Why is the universe QUANTUM? What if it isn't?

Background videos:

Chapters:
0:00 - The beginning of science
2:07 - Magellan offer
2:30 - Classical physics
3:40 - What is a black body?
4:24 - The Ultraviolet catastrophe
5:30 - Solution by Max Planck - Planck's law
7:03 - Why electrons should hit the nucleus
8:04 - The Bohr model of the atom
9:48 - A problem with Schrodinger's equation
10:30 - The Dirac equation and quantum field theory
11:51 - Is the universe Probabilistic or deterministic?
12:58 - What would a non-quantum universe look like?

Summary:
What do we think the universe is quantum? What if the universe was not quantized?
Classical mechanics was doing just fine after Isaac Newton reduced nearly all mechanical phenomena to a single powerful equation: F=MA, James Clerk Maxwell also solved the mystery of electricity and magnetism. Classical physics is continuous. This means you can always keep dividing things into smaller pieces. But scientists realized that classical physics had some major flaws because certain phenomena could not be explained, like the color of a hot glowing body.

In 1900, Lord Rayleigh and James Jeans had used experimental data to come up with a law for how all objects emit electromagnetic radiation. The problem was that according to their theory a black body will send out energy in any frequency range allowed by the temperature. But for very energetic objects at temperatures above 5000 Kelvin, their theory predicts that the object should radiate away all its energy until it reaches absolute zero. It is called the ultra-violet catastrophe.

The solution to this problem marked the end of the classical world and the beginning of the quantum world. In 1900, Max Planck had come up with an equation to explain black body radiation. He treated radiation as being quantized, released only in discrete quanta of energy. So the emission of radiation was limited to quanta of energy, proportional to a Planck's constant. E=hf, where the quanta of energy, E, is equal to the frequency f times Planck’s constant.

Another phenomenon that only quantum mechanics could explain was why an electron does not lose all its energy when orbiting a nucleus. If electrons orbit around the nucleus, then their circular motion means that they are constantly accelerating. But an accelerating electron means that it must be emitting photons, which means it must be losing energy. This would mean that the electron would continuously lose its orbital energy, and eventually hit the nucleus. So atoms could not exist.

Niels Bohr solved the problem by showing that only special orbits are allowed around the nucleus where the angular momentum of the electron is a whole number multiple of Planck’s constant over two pi. Light is only emitted or absorbed when electrons jump from one orbit to another.

Now to fully grasp our quantized world, we also need to account for special relativity. It was realized that the Schrodinger equation is wrong because it does not treat space and time equally. Paul Dirac fixed this problem by reformulating Schrodinger's equation to threat space and time equally. This became the =Dirac Equation.

His equation, and later others, do not quantizing objects, but they quantize fields. And this gave rise to quantum field theory, or QFT. In QFT, particles are treated quantizations of fields. This allows us to treat space and time equally such that it satisfies special relativity.

Another big departure from classical mechanics is the idea of probabilities. The wave function in the Schrodinger equation is related to the probability of finding the particle in a given location if you were to measure it. Prior to measurement, we cannot know in advance where it will be. So the outcome is not deterministic, but probabilistic. Only the probabilities of the alternative possible outcomes are deterministic.
#quantummechanics
#quantumuniverse
The world and the universe would be very different if it was not quantized. It would be a deterministic world where, theoretically the future would be predictable. But the world would not exist as we know it because, atoms could not form, quantum particles would not form. There would be no energy and no radiation. Without quantum mechanics, you could still have spacetime because general relativity does not require quantization. But this universe would be filled with nothing.

[ArvinAsh]

Background videos for better understanding of specific subjects discussed in this video:

[spark_y]

This guy's videos are so clear to understand any complexity that we have in our universe. You work hard to make this easier for us. 👍🏻

[nikivan]

The best video I've ever seen on the topic. Straight to the point and easy to understand. Thank you.

[mn-ruli]

In watch a lot of these types of channels, but Arvin Ash is the only one I wish was my friend. 💛

[naytchh]

"As we know, every object emits radiation."
You certainly don't talk down to your audience and that's very refreshing. At 43, I only recently learned about Black Body radiation and it's still blowing my mind. Another great video Arvin!

[AndrewJonkers]

There is an underrated (IMHO) paper by Lucian Hardy "Quantum Theory from Five Reasonable Axioms" relating to this. And yes, General Relativity is also compatible with these axioms (the first axiom being the principle of relativity dressed up as "measurability") Given the body of empirical evidence summarized by QM and GR, it is almost inconceivable that a different set of axioms that are not reducible to these five might also explain this data. The conclusion (by an almost overwhelming likelihood) is that quantum theory may be incomplete but not incorrect - that is if we find a unified theory, it will also be quantum in nature. Curiously the first 4 axioms in this paper come from classical probability theory. The last, which is what makes a theory "quantum" in nature, is simply a statement of reversible continuity between any two system states.

[catmate]

Great video Arvin, one of your best so far. Very well written and put together. Looking forward for more!

[ryantennyson]

Thank you. Your channel is always enlightening and the graphics are fab.

[n-da-bunka]

EXCELLENT video. from 9:21 through 11:30 timestamps perfectly resolves the alignment of the Schørdinger equation with relatively with Dirac's equation bringing my understanding of how MOTION plays into these distinctions. Although I have been immersed in this world for a while, until this episode I did not realize these dependencies and interactions with time and motion which had really been bothering me. I had been simply doing the "shut up and calculate" approach. No professors had shared this aspect to clarify that Dirac's equation HAD included that balance and how that led to QFT.

[Gamer-xbeo]

I really love the music at the start when the Arvin Ash logo comes up. Credits to the makers.

[duggydo]

Arvin, you explain these concepts soooo well. I wouldn't be surprised if one morning you premier a video announcing that you've developed your own Grand Unified Theory. Well done!

[reeboothemad]

I love videos like this – historical backgrounds of how we found out about things. They make it so much easier to learn and understand how we understand the universe as of today. If there were videos like these thirty years ago, I might have been much better in school.

[nathangarcia]

Found this channel on accident and I’m so happy I found it. I’m too curious a person to not try to understand some of this and this channel helps put some of that at ease

[chrisjudd-ucsh]

One of the very best explanations of where we are and how we got here. Thank you.

[harshalpurswani]

You explain v complex topics so smoothly. its just amazing, My Friend..!

[radiokid]

Arvin--you are a total treasure on the internet. Thanks!

[Rewski_Man]

I just want to say I appreciate your videos, most of the time it’s hard for me to retain information with other videos, but for some reason your videos I can retain it very easily

[david.thomas.]

The best video I've seen on quantum mechanics in a long time!, wonderfully explained with thanks.

[robertschlesinger]

Excellent video. Actually, I'd say it's a superb video, but many of your other videos are equally superb, so we have a tie with a plurality of your videos being in that superb position. Very interesting, informative and worthwhile video. A must see video for everyone. Arvin, you are enlightening the world.

[eatcool]

Greetings from New Zealand, I love this channel. Worth getting up at midnight to watch these videos!!