Feeling gravitational waves

04 December 2018

Question

If Earth was close to the source of a gravitational wave-causing event, assuming it survived the blast, what would it feel like?

Answer

Chris Smith put this question to astronomer Carolin Crawford...

Carolin - Oh very good question. So if you are near a big event, well let's, we’ll worry about how close in a minute, but let's work out what a gravitational wave does to you. And the thing about a gravitational wave is that these are travelling disturbances in the shape of space. They stretch and they squeeze space and everything in it. So if I am facing an incoming gravitational wave and imagine I stretch my arms out, I am going to be stretched and squeezed alternately from one fingertip to the other tip while being squeezed and stretched in the opposite direction from the top of my head to the tip of my toes. So I've got that action going on - stretching, squeezing - and there’s also another mode which will stretch and squeeze me from my shoulder to the opposite hip and then from the other shoulder to the opposite hip. So you are being shaken internally like a jelly, you've sort of been wobbled in all these different directions.

However this is happening to all of us all the time, right. And you're not feeling it are you?

And this is because, these gravitational waves, most of them are incredibly weak - I mean the kind of length you get stretched. Well we had, the first detection was two 30 solar mass black holes that collided with each other a billion light years away and that's one of the best detections we've had so far, the most sensitive detectors, and that stretched and squeezed a 4 kilometer long laser by a distance less than a millionth of an atom.

Okay so gravitational waves are very weak. So then. But you know Paul was very clever. He said what if you're close to a gravitational wave event and magically we’re somehow protected to whatever is, you know where these black holes are colliding. Then it starts to get a bit more interesting because gravitational waves, well the, you know how much you get stretched and squeezed depends on one over the distance. So if you had an event like two 30 solar mass black holes colliding somewhere where the sun is, so one hundred fifty million kilometers away, the amount of distortion you might feel is of the order of tens of nanometres. I don't know whether you'd feel that.

Ljiljana - My nano-diamonds might feel it, because they are very small.

Carolin - They're very small, but whether you might feel it in your body… So let's move the event a little bit closer and maybe it's a few thousand kilometers away, then the amount of distortion gets to be about a millimeter over the length of a body. Again whether you feel that or not, it's really got to be a few hundred kilometres before you start being stretched and squeezed over you know sort of 180 centimeters or so by one centimeter. Now I'm reckoning you would feel that. I also don’t think it would be very good.

Chris - Probably wouldn't be very good for you would it, for your body to have that?

Carolin - No. Would your muscles stay attached to your bone? What would it do to the brain? And you know the absolute last thing I haven't told you about is the frequency that these gravitational waves are doing the stretching the squeezing. This is on sub millisecond time scales.

Chris - Because they travel at the speed of light, gravitational waves, or thereabouts, don't they?

Carolin - They travel at the speed of light.

Chris - Therefore your body would be stretched and squeezed at the speed of light. So you probably wouldn't be, if it was going to squash and squeeze your brain at the speed of light then it probably would mean you weren't aware of it.

Carolin - No, the gravitational waves travel out from the disturbance at the speed of light. What I'm talking about is the, you know between the stretching and squashing, that would be, you stretch and squish on basis of you know less than, you know submillimeter seconds. So I don’t think your body would really survive that. So I think the answer to your question is you will be wobbled like a jelly, shaken from the inside out and I don’t think it would be very good news.

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