Listener Questions 45: Gravity and Black holes

Published Nov 28, 2023, 6:00 AM

Daniel and Jorge venture to the edge of human knowledge about black holes, gravity and magnetic fields.

See omnystudio.com/listener for privacy information.

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Hey Daniel, do you ever get tired of answering questions from listeners?

Not so far, But you know, ask me again in another five.

Years, Oh, what's going to happen in five years?

I might have a different answer you.

Sometimes you get the same questions over and over.

You know, there are a lot of common themes in these questions, that's true.

And do you have like pre preferred answers ready to go?

You can't really do that because every question's a little bit different. And for me, the fun part is figuring out, like what somebody misunderstood to lead to their confusion and helping them unravel that.

So you can't just like program an AI to answer questions for you, kind of like they do now with like customer service lines.

You could definitely program an AI to answer questions, but it would generate.

Nonsense sometings isn't the answer nonsense. In physics, especially quantum mechanics, the.

Most amazing thing about the universe is that it doesn't seem to be nonsense. It seems to actually make sense so far.

Maybe I'll ask you again in five years.

Maybe in five years you'll have replaced me with an AI.

Maybe in five years will all be replaced by AIS and only AIS will be listening to this, But maybe not II. Poorham Me cartoonists and the author of Oliver's Great Big Universe.

Hi, I'm a professor of physics and I do experiments at CERN, and I don't think that I'm an AI.

But you could be. Are you saying.

We never know philosophically where our consciousness comes from. We could all actually be AIS?

Yeah, we could. I guess there's several possibilities, right, Like we could be in a simulation or something and we could all be AIS. Or you know, if the people who are religious are right, then technically we are kind of artificial intelligence because we were made by another intelligence.

Yeah, it certainly could be. Or we could all just be cylons, you know, thinking we're humans, programmed to think we are humans, but silicon underneath. Yeah.

On a TV show that sadly got canceled.

That was a great show.

But anyways, welcome to our podcast. Daniel and Jorge Explain the Universe, a production of iHeartRadio.

In which we use our consciousness simulated or not, artificial or not, to try to understand the nature of the universe, whether it's real or not, and whether our subjective experience is organic or not. We think it's worthwhile to try to understand what's out there, to try to make it all work in our minds, to ask questions and seek answers, and that's what this podcast is all about.

Yeah, sometimes I feel like by intelligence is simulated, Like I'm just pretending to be intelligent.

What's the difference between being intelligent and effectively pretending?

Oh, good point, that's an intelligent answer. And I don't mean that in a fixed sense. I mean that in an unartificial sense.

Sometimes I feel like the most important function I serve for my students is asking them dumb questions about their research, and often I see it spark good ideas. They're like, well, that doesn't make any sense, but it gives me a good idea about something I could do.

So you're incentivized to ask dumb questions.

I just simulate knowing what I'm doing, and somehow the people around me get stuff done.

Tas like you could program an AI to ask dumb questions and then you don't have to do anything.

Yeah, exactly. I think often I could be replaced by a cardboard cutout of myself. Just having students explain what they're doing to me helps them understand what they're doing wrong.

Yeah, and it has pre program answers like or questions like huh, that's really interesting. Have you checked the error bars?

Are you sure about those assumptions?

That's it. Just have it on loop. People come in, they press a button, You're in a Caribbean island doing nothing.

Yeah. Or they just pull a string behind me, or I'm just like a stuffed Teddy bear.

Yeah, there you go.

Maybe we should actually make merch stuffed versions of us. They pull the back of you and you go hmm, and they pull the back of me and it goes chuckle.

Yeah. Perfect, And then someone could just repidate the entire podcast without us there.

Just you could build a robot to pull the strings and we'd be done.

Yeah. Oh man, but I guess you need someone to press the record, butk.

I have students who can do that.

Perfect. But yeah, it is a pretty interesting and amazing universe. Whether it's simulated or not, or whether we are simulated or not, and whether we are artificial or not. We have questions about what's going on in this universe, about how it all works.

Scientists have questions and are busy doing experiments to try to get answers. But we're not the only ones with questions. Everybody out there asks questions about the nature of the universe since the first people have looked up in the night sky and wondered what all those twinkling lights were. Being curious is just part of being human, and looking for answers is doing science, whether or not you're getting paid for it.

Yeah, it's not just a job of scientists to ask questions about the universe. It's your job. It's everybody's job to look at the cosmos and wonder why it's all there and how it all works.

One of our goals on this podcast is to find answers for you, but also to stimulate your questions, to get you to think about the things you don't understand, to hear what we're saying and then try to click it together in your mind, and when it doesn't quite fit. We love if you reached out to us to ask us your questions about the universe. We'll always answer them to questions at Danielandjorge dot com.

Yeah, everybody has questions, the kids, adults, and every one in between, and sometimes we'd like to answer those questions here on the podcast.

We absolutely do, so feel free to write to us and ask us your questions. Sometimes I'll get a question that I think, hmm, I bet other people want to know the answer to that, and so we'll answer it here on the podcast.

Do you sometimes think, oh, nobody else wants to know the answer to that. Do you have the opposite feeling?

I do sometimes get personal questions about people's life path and stuff like that, and so yeah, that's individualized answers that don't need to be on the podcast.

Do you get a question that it's like so complicated you don't think anyone else would be interested.

I get a lot of people sending me their personal theories of the universe, and I'm not sure anybody else really wants to read those hundreds of pages written by retired engineers.

What if they're right? Oh, I see, But then it becomes your theory and then you want everyone to read about it.

No, I read those theories and give them critiques and if there was something to it then yeah, you'd hear about it well.

As Daniel say, we'd like to answer questions here on the podcast, and so on today's episode will be tackling listener questions number forty five, Gravity and black holes. That's the theme of the questions today, gravity and black holes.

Yeah, black holes and gravity seemed to be on people's minds the week that these questions came in.

Yeah, and so we have some great questions here about famous physics experiments, the event horizons of black holes, and what happens since the Big Bang. Pretty deep questions, deep in time and space. So we'll just jump right in. Our first question comes from Sean from Canada.

Hey, Daniel and Jorge, this is Sewn calling from Canada. I have a question about the observer effect. What would happen if the observer was on the inside of the event horizon of a black hole and the experiment being observed was on the outside of the event horizon of a black hole? Would it know that it's being observed? Would the waves collapse and all that stuff? Yeah, let me know.

Thanks, interesting question. I guess the gist of it is that what happens if you're inside of a black hole, can't tell what's going on outside of the black hole.

Yeah, there's a lot of really interesting stuff going on in this question. It's about black holes, it's about quantum mechanics, it's about all that kind of stuff. Of course, inside a black hole, you can see things that are happening outside a black hole, right. A black hole is where information cannot escape from, but new information can always arrive, Like photons can fall into black holes, and if you were inside a black hole, those photons could still reach you. So from within a black hole, you can observe things happening in the outside of the universe. But I love this question because it touches on this complicated quantum mechanical issue of observing things changing them right. Often in quantum mechanics we say that observation changes an experiment, And he's wondering, if you're doing that observation while you're inside a black hole, can you change an experiment that's outside a black hole?

Because I guess in quantum mechanics, once you observe something like it changes the wave function of it right.

Exactly, And this is something in quantum mechanics that's not very well understood. So often when you push the boundaries and come up with crazy thought experiments, the answer is we don't know or we have no idea because actually our theory quantum mechanics doesn't make any sense. So it's probably important to like sum up what is the observer effect in quantum mechanics what we're talking about here.

But I guess maybe I have many more basic questions about the setup here. So like the observer, you are inside the black hole, and this is I guess, assuming you survive going into the black hole.

Right, yeah, I assume. I assume you survive if you're observing the experiment.

Like if you have a camera and you take it inside of the black hole and you somemos survive, you would still be getting information from outside the black hole, but you wouldn't I guess. See, like the whole universe, like I think we've talked about this before, the whole universe would look like one pinpoint to you.

That's right, All light that arrives on the black hole would arrive to you at just one point, So like the entire event horizon would be collapsed to a single point in your vision. The rest every other direction from your perspective would be towards the singularity. Because remember that black holes are curvatures of space time, and so changing the way space is organized inside the black hole.

Okay, so now the scenario is that there's maybe like an electron just outside the black hole, and it's about to veer to the right or to the left depending on some magnetic field right because it has some quantum uncertainty about that. And then the question is could you see that, Like, could the photon from that electron reach you? Would it reach you?

So a photon from that electron definitely could reach you. If the electron gives off a photon that can fall in the black hole, and then it can reach you inside the black hole absolutely.

What about like time, doesn't time slow down at the surface of a black hole, or doesn't it stand still?

So time does get slowed down by gravity. Places that have strong curvature feel time going more slowly. So for example, if you're near a black hole and you're looking at the rest of the universe, your time is going more slowly. You see the rest of the universe going more quickly. Or if you're watching somebody fall into a black hole, you see their time slow down, so from the outside you can't actually see somebody fall into a black hole. You're right, if time slows down so much as they approach to the event horizon that it's not until like time equals infinity, that they actually fall in from your perspective. But if you're the person falling into the event horizon, then you just fall past the event horizon. You don't notice these effects.

The elector would just fly right in from your perspective inside the black.

Hole, inside the black hole exactly.

But really outside of a black hole, it wouldn't happen for infinity.

From the point of view of a distant observer watching you fall in, it wouldn't happen until time egals infinity, or until something else falls in the black hole and grows it so that it encompasses you. That's the reason that like real black holes in the universe can actually grow, that they don't have to wait until time equals infinity for things to fall in, because there's a whole series of things falling in. Each one grows the event horizon for the previous one.

Okay, so you can get information from inside the black hole, and I guess you're not really watching the electron. You just watching whether it veers to the rider to the left, right something to texts the electron going right or left or something.

Yeah, I think the setupiece interested in is like an electron is in a superposition of two possible states, like does it go left or right? And somehow you maybe use a photon to detect left versus right, and that photon falls into the black hole. And he's wondering if observing that photon inside the black hole collapses the wave function the experiment outside the black hole.

Right, Because I guess since you're inside the black hole, there's like no way for the electron to know whether you saw it or not exactly.

That's where this cool quantum mechanics black hole paradox comes in, because if you take away the black hole, we had the sort of classic observer effect that the electron can still be fifty percent chance left, fifty percent chance right until its wave function is collapsed. When does the wave function collapse? Well, nobody really knows the answer to that, but one ridiculous but standard description or quantum mechanics says that the electrons wave function is collapsed when it is observed by a classical object like a person or a big detector or something. So the photon can bounce off of the electron without collapsing its wave function because it's still a quantum object. But then when that photon carrying that information hits a screen or a detector or an eyeball or something like that, a classical object, it collapses the whole wave function. And that's when the electron decided, okay, I went left or okay I went right.

But does it collapse only for the observer or for the entire universe if you observe it. But I don't know what you observe. Is it still a quantum object to me?

Oh? Great question? And the answer to that depends on your quantum mechanics philosophy. So in standard quantum mechanics Copenhagen interpretation, it collapses for everybody, and it collapses instantly across space and time. Those two objects are entangled. The photon and the electron are quantum mechanically entangled, meaning that they share a fate. They're connected to each other. If the electron goes left, then the photon looks a certain way, And if electron goes right, the photon looks another way. So in your standard interpretation, as soon as you observe the photon, that collapses the electron for everybody. But in other interpretations to quantum mechanics, like Carlo Ravelli's relational quantum mechanics, then it only collapses for the person doing the observation. One person can collapse it for themselves, somebody else could have it be uncollapsed, and a third person can collapse it in another way. So there's different theories of quantum mechanics in the standard one that people typically think about and we complain about a lot because it doesn't make much sense. It's collapsed for everybody.

It also sort of depends on the idea of schrot Ainger's box, right, doesn't end like if I wrap a box or on you the observer and the electron, like, it's still a quantum object to me, no matter whose interpretation they think about. Does it the cat is both alive and dad and you saw it and not saw it at the same time.

Yeah, that is the paradox raised by Schrodinger's box, that things can be unobserved but still be classical. So in the standard Copenhagen interpretation we say classical objects collapse the wave function, and quantum objects do not. The problem with the standard quantum mechanics is that there's no definition of what's a classical and what's a quant to object, so it's sort of a useless distinction. But in the standard interpretation then you would still become collapse the wave function because you'd be a classical object and your observation collapses it. Even if I don't know what you've seen before. You're not a quantum object, so you can't be in a superposition.

Okay, so that this is an extra twist to it. Now, let's say that you're the observer and you're inside of a black hole and you saw the electron go right or left. I think Sean is asking, how does that affect things? Did the wave function collapse for the electron or is it still unknown for the rest of the universe.

This is a really great and very very difficult question, and before we answer it, I want to compare it to a similar complicated question, which is just about entangled objects that are really far apart right. A similar question you might ask is, well, well, if the photon is really far away from the electron when it's observed. How does the electron know to collapse? If the photon flies for a thousand light years before it gets observed, how does the electron then collapse instantly across time? These questions are related because they have to do with apparently sending impossible information. And this is a classic question in quantum mechanics theory, right, And this is the paradox posed by Einstein decades and decades ago. And he can plain that quantum mechanics makes no sense because it requires things to violate special relativity to collapse instantly across time. Things outside of each other's light cones somehow, causing each other to change.

So in the case of it entangled quantum particles, the answer is that it does travel faster than light in a way. Right, Like, once you collapse one half of it a few light years away, it sort of instantly changes what you have in front.

Of you exactly. And there's a really crucial subtlety here. You're totally right that the collapse happens instantaneously across space and time. So quantum mechanics is what we call non local, and that's because the wave function is broad. It doesn't just exist in one place don't think of it like one particle doing something to the other particle. It's one big quantum state, and you collapse it anywhere. It collapses everywhere simultaneously. That does happen instantaneously across space and time. But and this is the crucial nuance, it doesn't send any information. So collapsing the wave function with a photon really really far apart doesn't send information to the electron. You can't use it to like send signals faster than the speed of light. Though a lot of people imagine that quantum mechanics entanglement can do that, you can't actually send information. Just collapses the whole wave function simultaneously. It's not a mechanism for information transmission. I see.

I think what you're saying is that, like there's no rule to how big a quantum system can be or how far apart its parts can be. So like, even if I have one half here and another half, you know, millions of lighters away, it's just still one quantum system. There's no rule in quantum mechanics that says, oh, no, you're too far apart. Now you're two separate quantum systems. You're actually like still the same system.

Exactly, and quantum mechanics explicitly is non local stuff can happen coordinated across space and time. It doesn't have to be like this thing bumps into that thing which is right next to it. Very your property of quantum mechanics that we really don't fully understand.

Okay, And so now the question is, what if half of my system is inside of a black hole? Is it still one system?

Great question, and the answer depends on your theory of quantum gravity, because now this is a question that involves quantum mechanics. We're talking about quantum particles and wave functions, and it involves event horizons, so gravitational effects. And the truth is we don't know how to marry those two things. So I'm sorry, Sean, you're asking a question we don't really know the answer to because we don't have a theory of quantum gravity.

Meaning like we don't know how like if you distorted gravity a lot, like in a black hole, you don't know how it affects this idea of like a quantum system being the two halves even though.

They're far apart exactly, We just don't know it.

Might affect it or it might not affect it exactly.

We don't know if gravity collapses wave functions or not, or if gravity is fundamentally quantum mechanical and allows things to be in superpositions even as they cross event horizons. We don't have the answer to that, but I can speculate an analogy to the other scenario where you have two quantum particles really far apart, basically outside of each other's light cones, which is sort of like being outside of each other's event horizons. There's still the wave function does collapse, but no information is transmitted. So I suspect that what happens here is that if you observe the photon inside the black hole, it does collapse the wave function of the electron outside the black hole, but without transmitting any information and so not breaking that rule of black holes.

WHOA Does that mean you could now communicate from inside of a black hole to the outside of a black hole.

No.

In the same way that you can't use quantum entangled particle collapse to send information, you also can't send information from inside an event horizon using the collapse of a quantum object across that event horizon. That's the analogy.

So I wonder if like, practically speaking, you didn't really collapse it because you're inside of a black hole and nobody will ever know what you saw. So pretty much of the rest of the universe, the half that's outside, is still quantum unknown.

Yeah, that's very insightful because the reason you can't send information across quantum particles faster than light is that you can't know whether it's collapsed. Like if I have a quantum particle and you have a quantum particle and they're entangled, I can measure mine which would collapse yours if you haven't already measured it, But you can't tell if I've collapsed at All you can do is measure your particle and get like spin up or spin down, left or right. You can never tell that I've collapsed it. That information doesn't get transmitted. And so in the same way, if somebody observes that photon inside the event horizon, somebody else looking at that electron can't tell whether the electron's wave function has been collapsed or not.

Okay, So then the answer for ra Sean is that we have no idea a calling answer we give you in the podcast, because nobody knows how gravity or extreme gravity like black holes effects, quantum mechanics and quantum systems and wave collapse. But our best guess here on the podcast is that it probably does collapse it, but maybe it doesn't matter, so it doesn't really collapse it.

Yeah, that's a great summary.

Well, let's get to our other questions here today about black holes and about the big dang and magnetic fields. So let's get to those, But first let's take a quick break.

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Right, we're taking listener questions here today and at least trying to answer them. Although sometimes the answer is nobody.

Knows, sometimes the answer is great question I wish I knew the answer.

Yeah, if you have the answer, write it to us in one hundred page summary, and then Daniel will read it and let you know.

That's right. Really, the answer some of these questions requires us to develop theories of quantum gravity, and to figure out how to develop those theories. We want to look inside black holes, which is impossible, so we're a little bit stuck sometimes.

Well, you can look inside of a black hole, just can't tell anyone what you saw.

Yeah, that's right. That physicist who fell into a black hole knows the answers, just can't get any awards for it.

Yeah, that physicists solved everything. Let's throw a Nobel Prize medal inside the black hole for them. That million dollars also or million coroner. I'm sure they can find a good use for it in there. All right. So our second question comes from Ryan, who lives in northern Virginia.

Hello, Daniel and Hore. My name is Ryan and I live in northern Virginia. I have a question for you today about black holes, more specifically about their magnetic fields. If the Earth's magnetic field comes from its liquid outer core, where does a black holes magnetic field come from. Considering there's no liquid core in the black hole and nothing is supposed to be able to escape the event horizon, I'm left to guess that the accretion disk is the big driver of the magnetic field. But that's just a guess. Thanks for considering my question. Love the show.

Awesome great question, Ryan. You know I don't know the answer here, and I'm gonna guess. Maybe the answer is again, we don't know because it deals with black holes.

No, this one, I think we do know the answer to. Actually, well, I know physics knows something, all right.

Well, let's see Ryan's question is where does a black hole's magnetic field come from? Because I guess black holes have a magnetic field. Do we know that for sure?

Well, we do measure very strong magnetic fields near black holes.

How do we measure them?

How do we measure them? Great question? Well, we can see the effect, right. Sometimes black holes have enormous jets of stuff that shoot up and down on their north and south and we think that comes from the magnetic field, like funneling particles up and down, sort of the same way that the Earth's magnetic field causes northern lights. Charged particles coming towards the Earth, magnetic field get funneled up towards the north and south poles. Particles falling into a black hole sometimes will miss because the magnetic field will deflect them up and down, and you get these enormous, like thousands of light years long jets of stuff shooting out of black holes. We think from the magnetic fields.

Well, I guess, first of all, we think those are black holes and that those jets are coming from black holes. Don't, Like, we haven't actually seen the inside of what's inside of those jets.

We've study those jets in some great detail, and we have really pretty good models that predict those jets. Recently, we imaged a couple of black holes and saw ripples in the accretion disk around them, and so we're able to like really pin down the details of the magnetic field near the black hole. I mean, we'd love to send a probe near the black hole and measure those directly, but there's a lot of indirect ways to measure those magnetic fields, just by seeing the impact to have on char large particles near the black hole.

And I guess the other question is, how do he knows those magnetic fields are coming from the black hole enough from the stuff around the black hole.

Yeah, that's a great question, and we don't know the answer to that. We're sure that the stuff around the black hole can make a magnetic field. Those are charged particles. They're moving in a circle, so you have a current moving in a circle that always generates a magnetic field. The second part is thinking about whether a black hole on its own could have a magnetic field even without the accretion disc, even without the stuff swirling around it. Whether just the black hole itself can have a magnetic field is a really interesting question.

Do we know the answer? Does a black hole on its own have an inherent magnetic field?

So we can answer that question theoretically. We've never seen a black hole all by itself without an accretion disk and measured it. But according to general relativity, black holes can have magnetic fields. And that's because black holes can do two crucial things. One they can have an electric charge and two they can spin. And essentially anything with an electric charge that spins has a magnetic field.

But you're saying it's all theoretical though.

It's theoretical because we've never observed a black hole without any stuff around it. And measured its magnetic field. That would be an awesome test of this theory.

I see. So basically we don't know.

Yeah, that's true of lots of things. I suppose. We're not sure about it, but we do have a pretty good idea. And lots of our models of spinning black holes and black holes with charge have been tested indirectly. We've never done this exact test.

Okay, So you're saying theoretically black holes do have magnetic fields because they somehow preserve it. Right, even though you when you throw charge particles in it with spin in it and magnetic fields, presumably that doesn't get destroyed by.

The black hole exactly. And Ryan is asking about, like where that magnetic field might come from, because you can't see anything beyond the event horizon, so you can't have like swirling matter within the event horizon causing this magnetic field. Because the details of anything like that happening within the event horizon is shielded by the event horizon. You can only know a few things about what's happening inside the event horizon. You can know the total mass, you can know the electric charge, and you can know.

The spin, meaning even if there are a bunch of electron spinning inside of a black hole. The magnetic field they would generate couldn't leak out of the black hole. Is that what you're saying, Because the space would just be pointing inwards.

A lot of the details of what's happening to those electrons you wouldn't be sensitive to, Like if one electron zigs up or zigs down, you couldn't sense that from the outside. You can, however, sense that there are a bunch of electrons, and you can sense that they're spinning overall, because you can measure the total spin of the black hole and the total electric charge of the black hole. Like when electron falls into a black hole, just before it falls in, it has an electric field, and when it falls in, that electric field is now frozen on the outside of the black hole. Whatever happens to the electron after it falls in can no longer change to that electric field. It's sort of frozen there. So you can tell that something has fallen in and that it had charge, But the details what happens afterwards you're shielded from.

So then are you saying, like, to get the magnetic field of a black hole, you just multiply its charged by its spin somehow, and that gives you like what you would feel as a magnetic field outside of the black hole. But those are like overall numbers, not related to anything in detail inside of it.

Exactly in the same way that an electron has a little magnetic field. Right where does the magnetic field an electron come from. It doesn't have a magnetic charge. It has an electric charge, and it has quantum spin. Those two things combined to give the electron a tiny little magnetic direction, a magnetic dipole. And you can't tell what's going on inside the electron. We think maybe it's a fundamental particle. We have no idea. We can't see inside, and it doesn't matter. We know it has an overall charge and an overall spin, and so the overall charge and spin of a black hole similarly gives it a magnetic moment. It's a magnetic dipole.

I wonder if, like Ryan's question was more like, you know, how can a black hole have a magnetic field if nothing can escape it? You know what I mean? Like, if the electromagnetic field it has is due to the stuff inside of the black hole, how can its magnetism escape the black hole.

Yeah, that's a great question. We have a whole episode on how black holes can have magnetic fields and electric fields and digs into the details of this question. Very briefly, though, The answer is that the overall charge is essentially spread out on the event horizon. So something falls into a black hole, you're not getting information from within the event horizon, You're just getting information from the event horizon. So think about the event horizon itself as now having that charge and having that spin. There might be crazy stuff going on inside, weird quantum effects or singularities orringularities or whatever. You can't tell, but you can tell that something charged fell in, and you can tell without getting any information about what's going on inside the event horizon.

It sort of sounds like you're saying that the the black hole's magnetic field comes from its surface, Like it's the surface of the black hole that's basically you know, phasing out to the rest of the universe and you know, emanating an electric charge and an electric field, and that's why we can see it.

Yeah, that's a good way to think about it. The event horizon or the surface of it equivalently, has three properties mass, spin, and charge. And we can measure those and those have an effect on the rest of the universe right the same way, like the mass of the black hole, even though it's contained within the event horizon, still curves space outside the event horizon and can affect the trajectories of stuff. Think of that as the property of the event horizon. It doesn't matter what's going on inside behind the screen of the event horizon, if things are swirling around or not. All you know is the overall mass, the overall charge, and the overall spin. And that can create a magnetic field.

But again, time slows down almost to a standstill near the event horizon. How does the magnetic field ever get out? Don't have to wait to infinity to feel that magnetic field.

Yes, so time near the black hole is really really slowed down, not slowed down totally to infinity. So black holes can radiate information. For example, like of a black hole gets accelerated by another black hole, it can radiate a gravitational wave, or if it has charge, it will also radiate photons. Again, these are coming from the surface of the black hole, not from within it, so we're not breaking the rules of black holes. But they can radiate that information, And you're right that things near black holes are slowed down, and so it does take longer. And like those gravitational waves are slowed down by the time dilation of the gravitational field. So without that effect, those gravitational waves would look much crazier and the photons would be much higher frequency. But they're stretched out and red shifted by that time dilation, not all the way to infinity, because the curvature isn't infinity outside the black hole.

All right, Well, it sounds like the answer for Ryan is that a black hole's magnetic field comes from its surface. It's event horizon basically, or at least we can practically think of it as coming from the surface and the event horizon, and that's why we able to see it and experience it. We think, we think we don't know for sure because it's a black hole.

We don't know for sure anything. We don't even know if we exist or if this is a simulation. And also remember that most of the encneic fields we've measured probably do mostly come from the accretion disk of stuff around them. But as Wheree says, we don't know for sure.

All right, Well, let's get to our last question of the day, which is about the expanding universe and gravity and the Big Bang. But first, let's take another quick break.

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All right, we're answering listener questions here our favorite kind of episode where we take your questions that you send in and we try to answer them, usually with answers that are not we don't know.

We do our best.

Sometimes nobody knows which is an answer technically.

It is, And I wonder if that's like really satisfying because it means for the listener like, oh, I'm at the forefront of human knowledge, or really disappointing because like the rest of humanity is still left unsatisfied.

Yeah, I guess it is pretty exciting right to be to like come up against the boundary of human.

Knowledge, right, yeah, exactly, Like.

When I ask you a question you don't know the answer, I'm like, Wow, I am at the boundary of Daniel's knowledge of the universe.

Now, the boundary of Daniel's knowledge not the same as the boundary knowledge.

Let's not make it, or at least the boundary of the reason you've been able to do for the last hour before the podcast.

Exactly, I am at the boundary of human knowledge in one time is a little corner of the vast sphere of human knowledge.

Well, we're all here with you answer. Our last question of the day comes from Urgent.

Hight Daniel, I have a question. Ha just got me thinking while I was on the shore. We've come to understand that gravity is bending of space time, and that's objects which would normally travel in a straight line tend to take a curve path around the body, which is distorting space time. We also know that space itself is expanding in the universe for the last ever since Big Bang happened. Do we think the effects of gravity have changed over the last couple billion years or do we expected to be different later? Now that we know that space itself is expanding or stretching or thin out, whatever you may call it, will that effect the way gravity acts and behaves?

What has it changed over the years? What do you reckon?

Awesome question? What do we reckon? Daniel? Do we know the answer? In this case?

I reckon that Argent probably takes really long showers to have such deep thoughts about the whole history.

Of the universe, which is awesome. Thank you Argent for such a great question.

So, as usual, we have some ideas about how this works, but there's lots that we still don't understand, especially about the expanding and the accelerating expansion of the universe.

Okay, let me see if I understand Argent's question. He's in the shower and he's wondering, you know, the universe since the Big Bang has, first of all, it expanded really fast. Space itself expanded really fast, and space today is still expanding, and there's more and more space growing and being created. The universe is expanding due to something called dark energy. And I think his question is, like, is gravity affected by this? Expansion of space, like, do we know if gravity itself has been the same and for the last fourteen billion years, or is there an indication that maybe as a universe expands, it could be changing the effects of gravity like this gravity space dependent. What do you reckon then.

Is I think there's a crucial piece of the history of the universe that you sort of YadA yadded over there, because you're right, the things did expand very quickly in the beginning, but then then expansion actually decelerated because the stuff in the universe slowed down the expansion. And then later on, like six billion years ago, it started picking up again and it started to accelerate the expansion. So this is funny, sort of like zigzag in the history of the universe. It wasn't always just accelerating expansion. It was always expansion, but there was a period when that expansion was decelerating. I emphasized that not just because it's a cool zigzag, but underline something which I think is crucial to answering Argent's question, which is that's part of gravity. Gravity is not just things pulling themselves together, it's also space expanding. That is general relativity. Our framework that we have and what Argin describes as like moving through ben space time that allows for the universe to expand. That sort of part of what gravity is, Well.

What do you mean, like it's in the equations of our theories of gravity for the universe to expand due to dark energy.

It's part of general relativity that the universe can expand under certain conditions. So the broad answer to Argin's question is, we think the rules of the universe, the rules of gravity and space time and general relativity have not changed over the last fourteen billion years, and we can describe all of these weird epics of the universe using one consistent framework, and that's sort of amazing. So the rules haven't changed, even though the conditions do change from year to year and things get further apart and whatever. So there are specific conditions under which general relativity predicts the universe will expand and that that expansion will accelerate. If you have a bunch of energy inherent in space stored in a field that has high potential energy, then general relativity says space will expand and the expansion will accelerate. So we see that happening in the universe. Look back at the history of the universe. We see it's expanding. We see that expansion is accelerating, and so we say, oh, there must be some energy stored in the field somewhere with a lot of potential energy that's causing this. We don't know what that is. We don't know what that field is. We have no explanation for it. But general relativity can accommodate that.

Yeah, I think what you're saying is that physicists have a set of equations that explain the universe, and that set of equations has gravity in it, and it also has expansion of the universe in it. So it's like they're all actually kind of connected already into theories of physics.

Yes, exactly.

So it's not like one of them we don't know if one of them is doing something the other one doesn't know.

That's right. And when people say gravity colloquially, they mean like stuff attracting and things falling towards planets and whatever. When physicists say gravity, they mean the whole shebang. They mean the whole theory of space, time and all of its consequences. Because moving from like a Newtonian view of gravity as a force to a nine Steindian view of gravity as motion of stuff through curve space. Time has all of these consequences, not just oh, light is also bent by gravity, but also the universe can expand and it could also collapse. All of these things are consequences of this geometric view of the universe we have from general relativity, and we think that those rules have not changed. That the same rules applied in the very very beginning and in the middle point when things were decelerating, and now when things are accelerating. So in the broadest sense, the rules of gravity general relativity have not changed over the course of the universe.

I see, like the rules by which you mean the equations. But I wonder if Argin means like, you know, imagine there's a term in your equations for gravity. I wonder if he could mean that, you know, has that term the equation changed as the universe grew? Like could it that be something the equations don't take into account, or it could it be something we haven't noticed or what.

Yeah, there's a couple of ways in which that could be true. First of all, we assume that dark energy or whatever, this is this potential field that's causing the accelerating expansion in the universe. We assume that that's constant everywhere in space and everywhere in time, and mostly that works. I mean, it said that we can explain the whole history of the universe, and that's mostly true. But there are some discrepancies, Like we measure it early in the universe, we measure it late in the universe, and those two numbers don't quite agree. You can read more about that. It's called the Hubble tension. Essentially, that's predicting the rate of expansion, and different measurements don't quite agree. So that's quite interesting. So it might be that the dark energy is changing over time, and again that wouldn't mean a change in the rules, but it would mean some change in the conditions, which is affecting like your experience of the universe. And also, maybe more importantly, the second sense is that the density of stuff in the universe is definitely decreasing, Like things used to be really hot and dense and now they're very cold and very far apart, and so there's definitely like less sense of gravity in the universe. Because the mass density of stuff in the universe is going down, space goes up the amount of mass doesn't change, so the density decreases. Things get further and further apart. You're feeling less gravity from distant galaxies than you were before because they're now further away from you, and that's going to keep going.

Mmm.

So I guess technically you would be feeling more the gravity of Earth right as the universe gets more empty and empty, right, Like, I'm way more in the future regardless of what I do.

If you're relying on distant gravities to lift you up off the surface of the Earth and make you feel light, then I have bad news for you.

Yes, as my diet plan to get a z epic.

Yeah, exactly, I suggest hitting the gym instead of relying on distant galaxies. But I'm not a health expert. Don't take advice from me.

Well, I wonder if Argine's question then maybe more simply is like, you know, if I have a black hole the same mass and I see it bend light around it, is it going to bend light the same way now in the future, in the past when the universe was maybe more scrunched together, or is that light Can we be bent differently depending on what the universe is doing, especially like let's say we're going through a period where space is expanding faster and faster, or where we're hitting you know, in the zig Zager who we're hitting a point of maximum expansion, that light is going to band a little bit differently, right than it would in a period of not so fast expansion.

All right, So now you've pushed this into a corner of general relativity that we don't understand very well.

So the answers we don't know, we.

Don't know, and also fascinatingly because we don't even understand our own theory, Like general relativity has no problem with having black holes in an expanding universe, but we don't know how to do that calculation. Like Einstein's equations are nasty and they're very very difficult to solve. We can only solve them in very specialized, simplified settings, like you have a black hole in an otherwise empty universe that we can solve, or you have a universe that's expanding, but the matter in it is totally uniform, like dust sprinkled everywhere. We don't know how to solve the equations for a black hole in an expanding universe or a universe with like chunky stuff in it. So we have all these approximations. And so the specific question you just asked, like what happens to a black hole in an expanding universe? We don't know how to do that calculation, but we think that the rules are not changing, right, and so for a black hole, gravity is basically the same as it was a billion years ago and five billion years ago, Like the nature of space itself is not changing. It doesn't thin out, you know, as the universe expands, and that expansion accelerat, you just get more space, and that space behaves, we think, according to the same rules, and so Bend's light the same way as it always did.

All right, Well, then the answer origent seems to be that you don't think that the rules of the universe are changing, meaning like what the physicists would consider to be gravity, which is the whole set of equations. You don't think that's changing. But maybe the effects of a black hole might be changing, except you don't know how to calculate it.

That's right. And there was even this fascinating paper a few months ago about how like black holes might be driving the expansion of the universe. Right, that black holes might actually be like weird clusters of dark energy. So just to highlight like how little we understand the expansion of the universe and how tricky it is to do these kind of calculations in any sort of realistic setting.

Yeah, in a shower.

The shower is probably the best place to do these calculations.

Yeah. Well, if it wasn't for gravity, you couldn't take a shower. You definitely any gravity to shower.

Oh my gosh, wow, thank you gravity. We should be saying every time we have a shower.

Yeah, if not for gravity, we'd all be a little bit stinkier, or we'd all have to take baths in zero gravity, which I think is dangerous, isn't it, Like you would you would very quickly drown because the water would just englf you.

Maybe, but again, don't take health advice from this podcast. That is outside our expertise.

Yeah, don't take health applies from a cartoonist and a physicist.

Stay on your diet whatever it was before this podcast.

That's right. Listen to a real doctor, not the academic kite exactly.

You know what the best thing about getting your PhD is, there's the best thing. Every meeting is now a doctor's appointment.

There you go. I'm sure everyone loves doctor's appointments. Does that mean that this podcast is a doctor's appointment for the thousands of people who listen to us.

I guess so. Yeah, Oh man, keep your pants on, everyone.

Yeah, keep your pants. We're going to I'm not going to grab anything or ask you to cough.

No, the only things we're probing are the nature of the universe. The only black holes we're investigating are in the theoretical kind. That's right.

That's right, only physical dark matter that we're interested in. All right. Well, that answers Arjie's question and everybody's question again, and another interesting journey to the edge of human knowledge and the realization of how much we know and still have yet to know about the universe, which is kind of exciting.

That's right. So keep asking questions. You'll be surprised how quickly you can get to the edge of human.

Knowledge or the edge of a black hole.

Apparently, or the edge of this doctor's appointment.

Or the edge of this podcast. So we hope you enjoyed that. Thanks for joining us. See you next time.

For more science and curiosity. Come find us on social media, where we answer questions and post videos. We're on Twitter, Discord instance and now TikTok and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. More podcast from iHeartRadio. Visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.

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When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases, Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last sustainability to learn more.

There are children, friends, and families walking, riding on paths and roads every day. Remember they're real people with loved ones who need them to.

Get home safely.

Protect our cyclists and pedestrians because they're people too. Go Safely, California from the California Office of Traffic Safety and Caltrans

Daniel and Jorge Explain the Universe

A fun-filled discussion of the big, mind-blowing, unanswered questions about the Universe. In each e 
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