The Trouble with Gravity: Why Can't Quantum Mechanics explain it?

REFERENCES:

CHAPTERS:
0:00 - Deterministic to probabilistic universe
1:55 - Why must we quantize gravity?
6:22 - What is the central conflict with gravity and quantum mechanics?
9:09 - Why is quantizing gravity so difficult?
9:54 - Where do the infinities come from?
12:48 - String theory and LQG

SUMMARY:
In a classical universe, if we knew all the positions and velocities for all particles, we could predict the future and the past. Quantum mechanics is not like this. It shows that reality is not deterministic but is probabilistic. The precise location of a particle cannot be predicted in advance, even in principle. This is the most accurate theory we have. It can account for all the forces of nature, except gravity. Why is gravity so different? Why can’t gravity be modeled by quantum mechanics? Why is Quantum Gravity so difficult?

General Relativity is very accurate, so why must we quantize gravity? Because quantum mechanics works. And General relativity falls apart at quantum scales.

Quantum mechanics says that particles are not like little cannon balls but are like a wave described by a wave function. Particles are waves until some kind of interaction occurs, at which point the wave becomes localized like a particle. But prior to this, we can't predict the location of the particle.

But a photon or electron, just like any quantum particle must also have a gravitational effect because that's what General Relativity says. But if it's a wave prior to an interaction, and it could be anywhere until the moment we measure it, where is its gravitational effect located? General Relativity can't tell us where. We don’t know how this works because we don’t have a quantum description of gravity.

So the bottom line, we know quantum mechanics works well at the smallest scales. And we know that General Relativity works well at large scales. But the problem is that general relativity does not work at the smallest scales. This cannot be because gravity must work at the smallest scales, otherwise its cumulative effects would not work at large scales. This is why most physicists think General Relativity must be brought into the fold of quantum mechanics.

According to Quantum mechanics, all interactions between matter particles are mediated by the force particles. And all these interactions happen with space and time as the background. Gravity doesn’t fit this picture because in general relativity, gravityis due to a warping, or curving of the background spacetime itself. There is no force-carrying particle in general relativity which mediates gravity between matter particles.

This does not mean that General relativity is wrong, it is just incomplete. Why is quantizing gravity so difficult? The short answer is because we get infinities when we try to incorporate gravity in quantum mechanics equations. Where are these infinities coming from?

When an electron and a positron annihilate to create an energetic photon, which then converts back to an electron and a positron, quantum uncertainty is such that the photon on its way to turning into an electron and positron, can convert to any one of a number of different number of particles, for example it can turn into a top quark and anti top quark which annihilates, or it can turn into an electron and positron and back into a photon, or something else.

And it can do this 10 times, 100 times, 1000 times, or an infinite number of times before turning into an electron and positron again. When describing this mathematically, we have to take all the momentums of all the particles and all potential interactions between the various particles into account. There turns out to be an infinite number of combinations of interactions . This is where the infinities crop up in the equations of quantum mechanics. However, this problem can be solved in quantum mechanics by something called renormalization.
#quantumgravity
But renormalization does not work with gravity because instead of just considering all the particles that the photon can turn into, and their interactions, we also have to take into account all the gravitational effects. But just because we can’t solve it does not mean that solutions don’t exist. They probably do, but a completely new approach is needed. Two popular approaches are Loop Quantum Gravity and String Theory.

[Riogrande]

This is by far the simplest and clearest explanation of this issue that I've seen or read, period. Excellent, easy-to-understand graphics aid comprehension. Great job.

[rickpontificates]

Gravity always fascinated me because it's the only fundamental law that interacts with time. Unlike electromagnetism or the the strong and weak forces, we are living INSIDE gravity (spacetime), which might be why we can't properly study it, because we can't step away from it to analyze it objectively

[leighedwards]

Feynman was renowned for his ability to explain very difficult problems to others and so do you Arvin very well done.

[charlesblithfield]

Arvin, the graphics on your videos and editing consistently impress me. I learn a lot more because of how skillfully you integrate the text with images.

[Packwatch]

I have been looking for something about this topic for what feels like forever! it felt like people would just say "it breaks down once you try to include gravity" and leave it at that-- thank you for such a clear explanation!

[IQtichenor]

Arvin, you are absolutely my favorite science communicator. You don’t just make the explanations of the physics so clear, you also articulate the questions that demand more fundamental answers and the logic that gets us to new theories and solutions SO brilliantly. I learn so much from your videos, and the most valuable thing is how to *think* better. THANK YOU!

[projectw.a.a.p.f.t.a.d]

I hope I live long enough to see quantum mechanics and general reality come together.

[TheEncodedStar]

This is one of the simplest explanation on this topic I have ever come across on internet. The graphics in video are super explanatory. In Physics, it is very important to have clear understanding of the most fundamental concept and your tremendous efforts to simplify such difficult subject are commendable.

[shethtejas]

Like others have already mentioned, I have never seen a simpler video that deals with heavyweight topics like quantum gravity. Arvin, you are gifted seriously. Your videos are simple and easy to absorb by total amateurs but at the same time it also serves the right amount of details. Great job! I had read about these topics on wikipedia but never did I ever feel complete in my understanding. After watching your vid, I am now in a position to speak one line or two on why gravity doesn't work at quantum scales and why do we even need to merge the two realms.

[adamgm]

I've been looking for a video like this for a few months now. I can't get enough of quantizing gravity.

[Chon]

THANK YOU! Excellent explanation for people like me who doesn't know tons of physics (I'm a Pediatrician) but really like physics videos! Hope your Channel gets more subscribers, all of your team deserves it!

[jitulsarma]

Knowing something and teaching or explaining something are completely different things!
I really loved your explanation
Thank you!

[cooper]

It’s crazy how you don’t have at least a million subscribers, this video was very easily understood by me (a junior in high school with a C in physics), and very well put together as well.

[voidburger]

I've been binging these science videos recently and have been learning a lot about quantum mechanics, general relativity, and physics. Thanks so much for this great information

[LowellBoggs]

This is a great video Arvin, thanks! You always do such a great job of explaining things. I particularly like the fact that your graphics showed the Feinman diagram's with the gravity field distortions underneath them. I have heard similar explanations before but without that specific graphic, I did not grasp what the speaker was trying to say. Great job!

[Baghdadbatterymusic]

Amazing video. You explain things with so much clarity and in such simple to understand terms. You have a deep knowledge and understanding this subject and I'm thankful you took the time to break it down so well.

[jamesnasmith]

Nothing I have seen brought me closer to an understanding like this presentation. Thank you.

[chrisalvino]

Wow, that was an explanation of quantum gravity and its problems that even a lay person could understand. That was a super impressive piece of work!

[andoletube]

You explain very complex concepts in a way that is accessible to novice students. Great stuff!

[Chipchap-xupk]

Once again, another fantastic video. You've done so much to help the public understand science, Arvin. Thanks! The perfect balance of not over simplifying and not making it a tangled web of maths. You treat your viewers like intelligent people.