The discovery of quantum mechanics has fundamentally changed not just the field of physics but also our understanding of what reality is. Let’s take a look at just what makes quantum physics so weird and why it is so hard to reconcile with our perception of physical reality.Quantum mechanics is mysterious, but most of her examples could be applied to classical mechanics.
First she says that electrons and photons exhibit wave-like behavior that is hard to understand if they are just particles.
The same is true classically. Waves show interference, and that is hard to understand if they are just particles.
She says that if the position of a particle is known only to be a region in space, then that region of uncertainty grows over time. Yes, such regions grow classically also.
She says we do not see cat-states, where a cat is half dead and alive. But we do see coin tosses that are half heads and half tails, until you look. The quantum description of cats is not much different from the classical one.
Finally she says wave functions are not directly observable. Classical probabilities are not either.
She might have mentioned Heisenberg uncertainty, but classical waves exhibit something similar.
I would say that what really makes QM different is that observations are eigenvalues of non-commuting operators.
The stereotypical example is a particle that 3:49 is in several places at once, until you look and measure its position. 3:54 Then it’s suddenly in one place. Think again of that particle that came from the supernova and 4:01 spread out to a billion light years. How does this suddenly fit into a detector? 4:07 Physicists typically resolve this problem the way that Bohr approached it by saying 4:12 that it makes no sense to ask where the particle “really” was before you measured it.The classical picture is similar, except that we dare to say that the unmeasured particle has a position. Quantum mechanics is different in that a particle is not really a particle. It is some sort of wave that gets observed as a particle.
Similar points are discussed in another video:
The Quantum Frontier with Brian Greene and John PreskillAt 18:18, Preskill reveals that he subscribes to Everett many-worlds, saying it is the "simplest" interpretation. Supposedly it solves the measurement problem, but he admits that it does not explain why we only see one outcome to experiemnts. So it doesn't really solve the measurement problem. He says he is comfortable with Everett being the true reality, so he accepts it until experiments prove otherwise.Renowned Caltech physicist John Preskill joins Brian Greene for an in-depth discussion of quantum mechanics, focusing on where we are and where we're headed with quantum computing.
They agree on the big three startling ideas of QM: the world allows (1) probability; (2) superposition; and (3) entanglement.
randomness unpredictability is an 4:09 essential feature of quantum mechanics. So when we speak of probability, we mean 4:14 something different in the context of quantum mechanics than we do when we talk about probability in everyday life. 4:21 Usually we talk about probability like the probability that it's going to rain tomorrow in the sense that we're 4:28 ignorant of some of the features of the system that we're trying to predict. 4:34 Quantum mechanics is different because there is intrinsic randomness. Even if you have a complete description of a 4:41 physical system, you can't predict what you're going to see when you observe the system. That's really something new in 4:47 physics.No, we don't know that QM really has intrinsic randomness, or even if any such thing exists.
I am surprised that he says this, when probability and randomness pervades all of science, except that he subscribes to many-worlds, where they do not exist.
He says we could have quantum computers in 20 years.
It seems to me that if QM is so strange, then the leading popularizers could explain what is strange about it.
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