What are gravitational waves?

Scientists annouced the detection of these elusive ripples in spacetime but what are they? And why has it taken so long to find them?
19 February 2016

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Scientists annouced the detection of these elusive ripples in spacetime, 100 yearsGravitational waves after Albert Einstein predicted them, but what are they? And why has it taken so long to find them? With the help of David Marsh, Sheila Rowan, Jon Kaufman and Norna Robertson, Graihagh Jackson journeys through the cosmos to find out...

Scientist - Ladies and Gentleman, we have detected gravitational waves.  We did it.


Sheila - We did a lot of running from office to office telling saying have you seen this.  You know it's amazing so it was pretty fantastic.

Jon Kaufman - We'll have a whole new way to look at our universe which is pretty exciting.

Norna Robertston - It's as if we are now listening to the sky as well as seeing, so it's going objects that are different, that can't be seen in any other way.

David Marsh - And we can see some cool things like black holes colliding.  It's really opening up of a new era.

Graihagh - ...and given that this podcast is all about astronomy, it would be criminal of me NOT to be talking about it.

The case was made even more strongly when I got an email from Robin Laren...

Robin - It wasn't more than a week ago that there was lots of noise and brew ha ha about these gravitational waves and, of course, I didn't understand anything about it, about what's so significant about them.  But it seemed like something really nifty and something that Einstein said a hundred years ago that there shall be gravitational waves.  It made sense in my mind, and then I remembered another podcast where they tried to explain Einstein's general relativity that there isn't really such a thing as gravity.  And this comes from the same Einstein that sort of said that there is gravitational waves and then there is no such thing. So it's just curves in space and then it just doesn't make any sense to me any more.

Graihagh - You're sort of left feeling like what the heck is going on here!

Robin - It's the same guy - he was a genius so he sort of should have his facts straight.

Graihagh - So this month on Naked Astronomy - gravitational waves, with a hint of the big bang.

Okay.  Gravitational waves. Stripped down bare.  Here we go, they are like waves in the sea, or the ripples from a stone dropped into a pond. There's a ruddy great big event in space, like black holes colliding or even the birth of our universe at the big bang  - if we're continuing with our pond analogy, a massive stone, heck a boulder! - would have dropped in and you can immagine loads of ripples propagating out and this disturbance creates waves and ripples across the fabric of space time

So, if you dance around now, (like me) dun dun dun duner you're actually create gravitational waves!    David Marsh at Cambridge University and myself thought this idea was pretty cool.

David - Yes, it is pretty cool, yes.  Undetectably small but, nevertheless, you are creating gravitational waves tonight.

Graihagh - The effect of this ripple bends and stretch space and time and the one detected shook detectors for 20 milli seconds... so does that mean as a gravitational waves passes through me I get momentarily skinnier and taller? Or fatter and shorter? Albeit for a fraction of a second? Jon Kauffman, University of Caligornia, San Deigo...

Jon - Yes, but this is sort of the funny part -  and this what really makes LIGO so incredible is that the ways in which you would tell if you were taller, shorter, or fatter or skinnier would also stretch, so your ruler would stretch...

Graihagh - And that's in part why they've been so difficult to detect - massive huge ruddy great big things only distort spacetime by the width of a proton. Despite being seemingly unimaginable, TINY objects, Einstein somehow dreamt them up....

Nona - Gravitational waves are a prediction from Einstein's general theory of relativity which he produced a hundred years ago, and this was one of the final parts of his theory which had yet to be proven.  And now that we have made this dissection, we have proved that gravitational waves exist a hundred years after their prediction.

Graihagh - Very timely indeed isn't it.  Was that deliberate?

Nona - No, it's not deliberate, of course, but it is very timely and it is wonderful that it has happened on the hundredth anniversary.

Norna Robertson there from Caltech there (btw, we'll be hearing more from all of these guys later in the programme in more detail so, just bare with me)

Now, Isaac Newton explained how an apple fell from a tree - that objects with mass were attracted to each other - but not why. This is where Einstein comes in. Einstein said 'hey, the reason they're attracted to each other is because massive objects bend the fabric of space time... David Marsh again...

David - One way to think about it is that space time is like a mattress, if you wish, and if you place a very heavy object on this mattress, it will sink down and the mattress around it will bend upwards.  So that's what happens in gravity.  So if you have a heavy object (any heavy object) like me...

Graihagh - Or even something like the sun?

David - Or even the sun - even a little bit more heavy...

Graihagh - Just a little bit...

David - Yes.... bends space time around it.

Graihagh - This takes us to Robin's question about gravity, Einstein and Newton. Newton thought gravity was a force; Einstein thought that massive objects - sun, moon and stars and in David's analogy, a bowling ball - create a curvature in spacetime and this is what we feel as gravity.

In general relativity, gravity is not a force between masses. It's an effect of the warping of spacetime.

If we return to gravitational waves - when massive objects collide, they create ripples across the fabric of spacetime, just like if you dropped that bowling ball on a sheet. We have been trying to detect these things, these ripples, for donkeys years - people have spent their entire careers working on this! And this month, the LIGO team found them.


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