Why are quantum mechanics and general relativity incompatable?

Question

Thanks!!

Answer 1

Quantum mechanics makes extremely accurate predictions about what happens at atomic scales and smaller, while relativity does the same for macroscopic scales. However, neither theory can be derived from the other - and yet they *must* be related since they describe different aspects of the very same materials!

The relationship is so subtle it simply hasn't been figured out yet. Just as relativity was a fine-tuning of Newtonian mechanics, so must be the connection between relativity and quantum mechanics. This will be the next Great Leap Forward in physics, and hopefully will happen while I'm still here to appreciate it - and who knows what fantastic abilities it will give us?

PS: By the "extremely accurate predictions" made by quantum mechanics, I mean of course the predictions of probabilities. Sorry if this has caused some confusion.

Answer 2

According to relativity, if you know the position and velocity of everything in the universe, you can predict what the universe will look like afterwards. According to quantum mechanics, it is impossible to know the position, velocity, and any other properties of even a single particle simultaneously. This is because quantum mechanics is governed completely by randomness and probability. It is so random, that it even allows the law of conservation of energy to be broken. It also treats particles like waves and vice versa. Also, it is hard to reconcile the differences between the two theories because general relativity deals with huge things and their gravity, while quantum mechanics deals with subatomic things and gravity is virtually ignored.

And when you combine equations from the two theories, you often get answers that come out to be infinity. These answers are obviously wrong, and because real math doesn't actually work with infinity, combining the equations is useless.

Answer 3

Relativity predicts that nothing can travel faster than the speed of light. In quantum physics, particles are not actually particles but behave like both waves and particles. The classic demonstration of this is Young's two slits experiment. If you fire photons of light through two slits you get an interference pattern. You would expect light to behave like that from a wave source - not individual particles.

Anyway, this indicates that each particle somehow "knows" what the other particles are doing. When you observe a property of one particle in a pair of such "entangled" particles, it instantly affects the other one - regardless of how far apart they are. The information must travel at least as fast as 10,000 x the speed of light (300,000,000 metres/sec).

According to relativity, this is impossible. Anything with mass, approaching the speed of light acts as if it has infinite mass, so it takes an infinite amount of energy to reach light speed. We don't yet have a way of reconciling the two theories - both of which predict the known universe brilliantly.

Answer 4

in general relativity, or classical physics you can calculate everything exactly - like the orbits of planets.

at the quantum level - you can only calculate the probabilities of things happening. impossible to calculate exactly.

Answer 5

Probably I'm wrong here, so perhaps the previous answers will go go easy on a poorly educated old git.

But I thought string theory was  pulling these things together..

Answer 6

Quantum mechanics is little tiny stuff

Relativity is HUGE stuff

it's like getting a star to talk to a microbe. very hard.

anyway, not compatable YET.

Answer 7

General Relativity is a classical theory that deals with the distortion of  pace-time by mass-energy to create gravity. Within this theory, the distortion of space-time is continuous. Thus, a theorist using General Relativity can calculate the distortion of space-time down to a very small scale but not down to a singularity (where General Relativity breaks down mathematically).

However, within Schrodinger's and Heisenberg's quantum theories; changes in energy states are discontinuous and can only be calculated to a probabilistic expectation value of an observation. Furthermore, the Heisenberg uncertainty principle prevents quantum theory from accurately predicting two mutually exclusive variable such as momentum-position or energy-time. On the other hand, General Relativity allows many variables to be calculated with equal degrees of precision all at the same time.

Finally, General Relativity deals with the macro world of gravity whereas quantum mechanics deals with the micro world of elementary particles although it does have a macro world interpretation.

Answer 8

i thought that quantum does incorporate general relativity to an extent but the main thesis of it pertains to far more subtle energies than those described by Einstein.