• [deleted]

Steve, I think our views on particle physics are so far apart that I could not find any common ground, but thanks for your comments all the same.

  • [deleted]

I think it is all part of a bigger picture. Your QI work is still very interesting to me even if you don't see it going in the same direction.

  • [deleted]

Vladinir, yes we all spend many years in education and almost everything we learn is correct, so it is easy to put too much faith in the formalisms and extrapolate them to new areas as if things have to work the same way. To do fundamental physics it is important to have a good feel for why we believe in certain things. If it is because mathematical logic or experiment confirms that they are right, then that's fine. If it is just because we have grown used to the formalism then we need to question if it is the right way to go forward.

I think the choice between continuous and discrete mathematics in theories of quantum gravity is a good example. Classical physics gets us used to working with continuous functions and it is only when we do quantum physics that some discrete theory comes in. Even then the formalism is usually portrayed as continuous with discrete aspects arising from solutions to differential equations of bound systems. People who have come through a physics education tend to favour continuous formalisms because that is what they have always used

Some people who are less steeped in physics as their education take the opposite view and think that things have to be discrete ultimately because infinities are illogical. That is not really correct either. There is nothing logically wrong with continuous mathematics or infinity.

I don't know whether the laws of physics will ultimately be expressed using continuous or discrete mathematics but aspects of both seem to be important at our current level of understanding so it is important to keep an open mind.

The Fermi findings which rule out some discrete theories are an important clue that must be taken into account, but so is the holographic principle which seems to point in the direction of discrete bits. We have to find a way to incorporate all the indicators we get.

I don't think that string theorists as a group are close-minded. I don't personally fit the profile of a typical string theorist, but I have talked with some and read the work of others. They all seem willing to consider new ideas. They are just unwilling to look at alternatives to string theory that seem less promising.

There are some powerful arguments that string theory is the right road to quantum gravity. There must be some logically consistent description of the interaction between particles and gravity in the limit of weak fields and almost flat spacetimes. Perturbative string theory is the only solution we have for that. It's not a watertight argument, but until someone provides an alternative that achieves the same thing I think we will see people continuing to do string theory.

Personally I do like to look at the alternatives even if they do not succeed where string theory does. You have to understand the failures to know how to succeed. Loop Quantum Gravity does not work at a perturbative level but it provides some pointers about the kind of mathematics that applies to non-perturbative gravity and its origins are close to those of string theory. Its offshoots such as spin foams, group field theory, quantum graphity etc have some nice mathematics that may be clues about how to formulate string theory non-perturbatively. I find it more sad that people working on these things reject string theory than that string theorists reject the alternatives. Everybody seems to work in a very narrow band of ideas. Instead they need to stand back and take in the big picture, think about what works and what does not work in different approaches, then consider how lessons learnt can be brought together.

  • [deleted]

You should use the equation feature for your latex

[math]G_abcd\psi^a\psi^b\psi^c = 0[/math]

[math]M^{ab} = G^{abcd} \psi^b\psi^c[/math]

etc.

  • [deleted]

Hi dear Philip,

I just say my opinion.A critic can be constructive.

I recognize that said a real searcher.And I congratulate you for your essay.

Indeed the roads are different,but universality is universality.

Happy new year also.

Steve

  • [deleted]

About half the time the little preview feature does not work, so I can't check these. So if I use the TeX feature there is about a 1/3 chance that the final product will look like crap. That is a problem with these HTML TeX interfaces. So for simple equations using unicode seems easier.

Cheers LC

  • [deleted]

Phil,

Quantum gravity theories are based on two underlying assumptions:

1) classical gravitation can be quantized and consistently treated as a quantum field.

2) its effects become perceivable at some large energy defining the unification scale.

The question is: What evidence there is that these two assumptions are on the right track?:

1) there is no direct evidence for gravitational waves, let alone for gravitons. I am not talking here about indirect evidence from binary stars but observations from LIGO and similar detectors.

2) there is no direct evidence that gravitation survives as an interaction field below an experimental limit of about 50 um or so. It is only inferred from what we know today that this indeed must be the case. But how solid is this hypothesis?

We just started to explore physics on the TeV scale. At least from where we stand today, it seems to me that the two assumptions I listed do not have a strong experimental support.

  • [deleted]

Ervin, you make some very good points.

I would be very surprised if gravitational waves are not detected with a future generation of detectors. The indirect evidence from pulsars is very hard to explain any other way. When physicists applied for funding for the first LIGO runs they were a little optimistic about what gravitational wave sources there were likely to be. As a result LIGO is starting to look like it has failed. I think they just need more sensitivity.

Even if there are gravitational waves it does not mean that gravity necessarily has to be quantised, but it is difficult to work any other way. If quantum gravity is hard then having no quantum gravity is even harder. The assumption that gravity must be quantised leads to string theory. Where do you get if you assume it is not quantised? I think physicists have been happy to play around with any set of modified assumptions but they have ended up writing about the possibilities that lead somewhere and kept quiet about other choices they tried. What else can they do?

Although there is not much experimental support for these assumptions and extrapolations, the requirement to consistently combine quantum theory with gravity seems to be such a restrictive constraint that we can make some progress without it. I hope that when we fully understand the theory we will be able to make some predictions that can be tested without a Planck energy accelerator.

    • [deleted]

    Phil,

    But the lingering question remains: do we know enough about Terascale physics to hope that our current understanding will stand as is? History of physics has repeatedly taught us that new and unexpected layers of reality emerge as we probe deeper into its structure.

    How about if the fundamental premises of lagrangian field theory will break down around, say, 10 TeV or so? Early findings at LHC and elsewhere point to anomalous observations that cannot be easily accounted for with current theoretical models.

    How about if reality is a dynamic complex structure and a multifractal at its core, in accord with a wealth of findings in all branches of fundamental and applied sciences?

      • [deleted]

      The reply above is mine. Sorry about not entering my name.

      Ervin

      • [deleted]

      I know the feeling. It could be the case that physics above some scale is completely different from anything we have seen so far. Without much hope of experimental input we would never figure it out. We would have no more hope of understanding the laws of physics than a cat has of understanding quantum mechanics.

      I am more optimistic than that and I will tell you why. When we try to bring together quantum mechanics and gravity we find that black holes must have entropy and temperature that extends the laws of thermodynamics in an unexpected and surprising way. To understand how this can work we need to invoke a holographic principle that nobody would have anticipated any other way. When we look for a perturbative theory of quantum gravity we find that we can do it with a theory of strings which turns out to be something we can use to solve deep problems in pure mathematics. Then we find that in this string theory the holographic principle applies just as it needed to for consistency.

      If physics beyond some scale is something else then the existence of all these remarkable theories is just a crazy coincidence that has led us all astray. I think there is too big a set of coincidences for this to be likely. In the past when the maths has turned out to work it has later been verified by experiment. The Dirac equation which predicted antimatter is the canonical example, but Maxwell's equations and relativity were similar. With quantum gravity we have pushed out further into unknown territory with no experimental feedback, but the network of mathematical theories that hang together unexpectedly is also bigger and more impressive. I think this is because it is right and the assumptions that have been made were the correct ones. It is still incomplete but progress is still being made. I don't know how long it will take, but I think the signs are that it is something we have the capacity to comprehend.

      I think that results from the LHC are likely to help us understand a lot more. I'm not sure which anomalous observations you are referring to but it is early days. I hope the gap between these energies and the Planck scale can be bridged in some meaningful way with a combination of theory and experiment but it is impossible to know how far there is to go.

      • [deleted]

      Nice job! I especially appreciate your faith in the ability of M theory to produce a complete algebraic theory. I agree that continuous function physics is so ingrained in our consciousness that it's often hard to escape.

      Tom

        • [deleted]

        Phil,

        You say,

        "In the past when the maths has turned out to work it has later been verified by experiment"

        There are many counterexamples where reality either already fails or may fail to follow appealing mathematical frameworks: chirally symmetric field theories, SU(5) GUT, Deformed Special Relativity, preons, large extra dimensions, Fermi liquid behavior in strongly correlated compounds, spin-statistics connection in fractional Quantum Hall effect, Navier-Stokes equations of fully developed turbulence and so on.

        Is there comprehensive evidence that holography works as advertised in strongly coupled field theories? Is there convincing observational evidence that black holes and exotic thermodynamics are able to fully and successfully account for physics of the Terascale sector?

        I am not pessimistic when I pose these questions. I want to caution about extrapolations based on beautiful mathematical structures that may turn out to be wishful thinking after all.

        .. Still fresh from the old cyclotron...

        Philip,

        I think the highest and most fundamental symmetry is from the existence/non-existence duality. From this binary symmetry, I can see the Weizsaker approach exploding into a whole universe. Assuming we start with nothing, a whole universe may start to exist from this one cubit. But, as explained in my last essay, only an evolving time can exist at that "moment" and forever after without failing the rule of non-contradiction.

        p.s. for other essay writers: don't forget page numbering as this is important for reference in discussions..)

        Thanks,

        Marcel,

          • [deleted]

          " ... and reverse engineer the program that codes our universe."

          A couple of questions:

          (1.) What's the basis for assuming that the computation analogy is valid in this context? Does the fact that we think in terms of "programming" mandatorily mean the Universe does?

          (2.) NP-Completeness. This is an issue for quantum computation just as much as it is for classical. Could the proposed reverse engineering even be conceivable unless P=NP? Does it?

          • [deleted]

          Phil, if you just shrug your shoulders about the existence or non existence of SUSY partners, you are not doing physics. You have to decide.

          • [deleted]

          As I've said on vixra log I am quite optimistic about SUSY to be found at the LHC in some form, but it is far from certain. It would not directly affect any work I have done if it was not there.

          • [deleted]

          nikman, there is certainly not anything mandatory about the proposal that the world works like a computer. It is just a hypothesis that we can follow in our pursuit of better understanding. A statement like "the universe is a quantum computer" is currently just a motivational buzzphrase. It does not mean a great deal until we can find a concrete mathematical theory for which it is a reasonable interpretation. It may lead to the development of such a theory.

          The justification for considering it is certainly not that we think in such terms. Usually that is not a good guide. The justification is that the mathematics of information and especially quantum information comes up in string theory.

          It is not obvious whether or not an answer to the problem P=NP would have any implication for physics. Perhaps if we had a better idea of how the computer "analogy" is realised it would be possible to say more about it.

          • [deleted]

          I like it!

          So are you doing an essay this time round?

          Oh yeah I should have put page numbers, but you can see them in acrobat reader.

          • [deleted]

          Yes, some people want physics to be purely geometric. prefer it to be purely algebraic.