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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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Relativistic speeds are usually measured in terms of gamma, not meters per second. Gamma is a value that represents the amount of time dilation and mass increase an object has; if you're moving at 86% of the speed of light (~206257211 m/s) then gamma is ~2.0, meaning that time would run twice as fast for you, and to a relatively stationary observer, your mass would be double what it is at rest. Gamma is calculated thusly:
y = 1 / sqrt(1 - (v^2 / c^2))
Gamma can rise unbounded; as your velocity approaches light, gamma rises exponentially, reaching infinity when your velocity is equal to that of light. I'm assuming that the original paper used values of gamma for measurement, rather than meters per second.
More about gamma here.
"Destroy science and religion. Science would re-emerge exactly the same; but not religion." - Penn Jillette, paraphrased
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.
- Strong
- Electromagnetic
- Weak
- Gravitational
I even fetched a URL on a whim, just in case you disagree for some reason.How about this: a photon has zero rest mass. However, it is never at rest, but travelling at C. It does have energy, which translates to a very little mass and does warp space time, but when it hits something, that energy goes away, and so does the photon.
I wonder if a sufficient density of photons would collapse into a black hole.
"We returned the General to El Salvador, or maybe Guatemala, it's difficult to tell from 10,000 feet"
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.
Actually if I'm not mistaken, gravity and electromagnetic both exert a 1/r potential (1/r^2 force). So their distance range is the same.
So at short distances:
strong>weak>=electromagnetic>gravitationa l
but this isn't quite right because the strong force has two characteristics, a main force and a residual force. The main force is what keep quarks together in neutrons and protons. That's absurdly strong, and it's strength actually INCREASES with distance. However, the residual strong force is what keeps the nucleons together, and falls off really fast with distance, like the weak force.
At long distances things are a bit simpler:
Electromagnetic>Gravitational>strong>weak
The problem with this is that the electromagnetic and gravitational are relative. You can't go by the constants associated with the field, because they're defined by us (for example, what if mass were in terms of petagrams? Then G~10^19, and the force (in terms of petanewtons, I think) would skyrocket.)
Point is that it's all dependent upon the system you're talking about and the units you're talking about them in. We really can't compare them.
The best way to predict the future is to invent it.
To perform the experiment, numerous (probably several thousand) measurements are taken, but due to imprecision in the process of taking the measurements (imperfect measuring equipment, human error, etc) you get a variety of results. These answers could vary from well below c to well above it. If Einstein was right and nothing propogates faster than c, the higher results could only be attributed to imprecise measuremements, but you can't throw those measurements out if you are trying to be objective.
At the end of the process, you have something vaguely resembling a normal bell curve, where the height of the curve at a point along the x axis (velocity) is a measurement of the relative frequency with which that speed of gravity was obtained as a measurement. The total area under the curve will be exactly 1. In many cases, the curve may not be symmetric, but for an experiment such as this, you are unlikely to obtain an assymetric curve (Central limit theorem of statistics, or some such thing). A line right down the middle of the curve shows the measured average result (.95c).
A confidence interval is then picked (it is a shame that this interval is not mentioned in the article, but it is almost assuredly at least 95%, probably even 99%, or 99.9%). This percentage is converted to decimal (95%=.95, 99%=.99, etc), and a symmetric region around the average score with that area is blocked off. This blocked off area has a minimum X component of .7125c, and a maximum X component of 1.1875c, the difference between each of these and the average measured velocity being .2375c, which is 25% of .95c.
And that's where the 25% margin of error comes from -- for their desired level of confidence, the variance in measured results was off by no more than 25% of the value that was actually obtained as the mean.
Since the value of 'c' lies WELL within the bounds of the margin of error of the experiment, and pre-existing theories support the speed of gravity being c, this experiment supports those theories. It is important to note that this experiment did not prove anything, it only failed to disprove that the speed of gravity is anything other than something very close to c.
File under 'M' for 'Manic ranting'
- 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."
Strange, 20 years ago I was taught other people had experimental evidence agreeing with a prediction that the effects of gravity move at light speed:
In 1882 Simon Newcomb observed an excessive perturbation in precession of the orbit of mercury, to the tune of 43 seconds of arc per century. In 1915, Albert Einstein showed this could be explained by the propogation of gravitic wave effects at the speed of light...
But thanks for playing.....
And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.
point of note: a "nova" is what happens when fresh yummy hydrogen falls on a white dwarf. Boom! A "supernova" is what you were talking about. Confusing the two is a little dangerous, because they're two completely different processes.
Depends on the mechanism of acceleration, really. If it's merely "moving" at a Lorentz factor of 100, then no, of course not, because all you did was Lorentz boost the system, which you can always do. In the Sun's rest frame, it's fine still, of course. In the boosted frame, it's also incredibly flattened (like a pancake - by a factor of 100, no less) but amazingly enough, you can still work out hydrostatic equilibrium for it, and determine that yes, it is still in equilibrium, and not going to blow up. Beauty of relativity - laws of physics are Lorentz boost invariant.
However, if you're actually accelerating the thing, now that's a different story. You (still) won't make it go supernova, because you're NOT actually increasing the number of particles inside it, and that's what breaks hydrostatic equilibrium - pressure generated versus gravity, and BOTH of those change in the boosted frame - but you WILL screw it up really badly by sending pressure bubbles through the whole thing. Since the Sun isn't a rigid body, you'll probably strip the chromosphere right off of it, and leave the core bare. This, however, won't due much except really really confuse distant astronomers.
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.
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