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The Speed Of Gravity Revealed
redwolfoz writes "New Scientist is reporting that the speed of gravity has been measured for the first time. 'The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.' Researchers made the measurement of the fundamental physical constant with the help of the planet Jupiter. One important consequence of the result is that it will help constrain the number of possible dimensions in the Universe."
Light has mass? no it does not.. the energy of a photon has a mass equivalence, but it does not have mass.
You're confusion arises because you were taught elementary Newtonian physics. In general relativity, one learns that any "information" cannot travel faster than light. Gravity is considered information because if you feel a gravitational force on you, you know that there is a body out there acting on you. That is, you have information about it (you could even estimate its mass by measuring the tug it exerts on you).
In Newtonian physics, lots of things are assumed to happen instantaneously (like gravity) so they don't have a speed per se. But in general relativity, everything has a speed -- and that speed is no greater than the speed of light.
GMD
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Yeah, that's the real trick. For those who aren't aware, getting gravity to "play nice" with both general relativity and quantum mechanics is pretty tough. Relativity models gravity is a warping of space. But coming up with a quantum theory of gravity is mighty difficult. There are theories that gravity acts through particles (the so-called gravitons you always hear about on ST:TNG) but I don't believe this has been proven yet.
GMD
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What next? The speed of magnetism?
Yes, the speed of magnetism. The particle which mediates electromagnetic interactions is the photon which propagates at the speed of light. So if a magnet is suddenly given a push in one direction then there is a delay before distant particles notice a change in the field of that magnet.
This is an analogous result for gravity and the postulated graviton particles.
It's one thing to not understand something, we all have our fields of expertise. But assuming you know everything based on some limited high schooling makes you the saddest kind of idiot.
:wq
"In general relativity, one learns that any "information" cannot travel faster than light"
What about quantum pairs? Move them apart, and a change in one is reflected intantly in the other.
That's why I specifically said "In general relativity...". Quantum pairs are from the theory of quantum mechanics, not general relativity. Physicists have been working hard to try to combine relativity and quantum into a single unified theory. However, problems arise when the two theories predict different things -- such as the quantum pairs example you listed. According to relativity, there would be a finite time lag for the change to be reflected in the second entity of the pair whereas quantum would say that the change is instantaneous.
Incidently, I heard that a few years ago an experiment was performed on quantum pairs and, sure enough, the change was indeed instantaneous. Can anyone else corroborate this?
GMD
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Okay, I'll bite.
A photon delivers an impulse when it is fired or when it is destroyed on impact with matter - but when it is in transit in space it has no mass.
Imagine a giant cluster of light, like fired by a superlarge pulse laser. It will transfer momentum to whatever it hits, but it does not actually have mass, so when its in transit this massive ball of light will not suck in anything with its gravity.
It's accomplished via huge (4 ft. diameter, 2.5 mi. length) tubes in an L-shape. A laser is then bounced along the length of the tube, and measures its distance very accurately: to within 10^-16 (!) cm, or about one hundred millionth the diameter of a hydrogen atom. Any change in the distance is a possible indication of a gravity wave passing through from some distant, powerful source. The fact that gravity decreases exponentially with distance means that even gravitational waves from extremely powerful sources, like binary neutron-star systems, are very weak when they get to Earth.
Of course, other vibrations can screw this up, so these observatories are really isolated (both geographically and mechanically) and data is compared from around the world. Lots of information is available at the LIGO (Laser Interferometer Gravitational-Wave Observatory) website, where I got most of the specs listed here.
That's it. I'm no longer part of Team Sanity.
- the inverse square law implies that the ratio of these forces should remain constant with distance, but
- everyday experience and astronomical evidence seems to suggest that gravity grows stronger than the EM force at macroscopic scales
I think the key to resolving this conundrum is to realize that the EM attraction is proportional to the relative charge difference between two bodies.- At microscopic scales, one is often dealing with individual EM charges, so the relative charge difference at that scale is large and the force is strong.
- In macroscopic objects, it is difficult to separate macroscopic amounts of charge precisely because the EM force is quite strong, so macroscopic objects usually have relatively small charge differences and the macroscopic EM force seems relatively weak.
Compare this to gravity, which only has one type of charge--mass--which always increases as the size of the object increases."It take 9 months to bear a child, no matter how many women you assign to the job."
Gravity is a long-distance force that decreases as inverse distance squared. This is Newton's famous 1/r^2 law, and it remains unaltered by the theory of general relativity (after all, Newton's laws are just a limiting case of General Relativity.)
With a short-range gravitational force, decaying exponentially with distance, stable planetary orbits and galaxies, with their literally astronomical extent, could not exist.
Read this FAQ.