Why are Quantum mechanics and General relativity incompatible?

In summary: This makes the entire approach of QM rather difficult to apply to gravity.In summary, Quantum mechanics and General relativity are incompatible due to their differences in mathematical assumptions and conceptual approaches. This creates difficulties in applying QM to gravity and results in bizarre or contradictory predictions. A new theory is needed to better explain the behavior of particles and large objects.
  • #1
MagnusM
3
0
Why are Quantum mechanics and General relativity incompatible?
Probably some complicated mathematics going on? Right?
 
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  • #2
MagnusM said:
Why are Quantum mechanics and General relativity incompatible?
Probably some complicated mathematics going on? Right?
Your guess is correct. When both need to be applied at the same time (for example inside a black hole), mathematical nonsense results.
 
  • #3
Actually, it's not that complicated. The most obvious incompatibility is principle of superposition. Basically, Quantum Mechanics requires that equations that describe the system are linear. QM completely breaks down without this assumption. On the other hand, the field equations from General Relativity are non-linear.

Linearized gravity, which is basically Newtonian Gravity with added gravitomagnetic effects, can be quantized with some success. It's not terribly practical, but it makes some predictions about gravitational waves and gravitons.

Alternatively, you can do some limited quantum mechanics in GR setting, so long as effects of QM don't alter gravity too much. Hawking Radiation relies on this, for example.

So there are ways to make them work together in some limited situations, but overall, the equations are incompatible, and so a better theory is needed. My bet would be on QM being wrong and working only as a small-scale approximation, with true field theory being non-linear, same as that of gravity. But that's just a stab in the dark, really. Of course, we'll keep using QM in all likelihood regardless, due to it's great utility, same as we keep using Newtonian Mechanics despite its limitations.
 
  • #4
to put it rather simple , general realtivity aplies and correctly predicts the action of "big objects " while quantum mechanics tries to explain the unpredictable behaivor of small paricle , particles that everything is made of.
you can`t have a set of rules for big objects and another for the small particles that they are made of .
to me the most important factor why they can't go togehter , is called the uncertanity principle , it states that small particles can't be acuratly meaured without interfering on the particles behaivor and thus altering the result of the measurment. small particles simple seem to be to random to unpredictable , in fact they cannot even be meausered without being altered , and you can surely figure that this is not the case for any " big object "
big objects made of small particles seems to behave different from the particles that they are made of , that is way the general relativity and QM fails to "explain" things , when put together
 
  • #5
castro, quantum mechanics adequately explains all of the behaviors of "large objects" with gravity being the only exception.
 
  • #6
and gravity beeing a result of an objects effect on space time due to its mass , so if you can't predict the effects of spacetime on an object and viceversa , can you truly say that you have explained the objects behavior ?
i know that the problem is that quantum mechanics fails to quantized gravity , or explain for the particlee that would act as "transport" .
and the point that i wanted to highlight was the fact that small particle acts " ramdomly " and unpredicteable. i know that the obsevation paradox is a tricky problem , to say the least ,and i can't se how i would eventually get a slotion , becuase you can't meauser without measuring ... and i think that the same aplies for big objects as well , but its precissly their mass ,and their effects on spacetime that make them differ from particles, a person can't be at two place at the same time , simply because he weights to much ..
or maybe i got all this wrong ?
 
  • #8
In GR, the metric of spacetime depends on the stress-energy distribution. In QM, you never know the energy distribution exactly.
 
  • #9
Khashishi said:
In GR, the metric of spacetime depends on the stress-energy distribution. In QM, you never know the energy distribution exactly.
That's exactly the sort of thing that Quantum Field Theory is designed to deal with. And for other fields it does.
 
  • #10
Thank you for the replies. Getting some (although little compared to everyone else) understanding now. Still, I have one question remaining.

mathman said:
Your guess is correct. When both need to be applied at the same time (for example inside a black hole), mathematical nonsense results.

Mathematical nonsense as in bizarre answers and something very different from reality?
Like gravity being predicted to be too weak or something like that?
 
  • #11
MagnusM said:
Mathematical nonsense as in bizarre answers and something very different from reality?
Like gravity being predicted to be too weak or something like that?
The assumptions are incompatible. And by assuming a contradiction, you can derive absolutely anything. So in principle, any result you'd like, you can get. Want it to be too high, it can be made too high. Want it to be too low, it can be made too low. Mathematics is completely broken at this point.
 
  • #12
MagnusM said:
Why are Quantum mechanics and General relativity incompatible?
Probably some complicated mathematics going on? Right?

K^2 is giving you some good information.

QM and GR are also incompatible on a conceptual level- QM, even QFT, uses spacetime as a background upon which fields are described- for example, fields are written as U(x,t). However, in GR, spacetime is itself a dynamical field.
 

1. What are Quantum mechanics and General relativity?

Quantum mechanics and General relativity are two theories in physics that explain the behavior of the universe at different scales. Quantum mechanics deals with the behavior of particles at a subatomic level, while General relativity explains the behavior of objects at a larger scale, such as planets and stars.

2. Why are Quantum mechanics and General relativity considered incompatible?

Quantum mechanics and General relativity are considered incompatible because they have different mathematical frameworks and make contradictory predictions in certain situations. They cannot both be used to explain the behavior of the universe at the same time.

3. Can't we just combine Quantum mechanics and General relativity to create a unified theory?

Scientists have been working on combining Quantum mechanics and General relativity for decades, but have not yet found a successful unified theory. This is because the two theories have fundamental differences that make it difficult to merge them into one cohesive framework.

4. How does the incompatibility between Quantum mechanics and General relativity affect our understanding of the universe?

The incompatibility between Quantum mechanics and General relativity has significant implications for our understanding of the universe. It means that there are certain phenomena, such as the behavior of black holes or the beginning of the universe, that cannot be fully explained using our current understanding of physics.

5. Is there any research being done to resolve the incompatibility between Quantum mechanics and General relativity?

Yes, there is ongoing research and debate in the scientific community to find a way to reconcile Quantum mechanics and General relativity. Some theories, such as string theory or loop quantum gravity, attempt to bridge the gap between the two theories and create a unified understanding of the universe.

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