## Friday, June 22, 2012

### Q: What Would a Gravitational Wave Feel Like?

Several people who have found this blog were led here after searching for this question:

What would a gravitational wave feel like?

This is an excellent question so I decided to answer it directly in today's post...

HOW DOES A GRAVITATIONAL WAVE INTERACT WITH MATTER?

First, let us review what a gravitational wave does to matter as it passes by.  Gravitational waves will expand space in one direction and compress it in the perpendicular direction.  This stretching and squishing happen in the plane that forms the cross-section of the wave; this is perpendicular to the direction the wave is traveling.  For a gravitational wave traveling into your computer screen, a circle of stuff will be affected like this:

 Image from Wikipedia [gravitational waves]

HOW WOULD A GRAVITATIONAL WAVE FEEL FOR A SOURCE FAR AWAY?

Now let us consider what you are likely to feel here on Earth from what we here at LIGO consider to be "big" gravitational waves.  These waves are produced by some of the most violent, energetic things in the Universe like black holes colliding and stars exploding.  These sources are rare and there are none nearby us, say within a few light-years.  Since the strength of a gravitational wave decreases as the distance from its source increases, by the time these reach us here at Earth they are incredibly small.  A once every ten years gravitational wave will squish and stretch LIGO less than 1000x smaller than the diameter of a proton (< 1x10-18m).

Therefore, a person will not feel anything from even the strongest gravitational waves we expect to detect with LIGO.

HOW WOULD A GRAVITATIONAL WAVE FEEL FROM A STRONG, NEARBY SOURCE?

If we were near one of the huge, violent sources LIGO is sensitive to, the gravitational waves would be strong enough to rip us to pieces!  (So it is a very good thing that these sources are very far away!)  This is due to a phenomenon known as "spaghettification".  In a previous post, I described a gravitational wave as simply a change in the gravitational field moving out into the Universe like a ripple in a pond.  The gravitational field is a measure of how much a mass would feel at any given place.  So, if the gravitational wave is a changing gravitational field, then the force of gravity that a mass would feel as the gravitational wave passes should change too.  If the change is so large (due to a very large gravitational wave), then it is possible that your feet will feel a strong enough force, compared to your head, to rip your legs off your body!  And to make matters worse, the sides of your body would be compressed at the same time.

Below is a clip of Neil deGrasse Tyson giving a humorous description of spaghettification.  His description focuses on what would happen to you if you fell into a black hole, but the concept is the same for super-strong gravitational waves:

CONCLUSION

Given where the Earth is in the Universe and how far away sources of strong gravitational waves are, any passing gravitational waves will be so small that we will never be able to feel them.  But if we were close to a strong source of gravitational waves, they could tear us apart!

So it is both a blessing and a curse that the strong sources of gravitational waves are far away:  a blessing because we will never be harmed by them and a curse because they are so small, that we need huge, extraordinarily sensitive detectors like LIGO to detect them.