## Friday, November 27, 2015 ... /////

### Greene, Weinberg, Strominger, Vafa, González, Mathur debate GR

Reality since Einstein

This week, we celebrate the 100th anniversary of the final form of Einstein's equations. In July 2015, the World Science Festival also known as Brian Greene hosted this 100-minute debate of top physicists.

You will do fine if you increase the speed 1.5 times in the video options.

Brian Greene, Andy Strominger, Cumrum Vafa, and Samir Mathur are string theorists and they were accompanied by Steven Weinberg and Gabriela González, the spokeswoman of LIGO. (I didn't know the name and couldn't resist thinking about the variation of the great quote from TBBT: Here in California, I am a janitor. But back home in Louisiana, I am a physicist. Please don't take it too personally.) Brian started with some stories, e.g. his excitement about a lecture by Stephen Hawking whom he didn't know. And a physics book that remembers everything that Brian has eaten over the years.

Greene – using his wife's eyeglasses – asked the guests the most obvious question that physicists are always asked, namely what they would like to be tattooed on their skin.

There would be curved spaces all over González, black holes on Mathur's skin, Einstein's equations on Strominger's back, and geometrized forces on Vafa. It was quite an over-the-edge celebration of the equations. Only Weinberg reintroduced some sanity when he pointed out that GR was no big deal because it's just another effective field theory governing spin-two particles. You have tattooed your bodies with generic junk, ladies and gentlemen.

Weinberg also pointed out that Newton was a weirdo. For example, he was explaining tides but this Briton never in his life came to see what the sea looked like. Einstein's GR was an example of a theory-driven revolution. There were no data that would scream "Newton is wrong". In 1915-1916, Einstein was focusing on accelerated motion – a different attitude than how we teach GR these days.

Strominger is asked why Einstein could persuade the people. Andy points out that, according to Einstein's notes, the Mercury perihelion precession made Einstein himself sure that he has the new correct theory. The bending light during a solar eclipse was verified some 3 years later and Einstein became a global celebrity.

Weinberg puts a beautifully positive spin on the fact that the development of GR didn't rely on empirical data. Which spin? Weinberg pointed out that theorists are often not trusted because people know that they are always able to fit the elephant, fudge their theories so that they always agree with the pre-existing data, whether the theory is fundamentally right or not. Exactly because Einstein wasn't trying to fudge anything – he didn't talk about any particular empirical data – his theory was more convincing!

The Popperazi should try to understand this important point.

In the same way, you shouldn't trust the experimenters, Weinberg warns ;-), because they know the theory and they may "improve" their observations. Brian added that they could have thrown away bad-agreement photographic plates and the bad weather was just an excuse. So always trust the theorists, not the experimenters, they agreed. González stayed silent. ;-)

Cumrun Vafa clarified in what sense Einstein's theory deforms, and not contradicts, Newton's theory. The potential becomes a component of the metric tensor etc. They quickly show a slide about the limiting procedure. Vafa is also hired to clarify the application of GR to cosmology. Perhaps surprisingly for others, Vafa praises Einstein's own proof that the static universe could have been embedded in GR. The balance between attraction and repulsion may be achieved but it's unstable. Georges Lemaitre and Alexander Friedmann did something else – expansion was allowed. Lemaitre, a priest, pointed out that one could start with the "primordial atom" or "egg" (the big bang singularity, we say). Einstein didn't like it – your maths is OK but your physics makes me vomit, he replied. Weinberg notes that there was already evidence for an expanding universe.

Strangely enough, Weinberg didn't know that Einstein did originally reject the expansion. He would criticize himself for having introduced the cosmological constant, as Weinberg said, but it was only years later. BTW Edwin Hubble was trained as a lawyer but could still become an astronomer – so there is hope literally for everybody, Brian said. ;-)

Samir Mathur speaks about the uniformity of the universe and the inflationary cosmology. Evidence is there in the CMB, González excitedly says some things about it. Weinberg considers the cosmological constant problem (the smallness of the C.C.) the biggest unsolved problem in cosmology. He points out that a Texan company got lots of money from the investors for planning to extract the energy from the empty space. Don't invest there, Weinberg recommends.

Andy starts with the black hole topic. Black holes were speculated about even before GR but Schwarzschild showed that you couldn't avoid it. Einstein believed that no one would ever find an exact solution of the nonlinear equations; Schwarzschild only needed a few months to prove Einstein wrong. Greene shows a famous photograph, "the five blacks in a tunnel", and claims it to be a new NASA photograph of a black hole. Andy says that black holes were disbelieved (John Wheeler called their prediction "the greatest crisis of all time") and they're still totally crazy and not understood.

Strominger believes that the escape velocity on Earth is 7 kilometers per second. Sorry, Andy, but it's 11 kilometers per second. Yours is the orbital speed. OK, when the escape speed exceeds the speed of light, we have a black hole etc. Andy still remembers the times when people around him (perhaps as close as one meter, where Steven Weinberg was sitting) believed that black holes didn't exist (Weinberg later indeed says to be one of the people who opposed black holes before they were discovered). Now we observe them. If Andy fell to a black hole, he would scream "help me Brian" beneath the horizon but the message wouldn't get out. But you wouldn't feel anything special near the horizon yourself. And destruction near the singularity.

Greene and Weinberg say that Einstein disliked black holes because he disliked the possibility that his nice equations would have ugly singular solutions. Mathur explains that all black holes are alike, no hair etc. González objects that they have three hairs, the macro parameters. ;-) González talks about gravitational waves – something she is searching for – and Einstein disbelieved those, too. Einstein was wrong literally about every important consequence of GR, except one claim: that the waves were so weak that they wouldn't be measured for a huge time. LIGO still hasn't measured them, has it?

González talks about LASER beams in LIGO and they measure their interference. The beam is split into two mutually orthogonal ones that go through the two long tunnels, get reflected, interfere, and the phase is used to deduce the changes of the lengths of the tunnels due to the gravitational waves. Those 4-kilometer arms only change by thousandths of the proton radius. To be this precise, they need to shield it from vibrations, use correlations in several gravitational detectors, make the thermal noise irrelevant, guarantee a high coherence in the beam etc. etc. She claims that it was expected that they wouldn't have observed anything by now (in the previous run). But she offered no prediction when the discovery would materialize. When asked again, she believes it would be before 2020 (13th Brian festival).

She plays a nice telephone-like 1-second signal with an increasing frequency – that is how the assumed gravitational wave signal from the event would look like. Quite a distinctive signature.

Samir goes to the 1970s and what Hawking was thinking about. The vacuum has fluctuations. The horizon may divide the particle-antiparticle pairs. One of the pair goes out. The evaporation gets faster and ends with an explosion of a sort. When Hawking discovered that, Weinberg still didn't believe black holes. Samir has a heuristic explanation why the energy of the infalling particle from the pair is negative. Brian wants to turn those eaten negative-energy particles to a dietary investment in Texas. Mathur also talked about Bekenstein and the need for black holes to have some entropy. Mathur argues that it's a weird formula – a huge quantum-gravity-dependent value etc.

Brian introduced the Strominger-Vafa paper as perhaps the most important contribution of string theory to explanations of pre-existing puzzles. Vafa gives a short introduction to string theory. Strominger says that independently of the relevance of string theory for Nature, it had to be consistent with the Hawking-Bekenstein-like phenomena. So this test could have made people stop working on string theory – it could have proven it was wrong. Instead, they were able to calculate the right entropy that comes from the ingredients moving in extra dimensions (those vices became virtues, Vafa adds). Brian asked whether Andy and Cumrun had a romantic relationship after their big discovery – and they insisted it would always be their private secret.

Samir begins to talk about the information loss puzzle. The information about the credit card in your wallet apparently gets lost when the hole evaporates. In all other parts of physics, the information may be in principle recovered. Mathur quickly gets to the entanglement between the Hawking particle pair. Quantum mechanics could have broken. But people fought back. Some proposed remnants. Andy sells remnants as small trash cans that get compressed and keep the information as long as they don't disappear completely (tiny Planckian storages of information). The remnants have many problems and almost no advocates (Strominger estimates the number to be zero). Mathur says that in some sense, he is a remnant fan (because the fuzzball is a remnant of a sort), and Strominger uses it as a proof of despair and some other point he was making. ;-)

Maldacena and holography. AdS/CFT shows that the information is preserved, "people believed", as Brian puts it, but "there are doubts about it" today, he claims. That's just rubbish because the preservation of the information in the gauge theory is manifest. Mathur correctly says that there's no clearcut solution of the Hawking puzzle coming from that – like well-defined corrections to the Hawking calculation that get the information out. Mathur claims that the information theory "came to the rescue" and proved that the "problem is still there". I totally disagree with these claims in Mathur's monologue and papers.

Mathur sketches the ER-EPR, wormhole-entanglement, correspondence. Samir explains fuzzballs really as if they were examples of remnants. This is just weird because remnants are supposed to be small and carry the information from any larger black hole - the hole was previously larger. Fuzzballs are just causally different "models" of a black hole of the equal size. A big part of the confusion in these QG discussions is just due to people's sloppy usage of the language (or overlooking obvious mistakes).

Is spacetime a thing, a viewer asks via Twitter? Cumrun says that dualities may relate something that is there and something that isn't there. I am not sure what he means. If nothing is there at all, it has a one-dimensional Hilbert space with the "ket nothing" only so it can't be dual to anything that has nontrivial degrees of freedom. Dualities may only relate different "somethings". Weinberg says the spacetime can be emergent and believes that we know nothing about the Planck-scale physics. He even questions the existence of string theory – the different descriptions' looking like describing the same theory may be an illusion (that's just crazy). Weinberg says to be frustrated, not disappointed, by where we are in QG. It's too hard, we can't touch black holes, so he's more excited about LIGO. He is basically "softly" extending his disbelief about the existence of black holes!

Andy Strominger intervenes and says that the black holes are experimental. A project called The Event Horizon Telescope will try to directly observe the horizon of the black hole at the galactic center via 1.3 mm very long baseline interferometry.

What question you want to see answered in your lifetime?

Mathur – whether quantum gravity acts at longer length scales. González – can we measure predictions of quantum gravity? Vafa – what is the fundamental formulation of string theory? Strominger – how does the information get out? Weinberg – what are the laws of Nature and do they vary in a multiverse? Weinberg, a guru of the multiverse believers, says that he hopes that the multiverse is wrong. ;-)