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I more or less understand the complications and difficulties in combining the two theories, but what confuses me is why physicists have assumed that this combination makes sense in the first place.

It is not obvious to me, given the evidence that we have, that the two theories can be combined at all. It seems that the two are completely separate systems working in tandem.

Is there a critical piece of evidence that hints that the two can be combined? To me, it seems that we have assumed that this is possible without reason.

I haven't been able to find any justification for this decision, so any feedback is appreciated.

Qmechanic
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A.Z.
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  • Possible duplicates: https://physics.stackexchange.com/q/6980/2451 , https://physics.stackexchange.com/q/52211/2451 , https://physics.stackexchange.com/q/387/2451 and links therein. – Qmechanic Jan 04 '24 at 18:11
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    String theory combines QM and GR. Regardless of whether it is correct or not, it shows that this unification is possible. – Ghoster Jan 04 '24 at 18:22
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    We see both quantum and gravitational phenomena together in nature. We (perhaps arrogantly) like to assume that we can use mathematics to model anything that nature does. – John Doty Jan 04 '24 at 18:35
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    "Why have we assumed that quantum mechanics and the theory of general relativity can be combined into one theory?" - because the alternative is to accept that we will never ever be able to understand the physical world at its most fundamental level, which is a conclusion that most scientists find unacceptable. – gandalf61 Jan 04 '24 at 19:18
  • There are physicists who believe that it cannot be combined in a way to get to quantum gravity, but they think that some form of QFT + classical GR is at work. i.e. What you asserted as an assumption is not an assumption at all. However, it is absolutely necessary that some combination must exist, be it as ugly as the QFT+classical GR, because our universe has stuff that operate in the intermediate regime, and so we need something that can describe that. – naturallyInconsistent Jan 05 '24 at 03:51
  • In short, according to Einstein's equation, if the matter part is quantized, then the left-hand side should also be quantized。 – David Shaw Jan 05 '24 at 03:55

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