What is quantum mechanics really all about?

For the most part, physics is a pretty straightforward discipline. While the math might be complicated, physics is often about quantifying such things as “what goes up, must come down” and “don’t play chicken with a train.” Statements like those are intuitive and pretty much everyone can get their head around them. But, over the last century or so, physics has explored some very unfamiliar realms, like what happens when objects go incredibly quickly or what rules govern the universe for objects the size of an atom or smaller? That last one, the one involving the super small, has a generated an entire field of physics called quantum mechanics. Now if you have heard anything at all about quantum mechanics, there is a good chance that you’ve heard about such things as how it means that cats are alive and dead at the same time, or that nothing is real until we see it because we generate reality using only our mind. And, of course, none of these things are even remotely true. They're a series of outdated ideas that were invoked in the early years of quantum mechanics in attempts to explain the subject in simple ways. Each idea has burrowed its way into the subconscious of people who are interested in science, but aren’t quantum experts. So, what is quantum mechanics all about really? To understand that in any accurate detail takes a ton of study, but I thought I’d try to boil it down to its essence in a single video. Perhaps we should start with the name. The word quantum implies a discrete quantity of something. For instance, the Sahara Desert can go on, seemingly forever, with dune following dune. But, as infinite as the sea of sand seems, it has a quantum side. The quantum of a desert is a single grain of sand. Alternatively, the ocean is majestic, liquid, and in constant motion. There, the quantum is a single molecule of water. So that’s what quantum means. It means that we’re talking about the smallest bit of something. The term mechanics is much easier. It just means the behavior and motion of something. For instance, Celestial Mechanics means the motion of the planets and other heavenly bodies. Thus, quantum mechanics is the motion and interaction of individual objects of a bigger collection. By that definition, the motion of an individual car on a freeway could be described as being governed by quantum mechanics, and of course that’s not quite what we mean. In order to experience the counterintuitive bits of quantum mechanics, we have to look at individual things that are very small- generally smaller than an atom. This is because quantum theory uses the Planck constant, which is a tiny number. It’s this tininess that keeps the quantum weirdness constrained to the subatomic realm. When physicists first learn quantum mechanics as part of our training, we are studying the behavior of individual electrons surrounding atoms, or individual photons of light. By and large, that is the realm of academic quantum mechanics. In a subsequent video, I’ll talk about how the weirdnesses of quantum mechanics can manifest themselves on larger scales. But, for now, let’s talk about this world of the ultra-small, where we’re talking about individual subatomic objects. Okay, so now I need to introduce a key idea, called the wave function. At the subatomic level, a wave function is a mathematical function that describes what is going on. It is called a wave function, in part, because for individual photons or electrons that aren’t connected to an atom, the wave function looks like a familiar sine wave. There are other wave function shapes when electrons are near a nucleus, but the big ideas aren’t dependent on the details of their shape. For simplicity, you can simply think of wave functions as sine waves. Now that is the first curiosity. The sand grain or water molecule is a distinct and discrete object– what scientists call a particle. Waves don’t seem to be relevant. However, in the world of the super tiny, the wave function is the only way to explain what is going on. Now exactly what the wave function actually means is a lot trickier than you’d think. I’ll get to that in a little bit. Answering that question is a big part of the reason why conversations about quantum mechanics are so mind blowing. However, there are ways we can think about the wave function that are easier to understand. If we take the wave function and square it, which means to multiply it by itself, the squared wave function tells you where a particle can be found. Where the wave function is large, there is a large probability of finding the particle there, and where it is zero, the probability of finding the particle there is zero. So that’s not too bad. The wave function squared gives you the probability of where you can find the particle the wave function is describing. That’s the first big quantum idea. Now when we detect the particle, say the electron or photon, we don’t find it everywhere. We find it in one location. At the moment of detection, the location of the particle is determined. Scientists call this moment of detection the “collapse of the wave function.” There’s a reason for that particular phrase and it is because of a quantum weirdness. When you roll a fair die, there is a probability that each side will come up. It's difficult to predict which side we’ll see when it starts rolling, but if we knew the cube’s original velocity and the details of the surface it’s rolling on, we could reliably predict the top face of the die. In quantum mechanics, it’s different. In quantum mechanics, the electron or photon is actually in all locations allowed by the square of the wave function and it is at the moment at which the particle’s location is detected that the wave function changes from being in many locations to the one location where the particle is found. The wave function collapsed from being in many locations to just one. So those are the two key components of quantum mechanics. Now, before we get into just why quantum mechanics still seems so mysterious, it’s worth stating the things on which most scientists agree. There is a mathematical function called a wave function that, when you square it, tells you the locations that you can find the subatomic particle and with what probability. The subatomic particle is simultaneously everywhere the wave function says it can be, kind of like the electron or photon being spread out. And then, when the particle is detected, the wave function collapses and then we know with 100% certainty where the subatomic particle is. There's nearly universal agreement that this way of thinking about things gives accurate predictions. So just what is so confusing about quantum mechanics? Well it boils down to the very simple fact that nobody really knows what the wave function really is. I mean, we know how to use it to make predictions- that’s been called the “shut up and calculate” school of thought- but the true nature of the wave function is perplexing. The simple and honest fact is that, despite a century of thinking about it, nobody knows what it is. There are philosophers who argue about whether the wave function is an ontological or epistemological construct. And those are big words, because- you know- philosophers. Then, there are some people who claim that the waves are actually things that just push other things around. They’d say that the electrons are particles getting pushed by the waves like surfers on a gnarly curl at Oahu. The technical term in quantum mechanics for this idea is “pilot waves.” Another school of thought says that the wave function predicts what can happen and when you collapse the wave function, you literally create several universes, in which each one, one of the possible outcomes occurs. This is called the many worlds interpretation. So which one of those ideas is right? Nobody knows. It’s truly a head scratcher. And because science hasn’t figured it out yet, that opens up the conversation to weird ideas, like the mind/body problem of traditional philosophy because it’s been said that nothing is real until it's observed. Those things really aren’t connected to quantum mechanics at all. This is the bottom line. We know how to use the wave function to make accurate predictions of the subatomic world and yet we don’t have a fundamental understanding of the wave function and what happens when it collapses. This is a not-so-secret secret of physics. And it’s not for lack of trying. Many people have mulled things over and generated tons of ideas, but none of them have resulted in a definitive conclusion. It’s a dissatisfying state of affairs to be sure, but it is what it is and it’s going to take a genius to move us forward. For the time being, we’ll just have to wait and see. And hopefully, you now know enough to dismiss any explanation that invokes zombie cats that generate reality by looking at it. So- I hope that this video was eye opening in that it kind of gave you a sense of the key ideas of quantum mechanics. And hopefully you can see why books written for the public about quantum mechanics can be kind of sketchy. Physicists can use quantum mechanics, but don’t understand its underpinnings very well. If you like the video, be sure to like, subscribe and share. And be sure to click on the little bell icon so you’ll get notified about all of our upcoming videos on many physics topics. And why wouldn’t you? After all, physics is everything.

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Channel: Fermilab

Views: 255,814

Rating: 4.9210501 out of 5

Keywords: quantum, weird, educational, Fermilab, general relativity, proof, CERN, particle, example, physics, explained, Ian Krass, LHC, discovery, Physics, science, Don Lincoln, Gravity, quantum mechanics, quantum gravity, high energy physics, physicist, superstring theory, einstein, experiments, infinite, mass, space, everything, mysteries of the universe, learn, how, why, earth, expansion, universe, stars, subatomic, theory, mind, body, schrodinger, cat, schrodinger's cat, existence, wave, function, sine, mechanics

Id: K0VY9_hB_WU

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Length: 10min 18sec (618 seconds)

Published: Mon Nov 18 2019