If you have 94 spare minutes, you should watch this insightful and amusing panel discussion on "What is string theory", a public event that took place on Monday, at the end of the Strings 2015 annual conference in India.
Rajesh Gopakumar introduces the two main heroes, Milner Prize winners Ashoke Sen and Nima Arkani-Hamed.
They talk about all kinds of conceptual questions, what string theory and quantum gravity is, why quantum gravity is difficult, and so on. The monologues begin with a rather conventional Ashoke Sen's 20-minute introduction to modern physics, quantum field theory, and string theory.
There is room for questions (the audience is mostly local undergrads) at 22:00. The first question is about connections between Higgs fields and gravitational fields. Nima explains that those two are further than suggested by the pop-science talk about it. Most of our mass is from gluons, not the Higgs, anyway.
The second question is whether you can say anything quantitative about the weak force, buddies. You bet, pal! Nima traces the history of the weak force since the 19th century observations of the beta-decay. Comments about the galactic-size colliders. Even without those, we may do things to probe the high-energy regime, like rare processes (analogous to the old neutron decay, e.g. the future proton decay). Big experiments are analogous to building of temples – one needs stamina and character and isn't guaranteed to see God at the end, anyway.
What do higher energies improve about the experiments? We get to shorter distances – paradoxically, we need a very big machine for that. Doubled energy multiplies the rate of new particle production by 50 or so, a power law. Ashoke adds that with the identification of the right compactification, one may predict everything. A question about the range and strength of forces. Nima warns a young Gentlemen that many propositions seemingly reasonable at the level of words become unreasonable at the level of quantitative scrutiny. It's important to learn as much as possible to be able to disprove your own words as quickly as possible, that's what schools are good for. Very important comments – the broader laymen community around physics misunderstands these conceptual matters. Ashoke answers the same question (discourages a strange tunneling connection between the strong force and gravity) more "technically".
At 43:00, a young chap asked about the status of dark matter and dark energy; the "exact" meaning of supersymmetry; and "contenders" to string theory. Nima says how dark matter affects gravitational fields but emits no light. We may be close to the direct detection – perhaps next 5 years. One TeV particle zipping through this room. Dark energy is just a constant energy density of the vacuum. SUSY explained via an analogy with the Lorentz symmetry that implies antimatter (within quantum mechanics). Supersymmetry is the last thing like that to see; an update of spacetime that is more quantum mechanical, with new anticommuting numbers. Those imply another doubling of the world. A very concise story – surely not the first time when Nima said it.
Ashoke gives an alternative view on the dark energy – shows that the cosmological constant is the height of the minimum of the potential energy graphs in the landscape. Why is the dark energy so tiny is like why is the Universe so big? Ashoke's answer about the "contenders". He may be "biased" but he thinks that loop quantum gravity hasn't gotten anywhere – in comparison with string theory. It can't even explain Newton's force. Nima adds that the constraints demanded from good theories are harsh. To find even one theory that can predict the cross section of some graviton scattering at some energy and that is still compatible with the consistency requirements is tough. The passing proposals always turned out to have strings running inside. It's not a matter of sociology; if someone managed to find some actual non-stringy theory able to do the same thing, she would be extremely famous. Amen to all of that.
Another question at 57:00 was about Einstein's cosmological constant – yes, it's the same discussion (and Nima says that Einstein's goal was just the opposite, to make the Universe static, not accelerating) – and about the many worlds interpretation. Nima emphasizes that MWI is a different thing than the multiverse. Nima nicely and politely says that "concerning the many worlds interpretation, many of us think that there is not much substance in these debates." ;-) Nima moves on and suggests that it's plausible that the other regions of the multiverse could be unphysical, not simultaneously existing with our patch.
A guy asks why can't we just construct a model of dark matter by ourselves if we know all the particles. Ashoke corrects that – we only know light enough (and strongly enough interacting) particle species. And that's just not enough to identify the point in the landscape – and predict all the other particles. Nima adds why Planckian ultrashort distances become physically meaningless or very different – Planck energy colliders produce black holes that don't get smaller anymore. I missed that someone had asked the question. Ashoke adds comments.
Another participant asks what strings say about entanglement. Ashoke says that entanglement is generally everywhere in quantum mechanics. String theory allows entanglement to be linked to geometry in a relationship that is still emerging. Just as if he were me ;-), Nima attacks the widespread slogan that "entanglement is mysterious". Quantum mechanics has been around for almost 100 years. And it's really fundamentally different. Far from mysterious, entanglement is the engine that makes quantum mechanics special and produces all the new phenomena. Entanglement is only mysterious from the viewpoint of classical physics but not from the perspective of the right theory which is quantum mechanics. And physicists' job isn't to enslave our explanation to our bodies and minds as they evolved – to understand things classically. The laws of QM are beautiful, crystal clear, and leave nothing mysterious about entanglement at all. I encourage you to be skeptical about people who say that QM is mysterious etc. Nature just doesn't want to be described in this way and it's amazing that the physicists 90 years ago could have found the better way.
At 1:12:00, a charming girl made a long monologue but didn't use the microphone wisely so I didn't understand a word. But she entertained the panelists. She apparently said something like "it's not too important if the Higgs field gives masses etc., what important things does it do?" ;-) Does it solve the Israel-Palestine conflict? One may see that the interaction with the Bengalúru audience is spontaneous and entertaining – some of the responses are funny, indeed. Nima may be an applied string theorist but his defense of string theory – and why it maximally fulfills all the dreams of unification – is spontaneous and cool. The Higgs boson is strange – meaning the first spinless particles (that weren't known and may be assumed to be very heavy). The Hydrogen atom is also light and spinless but not elementary – the Higgs looks elementary. Ashoke says that the theorists knew the Higgs 50 years before the discovery. Half a century of alternatives have basically failed. Nima adds that decades ago, there seemed to be just-particle and not-just-particle camps in physics but string theorists have unified them. QFT and ST are much closer than both camps had realized. There are no camps anymore. String theory has unified everything. Due to the links to things already established, it's hard to imagine strings will go away.
Someone asks whether the dark energy and the vacuum energy are the same or if there are extra terms. Nima says it's an experimental question: other things may cause the acceleration, too. But we may still ask whether the vacuum energy is constant in time or not. It seems very constant, experimentally. Change at most by 10% since the early Universe. Next experiments will probe it up to 1%. If it were variable, the rate would probably be much higher. Another part of the question: the vacuum energy only makes sense when gravity is turned on because otherwise the energy may be shifted by an additive shift without changing physics. In the lab, we would hardly observe the effects of the C.C. – doubling each 10 billion years is too slow a process. Concerning the final question from that Gentleman, spin-3/2 particles are special because they can only exist if SUSY exists. And SUSY then determines all of its (gravitino's) interactions.
After another question, Ashoke says that "grand unified theory" may have different meanings. The unification of forces may be either the "straightforward one" (mathematics of GUT) or a more general one, but still internally consistent. Ashoke says that it would be inconsistent to combine a classical theory of gravity with the quantum theory of everything else.
Nima talks about the steady progress in physics since 19th century which was also making our very dreams different, deeper, and more mature. Quantum mechanics implied that our Laplacian dream (to predict everything deterministically) has "failed". But it's good that it did, Nature just works differently. Those changes aren't disappointments. The nature of questions keeps on evolving – and it always pushed us towards more striking ideas. Lorentz and other classical physicists could have been disappointed by QM but it's QM that allowed the wonderful new unification of waves and particles etc. The more we understand, the deeper simplicity and beauty of the Universe we uncover. The string landscape is analogous, Nima says: instead of viewing it as a failure, you must understand it as a sign of the amazing unified nature of string theory – its ability to view all possibilities as solutions to the same equations.
Later in the day, there were public lectures by Seiberg, Strominger, and Vafa. And one more Nima. Check the Indian institute's YouTube channel. At the top, you see lots of more popular talks by Witten, Maldacena, and others, and technical talks beneath them.
Thanks to Giotis