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been exposed
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A MORE IN-DEPTH STUDY
Quantum gravity, mathematical explanation: https://youtu.be/NsUm9mNXrX4
String theory vs loop quantum gravity: https://youtu.be/3jKPJa-f3cQ
How QM is ruining GR: https://youtu.be/S3Wtat5QNUA
General Relativity explained: https://youtu.be/tzQC3uYL67U
CHAPTERS
0:00 How gravitational models evolved
2:22 Is quantum gravity even necessary?
6:23 Bronstein 3D Cube
7:56 Why can't we quantify gravity?
11:19 How to quantify gravity
12:59 Why don't we integrate the other forces into General Relativity?
14:26 String theory and loop quantum gravity
16:52 Why should we care about quantum gravity?
SUMMARY
The universe appears to be quantum and not classical. But General Relativity is classic. When we try to use Einstein's theory to create a quantum model of gravity, we get absurd results. Why is quantum gravity the hardest problem in physics? Is a quantum model of gravity really necessary? why can't we integrate the other three forces into the framework of general relativity instead?
Reasons to quantify gravity: All other fields in nature are quantized. Why would nature make an exception only for the gravitational field? Furthermore, general relativity breaks down at the Big Bang singularity and inside black holes. A singularity of zero size seems absurd and probably not very physical. This probably means there is a break in the theory.
We can see a problem just by looking at the equation of general relativity. The left side describes the curvature of space-time, which is classic. But the right side is matter, which is quantum. So we have two incompatible types of mathematics.
A 3D Bronstein cube can illustrate what a theory of quantum gravity would look like. We either need to take general relativity and quantify it, or we need to take quantum field theory and incorporate gravity into it.
All quantum theories in the Standard Model take classical theories and make them quantum by taking certain variables such as momentum and transforming them into operators. This procedure doesn't seem to work with gravity, however, because we get all sorts of infinities that can neither be corrected nor renormalized.
What makes General Relativity unique is that it is a theory of space-time itself, not what happens in space-time, as is case of the three other fundamental forces. Gravity results from the geometry and curvature of space-time. The other forces describe events that occur in this background geometry, not in the background itself.
Another factor that makes it difficult to quantify gravity is that it is very weak, making it almost impossible to perform experiments. Thus, inventing a quantum theory of gravity becomes above all a thought experiment.
We can try to treat gravity as a field in the same way as other fields. An excitement in this area would be the graviton. The exchange of gravitons between two particles would result in an attraction. This is what string theory attempts to do, because a graviton appears in the mathematics of string theory.
The second way is to quantify spacetime itself. This is what Loop quantum gravity attempts to do. But this would mean that spacetime could exist in a superposition of different geometries. This is problematic because spacetime would be dynamic in quantum gravity. So we can't ask for example what the probability is of finding an electron at a certain location, because there is no objective way to specify which location we are talking about. Indeed, space-time itself would be in superposition.
Why don't we try to fit everything into the framework of general relativity instead of trying to fit general relativity into quantum mechanics? This has been attempted. In 1919, Théodore Kaluza proposed general relativity in 5 dimensions instead of 4. He found the laws of classical electromagnetism in his 5-dimensional equations. But this was obviously false, because we live in a 4-dimensional universe, 3 spatial dimensions and 1 temporal dimension, not 5.
Swedish physicist Oskar Klein suggested that the newly hypothetical 5th dimension may have been very small, so small that it could not be detected. This idea was shelved because, at the same time as he published his paper, quantum field theory was taking off.
#quantumgravity
One of the most popular approaches today for quantifying gravity is string theory and loop quantum gravity. This video discusses the difference between these two theories, as well as their pros and cons.
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