# Faster than light?

quasi426
I know light is the fastest thing in the universe. But what about forces, namely gravity, magnetism, strong and weak forces. Do these forces act instantaneously OR do they act as fast or slower than light, does the medium they are acting in matter or not?

Homework Helper
Nothing is instantaneous, that's for sure. All fundamental forces are described by bosons, gravity, electromagnetism and strong interactions are described by massless bosons, which means that they propagate wih "c". The weak interaction is mediated by massive bosons, whose velocity is obviously less than "c".

Daniel.

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Are gluons massless?

sniffer
gluons are "massless", but i don't know what "massless" means. maybe it is more tricky than that.

Gold Member
quasi426 said:
I know light is the fastest thing in the universe. But what about forces, namely gravity, magnetism, strong and weak forces. Do these forces act instantaneously OR do they act as fast or slower than light, does the medium they are acting in matter or not?
They all operate at the speed of light.

quasi426
Can someone describe an experiment or simply give me a link where an experiment showed that these forces travel at the speed of light. (Does the medium the forces travel in matter?)

quark
Sometime back, when I was actively reading material on astrophysics, I came across a paper which shows the speed of gravity much greater than light. The idea is that if the speed of gravity is equal to that of light, by Poynting-Robertson effect, the Earth would speed up and run away from the Sun. But no such evidences are perceived by the astronomists. The content of the paper seems plausible. It is worth reading http://www.metaresearch.org/cosmology/speed_of_gravity.asp [Broken]

I don't know of any papers, as I am no expert in Physics, which negate the above theory but I hope experts of this forum may suggest something.

Regards,

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Staff Emeritus
quark said:
The content of the paper seems plausible.

Here's a hint: If this "paper" only exists at this website and no references have been made to it appearing in any peer-reviewed journal, the "plausibility" degree drops WAAAAAY down.

Also, those people are called "astronomers", not "astronomists".

Zz.

quark
OK, thanks for the correction. Can you let me know what is wrong with the paper? Not appearing in a peer reviewed journal may be one reason.

Staff Emeritus
quark said:
OK, thanks for the correction. Can you let me know what is wrong with the paper? Not appearing in a peer reviewed journal may be one reason.

I will deal with JUST the first few lines in the abstract, which already contain something weird

Standard experimental techniques exist to determine the propagation speed of forces. When we apply these techniques to gravity, they all yield propagation speeds too great to measure, substantially faster than lightspeed. This is because gravity, in contrast to light, has no detectable aberration or propagation delay for its action, even for cases (such as binary pulsars) where sources of gravity accelerate significantly during the light time from source to target

Where exactly are these measurements that "all yield propagation speeds too great to measure"? The speed of gravity requires the ability to measure gravity waves, either from a supernova, a binary star, etc.. i.e. something that is producing a fluctuating gravity as observed on earth. Now unless I've missed something, such gravitational waves have NOT been observed YET! LIGO still hasn't made any conclusive detection, and it is being upgraded further to be even more sensitive.

The only valid attempt so far as showing such measure was done a couple of years ago [http://arxiv.org/abs/gr-qc/0206022] [Broken]. However, even this is still controversial, and people like Clifford Will has shown that the calculation/model is flawed [http://wugrav.wustl.edu/people/CMW/SpeedofGravity.html]

So judge for yourself.

Zz.

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ZA
All known forces travel at the speed of light. All known particles travel at or under the speed of light. All massless particles travel at the speed of light. The neutrino, which is now thought to have mass, travels at the speed of light too. The existence of certain particles called "tachyons" have been theorized, which are supposed to have a minimas speed at the speed of light and maximum speed at infinity. They supposedly don't inetract with matter and get more massive as they get closer to the speed of light from the opposite direction of mater. However, they have never been observed and may turn out to be a mistake. There is one way of random information traveling faster than the speed of light called entanglement, which means that we can find out the spin of an electron at any distance from us immediately by observing it's partner electron.

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ZapperZ said:
The only valid attempt so far as showing such measure was done a couple of years ago [http://arxiv.org/abs/gr-qc/0206022] [Broken]. However, even this is still controversial, and people like Clifford Will has shown that the calculation/model is flawed [http://wugrav.wustl.edu/people/CMW/SpeedofGravity.html]

A little background for interested readers. In late 2002, the measurement was performed by Fomalont and Kopeikin when a quasar passed behind Jupiter. Fomalont and Kopeikin claimed that this experiment showed that the speed of light and and the speed of gravity are the same. Cliffored Will disagreed, saying that Fomalont and Kopeikin's experiment didn't actually measure the speed of gravity.

In general relativity, the speed of gravity and the speed of light are the same, so an alternative to general relativity is needed to even talk about a difference. Steve Carlip has written a nice, thoughtful paper on this. According to his analysis, the interpretation of what was measured depends on the which class of alternatives to general relativity is used.

This is subtle stuff, and one of Carlip's conclusions is that for a certain class of models that have different speeds for gravity and light, measurements have yet to reach the sensitivity required to measure the difference.

Regards,
George

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quark said:
Sometime back, when I was actively reading material on astrophysics, I came across a paper which shows the speed of gravity much greater than light. The idea is that if the speed of gravity is equal to that of light, by Poynting-Robertson effect, the Earth would speed up and run away from the Sun. But no such evidences are perceived by the astronomists. The content of the paper seems plausible. It is worth reading http://www.metaresearch.org/cosmology/speed_of_gravity.asp [Broken]

I don't know of any papers, as I am no expert in Physics, which negate the above theory but I hope experts of this forum may suggest something.

Regards,

Meta Research is run by Tom Van Flandern, who is not a reputable source when it comes to such matters.

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Gold Member
In Newtonian gravity, the speed of gravity is infinite. Otherwise, planetary orbits would not be stable. Since we are here to have this discussion, we can safely conclude either the speed of gravity is infinite or the Newtonian theory of gravity is incorrect. We have not yet actually measured the speed of gravity [as noted by other posters], but we do know the Newtonian theory of gravity is incorrect. It has been replaced by the much better theory of gravity called general relativity.

In Newtonian theory gravity is treated as a 'force'. In general relativity, gravity is treated as a field [technically speaking it is more precise to say gravity is the geometry of spacetime, but that needlessly complicates the explanation]. In a field, the attractive force exerted on a body is not directed squarely at the source of that field. It is instead offset by an amount that depends on both the velocity and position of the source and body being acted upon. This almost exactly cancels the effect of the apparent propogation delay predicted by Newtonian gravity. This nearly complete cancellation effect is directly predicted by the field equations.

Now for the rest of the story: the field equations allow us to predict the slight propogation delay that is not canceled out. When a body accelerates in a field, GR predicts it will radiate away a small amount of energy [i.e., gravity waves]. This would change the orbit of the body by a predictable amount. And the effect has been verified in the orbital decay of binary pulsars [which earned a Nobel prize]. The decaying orbit effect would not be observed if the speed of gravity was infinite. It was another stunning triumph for the much better [than Newtonian] theory of general relativity, and another resounding defeat for the meta-nonsense of Van Flandern.

quark
Thanks guys. Point well understood and I will look into the papers you suggested.

SeaTea
Something more basic.

I'm a novice at all this, and I have a (perhaps) basic question.

If time slows as speed approaches the speed of light, shouldn't light (since it travels at the speed of light) get everywhere instantly (i.e. be infinitely fast)?

One plausible bit of reasoning is as follows:

1) To have infinite speed, a particle would have to have zero mass.
2) A particle with zero mass would not exist.
3) In order to exist, a photon has > zero mass, therefore travels at < infinite speed.

If so, doesn't the "speed of light" actually measure something just less than a perfect trade-off between speed and time?

Also, if 1), 2) and 3) are true, wouldn't this provide an easy way to measure the mass of a photon? Just calculate how much mass something would have to have to slow it down to the measured speed of light?

Anyway, thanks for any thoughts or answers.

SeaTea

reasonmclucus
Would we be able to directly detect something moving faster than light or would it be invisible or "dark"? Might something moving faster than light be only detectable indirectly through an affect that cannot be explained?

Gold Member
ZA said:
The neutrino, which is now thought to have mass, travels at the speed of light too.
No... just very close to it.

SeaTea said:
1) To have infinite speed, a particle would have to have zero mass.
2) A particle with zero mass would not exist.
3) In order to exist, a photon has > zero mass, therefore travels at < infinite speed.
1) Incorrect. To have a speed equal to that of light in vacuum, a particle has to have zero mass.
2) Incorrect. Mass is not a requirement for existence.
3) Incorrect. A photon has zero mass and thus travels at the speed of light (and light, of course, is photons).

reasonmclucus said:
Would we be able to directly detect something moving faster than light or would it be invisible or "dark"?
We detect things moving faster than light every day. The trick is that it has to be through a refractive medium rather than vacuum. 'Cherenkov radiation', for example, is the blue glow that is seen in uranium storage pools and is caused by massive particles exceeding the speed of light in water.

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ZA
neutrino

Are you sure the neutrino travels at under the speed of light? Everywhere I erad, it said it travels at the speed of light. So could you please give me your source for that? thanks.

Gold Member
Sorry, ZA... I can't cite a specific source. It's just something that's been around for decades. The reason that neutrinos were originally thought to travel at c was because they were believed to be massless. Once the Russians hypothosized that they had something like a 3eV rest mass, and it was experimentally verified, then their speed is of necessity known to be less than c.

Staff Emeritus
Check out this paper. It has practically all the references to experiments that have detected neutrino mixing/mass, from the first one at SuperK, to Kamland, to SNO experiments.

http://arxiv.org/abs/hep-ph/0503086

Zz.

Gold Member
Thanks for the link, Zapper. Cool stuff.

Charvell
Chronos said:
the effect has been verified in the orbital decay of binary pulsars [which earned a Nobel prize]. The decaying orbit effect would not be observed if the speed of gravity was infinite. It was another stunning triumph for the much better [than Newtonian] theory of general relativity, and another resounding defeat for the meta-nonsense of Van Flandern.

Frictional aspects within a binary system, (individual stars) tidal forces within the non solid (plasma) system causes bodies to gradually lose orbital stability. Close binary stars would have a major contribution to each other. Like rolling a car with flat tires vs inflated tires. Tidal forces would be proportional to the mass/distance between bodies. Coronal/solar interactions would extend far out from the surface. There's also the electro/magnetic aspects. A rapidly spinning star would make one hell of a dynamo. Any comments?

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flotsam
GR predicts gravity travels at the speed of light. Why the speed of light? What is it traveling through? What is the current speculation as to the make up of space/time and why matter warps it? Is its speed some evidence of the graviton?

sanitykey
Just wanted to say i enjoy reading threads like this, really interesting stuff. I'm also a novice at this, and also have perhaps a very basic question...

Assuming that the universe is expanding below the speed of light (at the moment), what would happen if you were at the very edge traveling at the same speed and stuck your hand outside that boundary?

Gold Member
There isn't an 'edge' to spacetime. It's finite, but unbounded. No matter where you are, you're in the 'centre'. Also, there is no 'outside'.

sanitykey
Thanks Danger, that's really got me thinking such as along the lines of "how are you always at the centre?" the answer is probably somewhere on these forums. I could ask here but i feel i might be deterring the thread too much

Gold Member
Think of it as a 4-dimensional analogue to a 3-D sphere. No matter where you are on the Earth, you're in the middle of the surface area. Keep going in one direction and you'll end up back where you started. I don't think that the human mind is capable of 'envisioning' it other than mathematically.

masudr
Danger said:
...a 3-D sphere. No matter where you are on the Earth...

The surface of the Earth is a 2-sphere.

Gold Member
That might be just a matter of definition, masudr. To my mind, a sphere is 3-D, a plane is 2-D, a line is 1-D, and the universe is 4-D. My apologies if this isn't proper topographic terminology; I've never studied that.

pallidin
Danger said:
'Cherenkov radiation', for example, is the blue glow that is seen in uranium storage pools and is caused by massive particles exceeding the speed of light in water.

True, however, the speed increase is relative to medium and NEVER exceeds the speed of light in a vacuum.
Therefore, the Cherenkov effects are superluminal with respect to the common mode within the medium, which IS LESS THAN C, but ARE NOT superluminal in a vacuum.

pallidin
In other words, the speed of light in a vacumm is approximately 299,792,458 meters per second. The Cherenkov effect DOES NOT exceed that. It only exceeds the speed of light common in that medium, which is about .75c.

Gold Member
Absolutely, pallidin. That's why I specified that it can only occur within a refractive medium. Nothing—unless tachyons actually exist—can exceed c in vacuum. Even in a refractive medium such as water or glass, the individual photons still travel at vacuum-level c. The atomic interactions limit the propogation speed, which can be exceeded by other particles that suffer less interference. (Not a proper scientific explanation, but the best that I can summon right now.)

A sphere embedded in $\mathbb{R}^{3}$ is very much 2-dimensional.
As Daniel correctly points out, the 3D sphere you speak of is embedded in $\mathbb{R}^{3}$ but you only require 2 co-ordinates to describe any position on the surface (e.g. $\theta, \phi$ form a common system). Since there are only 2 degrees of freedom in the tangent spaces of this system, there is no justification in calling the sphere 3D.