General Relativity vs Quantum Mechanics
General Relativity Vs Quantum Physics
Special relativity solved the first major conflict in physics between electromagnetic theory and classical mechanics and confirmed that that speed of light is invariant in all frames of reference.General relativity solved the second major conflict between special relativity and Newton's theory of gravity.
But in solving this, the biggest conflict in modern physics was created - general relativity vs quantum mechanics.
Quantum Field Theory
Quantum field theory incorporates special relativity into quantum mechanics. It results in us imagining every point in the universe as containing various fields. The particles we see are just excited states in the various fields and we describe interactions between particles as interactions between the fields. A photon is simply a point where the electromagnetic field is in an excited state. This means even empty regions of space are not actually empty, they actually contain fields with no localised vibrations which is why they appear empty.Understanding why this theory is incompatible with general relativity comes down to the uncertainty principle. There is actually another uncertainty principle relating energy and time. The uncertainty in the energy of a particle in a given state multiplied by the uncertainty in the interval of time that the particle is in that state is greater than or equal to ħ/2.
If you have a good idea of the energy of a particle, you don't really know how long it has that energy for.
If you know how long the particle is in a state, you don't really know how much energy is has.
Quantum Fluctuations
A consequence of the uncertainty principle in quantum field theory is that everywhere in space has quantum fluctuations because they actually contain fields. Quantum fluctuations allow what we call virtual particles to pop into existence for a very short amount of time before annihilating themselves and disappearing. This is because as you examine smaller areas of space for short amounts of time, the uncertainty in the energy of the quantum fields becomes increasingly large until the energy is enough to generate particles.In empty space there are fluctuations of all the quantum fields but these average to 0 and so when we look across large distances for long times, we see nothing.
If we want to unite quantum field theory then the gravitational field will have to act like the other quantum fields but this is where physics breaks down.
Quantum Foam
As you examine smaller areas of empty space over shorter periods of time, the fluctuations in the gravitational field become bigger and bigger and eventually result in violet distortions of space-time. John Wheeler called this "quantum foam". Here direction and time lose meaning and the laws of physics break down. The uncertainty principle conflicts with the smooth model of space-time and calculations done on this scale result problems such as infinite probabilities. This quantum foam appears when examining space below the Planck length 1.6×10-35m. |
| Quantum Foam |
Another problem with general relativity and quantum theory is that quantum theory seems to quantise everything and would suggest space-time should be made of discrete chunks. This directly goes against general relativity's picture of smooth space-time.
Physics rarely requires us to use both general relativity and quantum mechanics because relativity is used for very high masses and quantum mechanics is used for small objects. Most of the time one of the theories can be ignored and the calculations will work perfectly fine. When we want to look at the start of the universe or black holes however, you are dealing with tiny areas of space with a lot of mass. This is where theory uniting gravity and quantum theory is necessary.
One way physicists are trying to do this is with string theory and in the following weeks I will be posting a series talking about the theory.
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