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Renewed Gravity Research Could Soon Yield Results
t482 writes "Dr. Michelle Thaller has a nice article describing the current thoughts on gravity. Why is it so weak? Detecting gravity waves has turned into a bit of a cottage industry. "We are close," says MIT physicist Rainer Weiss, a pioneer in gravity wave research for more than 30 years. "I think sometime in the next two or three years we will see something.""
Gravity waves are bread and butter on Slashdot
Personally, I prefer to think that this universe may not be infinite, but an infinite number of the infinitely many parallel universes are!
Allright, IAAP (I Am A Psysicist), and I think it's good two debunk a common misconception here:
Gravity waves are not the same as gravitational waves
Gravity waves are matter density waves in fluidi (fluids or gases) caused by the interaction of two forces: bouyancy and gravity. Here, bouyancy is the upward-driving force, and gravity is the downward-driving force. The essence is that these waves require a medium to propagate (e.g. air).
Gravity waves can be found in the atmosphere, e.g. clouds which form in regular bands of cloud and clear sky, where the gravity waves carry momentum and energy from the troposphere to the middle and upper atmosphere Gravity waves can also be found on the surface of fuilds: think of the waves behind a boat. A good primer on gravity waves can be found here
Gravitational waves are a whole different ballgame! These waves have got nothing to do with matter densities as they don't require a medium to progagate: it is not matter that moves, and in that respect gravitational waves are like light (which, contrary to beliefs held at the beginnning of the century, don't require a medium such as "ether"). Gravitational waves are wacves in the spacetime-metric.
So what the hell does that mean? Well, in gravity waves, there is a wave in space (and time) in which the thing that changes over space and time is the density of matter. In gravitational waves, there also is a wave in space and time, but the thing that "wiggles" is not the density of matter (or the strength of electric and magnetic fields, like in light or EM radiation in general), but the properties of the fabric of space and time itself. You can think of it as if the coordinate system itself wiggles, so to speak. This "wiggling" results in the length of the arms of e.g. the LIGO interferometer to change ever so slightly, causing a phase shift between light beams send through both arms, which can (hopefully) be detected.
In more mathematical terms, the exact properties of space and time are called the metric. In a portion of space without any matter, the metric is flat (called the Minkovski metric), which means that the usual laws of geometry apply. In any circumstances with matter (and thus gravity) present, these laws to do hold up!
What?!, I hear you think. Yes sir, you've been lied to in geometry class! However, you've been lied to only very, very slightly. Example: if you measure the radius of a sphere (say: R), you expect to find a surface area of exactly 4/3 * pi * R^3. If the earth would be a perfect sphere (which it isn't), and you would be able to measure its radius and surface very accurately, you would find that the surface area is ever so slightly smaller than expected. Or, in other words, the radius seems to be a bit too large (in the order of 3 cm or 30 cm IIRC). Read more about space time curvature here/
A primer on gravitational waves can be found here. A more detailed description here.
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Effect is a verb. He just used it incorrectly. To effect = to do, as in "he effected a change", whereas to affect = to alter.
Even far away from the planet the lab would still be inside the gravitational field of the sun.
So to compensate for gravity you would need to place the lab in interstellar space, however it's much more practical to leave the lab on earth and compensate for the planet's gravity in the equations.
The professor looked startled: "This is an unfortunate turn of events. It appears that of the two people in the world ever to have understood General Relativity, one of them has forgotten".
Anyway, thing is, General Relativity is all about the fact that the presence of mass causes the curvature of space-time. What we call gravity is a consequence of that curvature.
Get your head around that one, and we'll talk some more.
Matt...
Save the Bottom Line
No, no, gravity is just spacetime curvature and spacetime curvature holds us on the Earth.
Why couldn't we put this lab in orbit?
The main reason is that the effect is so weak. A mission concept called LISA is being studied by ESA and NASA. The idea is to have 6 spacecraft orbiting the Sun, which together form a interferometer several million kilometers in size. The catch: Because the waves are so weak, the distances between these spacecraft would need to be controlled to within about a nanometer (!) to have any hope of detecting a signal. Needless to say a VERY challenging mission.
A lot of other interesting missions would be enabled by good formation flight technology. Look at NASA's Terrestrial Planet Finder mission, or the ESA's similar Darwin mission.