Picture the Solar system. Likely you have put the Sun at the center, motionless in your mind, with the planets whizzing all around it. Or maybe you've seen that viral video that breaks the common view and shows the Sun in motion through the galaxy. But isn't the galaxy moving to what's the right way to think about all this to assemble a mental universe? How fast are we going? And does that question even make sense? A new study
raises some interesting puzzles. Today we're going to dig into all of that and help you make sense of it. Welcome to Daniel and Kelly's Extraordinary Universe in Motion.
Hello, I'm Kelley Winer Smith.
I study parasites and space, and it doesn't matter what I am moving relative to, I am always moving pretty slow.
Hi. I'm Daniel. I'm a particle physicist, and I'm particularly interested in our cosmic context.
So, Daniel, cosmic context?
Yes, what in your mind would be the coolest spot for us to be in the universe? If we could know, like, should we be in the center, should we be at the edge, if we could be anywhere you wanted us to be? Where would we be?
Wow, that's a crazy question, And I can't believe nobody's ever asked me that before, and I have never thought about that before. You know, if the universe is infinite, then there are no places. Really, They're all the same. So then the question is only interesting if the universe is not infinite. And even if the universe is finite
and closed, there's still no special places. So the most interesting thing would be if the universe is finite and has an edge, in which case being near the edge would be amazing because the edge would have to be different in some way from other kinds of space, and it'll be like a different kind of lego brick that makes up the universe, and different is always fascinating because like at the edge, for example, momentum wouldn't be conserved.
You know, for example, like when you throw a ball against a wall, the ball bounces back, but also the wall gets pushed back a little bit, even if the wall is attached to the Earth, right, But if you throw a ball against the edge of the universe, it bounces back and the universe doesn't get pushed it can't, and so momentum is not conserved. So that would be pretty interesting to see momentum not conservation. Hey, was that the nerdiest possible answer I could have given.
You pretty close.
I'm trying to max out all the metrics here today.
Awesome.
All right, Well, let's see what other metrics Daniel can max out today as we discuss how fast we're moving through space.
Yeah, this is a really fun topic because we get to talk about not just like our cosmic context, where are we in the universe? How is everything slashing around? Give you like a sense for our neighborhood and the bigger picture of view of the universe. But also it touches on some really deep and basic but hard to grapple with concepts in relativity, like what does velocity even mean?
Man?
So get at your banana peels. We are going to smoke them today.
Oh, we're getting philosophical.
I guess we always get philosophical when it's physics. Episode amazing can't be avoided. So I was wondering what people thought about this question before we dive into the physics of it. So I reached out to our amazing group of volunteers who stand at the ready to offer their ungoogled opinions on questions of the day Today. I asked them how fast are we moving through space? Think about it for a moment. What would be your answer. Here's
what our group of experts had to say. Should be dependent upon the relative location and velocity of the observer.
We're all going back to the same location, so effectively, we haven't gone anywhere.
Ninety six thousand kilometers per second. They don't know how fast the Milky Way galaxy is moving.
We're moving at thousands and thousands and thousands of kilometers per second.
How fast we're going depends on our frame of reference. I'm sitting down zero miles per hour.
It must be thousands of miles an hour.
If the universe is infinity in size, then we're going infinity speed around something.
Relative to the galaxy in the universe.
I don't know, man, tough question. Speed is always relative, So the question is how fast are we moving relative to what? Velocity is relative?
When it comes to space time, we're moving all at the speed of light.
Relative to what's not fast enough, guys, hit the nitrous are side.
But on these are great answers. And having listened to you explain physics for over a year now, my guess was, it's got to be relative to something.
And then I was like, and that's all I know.
But I will hear a whole hour on this topic, so soon I'll be an experts And so, yeah, what did you think of these answers?
I thought they were great. They were very well informed and really funny. Also, but I have a question about your response, Kelly. Do you have a different relationship now to like the random physics question that might come up in conversation, Like if your kids ask you a random question about the universe, do you feel like more qualified to maybe answer it or dig into it with them than you did a year ago.
I do, and I'm much more likely to like interject physics information. Like Hata said something about an electron the other day, and I was like, did you know that we don't know if the electron is made up of other bits.
We've tried to figure it out, but we did. It doesn't look like it's made up of other stuff, but we just don't know.
Because we haven't been able to, like, you know, if we had more money, we could dig in more.
And she didn't. You know, I can't say she looked interested, but.
I was excited and so so yes, I do feel like now that I know more about physics. I'm more excited about physics. It feels less opaque and I'm excited. Yeah, life is good.
Okay, yeah, physician accomplished one person at a time.
Woo. Yes, you want a biologist over.
Well, then I'm one for two because I'm not sure Katrina feels the same way. Oh.
Ouch, And how many years have you been working there? Is that like twenty years now? Yeah?
Twenty six, twenty seven? Yeah? I know, right, Well, she definitely knows more particle physics than she did when she started, So I don't know if her enthusiasm has gone up, but her knowledge definitely has.
Well, how do you feel about poop in the fridge? I guess maybe your knowledge of what's happening with the poop in the fridge has gone up, but your excitement about it has probably not.
Yeah, so that's probably fair. Yeah, okay, comparison. All right, The day I'm excited to see poop in the fridge is the day I expect Katrina to be excited to talk.
About particles, all right, urriage in a nutshell.
Everybody likes to talk about the interdisciplinary work, but we're living it, baby.
That's right, that's right, it's messy.
All right, let's get back on track and talk about our emotion through the universe, not bowel motions that ended up in Daniel's freezer.
Amazing, all right, all right, so all motion, because I've been listening, motion has to be defined relative to something else. Yeah, And is there like an obvious thing to compare your motion too for this question, or do you can just pick anything?
No, the obvious thing to compare your emotion to is space. And people like to think about our motion through space. They like to think about the question how fast are we moving? How fast is the Earth moving? And they imagine that we are moving through some medium. But space is really weird. Space is not something that has a frame of reference. You cannot measure your motion relative to space.
You can only measure your motion relative to other stuff in space, which you know already opens the door to deep philosophical questions like well then what is space anyway? And I think that it's really hard to hold in your mind an idea of what space is because you know, on one hand, we talk about it is like having things in it like fields and matter, which is excitation of those fields, which are a property of space. Right, So it feels like there's stuff in space. I'm talking
about these fields, and they're oscillating, they're doing things. But on the other hand, I'm also telling you you can't measure your velocity relative to those fields. They somehow exist, they're out there, they're part of this medium. It's a kind of an ether theory because it's not an ether that provides a frame of reference. That's really the crucial thing. And so velocity is measured between two objects, and space
is not an object. It's kind of a thing, but it doesn't have this property that you can measure your velocity relative to that thing.
And if you really have smoked a lot of banana pills, you should listen to one of our early episodes What is Space, where you dig into that for a whole hour, and that is some trippy stuff.
And it's amazing that we don't really have a solid answer to this question. Like we have these theories and they work, and we can pull apart the philosophical implications of them. What does it mean that you can't measure your velocity relative to the thing in which light travels, for example, But we don't still really understand what it is and how it all works, and we can't pull it all together. And like quantum mechanics, view of space and general relative to view of space are very different.
And so somebody please give us a theory of quantum gravity which answers all of these questions. But you know, it means something to say that velocity is relative. It means that velocity is not a property of an object.
It's a property of a pair of objects. So any physics question you ask start out saying, say I'm in a ship and I'm going ninety percent of the speed of light, I'm going to ask you with respect to what it doesn't mean anything to say I'm going ninety percent of the speed of light, because you can be going ninety percent of the speed of light relative to one observer, and ten percent of the speed of light relative to another, and zero relative to another who's standing
next to you on the ship, right, Your velocity only means something if you say who's measuring it? Because again, there is no absolute frame, no preferred velocity in the universe.
And for those of us who have to admit that they had a little trouble remembering the difference between velocity and acceleration and motion and all of those terms when they started physics. Let's do a real quick recap motion, velocity, acceleration. What do these terms mean? Yeah, great for the purpose of this discussion.
Yeah, so let's start with location, right, because that's the basic thing. You know. Location also is relative, Like I can say, like if I live in a one dimensional universe, I can say, over here location x equal zero. Then I can measure how far I am from that location from zero. So I'm at x equals five. You're at x equal zero. I can say we're five units apart. Right, But somebody else could have put x equal zero somewhere else, So my location would be different if they're measuring it
or if I'm measuring it. So even location itself is relative because there's no like glowing tick marks in space. There's no like origin where the universe says this is zero or that is zero. It's just relative. So that's the basic measurement that's your location. Velocity is how your location changes with time. I was at x equals five one second later, I'm at xequal six. Then I'm at
xequal seven. That's my velocity. But that's why velocity is relative, because it's a measurement of how location is changing, and location is just relative.
Got it all?
That's great? And what that means, for example, is that any experiment you can do can't measure your velocity relative to space. So if I build some contraption and it does something the wiz bang experiment, and I run my experiment and does a wiz in a bang, cool, and then I put it in a box. Then I speed it up and I get it going really fast relative to Earth, and I do the same experiment, it should still whiz in bang in exactly the same way. The fact that it has some velocity now relative to Earth
doesn't change the physics inside the box. And it can't because if it did, then somehow I'd be measuring the velocity like within the box without measuring my distance relative to Earth. You can only measure your velocity relative to Earth by measuring your distance relative to Earth. So if the experiment just does the wiz bang experiment, if it does not like a ruler to measure the distance to Earth, then it should get the same result regardless of its
location or its velocity relative to Earth. And in fact, you can promote this to a general principle. You can say you can't measure your location or your velocity. If you're like trapped in inside a box with no access to the outside universe, there's nothing you can do to measure your velocity relative to stuff in the outside universe because your velocity only has meaning relative to that stuff, and if you don't have access to that stuff, you
can't measure your velocity. Does that all make sense?
So you could still measure your velocity with respect to stuff in the box, right, but not? Okay, got it.
But the amazing thing is that that's not true for acceleration. Right. So we talked about location, and we talked about how change in location is velocity. You can also talk about the change in velocity. So for those mathematically inclined, these are derivatives. Right. Velocity is the first derivative of location. Acceleration is the second derivative of location. It's how velocity is changing with time. Now, acceleration is something that's absolute.
If you are in a box, you can measure whether that box is accelerating or not. It's very easy. You just like draw a ball. If the box is not accelerating, the ball will float in front of you. If the box is accelerating, the ball will move to the back of the box. If the box has a negative acceleration, like somebody's putting on the brakes, the ball will move to the front of the box. It's just like having
a bowling ball in the back of a truck. You can use that bowling ball to tell if you're accelerating or decelerating. You can't use it to measure the velocity of the truck, but you can use it to measure the acceleration. So acceleration is absolute, but velocity is not.
Okay, I get that.
It still feels kind of counterintuitive because if it's the second derivative of location, it still feels like location should matter. But your box example, I understand how that works.
And you know, for those philosophically inclined, you might wonder, like, well, why is that Why it's taking one more derivative make it absolute instead of relative. And that's a whole digression. But very briefly, acceleration is philosophically very similar to curvature. Right. The effect acceleration is almost exactly the same as the effect of space time curvature. In fact, acceleration can create
event horizons. There are scenarios for example, where if you are constantly accelerating, you can outrun photons, effectively making like an event horizon relative to photons. That's a whole other episode we should dig into.
Wait wait, wait, whoa, whoa. Did you say something can go faster than light?
No, you can never go faster than light, but you can outrun a photon. So, for example, if you take off in your spaceship and you're moving slowly but you're accelerating constantly, if I then try to shoot a laser beam to catch you, it will never catch you. If you're accelerating constantly, you can outrun that photon. So it's not like if we have a race Kelly versus the photon,
that you'll go faster than the photon. You'll never go faster than light, but I can't catch you with a photon if you have left earlier and are constantly exciting.
Okay, all right, Yeah.
So acceleration is weird. It's kind of like curvature. It's a whole thing in general relativity. But the point is that velocity is relative, and this leads to some sort of weird things like, for example, what if it's just you in the universe and nothing else, imagine an empty universe with just you. What's your velocity? There is no velocity, not that your velocity is zero. Velocity has no meaning because it's a property of pairs of objects. In an empty universe, there are no pairs of objects.
It sounds very lonely.
It's like being married to yourself.
Oh, I can't imagine to be around for very long either, in the vastness of space. If it was just you, it would be a short, lonely existence.
Always thinking about the practical aspects of weird philosophical hypotheticals, love it.
Always thinking of death is pretty much where it goes to.
And conversely, you can never imagine a scenario where you are at rest relative to a photon because photons always have velocity of the speed of light relative to everything. So you can never pull up alongside of photon and say, oh, look, this is what a photon looks like when it's at rest, because the photon is pure motion.
If you pulled up alongside a photon, what kind of music do you think it would be?
Listening to?
Classic rocks?
Ah?
Nice, Yeah, all right, let's go with that.
Yeah, or quantum punk. I'm not sure actually, so What this means is, you know, space has no texture, there's no reference frame. It feels like space is a thing, but it's not, and it's the same thing everywhere. We mentioned earlier the idea of space having a boundary and its connection to conservation and momentum, and this tells you that the idea that space has no reference frame has
really important consequences. Like Nother's theorem is what tells us that space being the same everywhere leads to conservation of momentum, which is why space have an edge to it would lead to a violation of the conservation of momentum. So the fact that we never see violations of the conservation of momentum tells us space really is the same everywhere, and we've never noticed that. Like if you do an experiment here and you do an experiment there, you get
different answers. Your whiz bang experiment always whizzes and bangs the same way no matter where you are. So velocity and location are purely relative, right, And this means that, for example, it doesn't mean anything to ask are you motionless? Can we be motionless with respect to space? Right? Because you can't measure your velocity relative to space.
At all, because there's no space, man.
Yeah, exactly. No space is a thing. It just doesn't have a velocity. So in one sense, it's like trivial to be motionless. You just say, well, I'm gonna choose my frame of reference to be me, and I'm gonna measure my velocity relative to myself. Okay, look, I'm going zero.
Yay.
It's also kind of trivial to have a velocity near the speed of light. Choose any of the zillions of cosmic rays that are approaching the Earth at nearly the speed of light and say that's my reference frame.
I do go fast.
Yes, And from the point of view of those cosmic rays, Kelly, you are moving towards them at nearly the speed of light, So buckle.
Up, way to go me. I was also fast that time I jumped out of a plane that.
Was fast too ooh fast, and then slow I hope.
Yeah, yeah, yeah yeah yeah, and then not split not very slow, not fast and slow than very slow.
But anyway, all right.
So we have learned that space is confusing and you need to be careful what you're talking about your speed relative to And when we get back from the break, we're going to talk about our speed relative to lots of stuff in the universe.
All right, we're back.
Let's start by talking about our speed relative to let's say, my favorite planet in the Solar System.
Earth.
Yeah. Earth is already kind of complicated. You might think, well, I'm standing on Earth. My velocity is zero, right, but the Earth itself is spinning, right, So which part of the Earth are we talking about? Yes, if you say the part of the Earth that's under your feet, your velocity is zero, that's kind of boring. But if you say the center of the Earth, then at the surface
of the Earth you're already moving quite fast. Now, at the north pole you're not moving at all due to the spin, But at the equator, the Earth is spinning at sixteen hundred kilometers per hour. Woa, Like, that's not a small amount of motion.
No, you know, I realize that we don't feel that, but it is kind of amazing that we don't feel that.
That is pretty quick.
Yeah, And it's a little counterintuitive because this is acceleration. Right. To move in a circle at constant velocity requires acceleration because velocity is a vector, and if it's changing direction, even if its magnitude is constant. Then moving in a circle, your velocity doesn't change, but the direction of your velocity changes,
and that requires acceleration. Like if you have a rock moving through space at ten meters per second and you want it to be going a different direction at ten meters per second, you've got to give it a push. That's a force, that's acceleration. You might think I should be able to measure that, and you can. Actually you can measure the rotation of the Earth because it affects how much you weigh, Like at the equator, you weigh a little bit less than you do at the north
pole because you're being flung out a little bit. Like if you are on a merry go round and somebody spinning it faster and faster and faster, there's this apparent force that's pushing you away from the center. Right, that's due to the acceleration, which, as we were saying earlier, is something that you can measure. And so you can actually measure this acceleration.
How long did it take for us to like figure out and accept that, because it's like that's all pretty counterintuitive.
Yeah, it is counterintuitive, and it took a long time before people accepted this. When it was first proposed, people were like, what, that's crazy, And it took like having fairly precise experiments in order to be able to measure it. There are a few different effects from this kind of rotation. There is the fact that you weigh less at the equator, and there's also this Coriolis force, which is effectively like
a sideways force. If, for example, you drop something from like one hundred and fifty meters, then it doesn't go straight down. It departs from straight down by like a millimeter or so, so it takes kind of a precise measurement. Actually, that was a question I was asked during my oral exam in grad school to like calculate that on the spot on a chalkboard in front of the professor's I remember that. Oh my god, that was terrible, But I think I got it right.
They let you through one way or another.
They did. Yes. In eighteen fifty one, the foll called pendulum, which is basically like a lead filled brass sphere suspended on a really long string, in this case sixty seven meters. You can see it rotate at a rate depending on its latitude, so it's like a pendulum that swings back
and forth, but it also processes. If you ever see these pendulums in like a science museum, like a really long string, and it doesn't just go back and forth like it goes back and forth, but also the back and forth itself sort of rotates around and like leaves marks in the sand. That's because of the rotation of
the Earth, and that depends on the latitude. So if you do that experiment at the equator or you do that at the North Pole, you get a different answer because of the difference in tripetal acceleration.
This kind of stuff is amazing and like, so I have you know, I got to be honest, like I wouldn't want to spend my life creating precision weights or precision.
Instruments and stuff. But like, so you know when you've said.
If you took your weight at the equator and at the North Pole, they'd be different. But of course you don't actually mean like you weight, because the journey from the North Pole to the equator, you know, you'd probably like change your weight. You know, you'd eat a big meal in between balla.
So so you're like, I'm sure what you mean.
You're like you'd have like a you know, twenty kilogram weight or that you'd you know, compare at both locations and like.
And you wouldn't feed any snacks to your twenty kilogram weight along the way.
Yeah, exactly right.
No Cheetos, No, none of those stroop waffles that they're giving on the flights now, which I am so excited that that's a new thing you can get on flights.
Are you saying I couldn't go from the Equator to the North Pole without resisting a stroop waffle?
I could not, But but what I'm saying is.
I couldn't either.
Yeah, So eighteen fifty one is one of the experiments you were talking about, And so back then it wouldn't just be.
A couple hours to get from the equator to the North Pole. It would be you know, like half a year or more or something. You make that journey, so you change your weight during that time, but your twenty kilogram mass might not. But like, there's so many things we learned about the world by getting really good at making precision measurements, and like, yeah, we're such a boring and amazing species, Like, way to go us for managing to do that kind of stuff, and anyway, props to props to humans.
Yeah, props to nerds, you know, people who are like, I'm super interested in this. I'm going to get really good at that, and I'm going to somehow manage to take my twenty kilogram weight from the equator to the north pole without getting caramel smeared on it or something. Yeah, right, right, thank you nerds.
That's right.
Yeah. And it's because of the diversity of people's interest in weird stuff that we know all these amazing things about the universe. So yay. Yeah, So that's how we know that the Earth is spinning. Of course, we can also see it from space, et cetera. So the Earth is spinning at sixteen hundred kilometers per hour at the equator.
The Earth itself, of course, is moving relative to the Sun. That's thirty kilometers per second, right, So any fast again relative to the Sun. And this again is motion in a circle, and motion in a circle requires constant acceleration to maintain the same magnitude of velocity. So this is also something you could measure, like even if you were in a box, if you were orbiting a star, you could tell you were orbiting a star because that is an acceleration.
Well, now I want to know when and how we figured that out, but it's not on your outline, so I'm putting you on the spot.
Well, this all goes back to geocentrism and heliocentrism, right. We had these two theories of the organization of the solar system, one in which the Earth was at the center and everything moved around it, another one where the Sun was at the center and everything moved around it. And I think it's fascinating that the Greeks actually had the idea for heliocentrism. People often say, oh, the Greeks just assumed that the Earth is at the center. They
considered heliocentrism. They considered the idea that the Sun was at the center, and they even had an idea for how to check. They thought that they could look at the stars and if the Earth was moving around the Sun, they would see the stars wiggle in the sky. And they were right, they should be seeing that. That's called parallax. But they were wrong about the distance to the stars. They thought the stars were pretty close so that they
should be able to see the parallax. And when they didn't see the parallax, they concluded incorrectly that the Earth wasn't moving. If they had known the stars were so far away, they would have realized that you can't use parallax to discover the motion of the Earth unless you have really fine telescopes. And we weren't able to do that until like the eighteen hundreds.
Oh man, how frustrating. They were so close.
I know, they really on the edge of this. They
made the wrong assumption, led them down the wrong path. Anyway, a couple thousand years later people figured out that the Earth is moving around the Sun, but not by measuring the acceleration the local acceleration of the Earth, but by seeing the phases of Venus and then also getting more precise measurements of the motion of the planets, so we could see the geocentrism didn't really work, though it worked surprisingly well even with cycles and epicycles.
But then how did we figure out the thirty kilometers per second figure?
Oh? There, you could just use Kepler's laws, like if you know the period of the planet and the distance to the Sun, then you can figure out our local velocity. It's just basic kinematics. So we've known that for hundreds of years.
Folks, you should all know that Daniel didn't write that down in his notes. He just has the soul in his head. He's a smart guy, all right, keep going.
This is basic stuff, Kelly. You didn't have to be a jerk face.
I was just something nice.
I'm just trying to deflect your compliment. Thank you very much, all right.
Yeah, that's what you're supposed to say, Daniel.
And so there are these videos out there that try to break people out of the mental image of the Earth moving around the Sun and the Sun being stationary. And I think that's cool, because it's true that the Sun is not stationary with respect to the galaxy. But it doesn't really mean anything to say the Sun is moving through space, right, It's always relative to something. So even if you have like a stationary image of the Solar System, if you're approaching the Solar System with the velocity,
then the Sun is in motion relative to you. Like if you are on three I at lists the Interstellar Visitor, then the Sun is in motion. Or if you put yourself at the center of the galaxy, then the Sun is in motion. So it's true that you can move to a frame where the Sun is in motion instead of a frame where the Sun is at rest. But it's a little misleading to suggest that, like, oh, really,
the Sun is moving through space. It's moving relative to something, and the most interesting thing it's moving with respect to is the center of the galaxy.
And how fast is it moving with respect to the center of the galaxy.
It's moving eight hundred thousand kilometers per hour around the center of the galaxy, which is like a big number, but you know, it doesn't really affect your life because
think about it the other way. It means that the center of the galaxy is moving eight hundred thousand kilometers per hour relative to us, but it's really far away, so like, who cares how fast it's moving relative does doesn't mean anything for us here on Earth, though it is fun to think about, like how long it takes the Sun to go around the center of the galaxy. It takes like a couple hundred million years for the
Sun to do one orbit. So if you think about that as like a galaxy year, the way that like the Earth takes one Earth year to go around the Sun, the Sun takes one like galaxy year to go around the center of the galaxy, then our solar system is about twenty galaxy years old.
She's almost old enough to drink. I wonder what her preferred drink's going to be. It might be tea, maybe she's not into alcohol.
Cosmopolitan maybe, oh, cute love it, that's probably what it's going to be.
Yeah.
I think that's pretty cool because it means the galaxy has not had that many rotations. It's already formed all of the structure and the spiral arms and all that stuff without spinning more than fifty times. Right, that's kind of mind blowing.
That is kind of mind blowing. How many turns around the galaxy is it going to have? Do you know?
Well, that's a great question. Well, we're scheduled for a collision with Andromeda in a few billion years, and that's only another like ten ish or fifteen ish rotations.
Change the subject all messed up again? No, no, I don't want to hear this all right, moving on, And.
Of course you can ask, well is the galaxy in motion? And you have to ask is it motion relative to what? And so there's a bunch of different choices you can make here. You could choose the local galaxy cluster and say we're orbiting around the center of mass and the galaxy cluster. But I think here we should like skip forward to the biggest picture question and say, like, is there any kind of frame out there you could use
that's like the center of stuff? Because on one hand, there is no preferred frame in the universe for space. You can't say I'm moving through space with respect to anything, But there is a bunch of stuff in the universe, and you can ask like how fast am I moving relative to all the stuff in the universe. So this
feels like a weird kind of card trick. You have no velocity relative to space, but I can then fill space with a bunch of stuff like space was filled with the hot dense plasma a few billion years ago, and that stuff has no velocity relative to space either, right, Because you can't have velocity relative to space, but it doesn't mean that Now I can ask like, how fast
on going relative to that stuff? Right? So it's sort of like I put wallpaper on the wall and I said, you can't ask your velocity relative to the wall, but you can ask your velocity relative to the wall paper. Right.
Yeah, it feels like cheating.
It does feel like cheating, but in this case.
The wallpaper isn't there, right, or because the plasma's not there right, or it was a long time ago, but it's not there now.
Yeah, so the plasma's not there anymore. And really, you know, the wallpaper has no velocity relative to the wall. It's just like there's something else in the room with us. Now, so we can finally ask do we have a velocity relative to something? It's like, go back to that empty universe. You have no velocity. It means nothing to have a velocity. Then I fill that universe with stuff. Now you have
a velocity relative to all of that stuff. And that stuff has no velocity relative to anything else in the universe except for you or itself. It has no velocity relative to space. Right, But there is a preferred frame in the universe. That frame is the frame of all the stuff in the universe. And so you can't ask about your velocity relative to space, but you can ask about your velocity relative to the stuff in the universe.
And why is that the preferred Why is the frame that doesn't actually exist and is a mind trick the preferred frame.
Because it's the only one we can think of.
Okay, all right, there's no other option, right, digging the honesty, you can either.
Give up or you can choose this one, and neither a satisfactory.
And how fast are we going relative to the space wallpaper?
The answer is, we don't know. We actually have two different measurements, and they disagree by a lot.
What All right, let's take a break, and when we come back, boy, things even more confusing.
You thought this was going to be a simple episode, didn't you.
I hope so.
And we're back, and we should have saved this episode for Halloween because we're going to talk about this spooky radio dipole anomaly.
So we're interested in the question of how fast are we moving relative to all the stuff in the universe. Because you take an empty universe and you PLoP a bunch of stuff into it. Now there's a frame, right, the frame in which that stuff is at rest, and how do you measure that? Well, we're going to talk about two different ways of measuring it. One tries to measure our velocity relative to the stuff in the early universe.
So brief history of the universe, A bunch of stuff happened that we don't understand at all, but somehow led to a universe filled with a very hot dense plasma. That plasma is a and then it's cooling as the
universe expands. Suddenly it cools enough for the protons and electrons to get together and make neutral hydrogen, and the universe suddenly becomes transparent, which means all the light that previously was being absorbed just after it was emitted now flies free, free lights, and the universe has been transparent ever since then, which means we can still see that light. We can see evidence of that hot dense plasma, and
that's what's called the cosmic microwave background radiation. It's microwave because that's the frequency we see it at. It was a very very high frequency because very hot plasma's emit a very high frequency light, but it's been stretched out by the expansion of the universe to very long wavelengths and low frequency. So we can still see that light
and we can see Doppler shifts in that light. Okay, so we can still see that light, which means basically we can look around and see that plasma right And you know, it's a little bit confusing to think about what part of the plasma we're seeing. If you look in one direction from Earth, you're seeing light that's been traveling basically fourteen billion years since it was emitted and
just got to Earth. If you look in the other direction from Earth, you're seeing light that traveled to fourteen billion years since it was admitted and just got to Earth. So you're looking at bits of plasma that were like across the universe from each other. If you look all around the Earth, you're seeing light from a shell of plasma that emitted light in the direction where Earth was
going to be and just arrived here now. And as time goes on, we see light from a different shell of plasma, a larger and larger shell, further and further away from us. But we're always going to see CMB light because the universe was filled with this plasma. So it's a great way to measure our velocity relative to the stuff in the universe back when the universe was filled with this plasma that conveniently all gave off this light.
But so everything you just said made sense, but we're talking about in an anoma, So I feel like I wasn't supposed to understand all of it.
No, you're still supposed to understand it, And the way we measure our velocity relative to that light is pretty simple. We just look to see if the light is blue shifted or red shifted. Because if you're moving towards something, frequency goes up. If you're moving away from something, frequency
goes down. Wavelengths to get longer. And most of the time, when you look at a picture of the CMB, like on the internet, it looks like all these little red and blue dots, and what you're looking at there is not the raw CMB light. What you're looking at is that when they remove our velocity effect, if you look at the raw CMB, it's like blue on one side and red on the other. Because we are moving relative to the CMB, we're not at rest relative to it,
so they usually subtract this part out. Because we're moving at three hundred and seventy kilometers per second through the CMB, so pretty fast.
Yeah, look at us go.
So that's sort of the closest we can come to saying how fast are we moving through this stuff in the universe. That's a big number, and people would like to know is that really correct? Are we making a mistake? And you know, what we're measuring there is our velocity relative to the stuff that used to be in the
universe fourteen billion years ago. We can do a cross check by looking at the stuff in the universe now and asking, well, how fast are we moving relative to like distant galaxies for example, and all that stuff, And
so we can make the same measurement. We can look for galaxies and we can ask, like, is there like blue shifted or red shifted and that should make a map across the sky of like blue shifted versus red shifted galaxies and tell us which direction are we going and how fast relative to all the galaxies out there in the universe. Not relative to space again, but relative
to the stuff in the universe. And we should get the same answer because the galaxies out there in the universe, they came from this CMB stuff, right, that's stuff clumped together and made structure and eventually formed galaxies which spun for a few times before they collided with other galaxies, et cetera. So we should get the same answer, right.
Well, so then why did we need to do all this CMB stuff if we could have just done it with the galaxies or it's just nice to double check.
Replication is good.
It's nice to double check, and you think you should get the same answer in two different ways, so you always got to do it both ways, right, got it? That's the worrier in you, like, h what have we got it wrong? Let's double check?
Yeah, anxiety.
And so they recently made this measurement. It's tough to do. You got to look at galaxies in the radio you have to subtract all sorts of other effects. Galaxies are not as spread out evenly as the CMB is, and so fascinatingly what they find is they measure the same direction of motion. So they agree. The CMB measurement and this radio galaxy measurement agree in the direction. But the radio galaxy suggests that we're moving two to five times faster than the CMB measurement.
Says, oh, that's the anomaly.
That's the anomally. It's called the radio dipole anomaly. A pretty recent measurement, and it's not understood like either. It means there's something very wrong with our understanding of the universe and how it's evolved over time. But that would be pretty surprising, because you know, we really think the CMB was there and it filled the universe and it was mostly smooth, and the universe has expanded isotropically, so
it'd be pretty hard to explain how this happened. Most likely this is due to something boring like these radio galaxies have to be seen through a telescope. They're all in different locations, and the telescope isn't as good at spotting them in different locations, or the measurements of their velocity are not quite as accurate, or the calibration changes over the sky and maybe drifts as you move the telescope.
So people are hunting for an explanation, and you know, usually what you do is like look for a boring explanation, like oh, every time somebody made ramen in the microwave in the breakroom, or data shifted by two points, right, that was an actual explanation for an astrophysical anology in Australia. Yeah,
it was burritos in the microwave. So you always look for those kind of things before you conclude everything we know about the universe is, which is how the clickbait of many of these articles started out.
Yeah.
Well, also, you know, I think what we should conclude is that we need more money for better telescope. So that we can make all of these measurements more accurately.
Exactly, And you should look at not just radio galaxies, but other kinds of galaxies and other stuff in the universe, and we should measure this kind of stuff in lots of different ways, because this is how you make discoveries. Folks. You think you understand something, you double check it, you get an answer that's not what you expected, and that
forces you to reconsider your ideas about the universe. And you sometimes hear out there on the internet that, like, scientists will protect the narrative because they don't want to upend the apple cart in order to get grants, and like, no, absolutely not. Scientists love to upend the narrative. I know. That's why you see these crazy clickbait articles. It's a scientist trying to upend the narrative with their latest study which blows up our understanding of the universe.
That's how you get the awards.
Exactly. Scientists are not working together to protect some narrative. They're all working against each other, desperately using every trick they can to prove their friends and rivals wrong.
Yeah, they're varying degrees of cutthroat.
I don't want to make everybody sound like a jerk, but yes, I agree. If you can prove the dominant narrative wrong, that's when you get all the awards. All right, Well, Daniel, I am exhausted from all of this moving. Can we talk about Are there ways to not move? Ways to stay motionless in space?
Yeah? So it doesn't mean anything to be motionless with respect to space, right, And people sometimes write to me about this question when they read time travel fiction and they're like, if I went back in time to fourteen ninety two, wouldn't the Earth be in a different place? And how come this science fiction novel I read didn't account for that? And you're giving me a funny look.
This is a very common question. People think that they gotcha, and like, well, number one, time travel breaks a lot of physics anyway, and so maybe you should give the author a little bit of leeway on this detail. Also, it's question doesn't have any meaning, Like the Earth's location over time only means something relative to some axis, to some frame. So if you choose the frame of the Earth, then like you go back in time and you're still
on the Earth. So yes, if you choose the frame of the Sun, the Earth would be in a different location in fourteen ninety two than it is now. But the question doesn't really have meaning.
Oh, I see what they were getting at. Now, you would your time machine would dump you in the vastness of space instead of on Earth, then that.
Would be bad.
Okay, okay, all right, okay, But we've decided let it go. Enjoy your fiction.
Number one, let it go, which is my approach when I read time travel fiction, as I rarely do for that reason. And number two, the question assumes some absolute reference frame, right, which just doesn't exist. And you might think, well, what is the right reference frame? And there is none, right, And the problem here is that time travel doesn't make any sense anyway, and so there's no like hard physics
way to ask that question. But you can be motionless with respect to other stuff, which can be cool, like, for example, you can be motionless with respect to a point on the Earth's surface, right, like.
A point on the Earth's surface.
If, for example, you want to build a space elevator, you want that space elevator to be connected to the Earth by a cable, and so you'd like it to be effectively in geosynchronous orbit above someplace, hopefully on the equator that you built this thing, and so that's geosynchronous orbit. We have no motion relative to the Earth's surface. You're still in orbit, and it's still acceleration your velocity relative
to the center of the Earth. But you can have no motion relative to the Earth's surface.
Can I tell you a fun story about Yes, you can space elevators.
I'll try I'll keep it. I'll try to keep it short. No do so.
The bit the hard thing about making a space elevator is trying to come up with a material for the cable that is strong enough to hold the elevator, but also isn't so heavy that you're just like lifting the cable a whole time, right, And so carbon nanotubes might one day maybe work. What we have it made a continuous one that's long enough. But even if you did, what you'd really have to worry about is if it ever got struck by lightning, that could, like you the
game over, Like it could destroy the cable. And so I was talking to somebody about what to do about that, and their answer was, well, you know, there's this part of the ocean that's never recorded.
A lightning strike got to be kidding me.
You know that meme where they have the airplane and like airplanes that return only have hits in these locations, like the survivor effect meme.
Yeah, it's basically that one.
Wow.
I think that wasn't just a meme. I think that was actually like a study during World War one or two. Yeah, yeah, yeah, yeah, okay, anyway, yeah, yes.
Yes, that cracked me up.
I'm like, all right, fingers crossed, there's never any lightning here again. And then I think you also wanted the ability to be able to move it a little bit in case a storm came through.
Well, I think the terrifying thing is you guys pointed out in your book soon is is what happens when the cable snaps, because basically, now you have this like kill wire moving at high velocity like somehow or us the Earth's surface, and like yikes.
Yeah right, or if somebody decides to snip your wire, forget if there's lightning, just like a terrorist cutting your wire. Okay, anyway, no more doomsday scenarios relative to space elevators. Let's talk about lagrange points because I always found these slightly confusing, but you can clear it up for me.
Well, what I'm confused because I call them lagrange points.
Look at you, nobody. Nobody expects me to say it right. They expect you to say it right. That's your responsibility, not mine.
Well, it depends. Are they named after, you know, Jean Louis Lagrange or are they named after the city in Texas or the Zzy Top song or you know, I.
Think the zz Top song probably.
Nice.
Now that we've both done that together, I think we've made this joke before and we'll probably make it.
Again, probably because that's how our memories work.
I look for.
Yeah, there are these fun points where you can be at rest relative to the Earth Sun system. Like, for example, if you are the James Webb Space Telescope and you don't want to be in orbit or on the Earth because you need to be super duper cold, you can choose to be at lagrange point two, which is on the other side of the Earth from the Sun, where all the gravitational effects of the Earth and the Sun cancel out, and so you could just sort of like hang out there.
Yeah, so I guess so when I look, all right, so I'm looking at the diagram that you provided, and so you've got this spot that's on the far side of the Earth and on the far side of the Sun.
And I guess I always kind of feel like, why doesn't.
It just drift away? But I guess you still have the gravity pulling it in. I'm not sure I have a really great question to ask here, just kind.
Of Well, so imagine that you wanted to be in the Earth's orbit. Right, So you have Earth and it's orbiting the Sun, and you're like, hey, I want to join the Earth's orbit. Where could you be in Earth's orbit? Well, if you're right next to the Earth, then you and the Earth would pull on each other and you would smash into the Earth. So try to get a little bit further from the Earth. Well, how far away from the Earth you have to be? Well, one solution is to be on the other side of the Sun from
the Earth. So that's the lagrange point three. If you and the Earth are on opposite sides of the Sun, then you won't pull on each other and you can share an orbit with the Earth, right, Okay, So you can both hang out in the same orbit without crashing into each other. So that's what you want. It turns out there are two more points there's a leagarnge point four and five, which are like thirty degrees ahead or
behind the Earth. It can also be in those points where everything balances out and you can both be in orbit together. And this is why Jupiter, for example, has a big cluster of asteroids ahead of it and behind it in its orbit. Right. I think they're called the Trojans and the Greeks.
Cool love it. I hope they get along.
Ironic foreshadowy. And another one is to be between the Earth and the Sun, so closer to the Earth because the Earth has less gravity, but between the Earth and the Sun, that's the lagrange point one. Lagarage point two is on the outside past the Earth's orbit, and it's basically in a line from the Sun to the Earth and then past it. And so you can be out there where your orbit is not disturbed by the Earth. You could also just be in lagrange point two in
orbit around the Sun and be fine. But then if you're not synced up with the Earth, the Earth is going to disturb your orbit every time it passes you.
But if you're synced up with the Earth, so you're always keeping the Earth, Sun and your telescope in a line, then everything is stable, and that's where the James web Space Telescope hangs out because the Earth provides a shade from the Sun and the telescope needs to be super duper cold so that it doesn't emit radiation in the same wavelengths that it's observing.
Okay, so it's constantly in motion relative to the Sun, but not the Earth. But not the Earth, and have humans put things in all of these lagrange points.
So we've used lagrange points one and two. One is the one that's closer to the Sun, and two is the one that's further from the Sun. And two, for example, we have the James Webs Space Telescope, and it's not actually at too, it's orbiting too, so you can put other stuff there. And there are things that one which is pretty clear to Earth. For example, there's like the Solar and Heliospheric Observatory is there, and other stuff likes to be there, and we haven't put anything at four
and five. They're kind of a little bit far away. That James Wes Space Telescope is already like kind of far away for an observatory, and four and five are even.
Further and three is super far, so we have we put anything at three.
Yet No.
Three is super duper far away. It's on the other side of the Earth. And that wouldn't be terribly convenient because we couldn't communicate with it very easily because the Sun would be in the way. Yeah, and so that's basically all you can do to be like motionless in the universe. You could try to be like motionless with respect to the CMB frame if you'd like that wouldn't
like feel very special. You could try to be motionless with respect to the Earth and hang out in space at one of the lagrange points, and that's kind of special and you can see things about the universe. That's cool. But in general, motion is something kind of slippery because it requires you to measure your velocity with respect to something else in the universe. A lonely concepts requires you to be paired up with something out there in the universe.
So there is no solo motion or solo location in the universe. If you want to be in motion, you gotta find a friend.
Ah.
That's a nice note to end on, all right, extraordinaries, thank you for moving through the universe with us.
We appreciate you. Spending time with us.
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