Brian Greene Answers Quantum Physics Questions

[00:00] - From Doktor J: "Boy, are you quantum gravity? I have no idea what you're referring to Hi, I'm Brian Greene, physicist, professor, author,

[00:15] to answer your questions from the internet. [upbeat music] @peleenmziry: "Do people actually understand quantum physics

[00:31] We understand the core ideas, that we would think should happen in the universe

[00:43] So we have to reorient our thinking but it is something that those of us So yeah, we do understand a lot about quantum physics.

[01:00] "Has some grizzled professor," like me, to explain wormholes ever made anyone go 'aha!'? Still seems like an incomprehensible magic to me."

[01:16] I can draw a line connecting them, So imagine this is in space. and this is our Milky Way Galaxy.

[01:28] It can warp so that these two locations that were very far apart are now very close together. you now see that the orange between the two locations,

[01:44] We don't know if wormholes are real. but if they do exist, from one location to another.

[01:58] From Do0ozy: "If I ran into a wall over and over again, to eventually pass through the wall? I would need to run into the wall to pass through?"

[02:11] Now look, we don't do quantum mechanical experiments with macroscopic beings like a human body, but we do a version of you running into a wall

[02:25] with a particle like an electron or a neutron or a proton that we fire at a barrier over and over and over again. we do find it passes through a barrier

[02:38] If you scale this up, yeah, in principle, could undertake the same experiment.

[02:50] You'd have to hit this barrier enormous number of times, exponentially large in the age of our current universe. the probability that you will pass through the barrier is.

[03:06] AssusReamus: "Is our universe the only universe?" because it's the only one that we can observe But there's mathematics suggesting

[03:21] that the Big Bang may not have been a one-time event. giving rise to many expanding universes that all populate a grand multiverse.

[03:34] where the individual bubbles are universes like our own, many of these universes.

[03:47] they say, "You're talking nonsense. They have a point, this is an important criticism, Number one, in principle, this may be a testable idea.

[04:00] If two of these bubble universes were to collide, could leave an imprint We've not yet seen it at all,

[04:14] this would be circumstantial evidence The second point I would make is a theoretical statement. For instance, the Big Bang idea has been tested

[04:32] It has confirmed the predictions, which is wonderful. But if the math then says there should be other universes, because we have confirmed the parts

[04:48] of the mathematics available to our observations. We don't, but we are willing to consider it suggests that this really may be true.

[05:02] @Joonies_Girll: "Do we have any updates on time travel? what was happening on planet Earth a million years from now, for how in principle you could do that.

[05:18] Say you go six months, You will be one year older, of course, your clock was ticking slow

[05:32] So when you step out of that spaceship, it may be 2 years or 5 years or 10 years or 100,000 years, all dependent on how close to the speed of light

[05:47] But many of us are more interested And that's the one where we just don't know. using exotic things like wormholes

[06:04] or interesting kinds of motions around cosmic strings, we don't have an answer. And so we suspect that travel to the past is not possible,

[06:17] but as of today, nobody has definitively ruled it out. Why is teleportation not possible?" It's not possible for macroscopic objects like you and me,

[06:35] routinely teleport individual particles You set up two entangled particles that in some sense

[06:47] You bring in whatever you want to teleport And through the magic of entanglement, This person can manipulate that particle

[07:01] and extract an exact copy of the original. from one location to another. @sinonghostriley: "What is reality made of actually?

[07:17] Way back in the early days, people though it was earth, air, fire, and water In the early years of the 20th century,

[07:30] that matter is made of fundamental constituents, And as we examine atoms with ever greater precision, that make up atoms.

[07:44] that are in orbital clouds around a nucleus And the neutrons and protons themselves we know are made of yet smaller particles known as quarks

[07:57] They are the bulk of matter as we know it. atoms making up the fabric of space itself.

[08:10] I think there will be fundamental constituents of space What those ingredients actually are, but my guess is that that is

[08:23] where our understanding will lead us. "When does quantum mechanics get weird?" Quantum mechanics tells us

[08:38] of different mutually incompatible states of being. It tells us that distant objects can be weirdly connected It's weird from the beginning and yet it works.

[08:54] in terms of billiard balls bouncing around according to ideas and laws that he was able to intuit. the micro world doesn't behave that way.

[09:08] called quantum physics. we can extract them from quantum physics. it's quantum mechanical through and through,

[09:24] and quantum mechanics suppresses the weird stuff making the world appear to play But fundamentally, everything is quantum mechanical

[09:37] "If nothing can escape a black hole, Stephen Hawking determined way back in roughly 1974

[09:51] as the mathematics Particles in a sense can leak out of a black hole quantum mechanically.

[10:04] And as particles slowly radiate from a large black hole, which means the intensity of the radiation until at the very end there's a gigantic burst of radiation,

[10:19] which in essence is the death throes of a black hole. will take about 10 to the 68 years that the black hole will shrink down

[10:33] But what happens then? Nobody knows. "If an electron can be two places at the same time, how do we know those are not two different electrons?"

[10:48] We fire a single electron into an experimental context in which the results that we find can only be explained the electron took two different pathways

[11:05] Since we only put one electron into the experiment when we find in the intermediate stage that the electron it's only one electron that's doing that, not two.

[11:22] "I just learned about the block universe theory, It's funny that you're having an existential crisis.

[11:34] To me, the block universe idea is very comforting from an existential standpoint. The idea is rather than thinking of the past as being gone

[11:47] we imagine that all of time is out there within this block that has all of time and all of space. From that standpoint, we always exist

[12:02] because the block itself containing all events, So in that sense, nothing ever dies. always occupies that position in the block.

[12:16] From @miniapeur: "Is string theory dead?" you might get the impression that string theory is dead. string theory has been making enormous strides

[12:31] and understanding the nature of space and time. that we can test in the laboratory, Physics is trying to understand a coherent description

[12:48] We don't have such a coherent description. but string theory is the best developed together into one consistent framework.

[13:03] On that front, string theory is absolutely an enormously important development in the history of ideas. "If everything has a beginning, then when did time begin?

[13:17] what was here before that beginning?" who does know the answer to that question. Let me just throw out a couple of them to you right now.

[13:31] It could be that there is a beginning to time. You know, if you're walking on planet Earth "Hey, can you point me in the direction of further north?"

[13:47] Yeah, point me in the direction of north. When you get to the North Pole and you say to someone there, "How do I go further north?",

[14:00] "The concept of further north The North Pole is where north begins." that you can imagine going back a hundred years,

[14:14] When you get to the Big Bang, it could be that the concept of going further back in time than the North Pole.

[14:26] It could be that time itself simply began There are other ideas that people have developed In fact, there are some theories

[14:41] is simply one expansion of space event has undergone this kind of rapid outward swelling,

[14:54] other universes populating a grand multiverse. From Jerswar at the explain like I'm five subreddit: "What is the big deal with quantum computers?"

[15:08] a particle can be in multiple places at the same time. A quantum computer leverages that multiplicity to carry out multiple calculations in essence in parallel,

[15:22] on how fast the computer can in principle get to an answer. But there's a second part of quantum computation,

[15:34] How do you find the answer among that multiplicity And so for a certain class of questions, a quantum computer can give rise to an exponential speed-up.

[15:49] that works for other problems won't be applicable. And so my suspicion is that quantum computers will be a very niche-oriented kind of device

[16:05] like simulating quantum systems or factoring numbers into their prime factors, that sits on your desk or on your lap

[16:19] the kinds of things that we are familiar with. From @kero_sock: "What the [beep] is quantum entanglement?" we know that they can act independently of one another.

[16:35] But in the quantum world, two things can be far apart, from a behavioral standpoint, Imagine you've got a particle whose spin, we call it,

[16:49] And you can have two of these particles. a measurement on one of these particles The other particle, if they're quantum entangled,

[17:03] and acquire a single definite reality. quantum entanglement bridges the gap between them and allows their behaviors to be tightly choreographed

[17:16] in a way that we're still struggling to fully understand. What you do here quantum mechanically if the two objects are quantum entangled.

[17:29] "Schrodinger's cat seems like a dumb concept." of Schrodinger's cat then,

[17:41] Back in the 1930s you might have tried to argue that the weirdness of quantum mechanics

[17:53] and we simply don't need to think about it Schrodinger's cat was introduced by Schrodinger And he basically said, look, if you have an electron,

[18:10] of being here and being there, I'm willing to accept a particle in some mixture because it's in the microscopic world

[18:23] We can amplify the weirdness All we need to do is set up a little gadget which has poison that will be released into this box

[18:38] but it won't be released into the box And then if we put a cat in the box, the poison is in a 50/50 mixture of being released

[18:50] which means the cat must be in a 50/50 mixture Quantum physics, he was saying, whereby it can infect the everyday reality

[19:06] @ResonanceEnter1: "Does observation create reality?" There are physical systems where if there is no observation,

[19:18] the system evolves and behaves one way. that interaction does change how the system behaves. if it sufficiently impinges on a particle like an electron,

[19:35] that photon can coax a definite reality to emerge. that we have not yet fully understood is why is it that an observation or a measurement

[19:50] on a quantum mechanical system? Somehow, intervention, environmental influence, but exactly how this happens,

[20:05] still struggling to understand. El-Kabongg: "Why do atoms not run out of energy?" We've known since the 1800s that as electrons accelerate,

[20:18] And if they radiate away energy, that electron should spiral into the nucleus and crash. The answer, according to quantum mechanics,

[20:31] is that there is a lowest energy state that's available to a system like an atom, for the atom to achieve a lower-energy configuration.

[20:47] on how close the electron can get We can actually calculate that lowest energy state

[20:59] and then we can compare it to the energy states of atoms. It can come in chunks that can't be subdivided. From the Manuko subreddit:

[21:13] At the normal level, we have relativity. Is there another set of laws for super big things?" but I would say for the super big,

[21:27] That's Einstein's theory of gravity. when things are ever larger, for stars and black holes. We are the in between, the very small and the super big,

[21:42] And so there are things in the world where quantum mechanics rears itself There are other things that we encounter

[21:56] that have general relativity flexing its muscles, but we haven't found that there's a new set of laws Instead, our challenge

[22:09] so the calculations that we human beings we want those calculations to make sense And that is a blending that we have been trying to do for,

[22:25] From @ignorethatdoor: "Why does anything exist? so how did something come from nothing?"

[22:37] All we physicists can do Energy, matter, a configuration And then from that point forward,

[22:52] we analyze what happens to that something, that then come together to yield stars, galaxies, But if you push further back and ask,

[23:06] nobody really has an answer yet for that. A state of absolute nothingness may be unstable.

[23:20] may quantum mechanically fall apart We don't, but at least it's an interesting idea for how you could start with nothing and yield a something.

[23:34] @troll_on_patrol: "What happens after the universe dies?" in this little stack under here, "Until the End of Time."

[23:46] but one dominant possibility is that in the very far future, the universe will continue its spatial expansion. material objects will continue to disintegrate.

[24:02] Even black holes disintegrate into a bath of particles. And so that suggests that in the very far future,

[24:14] the universe will be nothing but a bath of particles ever quieter, ever colder cosmos. existed in the early universe,

[24:28] Those particles come together for a brief period of time, and on this planet people as well. And then in the far future, it all disperses once again.

[24:42] So everything that we know about, everything we care about, may just be a momentary aggregate of particles and then for effectively an eternity,

[24:59] there's nothing but particles wafting through the void. All right, that's all for today. Thanks for joining us. This is Quantum Physics Support.