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Hacking Cassini To Detect Gravity Waves
lennon writes: "With some upgrades to the tracking equipment, NASA is going to try to detect gravitational waves by tracking the speed of the Cassini probe. They've tried this with other spacecraft, but the sensors have evolved since then. Complete press release is here. Looks like a neat hack."
I understand the methodology (well, as much as can be deduced from a press release...) of the measurement, but could other factors cause apparent--or real--shifts in relative velocity? For example: mini planets, large asteroids, or lopsided planets could cause variation in the gravitational force exerted on Cassini and said object, altering the relative velocity of the probe. Someone enlighten me!
"What is the sound of one belly slapping?"
What I find truly interesting about all this is not just that they are measuring the velocity changes (the acceleration) of the Cassini probe, but that after getting back the information of the forces at work, they will have to somehow determine exactly where the gravitational waves are coming from.
Think of it this way: If two planets are on opposite sides of the probe, and both are emitting gravitational forces, then the probe will be subjected to the net forces of the two planets. The equation for relative force of gravity comes to mind here, and I assume they will be using it when calculating multiple sources of gravity.
(GmM)/(R^2) gives the acceleration of the system for two masses in space, so any resultant force must take into account that it could come from several different masses.
JPL engineers have carefully instrumented a large dish antenna at the Deep Space Network's Goldstone complex near Barstow, Calif., to send and receive the higher frequencies with unprecedented Doppler sensitivity. The upgrade includes refined pointing capability needed to exploit the higher frequencies, said Sami Asmar, supervisor of JPL's Radio Science Group. Other new equipment at Goldstone will allow researchers to correct for the atmosphere's distortion of radio transmissions and improve performance of the search.
As I see it, the trickiest thing here will be taking the "exquisitely accurate measurements" and turning them into real, useable models of gravity given off by our neighboring planets.
Side info: If you held open a matchbox in space, it would contain about 6 hydrogen atoms.
Security through promiscuity is no better than security through obscurity.
People always want to talk about string theory at the drop of a hat. But there is so much fascinating stuff in physics that holds a possibility of actually being true. :)
Maybe string theory enters into the picture on the Planck scale, or when you're going to talk about individual gravitons, but it's completely irrelevant as far as this experiment goes. Gravitational waves are a classical phenomenon, predicted by GR (which is a classical theory). They have not been detected as of yet because they are so weak. The coupling coefficient is c^4/(8*pi*G), which is really large. So space time is elastic, but it is extremely stiff. It takes a lot of force to warp it even a tiny bit. The earth emits something on the order of 1 watt of gravitational radiation as it orbits the sun. Jupiter emits something like 30 watts. (Don't ask me for a source on those numbers- I think I read them on the Internet somewhere.) But any laboratory source won't emit anything that can be measured. Gravitational waves are even more esoteric than neutrinos, since we know how to detect neutrinos that we have created. The only sources of gravitational waves that are even remotely detectable are binary star systems, where two neutron stars are in a close orbit. The orbital periods of some of these systems have been determined to be decreasing in a manner characteristic of energy loss from gravitational radiation.
Personally I've always thought it's a bit premature to be speculating on the stringlike nature of gravitons when we can't even detect gross macroscopic things like gravity waves or even gravitomagnetism. It's as if we're blind snails wanting to talk about photons.