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Why would we want to unify general relativity and quantum mechanics?

  1. May 1, 2013 #1


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    The question is pretty simple, why would we even want to combine the two big theories to create a unified theory of everything? Is it not better to simply say that the laws of physics behave differently at these scales and that a theory of one doesn't necessarily have to work with another? Granted we need to work towards a theory that explains all the forces of nature, but perhaps logic and reason from one scale simply can not be applied in the other, such as the assumption that particles behave like solid objects like atoms or molecules and are bound by our laws of nature, for example. Is it not a fallacy in logic and arrogant of us to expect one single theory to explain everything we observe (and don't observe) in the universe around us?
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  3. May 1, 2013 #2


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    Staff: Mentor

    The problem is that there are areas where the two overlap. There is nothing in the structure of GR to tell us that it shouldn't apply at arbitrary small scales, and likewise nothing in the structure of QM to tell us that it shouldn't apply to large masses. So which theory do we use when we have a large mass concentrated in a small space?
  4. May 1, 2013 #3


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    IMO two reasons were given by Newton, in his "rules for how to do science" published in "Principia":

    1. The same laws of physics apply everywhere in the universe, and
    2. There should be the minimum number of laws.

    If you don't believe in rule (1), you might as well not try to do physics at all, and rule (2) goes back a lot further than Newton - at least to William of Occam, and probably to the ancient Greeks.
  5. May 1, 2013 #4
    The snarky answers is that most people dont want too. Most scientists only give it a glancing thought and most physicists dont research it.

    For the few that do research it, they dont know that a unified theory is possible but they are trying anyway. Other disparate fields have been unified before and if these two can be then it will be exciting, thought provoking and insightful to the workings of the universe. Beyond philosophical satisfaction, it is also generally assumed that today's useless science is tomorrow's useful science. How often that is actually true, I wouldn't chance a guess.

    It is true that many of the physicists working in this area do have a disposition, and I would say prejudice, towards "beautiful" theories and "simple elegance" (whatever that means). You can see it in the quotes from the greats that are taken to heart. I cant help but see this as embraced confirmation bias. They only look for beautiful, simple and elegant theories and thus they only find beautiful, simply and elegant theories. Meanwhile, other scientists are modeling whole classes of phenomenon without this prejudice and without the first principles of physics.
  6. May 1, 2013 #5


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    To emphasize Nugatory's response, what goes on inside a black hole cannot be adequately described because quantum theory and general relativity don't work together.

    The goal is not so much unification as such, but a reconciliation of quantum theory and GR.
    In the Standard Model of quantum theory, the strong force has not been unified with the electroweak force, but this is not considered a major problem, since there is no apparent conflict.
    Last edited: May 1, 2013
  7. May 1, 2013 #6
    Wouldn't everything bar that which is confirmed by replicated experimentation be a theory? What I mean is that these two points might not be immutable. The new science often uproots the old science.
  8. May 1, 2013 #7
    That's the thing, they don't behave differently at different scales. Some quantum effects are crossing over from the microscopic scale to the macroscopic. Experiments have shown that buckyballs (carbon 60 molecules, clearly macroscopic) follow quantum principles, and some crystals entanglement ridges that were half an inch. So where do we place the divide between the quantum (microscopic) world and the GR (macroscopic) world? There is no divide, it's absolutely ridiculous. That's why we need a single TOE.
  9. May 1, 2013 #8
    Thats a bold and unsubstantiated claim. It feels good and intuitive, but that is not enough.
  10. May 1, 2013 #9


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    That's a common misunderstanding. A theory is something that has been confirmed by replicated experiments (or more accurately, has not been disproved by experiments - experiments don't prove theories, they disprove competing theories).

    It's also very seldom that the new science uproots the old science; much more often it builds on it. Perhaps the most striking example is the way that special relativity reduces to classical mechanics if you take v/c to be negligibly small; SR didn't uproot classical mechanics, it augmented classical mechanics by properly handling velocities that are not small compared with the speed of light.
  11. May 3, 2013 #10
    Thanks Nugatory I realised my mistake after reading the introduction in the introductory physics material.
  12. May 6, 2013 #11
    Einstein himself notes the need for a theory of quantum gravity - it's exactly the same as the reason we needed a quantised theory of electromagnetism; atoms would radiate constantly and thus be unstable. That is to say, by GR, atoms would release tiny amount of gravitational radiation, perpetually. They would be unstable, which is very much at odds with what we see around us. Thus one needs to quantise gravity.
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