To Peter, I continually wonder what physicists you hang out with. None of this stuff is secret. I was at a department wide talk which brought this stuff up in the past few months. Again, I’m rather curious what else you would have us tell the experimentalists to look for.

I wasn’t under the impression that the degree of fine-tuning needed if the bound on the Higgs goes up by 5-10 Gev was such that it would cause people to give up on supersymmetry. If so, one might not even need to wait for the LHC if the luminosity at the Tevatron improves sufficiently.

There are at least two reasons why a lot more honesty about the failings of low-energy supersymmetry would be a healthy thing (and I don’t see a downside since I don’t think there’s any danger the experimentalists at the LHC won’t look for superpartners).

1. Experimentalists deserve an honest appraisal of how likely various possibilities are so they can make intelligent allocations of their resources.

2. The dishonesy and over-hyping of supersymmetry is part of a larger problem in particle theory. The amount of this in superstring theory/M-theory is vastly greater, and no experiments are likely to save us from this there. Theorists should start behaving a lot more like scientists, with a lot less hype and a lot more healthy skepticism about ideas that don’t work.

Sure, you can add other gauge groups, but if the particles we see have charges with respect to them, they’ll experience a new force and will behave differently, so there are all sorts of bounds on such a thing.

If you assume that the particles we know about are all uncharged with respect to the new gauge group, then it is a “hidden sector”, and its effects will be hard to observe (perhaps only through gravity).

The “Planck Scale” is only the scale of quantum gravity if you assume that however you calculate the effective Newton’s constant, you get dimensionless numbers of order unity and dimensionful ones coming from the scale. If the calculation of the effective Newton’s constant contains an exponentially small dimensionless number, the scale could be quite different.

I don’t see any reason to try and get the standard model forces as induced from some other theory, but people have speculated that quantum gravity is induced from the other gauge forces. One intriguing fact about 2d WZW models is that they can be formulated as theories with a gauge (or loop group) symmetry, but the Sugawara construction shows they automatically also are non-trivial reps of the diffeomorphism group, so in some sense one is getting for free a theory of gravity from a theory with just a gauge symmetry.

G = <T>

the expectation value of the stress energy tensor.

The problem with keeping gravity classical is that you have to couple it to quantum stress energy. The obvious way to do that

G =

has problems. I’m open for suggestions on other ways to do it. Sakharov’s induced gravity gets things off by orders of magnitude as I remember it, but I don’t remember it very well.

Epicycles are completely irrelevant here. When you add epicycles, it means you’re adding parameters to your theory to account for the fact that the theory doesn’t fit the data. The standard model fits the data amazingly well.

There are theories where gravity unifies at the same point as the other forces. GUTs are going to have massive gauge bosons at the scale of the breaking. You can break large gauge groups in stages E8 -> E6 -> SO(10) …, for example. There’s all sorts of stuff that can happen, really.

The almost convergences of the couplings was not imposed in any form. It follows directly from the formulae.

Finally, I doubt the electroweak forces and QED are “induced” in any way (I’m not sure what that would mean, really.) The theories that we have for them now work just fine.

I never quite understood exactly why the scale for a quantum theory of gravity has to be taken at the Planck scale specifically. Are there any rigorous and/or mathematical arguments which shows that this always has to be the case? Why can’t the quantum gravity scale be at something like several million or billion TeV’s, or for that matter several billion or trillion times larger than the Planck scale itself? Is the Planck scale for quantum gravity just accepted on faith or by fiat decree?

Is there a possiblity that gravity is just strictly a classical force with no quantum counterpart? Are there any experiments that have confirmed any quantum gravity phenomena, such as the Unruh effect? (Would the Unruh effect even be a good candidate to search for in a tabletop experiment?) Is this belief in quantum gravity just an extrapolation of the three other forces being quantized? Could gravity just be nothing more than a “residual force” produced by higher order quantum effects, in the style of Sakhrov’s induced gravity?

On the surface, adding in more particles, symmetries, and other “junk” into the standard model feels a lot like adding in more “epicycles” and free parameters into an already grotesque looking theory. (Well, maybe not completely “grotesque” looking.) Could the electroweak and qcd forces be an “induced” force of some sort, in the spirit of Sakharov?

With the idea of three coupling constants merging at one point at some high energy scale, why doesn’t gravity also converge too to that exact same point as the other three coupling constants? Or for that matter if there’s a 5th or 6th force that we aren’t aware of yet, would their coupling constants also converge to this exact same point too? Is this convergence point just something that was noticed by running the coupling constants to higher energy scales, or was it imposed a priori by fiat decree?

The other answers are a bit more technical. One is that, as has been discussed a bit here, the standard model isn’t natural. There are two types of naturalness, actually. What it really comes down to, however, is that the scale of the electroweak interaction is much, much smaller than the Planck scale. We don’t know why this is true. This is dissatisfying.

There’s also technical naturalness which says that the Higgs mass is naturally at the Planck scale because of loop effects. Supersymmetry stops this from happening. So, why it doesn’t explain why the weak scale is so much smaller than the Planck scale, it does stabilize the Higgs mass.

A third reason to believe that there might be something beyond the standard model is that if you were to draw three random lines on a piece of paper, there’s little chance they’ll intersect at a simply point. Nonetheless, if you look at the runnings of the coupling in the standard model, you get three lines to come reasonably close to intersecting. This is a coincidence that is not explained by the standard model. It could just be random, of course, but it could also be a clue.

As for why finding just the Higgs would be depressing, where would we go, then? Building a larger accelerator is unlikely. It means that this dataless purgatory we currently inhabit is likely to last for the foreseeable future. That would suck.