THE HARDEST Problem in Physics Explained Intuitively: Quantum Gravity

Arvin Ash
Arvin Ash
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Publicado 2024-07-26
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FURTHER STUDY
Quantum gravity, mathematical explanation:    • The Trouble with Gravity: Why Can't Q...  
String Theory vs Loop Quantum Gravity:    • String theory vs Loop quantum gravity...  
How QM ruins GR:    • Quantum Gravity: How quantum mechanic...  
General Relativity explained:    • General Relativity Explained simply &...  

CHAPTERS
0:00 How gravity models evolved
2:22 Is Quantum Gravity even necessary?
6:23 3D Bronstein Cube
7:56 Why can't we quantize gravity?
11:19 Ways that we could quantize gravity
12:59 Why don't we fit 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 seems to be quantum not classical. But General Relativity is classical. When we try to use Einstein's theory to make a quantum model of gravity, we get nonsense results. Why is quantum gravity the most difficult problem in physics? Is a quantum model of gravity even necessary? why can’t we fit the other three forces into the framework of General Relativity instead?

Reasons to quantize gravity: All the other fields in nature are quantized. Why would nature make an exception for only the gravitational fiield? Also, General Relativity breaks down at the singularity at the Big Bang and inside black holes. A zero size singularity seems absurd, and likely unphysical. It probably means there’s a breakdown in the theory.

We can see a problem just by looking at the equation for General Relativity. The left side describes curvature of spacetime, which is classical. But the right-hand side is matter, which is quantum. So, we have two incompatible types of mathematics.

A 3D Bronstein cube can illustrate what a quantum gravity theory would look like. We need to either take General Relativity and quantize it, or take quantum field theory and incorporate gravity into it.

All the quantum theories of the standard model take classical theories and make it quantum by taking certain variables such as momentum and turn them into operators. This procedure however doesn't seem to work with gravity, because we get all kinds of infinities, that cannot be corrected or renormalized.

What makes General Relativity unique is that it is a theory of space-time itself, not stuff happening in spacetime, which is the case with the other three fundamental forces. Gravity results from the geometry and curvature of space-time. The other forces describe events happening within this background geometry, not the background itself.

Another factor that makes quantizing gravity difficult is because it is very weak, making it nearly impossible to do experiments. So inventing a quantum theory of gravity becomes mostly a thought experiment.

We can attempt to treat gravity as a field just like other fields. An excitation in this field 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 emerges in the math of string theory.

The second way is to quantize spacetime itself. This is what Loop quantum gravity attempts to do. But this would mean spacetime could exist in a superposition of various different geometries. This is problematic because spacetime would be dynamic in quantum gravity. So we can’t ask for example what is the probability of finding an electron at a certain location, because there is no objective way to specify what location is that we are talking about. This is because spacetime itself would be in a superposition.

Why don’t we attempt 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, Theodore Kaluza, came up with general relativity 5 dimensions instead of 4. The found the laws of classical electromagnetism in his 5 dimensional equations. But it was obviously wrong, because we live in a 4 dimensional universe, 3 spatial dimensions and 1 of time, not 5.

Swedish physicist, Oskar Klein suggested that perhaps the newly hypothesized 5th dimension was very small, so small that it could not be detected. This idea was put on the back burner because at the same time he published his paper, quantum field theory took off.
#quantumgravity
To of the most popular approached today to quantizing gravity is String Theory and Loop Quantum Gravity. This video discusses the difference between these two theories, and their and pros and cons.
Todos los comentarios (21)
  • @ArvinAsh
    Gravity is a Force. Let me explain. Though it's very popular to say, don't let people who have read a few articles on General Relativity tell you that “according to general relativity gravity isn’t a force”. It just isn't true. Einstein himself, in his writings, always referred to gravity as a force, and even specifically warned against its “geometric interpretation,” considering it a mental aid, rather than the true nature of gravity. He called it a "crutch" in German. www.sciencedirect.com/science/article/pii/S1355219… While it is true that gravity can be modeled using a geometric theory, i.e., general relativity, the other forces can also be represented using geometry. The only difference is that since gravity applies to anything with mass or energy, it applies to EVERYTHING, so the geometrization is universal. But the geometrization of other forces, for example, electromagnetism, only applies to electrically charged particles. Just because we don't have quantum theory of gravity yet, does not mean that it is not a force. Most physicists believe that it will be brought into the fold of the other three fundamental forces of nature, which can be quite accurately modeled in quantum field theory. This video presents the challenges to doing that.
  • @LeopoldoGhielmetti
    "In this book we will describe the General Relativity and the Quantum Mechanics, those two theories can be fused together to a simple Theory of Everything, but because it's not the purpose of this book, the unification is left as an exercise for the reader."
  • @radupaulalecu4119
    4:15 "Einstein didn't know that matter needs a quantum description". How come? In 1905 he explained the photoelectric effect exactly by quantum nature of light!
  • @vedsorbit9686
    Arvin almost reaching 1 million good luck my friend
  • @bendybruce
    When I first learned the basics of general relativity I was like "Aha! Gravity is not a force!" Then I found most physicists and scientists were unwilling to state this as a fact outside of describing the model of GR. It's at this point when you realize science is not about describing reality. It is about creating models that resemble the behavior of reality in such a way that we can gain predictive power. We must resign ourselves to the fact we will never know the ultimate truth relating to our existence but what science does give us is an ability to understand what that reality is not as we constantly seek to disprove our own models and look for something better.
  • @NomenNescio99
    I did not know about this channel before, but youtube has been recommending it to me for the last two days. Today I finally caved in and watched it. Turns out youtube has me figured out pretty well well, this was an awesome video. Subscribed.
  • @saelesbonsazse9919
    "Why should we care?" is as an unanswered question as "how to solve quantum gravity?"! If someone doesn't care about these topics, out of curiosity alone, there's little hope one could convince them otherwise!
  • @kallebendtsen7253
    Well, the problem seems to stem from the fact that people are trying to quantize an effect rather than the force causing the effect. Gravity only affects spacetime causing spacetime to warp/curve which moves objects closer together. Gravity doesn't interact with the objects themselves, only the spacetime containing the objects. At least that's what I'm getting from your explanation
  • @picksalot1
    There are two problems with understanding Space-Time. One is Space, and the other is Time. I think Time should be replaced with Entropy, as it is always the Present everywhere in the Universe, all change takes place in the Present, and Past and Future are just Entropic descriptions. That may help get rid of "Observer" related biases and anomalies based on describing temporal events. Also, Observers should not be Agents. Regarding Space, a "smallest size" implies that Space could be quantized, but what would separate two of the smallest units of Space from one another? Certainly not Space.
  • @Exen88
    This was beautifully narrated, my friend.
  • @crazieeez
    I love your video because you are clear in explaining problems
  • @nowinnablewars4480
    Great video! Neil Turok and Latham Boyle's Minimal SM/LCDM Cosmology deserves a mention. They found a way to add gravity to the standard model that is incredibly minimalist, and yet overcomes the problems with quantum gravity infinities, deals with the big bang singularity and more, and most importantly - makes testable predictions. It sounds incredibly promising.
  • @VIRCI
    Great video thanks