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Gravity as space-time curvature and the need to unify it with quantum mechanics

  1. Dec 23, 2011 #1
    Hi there.

    This is my first posting to this forum, and in fact to any forum in many years, so please excuse me if I have not followed the rules correctly or chosen the correct forum.

    I have been watching a lot of Neil DeGrasse Tyson-related videos and it has lead me to a line of thinking that I am wondering about. Perhaps someone here would be kind enough to help me.

    So, if gravity is just the curvature of space-time, then is it possible that it is not a "real" fundamental thing? If it is just curvature, then why do we need to unify gravity with the other forces at a quantum level? Would it not make more sense to try and find a quantum description of space-time distortions (liek curvature due to mass) or a quantum description of space-time itself?

    I mean, could the effects of gravity, just be a property of matter itself, rather than an external force that acts on matter? How do we know gravity is a fundamental force that need unifying? It is, after all, quite different from the other forces that have already been unified.

    My assumption is that since physicists do not seem to be pursuing this line of thinking (that I know of), that I am missing some key concept(s) that invalidates my line of thinking. But, I wonder if someone here might help me to understand what I am missing. Or, if this is a possibility, then what is the current state of this line of thinking and who is working on it?

    I will close by saying that my background is computer science, so I have some physics and calculus, but I have forgotten most of it. So, a layman's level explanation would be much appreciated.

    Thanks very much.

  2. jcsd
  3. Dec 24, 2011 #2
    I had a similar question that I posted a few days ago. Quantum mechanics is good at explaining the sub-atomic, and general relativity is good at explaining the cosmic. Quantum field theory is theorizing gravitons as the mediators of the gravitational force. You suggest that gravity is just the curvature of spacetime, but something needs to be responsible for curving the space time. What I believe is that gravitons emitted from massive bodies cause gravitational effects that we observe as the curving of spacetime. Since mass is only compacted energy thought to be caused by the Higgs Field, there is no reason for us to believe that mass would simply curve space around it. Now this is just what I have absorbed from reading about physics, and I am only in high school so it is quite possible that your idea could have merit, I am just not familiar with the line of thinking. I hope that this helped a little
  4. Dec 24, 2011 #3
    After reading your post again I could simplify my answer to this: There is a need to unify GR with QM because neither theory can explain everything in physics. QM breaks down as the particles increase, and GR breaks down when talking about the sub atomic. Physicists aren't happy having two theories for the universe, so they are looking for new explanations (such as string theory) to simplify. Although General Relativity accurately describes gravity as we observe it, it fails to predict the movements of the sub-atomic so thus the search for a Grand Unifying Theory (or GUT as you may see on this forum) goes on
  5. Dec 26, 2011 #4
    Sadly, String Theory doesn't serve to simplify anything. It's an abortion of a theory.
  6. Dec 26, 2011 #5


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    It's not that QM breaks down as the size of the system increases, the theory just somewhere along the line goes back to ordinary Newtonian mechanics.

    The real rub is that QM doesn't take into account gravity. This is precisely because gravity is so utterly insignificant on the typical scales we talk about particle physics -- all the other forces are something like 10^30 times stronger. However, some situations (gravitational singularities, such as those found in the center of black holes or the big bang) actually have gravity as an important force, and so we cannot ignore it in our quantum mechanics. THIS is why we need a quantum theory of gravity.

    I'll just quickly say there are two ways to go about this. You can take QFT to be your starting point, and try to represent gravity as a field and quantize it in some manner. This is what is done in string theory essentially. The other way to go about it is to take GR as your starting point, and try to somehow coax out quantum field theory from some dynamic theory of geometry. This is the approach taken by such schemes as Loop Quantum Gravity. (Much less familiar with this one, so someone else who is might want to come correct me or add more detail).
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