Sir Isaac Newton: Truth is ever to be found in simplicity , and not in the multiplicity and confusion of things (string theory’s 40+ dimensions, multiverse mania).

Albert Einstein: When the solution is simple, God is answering.

Einstein: . . . all physical theories, their mathematical expressions apart, ought to lend themselves to so simple a description that even a child could understand them. (quoted in conversation with Louis de Broglie).

Sir Isaac Newton: Nature does nothing in vain when less will serve; for Nature is pleased with simplicity and affects not the pomp of superfluous causes.

R.P. Feynman: You can recognize truth by its beauty and simplicity. When you get it right, it is obvious that it is right—at least if you have any experience—because usually what happens is that more comes out than goes in. … The inexperienced, the crackpots, and people like that, make guesses that are simple (matter is a bunch of tiny strings! we live in one of an infinite array of universes!), but you can immediately see that they are wrong, so that does not count. Others, the inexperienced students, make guesses that are very complicated, and it sort of looks as if it is all right, but I know it is not true because the truth always turns out to be simpler than you thought.

Albert Einstein: But before mankind could be ripe for a science which takes in the whole of reality, a second fundamental truth was needed, which only became common property among philosophers with the advent of Kepler and Galileo. Pure logical thinking (or even illogical multiverse musings) cannot yield us any knowledge of the empirical world; all knowledge of reality starts from experience and ends in it . Propositions arrived at by purely logical means are completely empty as regards reality. Because Galileo saw this, and particularly because he drummed it into the scientific world, he is the father of modern physics—indeed, of modern science altogether.

Nobel Laureate Physicist Max Born: All great discoveries in experimental physics have been made due to the intuition of men who made free use of models which for them were not products of the imagination but representations of real things.

Sir Isaac Newton: Rule I. We are to admit no more causes (multiverses, strings, loops, inflation) of natural things than such as are both true and sufficient to explain their appearances.

To this purpose the philosophers say that Nature does nothing in vain, and more is in vain when less will serve; for Nature is pleased with simplicity (dx4/dt=ic), and affects not the pomp of superfluous causes (inelegant strings, pseudoscientific M-theory, multiverses, hidden dimensions, bubble universes) .

Rule II. Therefore to the same natural effects (relativity, quantum mechanics, time and all its arrows and asymmetries, the second law of thermodynamics) we must, as far as possible, assign the same causes.

I highly recommend that every professor empower their students with the exalted wisdom of the Giants upon whose shoulders we stand.

]]>Personally I don’t think theoretical physics is getting healthier. The changes in recent years have been a mixed bag. One positive development is that there’s a lot more skepticism in the physics community about string theory, with the issues that Lee and I were raising moving from a marginalized point of view to a rather conventional one among physicists. This potentially creates space for people to work on other ideas. Unfortunately, there has been a backlash against sophisticated mathematical approaches, with too many physicists believing that it was mathematics that led string theorists astray. As a counterpoint, such mathematical research remains alive, partly due to significant funding from Jim Simons.

Among string theorists themselves, a sizable number have basically given up on conventional science, using the multiverse as an excuse to make string theory immune to challenge from experiment. Many if not most have moved on to studying other topics, from condensed matter physics to the kind of quantum gravity going on at the KITP. I have no expertise on the condensed matter stuff, but as indicated here I’m skeptical about the quantum gravity ideas and how they are being pursued. This seems to bring together the worst of the string theory fondness for hype and the worst of the long tradition of empty quasi-philosophical speculation.

]]>I have read with great joy Peter’s book “Not Even Wrong” as well as Lee’s book “The Trouble with Physics”.

Do you think that this change is a sign that theoretical physics is slowly becoming healthy again (using Lee’s words) and that this will take physics out of its present crisis and lead it back to the exciting and glorious years of the past centuries?

Thanks,

]]>The Plebanski action is indeed polynomial and cubic, but the action proposed by Krasnov is neither, due to the presence of the square root in the action (2) (also obvious in (20) in his paper). I don’t really see the benefit of this for the spinfoam models. In particular, the absence of the tetrad fields makes it hard to couple fermionic matter later on, just like in the Plebanski case.

There is also the approach based on the Poincare 2-group (I’m shamelessly advertising myself here… see arXiv:1110.4694), with the constrained BFCG action — also polynomial and cubic, similar in structure to Plebanski in that it has a topological sector plus the simplicity constraint. Its advantage is that tetrad fields are explicitly present in the topological sector, which allows for the straightforward coupling of fermions. Also, the constraint has a much more transparent geometric interpretation. Finally, if one adds the cosmological constant term, one can show that the MacDowell-Mansouri action can be recovered as a second-order theory from this action (by substituting one of the algebraic equations of motion back into the action).

IMO, there are many reformulations of GR action in terms of various variables, from the historic ones (Palatini, Einstein-Cartan) to these modern ones (including the teleparallel gravity, both historic models and the recent Baez-Wise version). Each reformulation has certain appeal and benefits, as well as drawbacks. Krasnov’s approach is certainly interesting, but I honestly don’t see why it deserves so much hype. What problem does it solve, that other proposed actions don’t? And is that solution worth the price of the nonpolynomial simplicity constraint?

Best,

Marko

I’d be really curious why you think “ER=EPR makes absolutely no sense”. My understanding is that it gives a geometric description of states in QG (very entangled black holes) that had no prior understanding. It is an extension of the AdS/CFT description of the eternal BH/thermofield double, and you can argue for it starting from Ryu-Takanayagi (Section 1 of http://arxiv.org/pdf/1412.8483.pdf). These may not be overwhelming evidence, but why is the idea nonsense?

]]>I partly agree, but there are three important aspects of gravity expressed in connection variables that MacDowell and Mansouri and Kelle and West missed. These are the fact that general relativity in 3+1 dimensions can be understood as a constrained topological field theory and that when doing so there is a redundancy in the equations of motion that can be removed by making the theory depend just on the chiral half of the spacetime connection. That is you can write a topological field theory for the chiral left handed SU(2) left space time connection, and constrain the action in the simplest possible way and find that general relativity emerges.

The third fact is that the action is most directly expressed as a function, not of the metric, and not of the frame fields, but of a self-dual two form. (The constraints that I mentioned yield the frame field as an integration constant.)

When one combines these three insights one has the Plebanski action, (whose Hamiltonian formulation was discovered by Ashtekar), which was also rediscovered by Capovilla, Dell and Jacobson. This action is not just polynomial, it is cubic in the fields, making it the simplest possible action that GR can have. The important work of Kirill Krasnov extends and deepens these insights.

These insights are also at the heart of loop quantum gravity and spin foam models. Non-trivial results are possible for a non-perturbative quantization because the action is that of a diffeomorphism invariant gauge theory with a cubic action, closely related to topological field theory.

Macdowell-Mansouri and Stelle-West build on a different insight, that (in the modern language) you can relate GR to a broken topological field theory of the deSitter or anti-deSitter group. This is compatible with the Plebanski formulation and can be incorporated into it.

Indeed by combining these insights there emerged also a connection with Chern-Simons theory induced in 3 dimensional boundaries such as horizons; and this led to an understanding of the role of the cosmological constant in quantum gravity, as an infrared cutoff that is imposed as a quantum deformation of the chiral SU(2).

Thanks,

Lee

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