Gravitational waves explained a little deeper


This is an experimental video.
I’m joined today by Lianne. It’s an experiment, because it’s different than normal sorts of videos I would make. This is gonna be a discussion between two of us. We have discussions all the time. They’re very interesting. And often I turn those interesting discussions into YouTube videos that I make on my own, which is a sad and lonely thing to do. So we can actually just film the discussion this time. As a caveat, when I turn our discussions into YouTube videos, I do lots of research and
make sure my facts are straight. So this is gonna be a bit looser. Facts might not be quite right, but the general gist of it… anyway… Also, please do correct me in the comments but I will try and figure out after we filmed if anything is terribly wrong. I’ll fix it with subtitles or something like that. – Cool. So anyway we’re gonna talk about…
– Gravitational waves. …gravitational waves.
– Because I want to know about them. Yes, it’s in the news at the moment. And I was thinking like that there are a few explanations. There’s this stock explanation that scientists will give on the news. Which is really quick, gets it out there, but it’s maybe unsatisfying for people who have got a bit more curiosity about it. – That’s what I found. I’ve been reading articles and it’s, if I come… – I’m kind of getting it but not quite. There’s not enough. By the way part of the reason we have interesting conversations is that Lianne has a low threshold for curiosity. And as a science vision, that’s brilliant. I’ll be like: Did you know that water usually boils slightly higher than boiling point. And she’d be like: – WHAT? – I might start by saying what I think they are.
Yeah, great. – I realise this is a very… – Okay, let’s start with the space-time continuum?
Okay. – Right, we can imagine it as a tarpaulin
Okay, yeah. – Right, I’m gonna put a big bowling ball in that tarpaulin. – That bowling ball is for example Earth. And gravity is — crudely — the effect that that bowling ball has on the tarpaulin. So things that approach the kind of… Let’s just describe what happens when the ball goes on the tarpaulin. Let’s call it a rubber sheet instead of a tarpaulin.
– Yeah, that’s fine. Rubber is better. Okay, great. So what happens?
– You get a rubber sheet, you put the bowling ball on and it kind of sinks in and creates a dip in the sheet around the bowling ball. Link here to a video of that.
– And, if you throw some… I don’t know, – If you drop a marble onto the rubber sheet then it will fall towards the bowling ball. – If you actually kind of throw a model on at the right angle it will kind of spin around. Yeah, because you create this sort of funnel shape like that and it’s gonna spin around. – And that is a crude way of explaining what gravity actually is. It’s the warping of space time.
– The warping of space time, brilliant. By the way, I really like that as a visual aid.
It’s very simple. What I don’t like about it is:
It’s an explanation of gravity that requires gravity. Because, you’ve got this rubber sheet
and you put a ball on it The ball is pulled down by gravity and the balls that you then put on it, are pulled down by gravity. You see what I mean? – It’s quite difficult to do an experiment to demonstrate gravity in a place with no gravity. It is. It is. – How do you want this experiment to be done?
You wouldn’t. But yes, well as the warping of space-time is more like a straight line now becomes a curved line. So an object that curves around the
Earth, in some senses isn’t accelerating even though it’s changing direction Anyway, it’s a nice visualisation, but I think it skirts over what’s really going a little bit. O, I did find a really good video of someone demonstrating how curved space leads to gravity. He built this amazing thing. I’ll see if I can find a link to that. So, that’s on a gravity waves. – Okay, so this is just… I don’t know if I’m right here. So you can correct me if I’m wrong. – But my thinking is: a gravitational wave… So I gather it’s when there is a big collision of two colossal objects – I am going to say object.
Yeah, it’s all right. – What I am thinking of here is black holes. Two black holes collide. A black hole is an object, okay.
– Okay, great, a black hole is an object. – An object. The object. – Two of these objects collide. And the force of the collision sends ripples out in the fabric of space-time. And those ripples are gravitationally waves? It’s not bad. I mean, yes, except that, if we were to walk around each other we would create gravitational waves. So it’s just that they’re so tiny.
– Colossal, yeah, the ones he ones that are created by the collision of the black holes are so big that you can actually detect them. But the one we create are tiny. So, the way to think about gravitational waves..
There’s a couple of nice ways. One way is to think about it in terms of electricity and magnetism and using that as an analogy. – Okay. Okay, so… an electron has an electric field. Is that an idea you’re familiar with? – Yeah. All that means that it has an area of influence around it. So it influences charged particles that are near it. – Yeah. So we call that it’s electric field. That’s the way of thinking about that. Instead of saying an electron has an electric field you say there is an electric field. There is one electric field that permeates the universe and electrons cause a disturbance in the field. – Ok, I like that.
So an electron is sitting in this field and it causes a, like a lump, if you like.
– Okay. – That’s nice. It’s really nice. So what happens when you move
the electron? The field moves as well. – Yeah. The field doesn’t move instantaneously.
It moves at the speed of light. – What? Ooooh. Ok, so imagine I’ve got an electron here and you’re really far away but you can feel… You can feel the force of the electron, right, and I move the electron over here. – Yeah. And you can detect that the election has moved. Suppose the electron has moved away from you. – Okay. You can detect that it’s moved away from you because the force got weaker. But you don’t detect it at the moment the electron moves, you detect it… – …at the speed of light.
… at the speed of light. So the time it takes for the change in the disturbance in the field to get to you.
– Yeah, okay. So what happens if I take that electron now and I move it back and forward like that, so that change in the field, that disturbance in the field is kind of… At this point it’s getting bigger and smaller, bigger and smaller, and that disturbance then travels through spaces at the speed of light.
But here is a cool thing. When you disturb an electron like that, it doesn’t just create a disturbance in the electric field that permeates the universe, it also causes a disturbance in the
magnetic field that permeates the universe. And that disturbance, those waves that travel away from it. They travel in a packet of waves. And that packet is a photon. A photon is a disturbance in the electric field magnetic field. – …at the same time.
They travel away from electrons when you jiggle them. – I mean it’s time to talk about how this relates to gravitational waves. So let’s think about… We could talk about Einstein’s general theory of relativity, and theories in general, because… So Einstein came up with this theory of general relativity, which describes gravity. But we already had a theory of gravity. Which is Newton’s universal law of gravity.
-Right. Newton’s universal law of gravity describes gravity really well. The reason it’s called the universal law of gravity is because… And this was his genius. He realised that an apple falls to the ground for the same reason that a planet orbits the sun. – Right. You call it universal because it’s everywhere and that’s great. So you’ve got a theory that describes everything. But when you have a scientific theory and it works. It describes everything, you don’t just stop at that point.
-Yeah. What you do is — and this is in an ideal world — this is like an idealised scientist. This is what a perfect scientist will do.
They would then say: Okay, I’m now going to try and disprove my theory.
– Right. So a good theory is one that is disprovable.
What we say is: a good theory in physics is falsifiable. What that means is the theory makes predictions, that we can test and hopefully prove to be wrong. If we find out that we can’t disprove a theory then that theory is now even stronger than it was before. – Okay. So, Newton’s theory of gravity explains everything you see. But can we use the mathematics to come up with a description that we haven’t seen yet. And then try and look for that thing. According to Newton’s theory of gravity, if you have a planet which is in an elliptical orbit… So imagine you’ve got the sun here and you’ve got a planet which is going like that. An elliptical orbit like that. That ellips won’t be stationary, but it will come slowly move around. – – According to…?
Newton. – Right. At a certain speed… It’s called the procession. So Mercury has an elliptical orbit like this. The ellipse processes around. – Right.
But it doesn’t process at the right speed. By testing Newton’s theory of gravity we find out that it doesn’t describe something. – Right. So, maybe there is a problem.
It’s called the anomalous procession of Mercury. Einstein came up with the general relativity theory, which describes gravity and it correctly describes the speed of Mercury’s procession. – Nice work, Einstein.
Great, that’s really good. But there is something else in the theory that we can use to now continue to test it, to try and disprove the theory of general relativity and one of the predictions of Einstein’s theory is gravitational waves. So looking for gravitational waves is a really important thing to do in science, because you’re probing.
You’re really thinking: Is this theory definitely true? And the answer is never “yes”, but..
– We proven one more part of the theory. – It’s even more likely to be true. Exactly. So that’s why we’ve been looking for gravitational waves. That’s why it’s so important we found it.
– We still haven’t got to what they are. No, we haven’t.
So in Newton’s theory of gravity… you feel the effects of gravity instantaneously.
So if the Sun were to suddenly appear, you would instantly be attracted to it,
no matter how far away you are. – And yes I know that we now know that gravity travels at the speed of light. – Or the effects of gravity are felt at the speed of light. Well this is one of the predictions of the general relativity. So in Einstein’s theory of relativity… gravity isn’t instantaneous. It travels at the speed of light. – Yes. So in the same way as an electron is a disturbance in the electric field that permeates the universe a massive object creates a disturbance in the gravitational field that permeates the universe. Einstein was a G! Sorry, it’s just hitting me again, but go on. Yeah, yeah (uppercase G not lowercase G).
So for example, do you know why he got his Nobel prize? What field was it in?
– I know that is not what I would expect. What was it? He got it for quantum mechanics. The two biggest theories in modern physics: relativity and quantum mechanics. He was doing both, so… What were we talking about? Gravitational waves. You can either think of the mass object as having a gravitational field or you can think of it as a disturbance in the gravitational field that permeates the universe. -Yes. Except that, instead of it being the gravitational field that permeates the universe, it’s the fabric of spacetime itself. So in the same way that when you wiggle an electron, it produces disturbances in the electric field that permeates through space at the speed of light, when you wiggle a massive object it produces permutations in space-time that travel at the speed of light. – In the same way, right. So find a massive object and just move it backwards and forwards. That will produce gravitational waves in the same way that you would get electric waves when you would wiggle an electron. Does that make sense? – Yeah.
So there you go, gravitational waves. Is that satisfying, or…? – It’s satisfying.
There is still some stuff that I don’t understand. – So, one of the articles I was reading was talking about the scientists who are looking for the gravitational waves listening to two black hole circling each
other for twenty millions of a second or something. Yeah. – How are they detecting it? – What are they listening to? What does this mean? So imagine you wanted to detect gravitational waves. And remember a gravitational wave is a disturbance in space-time itself. – Yeah. So, you know, it might be a lengthening and contracting, lengthening and contracting… – Of space-time? Yeah. So what you do is you get like a rod of metal and hopefully there’s waves hitting it from this is binary black hole that’s spinning around itself. And then you measure it. And is it getting bigger and smaller. – The rod? Yeah. Is that going to work?
– What measuring the rod? – From the way you’re saying, I’d say no. – I don’t know why.
Well because your ruler is doing the same thing. – Right! Yeah, obviously. O, good I like that.
Tricky. How do you handle it? You use light. Because light always travels at the speed of light. Now what you do is, you send a laser light down a massive tunnel and then when it comes back… What you could do is time how long it takes. Because if space stretched than it would take longer. Because the speed of light is a constant.
– Ahh, the speed of light is constant of course. Right. And that is the one thing we are sure about. If space contracted it would take longer to get back… But that’s such a hard thing to measure that difference in time. It would be impossible. So what you do instead, is you something called interferences So if I send light… If I used a special thing to split the light in two, so the half the light goes that way.
– What do you think is half the light. Half a photon? Or… Well, yeah, ’cause is quantum mechanics.
So it’s a bit like the double slit experiment where the photon goes down both paths.
– O, god, yeah, I love that. So, okay, you split the light in two. So, you’ve got a beam coming in here Half a beam goes that way, half a beam goes perpendicular. – Right. And then they come back. And you’ve made it so…
– Ohh, yeah, OK, go on. When they come back, you’ve arranged it so that the distance they have to travel… means that when they get back
— and remember light travels as a wave — you do it so as the peak of the wave coming this way, exactly matches the trough of the wave coming this way. They cancel each other out. Then you get no light.
That’s destructive interference. Now this gravitational wave that comes in, suppose it’s coming from directly above. It has the effect of stretching in this direction, and shrinking in this direction. – Right. Now what that means is, a tiny, tiny, teeny, tiny change in how far this one has to travel, versus how far this one has to travel. And it could be, you know,
the width of a wavelength of light. which is like hundreds of nanometres or whatever. It all depends on the laser you are using, obviously. You will go from having destructive interference where a peak meets a trough, to a peak meets a peak. – It’s like when you get light. When you see light, you’re like “hey presto!” There is a gravitational wave. Except that because it’s like seismic things you are constantly… like a myriad little… This is LIGO by the way, this is the name of the experiment. You have like seismic shifts which means that that distance is changing anyway. So you get loads and loads of noise. So you have to try and find this weak signal in loads of noise. and what they did was, they built a LIGO at one side of the US and a LIGO at the other side of the US where the noise is different and they can somehow pick out the signal from the noise. And that’s what they did. That’s how they discovered the gravitational waves. – Okay, should I sum up, what I’ve understood?
Go on. – A gravitational wave is a disturbance in the fabric of space-time itself. – Gravitational waves are caused by… the acceleration of any sort of object – But they’re usually so tiny, you wouldn’t be able to detect them. – So the gravitational waves that have been detected by LIGO… “lygo”, “leego”? I say “lygo”.
– I say “potayto”. What do you say? I also say “potayto”. – And “tomayto”?
I also say “tomahto”. – So the gravitational waves are being detected by…
– Let’s call the whole thing off. – The gravitational waves detected by LIGO were caused by the acceleration and the collision of two black holes Yeah, because the collision involves…
When they finally collide, it is going so fast. And producing such mad waves. – Oh, I see, in my minds it’s like they collide and massive bghgh… like sending massive waves out. Ah, it’s not no. But, brilliantly, as they got closer, they spun faster and faster and faster so the frequency of the waves that are detected, got faster and faster and faster. Until they vanished. – Ah, so that’s is what they are saying when they say they listened to these… black holes circling each other. Where we?
– We are summing up. – Shall we just maybe explain…
So this is Lyra. – This is Lyra. – She woke up from her nap. So she has come to help. – Any object that is accelerating through space-time will create a gravitational wave. Not any object but acceleration is a requirement.
– Okay, so what objects might not? Well, a spherical object won’t. Well, a spherical object which is spinning, so to say. So imagine if you had a ball bobbing up and down in a pond, and you span the ball. If you did it just right you wouldn’t see waves coming off. But if you had a dumbbell [two weights connected] and you span it in the pool, every time a dumbbell swept around, it would create a ripple. – That would work on my tarpaulin, in my rubber sheet analogy. It would, it would.
– Which I still think is really rather good. Yeah, okay, fine. If you span a heavy ball on your rubber sheet you wouldn’t get waves. But if you span a dumbbell on a rubber sheet…
But I see three clowns. On, two, three. If you span a dumbbell on a sheet, you’d get ripples spinning out towards the… – I like the way that I can visualise that. – So most objects accelerating through space-time apart from spherical objects for example, and cylindrical objects… – Almost all objects, as they accelerate through space-time, will create gravitational waves. – However in most cases those gravitational waves are so very tiny that they are impossible for us to detect at the moment. So the gravitational waves that were detected by LIGO were created by two black holes that were in the process of colliding with one another, and in that process they were spinning around each other really really fast. – Kind of like the two dumbbells that you described. Another way of thinking about it is… Einstein’s theory of relativity is a mathematical description. It’s a theory which has got a lot of hard equations. and these equations are really really tough to solve but what we hope is that our universe is a solution to that equation. But you can even see it like…
– Wait… I know, it’s nice… The Earth going around the Sun is that a solution to Einstein’s equation of general relativity? And, what we find through observation is that it is. So if I could come up with an equation for how a guitar string moves, When I pluck the guitar string, it will move in all sorts of ways but the way that it moves will be a solution to that equation. Okay, and in the same way Einstein’s equations of general relativity are general. But solutions to those equations describe particular things, which is where the idea of a wormhole comes from.
We don’t have wormholes that exists, but we know that wormholes are a solution to Einstein’s theory of relativity. – Can we detect wormholes? Are we looking for them?
We should be able to, and I doubt if we are looking. But we should…
– Can we start looking? Yeah. I don’t know. I haven’t seen one right there.
Can you…? We’re blocked by the neighbours there.
– My eyes aren’t that good actually. So maybe they don’t exist. Anyway they are allowed by Einstein’s theory of relativity. I hope you enjoy this video, as well I hope you enjoy the format of it. We’ll see you next time.
– Bye! [subtitles: Ger Hanssen] Can you say LIGO.
– ….. Can you say photon? Yes that is a photon, coming out of the light. She is very clever.

100 Comments

Add a Comment

Your email address will not be published. Required fields are marked *