- 15 hours ago
Theoretical physicist Professor Brian Greene joins WIRED to answer the internet’s burning quantum physics questions. Is String Theory dead? Is our universe the only universe? What happens after the universe dies? Answers to these questions and many more await on Quantum Physics Support.
Category
🤖
TechTranscript
00:00From Dr. J.
00:01Boy, are you quantum gravity
00:04because I'm relatively attracted to you.
00:07I have no idea what you're referring to
00:09or what you're talking about, but thanks.
00:12Hi, I'm Brian Greene, physicist, professor, author,
00:15and I am here today to answer your questions
00:18from the internet.
00:19This is quantum physics support.
00:26At Palinemisery, do people actually understand quantum physics
00:31or do we all just pretend that we do?
00:34We understand the core ideas, but they're very different
00:38from the things that we would think should happen
00:43in the universe based on our common experience.
00:45So we have to reorient our thinking
00:48to align with the discoveries of quantum physics,
00:50and that's not easy, but it is something that those of us
00:53who immerse ourselves in these ideas are able to do.
00:57So yeah, we do understand a lot about quantum physics.
01:00Next question from at Stuxman.
01:03Has some grizzled professor, like me, poking a pencil
01:07through a folded sheet of paper to explain wormholes
01:10ever made anyone go, aha, still seems like
01:12an incomprehensible magic to me.
01:16Take two points on a piece of paper.
01:18I can draw a line connecting them.
01:20And you can see that that line is kind of big.
01:22So imagine this is in space.
01:24Maybe this is the Andromeda galaxy,
01:26and this is our Milky Way galaxy.
01:28The fabric of space can be manipulated.
01:30It can warp so that these two locations
01:34that were very far apart are now very close together.
01:38So if I stick a pencil through, you now see that the orange
01:43between the two locations, that would be a wormhole.
01:46We don't know if wormholes are real.
01:48They emerge from the mathematics of general relativity.
01:51But if they do exist, in principle, they could provide
01:54a shortcut from one location to another.
01:58From Doozy, if I ran into a wall over and over again,
02:02would quantum tunneling allow me to eventually
02:04pass through the wall?
02:06And if yes, can we calculate or estimate how many times
02:09I would need to run into the wall to pass through?
02:11Yes, exactly.
02:12That is what would happen.
02:14Now look, we don't do quantum mechanical experiments
02:17with macroscopic beings like a human body.
02:21But we do a version of you running into a wall
02:25with a particle like an electron or a neutron or a proton
02:29that we fire at a barrier over and over and over again,
02:33a small fraction of the time.
02:35We do find it passes through a barrier
02:38that classical physics says it can't.
02:41If you scale this up, yeah, in principle,
02:44a many particle system like your body
02:47could undertake the same experiment.
02:49You'd have to hit this barrier an enormous number of times,
02:55exponentially large in the age of our current universe.
02:59That's just how ridiculously tiny the probability
03:02that you will pass through the barrier is.
03:06Asus Remus, is our universe the only universe?
03:11You would think that our universe is the only universe
03:13because it's the only one that we can observe
03:16with powerful telescopes.
03:18But there's mathematics suggesting that the Big Bang
03:22may not have been a one-time event.
03:25There may be many bangs giving rise to many expanding universes
03:30that all populate a grand multiverse.
03:34I like to think of it as a giant bubble bath,
03:36where the individual bubbles are universes like our own,
03:41but the grand picture contains many of these bubbles,
03:44many of these universes.
03:47Some people, when they encounter this idea, they say,
03:50you're talking nonsense.
03:51You're no longer talking science.
03:53They have a point.
03:54This is an important criticism, but let me just note two things.
03:57Number one, in principle, this may be a testable idea.
04:00If two of these bubble universes were to collide,
04:04there is mathematics suggesting that the collision could leave an imprint
04:08on what's known as the microwave background radiation.
04:12We've not yet seen it at all, but in principle, if we did,
04:16this would be circumstantial evidence in favor of this idea of multiple universes.
04:22The second point I would make is a theoretical statement.
04:25We theorists use mathematics to try to understand reality.
04:28For instance, the Big Bang idea has been tested by detailed observations of space.
04:34It has confirmed the predictions, which is wonderful.
04:38But if the math then says there should be other universes,
04:43we are willing to go where the math takes us,
04:45because we have confirmed the parts of the mathematics available to our observations.
04:52Do we know that that's the case?
04:53We don't.
04:54But we are willing to consider it because of the fact that the mathematics suggests
04:59that this really may be true.
05:02At Junesy's Girl, do we have any updates on time travel?
05:06Calls please take me back.
05:07If you wanted to see what was happening on planet Earth a million years from now,
05:13Einstein himself laid out a blueprint for how, in principle, you could do that.
05:18You travel out into space near the speed of light, say you go six months,
05:22you turn around, you come back for six months, you will be one year older, of course.
05:28But because your motion was near the speed of light, your clock was ticking slow,
05:32compared to clocks on planet Earth.
05:34So when you step out of that spaceship, it won't be one year into the future on planet Earth,
05:39and maybe two years, or five years, or ten years, or a hundred thousand years,
05:43all dependent on how close to the speed of light your ship was able to travel.
05:49But many of us are more interested in traveling back in time.
05:54And that's the one where we just don't know.
05:58There are proposals that people have put forward using exotic things like wormholes,
06:04or interesting kinds of motions around cosmic strings.
06:09But whether any of those ideas really work, we don't have an answer.
06:13And so we suspect that travel to the past is not possible.
06:17But as of today, nobody has definitively ruled it out.
06:22At Calupsy asks,
06:24Physics was up?
06:25Why is teleportation not possible?
06:28It's not possible for macroscopic objects like you and me.
06:34But physicists who work on teleportation routinely teleport individual particles from one location to another.
06:43You set up two entangled particles that, in some sense, act as one, even though they're far apart.
06:49You bring in whatever you want to teleport, and it co-mingles with one of the entangled particles.
06:54And through the magic of entanglement, its features get imprinted on the distant particle.
06:59This person can manipulate that particle and extract an exact copy of the original.
07:06And in that way, effectively, you have teleported a particle from one location to another.
07:11At Sinon Ghost Riley,
07:14What is reality made of, actually?
07:17Like, what is this universe?
07:19Way back in the early days, people thought it was earth, air, fire, and water as the basic ingredients of
07:26everything.
07:27In the early years of the 20th century, people got on board with the idea that matter is made of
07:34fundamental constituents made of atoms.
07:37And as we examine atoms with ever greater precision, we realize that there are other particles that make up atoms.
07:44You've got electrons that are in orbital clouds around a nucleus that typically has neutrons and protons.
07:50And the neutrons and protons themselves, we know, are made of yet smaller particles known as quarks, arranged in correct
07:58configurations and electrons.
08:00They are the bulk of matter as we know it.
08:04My guess is, at some point, we will talk about atoms making up the fabric of space itself.
08:10I think there will be fundamental constituents of space and fundamental constituents of time.
08:16What those ingredients actually are, we are still struggling to figure out.
08:21But my guess is that that is where our understanding will lead us.
08:26All right, next question is from Sparsh Mather.
08:30When does quantum mechanics get weird?
08:33Well, it gets weird almost from the get-go, right?
08:36Quantum mechanics tells us that the world can be in a blended mixture of different, mutually incompatible states of being.
08:44It tells us that distant objects can be weirdly connected through something known as quantum entanglement.
08:50It's weird from the beginning, and yet it works.
08:54Isaac Newton described the world in terms of billiard balls bouncing around according to ideas and laws that he was
09:02able to intuit for reasons that we do not understand.
09:05The micro world doesn't behave that way.
09:08Instead, it plays by these different rules called quantum physics.
09:12And here's what's remarkable.
09:14The rules that Isaac Newton wrote down, we can extract them from quantum physics.
09:19So perhaps the real way of talking about the world is it's quantum mechanical through and through.
09:24And quantum mechanics suppresses the weird stuff in the micro world as things get larger and larger,
09:31making the world appear to play by the rules of Isaac Newton.
09:34But fundamentally, everything is quantum mechanical through and through.
09:38All right, a user at Quora asks,
09:41If nothing can escape a black hole, how do they eventually die out?
09:46Stephen Hawking determined way back in roughly 1974 that black holes may not be as black as the
09:55mathematics of Einstein's general relativity suggests.
09:59Particles, in a sense, can leak out of a black hole quantum mechanically.
10:04And as particles slowly radiate from a large black hole, the black hole gets smaller and smaller,
10:10which means the intensity of the radiation becomes bigger and bigger,
10:14until at the very end there's a gigantic burst of radiation,
10:19which in essence is the death throes of a black hole.
10:23A black hole with the mass of the sun will take about 10 to the 68 years to radiate enough
10:30matter that the black hole will shrink down to its final state of nothingness.
10:35But what happens then?
10:36Nobody knows.
10:37From explainlikeim5 subreddit,
10:41If an electron can be two places at the same time, how do we know those are not two different
10:47electrons?
10:48We fire a single electron into an experimental context in which the results that we find can
10:57only be explained if en route to the detector the electron took two different pathways from start to finish.
11:06Since we only put one electron into the experiment and we only get one electron out from the experiment,
11:12when we find in the intermediate stage that the electron is in some sense at two places at the
11:17same time, it's only one electron that's doing that, not two.
11:22From at Drink More Gravy,
11:24I just learned about the block universe theory and now I'm having a major existential crisis.
11:31It's funny that you're having an existential crisis.
11:34To me, the block universe idea is very comforting from an existential standpoint.
11:42So what's the basic idea?
11:43The idea is rather than thinking of the past as being gone and the future is yet to be,
11:49we imagine that all of time is out there within this block that has all of time and all of
11:56space.
11:57From that standpoint, we always exist because the block itself containing all events,
12:06it never changes. So in that sense, nothing ever dies.
12:10Everything that exists at a given moment always occupies that position in the block.
12:16From at Miniapure, is string theory dead?
12:20Spend too much time scrolling on the internet, you might get the impression that string theory is dead.
12:25But the fact of the matter is, string theory has been making enormous strides in understanding
12:32black holes and understanding the nature of space and time.
12:36String theory has not been able to make any predictions that we can test in the laboratory.
12:41But please bear in mind, physics is more than that.
12:45Physics is trying to understand a coherent description of the external world.
12:50We don't have such a coherent description. Gravity and quantum mechanics are at loggerheads,
12:56but string theory is the best developed at putting the two pillars of physics together
13:01into one consistent framework.
13:03On that front, string theory is absolutely an enormously important development in the history of ideas.
13:11From Devon Scanadore,
13:14If everything has a beginning, then when did time begin?
13:17And if time had a start, what was here before that beginning?
13:22There's nobody on our planet who does know the answer to that question.
13:26We do have ideas. Let me just throw out a couple of them to you right now.
13:31It could be that there is a beginning to time.
13:36An analogy helps grasp what that would mean.
13:39You know, if you're walking on planet Earth and you want to go further north,
13:43you can pass by someone and say, hey, can you point me in the direction of further north?
13:47They point north where you keep on heading, you pass somebody else.
13:50Yeah, point me in the direction of north.
13:52They point you toward the north pole.
13:54When you get to the north pole and you say to someone there,
13:57how do I go further north?
13:59They'd look at you kind of funny and say,
14:02the concept of further north than the north pole is meaningless.
14:06The north pole is where north begins.
14:10Similarly, it could be the case that you can imagine going back a hundred years,
14:14a thousand years, a billion years.
14:16When you get to the big bang, it could be that the concept of going further back in time
14:22is as meaningless as going further north than the north pole.
14:26It could be the time itself simply began with the big bang itself.
14:33There are other ideas that people have developed where there is a prehistory to the big bang.
14:39In fact, there are some theories that envision that our big bang is simply one expansion of space event
14:47that is one of many times that the universe has undergone this kind of rapid outward swelling,
14:53which means there could be other big bangs, other universes populating a grand multiverse.
15:06In quantum mechanics, a particle can be in multiple places at the same time.
15:13A quantum computer leverages that multiplicity to carry out multiple calculations in essence in parallel,
15:22which gives you a multiplier effect on how fast the computer can in principle get to an answer.
15:30But there's a second part of quantum computation, which is,
15:34it's not enough that it does all these calculations in parallel.
15:37How do you find the answer among that multiplicity that you are looking for?
15:42And so for a certain class of questions, a quantum computer can give rise to an exponential speedup.
15:49But for other questions, the exponential speedup that works for other problems won't be applicable.
15:55And so my suspicion is that quantum computers will be a very niche-oriented kind of device
16:04that are really good at a certain category of questions, like simulating quantum systems or
16:10factoring numbers into their prime factors, but unlikely to be an all-purpose machine that sits
16:16on your desk or on your lap and does the kinds of things that ordinary computers do, the kinds of
16:23things that we are familiar with.
16:24From at Kiro Sock, what the is quantum entanglement?
16:29In the everyday world, if two things are far apart, we know that they can act independently of one another.
16:35But in the quantum world, two things can be far apart, and yet they are locked together
16:41from a behavioral standpoint, even though they are far apart in space.
16:45So imagine you've got a particle whose spin, we call it, is pointing up or down.
16:50It's in a mixture of both of those.
16:52And you can have two of these particles if they are quantum entangled.
16:56A measurement on one of these particles coaxes a single definite reality.
17:00The other particle, if they're quantum entangled, will also snap out of the fuzzy haze and acquire
17:06a single definite reality. Even though they're far apart, quantum entanglement bridges the gap
17:12between them and allows their behaviors to be tightly choreographed in a way that we're still
17:18struggling to fully understand. But the laboratory experiments are clear. What you do here quantum
17:24mechanically can have an impact over there if the two objects are quantum entangled.
17:29Next question is from the ChangeMyView subreddit.
17:32Dr. Schrodinger's cat seems like a dumb concept.
17:36Maybe you don't fully understand the idea of Schrodinger's cat then, because it's anything but
17:42a dumb concept. Back in the 1930s, when quantum mechanics was being developed, you might have
17:48tried to argue that the weirdness of quantum mechanics can be sequestered into the microscopic
17:55world and we simply don't need to think about it or worry about it in the macroscopic world.
18:01Dr. Schrodinger's cat was introduced by Schrodinger to lay that thinking to rest.
18:07And he basically said, look, if you have an electron, a little particle that's in a 50-50
18:12mixture of being here and being there, you might say, well, okay, I'm willing to accept a particle
18:18in some mixture of being two places at the same time, because it's in the microscopic world and
18:23I don't need to worry about it in the macro world. And Schrodinger says, no, we can amplify the weirdness
18:28of the micro world into the macro world. All we need to do is set up a little gadget,
18:34which has poison that will be released into this box if the electron's on the right-hand side,
18:40but it won't be released into the box if the electron's on the left-hand side. And then if we
18:45put a cat in the box, the poison is in a 50-50 mixture of being released and not being
18:51released,
18:51which means the cat must be in a 50-50 mixture of being dead and alive. Quantum physics, he was
18:59saying, its weirdness necessarily has a trajectory whereby it can infect the everyday reality that we
19:07experience. At resonance, enter one, does observation create reality? There are physical systems where if
19:16there is no observation, there is no measurement, the system evolves and behaves one way. But if there
19:23is interaction and involvement, that interaction does change how the system behaves. A single photon,
19:31if it sufficiently impinges on a particle like an electron, that photon can coax a definite reality
19:38to emerge. One of the deep questions in quantum mechanics that we have not yet fully understood
19:45is why is it that an observation or a measurement can have such a radical impact on a quantum mechanical
19:55system? Somehow intervention, environmental influence, observation or measurement is important
20:02to this transition. But exactly how this happens, this is something that we are still struggling to understand.
20:14We've known since the 1800s that as electrons accelerate, they radiate away energy. And if they radiate away
20:22energy, that electron should spiral into the nucleus and crash. So why doesn't that happen? The answer,
20:29according to quantum mechanics, is that there is a lowest energy state that's available to a system
20:38like an atom, such that there isn't any way in the world for the atom to achieve a lower energy
20:45configuration. And so quantum mechanics itself places a limit on how close the electron can get to the
20:54nucleus of the atom. We can actually calculate that lowest energy state and then we can compare it to the
21:01energy states of atoms. Energy can be quantized. It can come in chunks that can't be subdivided,
21:09they can't be diminished. From the Manuko subreddit, at the smallest level we have quantum physics,
21:16at the normal level we have relativity. Is there another set of laws for super big things? You're
21:23right, for the very small we have quantum mechanics, but I would say for the super big we have general
21:28relativity. That's Einstein's theory of gravity. And gravity becomes ever more important when things
21:34are ever larger for stars and black holes. We are the in-between, the very small and the super big,
21:42and we are necessarily a blend. And so there are things in the world that we are familiar with where
21:48quantum mechanics rears itself and flexes its muscles more strongly. There are other things that we
21:55encounter that have general relativity flexing its muscles, but we haven't found that there's a new
22:03set of laws that we have to introduce to understand the in-between. Instead, our challenge is to put
22:10general relativity and quantum mechanics so the calculations that we human beings in the middle
22:14actually do, we want those calculations to make sense and to make sensible predictions. And that is a
22:21blending that we have been trying to do for, I don't know, 80 years. From at ignore that door, why
22:30does
22:30anything exist? In the beginning there was totally nothing, so how did something come from nothing? We don't
22:37know why there's something rather than nothing. All we physicists can do is we assume that there was
22:44something, energy, matter, a configuration that can give rise to the big bang, and then from that point
22:52forward we analyze what happens to that something, how that something manifests as particles that then
22:59come together to yield stars, galaxies, and so on and so forth. But if you push further back and ask
23:06why is
23:07there anything at all, nobody really has an answer yet for that. There is one idea that I can just
23:14throw
23:15out. A state of absolute nothingness may be unstable. A state of absolute nothingness may quantum
23:22mechanically fall apart into a something and an anti-something. Do we know that that's the case?
23:27We don't, but at least it's an interesting idea for how you could start with nothing and yield a something.
23:34At Troll on Patrol, what happens after the universe dies? I wrote a whole book on it. In fact,
23:42it's the bottom book in this little stack under here, Until the End of Time. We don't know for sure,
23:48but one dominant possibility is that in the very far future the universe will continue its spatial
23:55expansion. As it expands ever more widely, material objects will continue to disintegrate, stars will
24:03disintegrate, planets will disintegrate, even black holes disintegrate into a bath of particles. And so
24:10that suggests that in the very far future the universe will be nothing but a bath of particles
24:18wafting through an ever larger, ever quieter, ever colder cosmos. Long before any of these structures
24:26existed in the early universe, it was just particles. Those particles come together for a brief period of
24:32time, giving rise to the stars and galaxies and planets and on these planet people as well. And
24:38then in the far future it all disperses once again. So everything that we know about, everything we care
24:46about, everything that matters to us, may just be a momentary aggregate of particles that comes together
24:54for a brief period, then falls apart, and then for effectively an eternity, there's nothing but
25:00particles wafting through the void. All right, that's all for today. Thanks for joining us. Hope you
25:08learned a little bit about quantum physics and the universe. This is quantum physics support.
Comments