# Measuring gravitational waves

1. May 2, 2006

### ragingsamster

I'm trying to get my mind around this one. (stretching thin in the extreme)

Black Holes Collide, and Gravity Quivers
New York Times 05/02/2006

In the most precise effort yet to detect gravitational waves -- the quiverings of space-time predicted by Einstein's theory of general relativity -- the National Science Foundation in the late 1990's carved two large V's, one in the barren landscape of central Washington State, the other among the pines outside Baton Rouge, La.

How do you measuer something that affects both space and time simultaneously? It would seem like any gravitational wave would distort both time and space equally and would escape detection.

I saw this article and wanted to find answers - a scientist's web page referred to this forum so I thought "what the heck" might as well look for answers here!
Thanks in advance for the application of generous quantities of ignorance remover

2. May 2, 2006

### robphy

Last edited by a moderator: May 2, 2017
3. May 2, 2006

### ragingsamster

From the site:
Einstein predicted gravitational waves using General Relativity in 1916. These waves are emitted by any object undergoing rapid acceleration, but only gargantuan masses, like colliding black holes or exploding stars, produce waves LIGO can detect. As they travel, these ripples literally warp space; they shrink it in one direction and stretch it in another. The farther they roam, the fainter they get. By the time they reach Earth, their already tiny warp is barely measurable.

What I was questioning was that they are saying their measurements would appear to discount the affect that gravity would have on time - if a gravity wave crosses one side won't that wave also distort time and as a result be unmeasurable. or am I linking space/time/gravity when I should not.

4. May 2, 2006

### pervect

Staff Emeritus
Gravity does have an effect on time, but it's a very small effect. For instance, look at the equations for gravitational time dilation

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html

The numerical result is instructive - Time at the surface of the Earth runs slower by a factor of .7 parts per billion compared to an observer at infinity.

Now the gravity of the entire Earth is quite large in the sense that it's something that's easy to measure. Gravity waves are expected to be so weak that it will take our most sensitive instruments (Ligo II) to be able to detect them. The effect on clocks of such weak gravity will be for all practical purposes negligible.

So gravity does have an effect on time, but for some reason, you are greatly overestimating it. I'm not sure exactly what you've read to cause this overestimate.

5. May 3, 2006

6. May 3, 2006

### Creator

Quote from article:
"Gravity is a familiar force...... It keeps beer from floating out of our glasses." Hmmm....I knew there was a reason God created gravity, but somehow that one slipped right by me.

And I really like this one:

"Eanna Flanagan, Cornell associate professor of physics and astronomy, has devoted his life to understanding gravity since he was a student at University College Dublin in his native Ireland. "When LISA flies we should see hundreds of these things," noted Flanagan. ...... It will be very exciting to finally see that relativity actually works."

Duh! Nobody in Dublin has ever seen relativity actually work?

Surely God created liquor to prevent the Irish from taking over the world.

Thoroughly amused,
Creator :)

Last edited: May 3, 2006
7. May 3, 2006