| From: John E. | 8/07/99
22:56:41 |
| Subject: Gravity Wave Detectors are blind | post id:
22718 |
Hi,folks!How is it that GWDs can detect gravity waves when space AND time are effected by GWs?Is not space and time one and the same "spacetime"? When a GW passes through a detector it will compress the laser beam in length but will it not also compress time?I know the speed of light (in this case the laser beam) is a constant but time is not, yes?How could there be an interference pattern?I understand the principals of GWDs but not (obviously)the dynamics.I've no reason to doubt GWs exsist.They are part of classical physics,but isn't it a little bit like trying to catch yourself blinking in the mirror?Could the answer be simply that the waves of the laser are compressed on one arm through the beam spliter and not the other?Even if the waves (laser)are compressed so is time, yes?Dosn't that mean in effect the laser in question has trevelled the same distance as its twin and at the same frequency? I know I'm missing something FUNDAMENTAL!Pleeeeeeease heeelllppp meeeeeee! Fundamentaly John E. | |
| From: Damion | 9/07/99
10:20:14 |
| Subject: re: Gravity Wave Detectors are blind | post id:
22780 |
| Since no-one else has responded
yet, I will try. I thought, time is only affected/altered as you (object) approaches the speed of light, so unless the gravity wave or the testing instrument is travelling near light speed (not the speed of the laser) the gravity would only affect space. mmmm ? | |
| From: Chris (Avatar) | 9/07/99
10:22:11 |
| Subject: re: Gravity Wave Detectors are blind | post id:
22783 |
Gravitational waves are predicted by Einstein's general theory of relativity (GR). They arise when any massive object is accelerated, in much the same way that electromagnetic waves are produced by accelerating charges (there are quite a few such similarities in the formalisms of GR and classical e/m). In GR gravity is described as the bending or warping of space-time. Space-time in this sense is a 4 dimensional coordinate geometry in which the notions of 3D "distance" and 1D "time" are interlinked. We should spend a moment to make sure that this is understood. Sometimes space-time is best understood if we compress the three dimensions of space into one, and plot this on an x - y axis against time. In essence we are simply ignoring two extra dimensions for the sake of understanding. Relativity, in particular GR, talks about space-time intervals, which are easy to see on the diagram we've drawn. You can draw a line which is parallel to the y axis (time) which is a timelike interval in space-time. This might correspond to you standing still for an hour - you've moved through time, but not space. You could also draw an interval parallel to the x (space) axis which represents a spacelike interval. This would correspond to you moving through space, but not time - which relativity tells us is impossible. Most intervals will involve a little change in both time and space (eg as you go for a walk) - and the two are related. However it makes perfect sense to talk of an interval in one, two or three dimensions of 4D space-time without including the extra dimension(s). A gravitational wave is a "ripple" in this geometry. It involves the compression of space-time in one direction and corresponding expansions in other perpendicular directions. But these ripples are incredibly small, which makes them very hard to detect (imagine something along the scale of trying to look for something the size of a proton in something the size of a solar system!) Up until now no gravitational waves have been detected - although we can infer their existence by looking at the way binary pulsar systems behave. But there is hope. The world's largest gravitational wave detector is LIGO, the Laser Interferometer Gravitational-wave Observatory in the US. It consists of two installations, one at Livingston, Louisiana, and the other at Hanford, Washington (separated by about 3200km). Each one contains a large L shaped vacuum pipe about 1.2m in diameter with arms 4km long. Three test masses (one at each end and one at the vertex) are suspended in the pipe with mirrors attached. Extremely stable lasers are fired between the mirrors. Gravitational waves are expected to contract the distance between the mirrors in one arm whilst expanding it in the other. These changes in length will only be on the order of 10-18m across the length of either arm - hence the interferometry. For the purposes of the detection, we can simply ignore change in the time "direction" just as we ignore any change in the third spatial direction. The two sites will compare results so as to eliminate the contributions by micro-earthquakes, acoustic phenomena, etc which might simulate the waves. The detector then sits and waits for a cataclysmic cosmic event - such as the collision of two black holes or a massive supernova - to provide gravitational radiation on a scale which we can detect. Why? The definite detection of gravitational radiation would be yet another compelling verification for a GR. Keep in mind that these waves were predicted in 1916 by Einstein - but it is only now that we have the technology to be able to detect them Hope this helps! Chris | |
| From: Chris (Avatar) | 9/07/99
10:26:20 |
| Subject: re: Gravity Wave Detectors are blind | post id:
22785 |
For Damion Gravitational radiation affects space and time - so does every mass in the universe. The two aren't separable in terms of effect. But the detector is trying to detect a presence and two dimensions are sufficient for that. In addition gravitational waves do travel at (or very very close to) the speed of light. In a purely einsteinian universe (which is a fair approximation locally) the waves travel at the speed of light exactly. Cheers Chris | |