. It will take many more iterations of this exchange between theory and experiment before the case can be settled.

So far, both the experimentalists and the theorists have found the exchange fruitful. “We are going ahead full force with modeling the echoes theoretically, and finding better ways to search for them,” said Afshordi, the author of the original study. He also pointed out that another group has found evidence for echoes in the LIGO data, claiming a less than 1 percent risk of a false positive.

Meanwhile Ofek Birnholtz, a researcher with the LIGO collaboration, said that “there have certainly been tensions,” but the idea that black holes echo is “without a doubt worth looking further into.” A search for black hole echoes has become one of the official goals of the LIGO Scientific Collaboration.

We all dream the same dream, here in theoretical physics.

[SteveH]

Interesting, thanks for sharing

[equaviator]

There’s nothing like the universe to bring you down to earth.

[Getready]

From what little I have been reading...I have heard that black holes may be very very dense neutron stars or something else very dense...so all these theoretical calcs might be trying to analyze the tbings which dont really exist....speaking of unknowns, what caused the so-called cosmic egg...and what caused it to “exlode”? I am guessing that we dont have a truly testable scientific answer
..cuz humans didnt exist at that time (was there “time” anyway)?, and could not observe the events .also the physics we know today did not exist at the moment of the beginning and cannot be applied to that event....no?

[poconopundit]

The author is talented and should definitely write a book and appear on Coast-to-Coast AM. But perhaps the finest scientific research on black holes is being conducted in Italy: Are Black Holes The Key To Interstellar Travel? There’s good news and bad news for anyone who hoped that black holes could hold the secret to interstellar travel. Scientists put forth a new theory this week that black holes could be a doorway through spacetime, but you would be unlikely to survive the experience. For a long time, scientists have believed that any matter that enters into a black hole would be destroyed, the gravity inside being so dense that the laws of physics no longer apply. But that may not be true. According to The Independent, scientists at the University of Valencia propose that at the heart of a black hole "is a very small spherical surface" that could be a "wormhole" through time and space. Unfortunately, any traveller would become “spaghetiffied” - their body stretched so thin in order to pass through the doorway that they would resemble a spaghetti noodle or piece of string. Dr Gonzalo Olmo said, "Our theory naturally resolves several problems in the interpretation of electrically-charged black holes.” .... * * * I find Dr. Gonzalo's powers of analysis absolutely amazing. Imagine: astro-physicians say the closest black hole is 27,015 light years away!

[exDemMom]

I wonder how one assigns statistical significance to a single set of observations? And how do they really know what they are looking at?

This reminds me of why I went into biochemistry. Certainly, there is less to question about what is being observed, and there usually is never any trouble getting sufficient sample sizes to run statistics.

[Jack Hammer]

If Einstein was right, how do we explain starships that go warp ten....?

[Enchante]

The University of Valencia is in Spain.

[ctdonath2]

Take a deep breath, set aside some serious time, and read some modern physics & cosmology textbooks.
All this stuff does actually make sense (though still working out fringe stuff like this article reviews), though takes a few years to understand.

You’re not going to comprehend it in a few minutes on social media.

Let’s say:
You experience a tiny fragment of the totality of the universe. Everything follows the same rules, but it’s hard to deduce the rules because you see only a tiny range of their application. Be humble - just because we don’t understand something doesn’t mean it’s wrong.

[broken_arrow1]

Oh, the noise, the noise....

https://www.youtube.com/watch?v=UsMlsDgMXJM

[super7man]

OK, I get all that. But how do I get my wife to be on time for church?