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Can there ever be a Theory of Everything?

  1. Mar 15, 2015 #1
    This article triggered the topic: http://physicsbuzz.physicscentral.com/2010/09/hawking-mlodinow-no-theory-of_30.html

    In "The Grand Design", Hawking argues that there may never be a Theory of Everything (in spite of a movie by the same name).

    The article says "In a quantum world, particles don't have definite locations or even definite velocities until they've been observed. This is a far cry from Newton's world, and Hawking/Mlodinow argue that - in light of quantum mechanics - it doesn't matter what is actually real and what isn't, all that matters is what we experience as reality".

    So can physics find an objective reality? Is that the goal of physics? Or has objective physics merged with subjective philosophy? Is the goal simply to "explain" the world as we see it?
  2. jcsd
  3. Mar 15, 2015 #2
    Well, they are working on it. If its not possible then they wouldn't be working on it.
  4. Mar 15, 2015 #3


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    Please note that there is a misuse of terminology here. What you are describing is NOT the "Theory of Everything" that is being discussed in physics. The TOE in physics has nothing to do with "objective or subjective reality", which has a more philosophical leaning.

    I suggest you get a Mentor to re-title your thread with a proper title and not cause any confusion or misleading discussion.

  5. Mar 15, 2015 #4


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    Hard to get more philosophical than this, but we can say the following:
    Physicists make up mathematical models to fit and predict experimental data
    Then, they perform more experiments to test how well the models work.
    With more data, we may see that our models don't fit quite as well as they should, and new models are developed.
    Sometimes, these models are so good that they predict things to exist that haven't been discovered experimentally until decades later.

    So can physics find an objective reality?
    Only if we first assume that objective reality exists (whatever that means).
    Then, we could only say that those parts of reality that we can experimentally test appear to fit our mathematical models to so-and-so accuracy.
    At best, we can get a hazy idea of what those parts of objective reality that we can interact with experimentally is like to a certain precision.

    If I had to guess, I'd say the goal of physics is two-fold. First, it is to create predictive models (as good as possible) of observable phenomena that allow us to understand how things work and what they're made of, and second (much more ambitious) is to have a single model that covers all observable phenomena (a theory of everything).

    I'm no expert in the philosophy of physics, though, so you may want to consult better sources.
  6. Mar 15, 2015 #5
    From my understanding yes it's definitely possible since any universe (notably our own) must be observer based for scientific inquiry to emerge in the first place.
  7. Mar 15, 2015 #6


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    I think that until we HAVE a TOE, if we ever do, it might not be possible to say we can or can't get one.
  8. Mar 15, 2015 #7


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    The article is probably misinterpreting. Even Einstein, who was a realist, said "It is the theory which decides what can be observed." Even more interesting, he said that in support of realism.


    Heisenberg: "We cannot observe electron orbits inside the atom...Now, since a good theory must be based on directly observable magnitudes, I thought it more fitting to restrict myself to these, treating them, as it were, as representatives of the electron orbits."

    "But you don't seriously believe," Einstein protested, "that none but observable magnitudes must go into a physical theory?"

    "Isn't that precisely what you have done with relativity?" I asked in some surprise...

    "Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same....In reality the very opposite happens. It is the theory which decides what we can observe."
  9. Mar 15, 2015 #8


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    Hi Gort,

    It would be quite bold/stupid for a layman to say that the distinguished and very brilliant Stephen Hawking is wrong – instead I put forward this question:

    Does Hawking have any formal proof that ToE is impossible, besides current state of affairs?

    My guess is no...

    Then again, professor Hawking has a point; Quantum Gravity is far from solved, GR and QFT are mutually incompatible in regions of extremely small-scale and high-mass, Gödel's Theorem put limits to any formal theory, etc. I.e. there are BIG problems to solve.

    But this is the fun part!

    We live in an era of almost unbelievable scientific discoveries, accelerating every day. Enjoy the ride!

    I guess not. Physics is about building (approximate) models of nature for predictions/explanation, not finding the indisputable ultimate TRUTH.

    However, if you by "objective reality" mean the old Local Realism – i.e. particles that exist with predefined properties – I have to make you disappointed; it's a dead parrot that won't resurrect. If you want realism it has to be non-local, or you have to hold on too even spookier QM interpretations of many-worlds, or accept superdeterminism.

    Personally, I see not much difference between MWI and philosophy... ;)

    Not a bad goal if you ask me.
  10. Mar 15, 2015 #9
    I'd be inclined to agree with the majority of what DevilsAvocado said... semantically speaking EVERYTHING is possible on an infinite timescale, and given how shaky the big bang theory actually is (does it even have mathematical and scientific legs to stand on anymore given all the new observations we've found?) we're still not really sure IF the universe has a beginning and an end, if it's merely a continuous loop, goes on forever, or any other type of iteration in between.

    So in other words it really depends on what the OP is asking.. whether we can really even DEFINE what "real", "realism" implies or stands for (objective/subjective). So yea without getting too much farther into philosophy I think most people can say with confidence that a TOE is not really happening in our lifetime]

    As a relative layperson in physics I'll raise you one and claim that many contemporary physicists have already begun to accept the "many-worlds" interpretation given how much data we now have against the big bang theory?
    Last edited: Mar 15, 2015
  11. Mar 15, 2015 #10
    Would a ToE have to explain why the universe is quantum mechanical and why space has the ability to warp and bend? I suspect that this is what will be needed to derive quantum gravity.
  12. Mar 15, 2015 #11
    According to Penrose the quest goes even beyond quantum gravity and dark matter/energy: in the "Shadows of the Mind" book he asserts that consciousness needs a non-computable physical universe. I thought it was bordering on crazyness until they found that brain microtubules actually display quantum coherence last year: http://www.sciencedaily.com/releases/2014/01/140116085105.htm - exactly as he hypothesized. Then I read the book and was pretty intrigued.

    Just a fun thought...
  13. Mar 15, 2015 #12
    This is not my terminology - it's either Hawking's (which is worthy of comment) or it's the mass media reporting on Hawking's terminology (also worthy of comment).
    The New Scientist said "Three decades ago, Stephen Hawking famously declared that a "theory of everything" was on the horizon, with a 50 per cent chance of its completion by 2000. Now it is 2010, and Hawking has given up. But it is not his fault, he says: there may not be a final theory to discover after all." See http://www.newscientist.com/blogs/c...king-says-theres-no-theory-of-everything.html.

    Scientific American (quoting Hawking) says ""In our view, there is no picture or theory-independent concept of reality. Instead we adopt a view that we call model - dependent realism: the idea that a physical theory or world is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations. According to model - dependent realism, it is pointless to ask whether a model is real, only whether it agrees with observation. [this, in my interpretation, is "subjective observation" - as opposed to "objective reality"]. If two models agree with observation, neither model can be considered more real than the other. A person can use whichever model is more convenient in the situation under consideration." [Can't get more subjective than that!]
  14. Mar 15, 2015 #13
    "Now, if you believe that the universe is not arbitrary, but is governed by definite laws, you ultimately have to
    combine the partial theories into a complete unified theory that will describe everything in the universe. But
    there is a fundamental paradox in the search for such a complete unified theory. The ideas about scientific
    theories outlined above assume we are rational beings who are free to observe the universe as we want and to
    draw logical deductions from what we see.
    In such a scheme it is reasonable to suppose that we might progress ever closer toward the laws that govern
    our universe. Yet if there really is a complete unified theory, it would also presumably determine our actions.
    And so the theory itself would determine the outcome of our search for it! And why should it determine that we
    come to the right conclusions from the evidence? Might it not equally well determine that we draw the wrong
    conclusion? Or no conclusion at all?"
    From A Brief History of Time by Stephen Hawking.
  15. Mar 15, 2015 #14
    My understanding (from the "Grand Design") is that Hawking changed his views from "A Brief History..."
  16. Mar 15, 2015 #15
    I think many people would agree with those goals. But what about when separate models give us the same observational predictions? Case in point - wave/particle duality of light. QED, QCD, and other areas of particle physics predict the behaviour of photons. Quantum Field Theory (QFT) nicely predicts the behaviour of light as a wave (actually, a field excitation). Both give equally valid results. Some approaches are easier to use than others, but either CAN be used. Which is "correct"? Does it matter? That's what meant by "objective reality". Are there really fields, and are particles (including photons) merely excitations of those fields? Or are there bundles of energy (called photons) which have a probability amplitude to take arbitrary paths in Minkowski space, and have a probability amplitude to travel at any speed (not just c)? QED certainly models this view, as it sums up all possible paths and speeds, and comes up with a straight line and velocity c. Does this actually occur (all experiments verify it!), or shouldn't we even care? I guess that's the question - can we ever know what "actually occurs"?

    I think Hawking used to believe that once you know what "actually occurs" in Nature, you can then develop a ToE - one that would hold for all observation. Even if we developed new "senses" or created new "observables" - the ToE would hold true. Now, I think, he believes that we can NEVER know how Nature "actually works". So a ToE (if you can call it that) would always be dependent on conditions and observables. I tend to agree with this latter view. Changed from when I first started studying physics - when the implicit goal was to "understand Nature".
  17. Mar 15, 2015 #16


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    That is a very unusual definition of objective reality, and confuses all the important issues. In classical physics, one basically always assumes objective reality, but that does not mean that the theories of classical physics are really theories of everything. Newtonian mechanics, special relativity and general relativity all assume objective reality, but are not theories of everything because they are approximate descriptions of objective reality.

    In contrast, the wave function alone is very hard to consider objective reality, because it allows things like Schroedinger's cat. So in the usual interpretation, one does believe in the objective reality of one's self, one's measuring instruments and one's observations, but the wave function is not necessarily real. So quantum theory, unlike classical theory in which there is just one copy of your "consciousness" in objective reality, allows several views. If one adds hidden variables, then the wave function can be considered potentially real as in Bohmian Mechanics. If one does not add hidden variables, then one can still consider the wave function potentially real but there may be many copies of your "consciousness" in a single objective reality. Or one can try a sophisticated redefinition of reality like in Griffiths's consistent histories.

    In other words, it is fine in classical general relativity to say that in reality spacetime is really curved, but it is an approximate statement that should fail if we probe at sufficiently high energies and the concept of spacetime fails to hold. In contrast, in quantum theory, it does not seem to make sense to say that the cat is really dead and alive at the same time, even as an approximation.
    Last edited: Mar 15, 2015
  18. Mar 15, 2015 #17


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    As other posters, correctly alluded to IMHO, it boils down to what you mean by reality, really etc. Sorting that out is a philosophers game and physics ages ago, for right and proper reasons IMHO, divorced themselves from that. The reason physicists in general think of fields as real is certain no go theorems figured out by Wigner - you will find some of the detail in Ohanians textbook:

    If we are to require, as Nothers theorem implies, conservation of momentum and energy then they need a place to be stored for fields - the obvious place being the field itself. If the field has energy and momentum like particles then its only reasonable to think of them as real. That's the whole thing with physics and philosophy - physics takes a simple common-sense view - philosophy argues continually about the most basic of things and gets no-where.

    Regarding your comments about QFT its now known its a crock of the proverbial - the usual approach of Feynman diagrams, virtual particles etc are simply artefacts of the perturbation formalism used. So physicists sometimes can sort out what is real and not real - although it can take time

    Feynman has commented on this sort of thing. Sometimes physicists made progress by taking ideas from philosophy like positivism or whatever. But they only really work once. Physicists are awake to the trick after that and its one of the first things they try to make progress. If it worked the progress would have been made and we would be onto the next challenge. Since it didn't its pretty certain some new idea is required. I have zero doubt this idea of 'actually occurs' would have been tried, after all it was one of basic ideas of Newtonian classical physics. I don't think it worked.

    I believe the next phase of physics will rely on something startling and different to what's gone on before.

    Last edited: Mar 15, 2015
  19. Mar 15, 2015 #18


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    Penrose actually believes the equations literally are the reality - not describing reality where exactly what reality is, is left to philosophers to argue about - but is the reality. I used to believe that but changed my mind when I saw an interesting talk by Gell-Mann:

    You may think it a bit of a weird view, and thinking back on it I now think its a bit weird, but my sneaky suspicion is a lot of physicists of the mathematical physics variety lean towards it.

    That's part of what I mean when I say its pretty meaningless to ask questions like what is reality etc - it cant be the math of our equations etc etc. The thing is - yes it can - and some people actually believe it is.

  20. Mar 16, 2015 #19
    Maybe I've not made the relationship between objective reality and ToE which Hawking alludes to very clear.
    We observe apples falling from trees. We come up with a model which says apples fall from trees due to Newtonian gravity. That model is not expected to be a ToE. It simply predicts that, if we have pears, they'll also fall from trees. With the same acceleration. That said, any ToE must explain (as part of it) that apples do fall from trees.

    Now let's pretend we live in a cloud-covered planet. Like Venus. And we didn't have rocket technology. Then, our observations about gravity would be pretty much limited to apples falling from trees. We couldn't observe Mercury and its precession of perihelion. We couldn't observe the recession of galaxies or gravitational lensing. There was no Einstein. So no GR (with or without the cosmological constant). No need for it. Newtonian gravity matched observations. So it became entrenched in a ToE. But was it actually a ToE? No - suppose we suddenly invent rockets and the universe is opened up to us. Suddenly, we need a new "ToE".

    I think this is what Hawking has realized (again, this is Hawking - not me!). He decided that there may be no such thing as a ToE because we don't know (and probably will never know) exactly how Nature works. We'll always come up with a new form of measurement to yield new observations. And theories will have to be revised. The only valid theories (again, this is Hawking) are theories which match observation under a particular set of circumstances. If two theories do that (say, QED and QFT), then take your pick - there's no right or wrong.
  21. Mar 16, 2015 #20


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    Yes, but this has nothing to do with quantum mechanics and the difficulty that quantum mechanics has with objective reality. The entire reasoning would hold even if we only had classical theories. In fact the entire reasoning is conventional in classical physics.
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