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Thread: Quantum mechanics is to general relativity as electromagnetism is to gravity

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    Quantum mechanics is to general relativity as electromagnetism is to gravity

    I mean the thread title says it all.

    Any input?

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    Well, it's true that quantum mechanics is a theory about electromagnetism, like general relativity is a theory about gravity. Quantum mechanics also describes interactions of the strong and weak nuclear forces, so it covers more than just electromagnetism. I'm not sure how this is a significant statement, though.
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by Grey View Post
    Well, it's true that quantum mechanics is a theory about electromagnetism, like general relativity is a theory about gravity. Quantum mechanics also describes interactions of the strong and weak nuclear forces, so it covers more than just electromagnetism. I'm not sure how this is a significant statement, though.
    Oh..

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    Quote Originally Posted by Grey View Post
    Well, it's true that quantum mechanics is a theory about electromagnetism, like general relativity is a theory about gravity.
    While trivially true, the OPs statement doesn't even go far enough. One can draw an analogy between the equations of electromagnetic force and (Newtonian) gravitational force. But this is limited because charge can have positive and negative values, while mass is only positive, the Newtonian equation only holds in low energy conditions, the electromagnetic force can be blocked but gravity can't, etc.

    I'm not sure how this is a significant statement, though.
    Agreed.

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    Quote Originally Posted by Chunky View Post
    I mean the thread title says it all.

    Any input?
    By your response to Grey's post, I assume you're talking about General Relativity then? I'll change the thread title, let me know if that's not OK.

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    Quote Originally Posted by Grey View Post
    Well, it's true that quantum mechanics is a theory about electromagnetism, like general relativity is a theory about gravity. Quantum mechanics also describes interactions of the strong and weak nuclear forces, so it covers more than just electromagnetism. I'm not sure how this is a significant statement, though.
    Quantum mechanics is a catchall or framework. QED is the electroweak quantum field theory, QCD is the strong force quantum field theory.

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    Quote Originally Posted by grapes View Post
    By your response to Grey's post, I assume you're talking about General Relativity then? I'll change the thread title, let me know if that's not OK.
    Fine with me. I would of made a body... but.. I really didn't know where (or how) to begin. Thanks to greys input it makes me wonder if connecting gravity and electromagnetism. Would in itself create a unifying theory for quantum mechanics and general relativity. -noob statement noob noob noob-

    Because I've no idea..

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    Quote Originally Posted by Shaula View Post
    Quantum mechanics is a catchall or framework. QED is the electroweak quantum field theory, QCD is the strong force quantum field theory.
    Does this mean.. quantum mechanics in itself needs a unifying Theory also? -noob noob noob-

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    Something to consider is that general relativity does take electromagnetism into account, and can be fairly naturally coupled with Maxwell's equations. Electromagnetism is far less foreign to GR than gravity is to QM theories.

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    Quote Originally Posted by Chunky View Post
    Fine with me. I would of made a body... but.. I really didn't know where (or how) to begin. Thanks to greys input it makes me wonder if connecting gravity and electromagnetism. Would in itself create a unifying theory for quantum mechanics and general relativity. -noob statement noob noob noob-
    There are several theories out there that have tried to do that, perhaps the most famous being Kaluza Klein theory. So far this approach has worked only for toy universes and has not really worked well to describe our universe. There are plenty of other attempts to do this, but so far nothing robust.

    Does this mean.. quantum mechanics in itself needs a unifying Theory also? -noob noob noob-
    People are always working on Grand Unified Theories and so on. But it is perhaps more correct to say that QM describes quantum fields and how they interact - but it does so generically. QED and QCD use that generic formulation to describe forces we experience. The reason they are often split out separately is not because they are radically different theories that need to be unified but because QED is mainly about fairly weak interactions and is largely expressed as a perturbative theory. QCD describes much stronger interactions and hence uses the same underlying theoretical framework but a completely different computational approach to get answers. The difference is essentially that with QED you can take a simple, easy to calculate first order interaction and it will be close to the final answer. If you need more accuracy you can add in the second, third etc order effects to get closer to the answer. In QCD you cannot because the second, third etc order effects are as important as the first order effect to the answer you get.

  11. #11
    Quote Originally Posted by Chunky View Post
    ... it makes me wonder if connecting gravity and electromagnetism...
    I'm sure that gravity and electromagnetism are completely different things. At least because the electromagnetic field can be damped by screening, and the gravitational field can not be. This suggests that the gravitational effect occurs through some parallel space/dimension, in which there are some certain lines of force, the difference in energies that creates the acceleration of gravity. I now conduct experiments with a pendulum to determine the frequencies of such gravitational lines. Unfortunately so far unsuccessfully.
    Last edited by Nikolay Sukhorukov; 2018-Apr-15 at 07:31 AM.

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    Quote Originally Posted by Chunky View Post
    Fine with me. I would of made a body... but.. I really didn't know where (or how) to begin. Thanks to greys input it makes me wonder if connecting gravity and electromagnetism. Would in itself create a unifying theory for quantum mechanics and general relativity. -noob statement noob noob noob-

    Because I've no idea..

    okay, if you have no idea, then this tread is moot.
    closed, if there is any reason why this should be reopened, report this message.
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    As this "ATM" thread sparked some interesting discussions, I have moved this to S&T.
    Please discuss further, but keep away from ATM topics.
    All comments made in red are moderator comments. Please, read the rules of the forum here and read the additional rules for ATM, and for conspiracy theories. If you think a post is inappropriate, don't comment on it in thread but report it using the /!\ button in the lower left corner of each message. But most of all, have fun!

    Catch me on twitter: @tusenfem
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    I would say there is a fundamental flaw in saying the relationship between quantum mechanics and general relativity closely mimics the relationship between electromagnetism and gravity. The latter is a relationship between two classes of phenomena, with no hint of any incompatibility. Natural phenomena, almost by definition, are never incompatible with each other, they are what they are. Theories that attempt to describe them, on the other hands, are famously incompatible because theories can be generalized to all kinds of extreme situations, whereas natural phenomena cannot-- natural phenomena just are, so if they work differently in some other regime, then the phenomena is different in that regime. So I don't think one should ever make an analogy that looks like "theory A is to theory B as natural phenomena a (intended to be described by theory A) is to natural phenomena b (intended to be described by theory B." That analogy essentially confuses the difference between a phenomenon and a theory that describes said phenomenon, and they way theories relate to each other is fundamentally different from the way phenomena relate to each other.
    Last edited by Ken G; 2018-Apr-17 at 04:01 PM.

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    I'd agree that a much better analogy would be to flip it around and say something like, "quantum mechanics is to electromagnetism and the strong and weak interactions as general relativity is to gravity".
    Conserve energy. Commute with the Hamiltonian.

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    Right, and put that way it would seem to miss the original intention. Said that correct way, the analogy would only be saying "a theory has a certain type of relationship to the phenomena it describes," but that isn't likely to sound too surprising. I believe the intent of the original analogy was to speculate that the ways those theories relate to each other should reflect the ways the phenomena relate, both in terms of comparisons and contrasts. But one way to relate two phenomena is to consider what happens when both are present together, and then the ways phenomena relate are usually quite different from the ways theories relate, because when nature mixes two phenomena, you automatically get a third phenomena, which may or may not be the simple addition of the two. But theories do not come with instructions about how to combine them, and in some cases you may find it impossible to combine them because they don't share a common language, and in other cases you may think there is a natural way to combine them but the result would produce logical contradictions (such as GR and QM at the Planck scale).

    So for this reason, the relationships between theories generally do not reflect the relationships between phenomena, that's a kind of "incorrect algebra for relating theories to each other." Nature does not have the luxury of incorrect ways to interrelate her phenomena-- she has to figure it out somehow. One example of this is, Bohr had a way to talk about how individual electrons behave in an atom, which related some quantum mechanical concepts to some electromagnetic concepts. But it took a rather different and more complete theory to describe how two electrons behave in the same atom. The phenomena of how two electrons relate to each other does not show up in the relationships between any of the theories of single electrons, be they quantum mechanical or electromagnetic. Perhaps I could summarize by saying that theories have to built top-down, not bottom-up, because phenomena interrelate in a top-down fashion (you often need to see how the whole system behaves to know how the individual parts will, like ants in an anthill). Trying to relate theories to each other, and expecting the phenomena will relate similarly, misses this point.
    Last edited by Ken G; 2018-Apr-17 at 08:57 PM.

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