light at more than speed of light?

kabtn05 · Mar 15, 2005

Hi. I probably can find this answer in your posts but my time online is somewhat limited. I thought of something this morning that I've waited all day to ask. OK. If two light-emiting objects (stars?) are moving towards each other at a speed greater than light, does the light take on new properties? And, would time be affected? (The reason I ask is because I recently learned that if they're moving apart faster than the speed of light, they will not see each other.) Thanks.

yevaud · Mar 15, 2005

No. They'll be blue-shifted, which means each photon increases it's frequency. This is to say, the light received on each planet will be shifted towards the blue end of the spectrum. But, no, it can't move faster than light. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

dark_energy · Mar 15, 2005

Nothing can travel faster than the speed of light in the first place... <div class="Discussion_UserSignature"> </div>

yevaud · Mar 15, 2005

Gee, you think? <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

heyscottie · Mar 16, 2005

There really isn't a scenario where two stars can move CLOSER together at superluminal speeds, because universal expansion works in one way only -- outwards. For the sake of this post, however, we'll assume that the universe starts contracting faster than the speed of light, and that there are now galaxies coming at us faster than the speed of light. Here's what we'd see: Nothing. At least we wouldn't see anything until after the light source passes us. The light would build into a shock wave, similar to a sonic boom. We would then have an incredibly bright flash, and then nothing again, as the trailing photons are red-shifted into nothing again. This effect has actually been observed in various media. It IS possible to move faster than the speed of light in certain media, such as water. Light slows down, but the speed limit remains the same, and particles can still travel at the rate light travels in a vacuum. Particles moving at these "locally superluminal" speeds give off shock-waves similar to what I described. The effect is known as Cherenkov radiation, and a quick google for this term will turn up boatloads of information for you. Scott

yevaud · Mar 16, 2005

There really isn't a scenario where two stars can move CLOSER together at superluminal speeds, because universal expansion works in one way only -- outwards. Huh? That's dead wrong. Of course objects can have velocities and magnitudes towards each other. Ever seen pictures of one galaxy "consuming" another? If what you said is true, then that could never happen. (to clarify this, I DON'T mean that objects can move in a superluminal fashion. I'm arguing against the other point) It IS possible to move faster than the speed of light in certain media, such as water. Light slows down, but the speed limit remains the same, and particles can still travel at the rate light travels in a vacuum. Particles moving at these "locally superluminal" speeds... You completely misunderstand the mechanism of Cerenkov radiation. It occurs when charged particles pass through a dense medium (such as water) at a velocity that exceeds the speed of light in that medium. The charged particles polarize the molecules of water, knocking them back to their ground state. The lost energy is given off in a predictable output of photons. That's Cerenkov radiation. And at not any point does any particle or photon travel "superluminally." The limiting factor is the speed of light, not the medium. It's not "superluminal," in any context - locally or otherwise. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

Saiph · Mar 16, 2005

Heyscottie: Good post, correct through and through. Sorry Yevaud. Yevaud, heyscottie describes them as "local superluminal" velocities because: <blockquote>In reply to:<hr />velocity that exceeds the speed of light in that medium<hr /></blockquote> The local speed of light is slower, thus the object going faster is going faster than the local light, so it's valid to call it a local superluminal speed. He also states in his post that the particles are not going faster than the universal speed of light "c". Btw, that quote, is from your post. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

yevaud · Mar 16, 2005

You'll note that he said ...and particles can still travel at the rate light travels in a vacuum... - Which is clearly wrong, since nothing with mass moves at the speed of light. And he describes Cerenkov radiation in terms of the particle trying to still move at the speed of light in a vacuum. It certainly does so in the context you mentioned ("locally superluminal"), but not as he stated, which is why I took exception to it. Otherwise, I suspect that I've heard too many people saying "can too travel faster than light," so I'm getting a leeeettlllee touchy on the subject. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

Saiph · Mar 16, 2005

that comment actually is at odds with the rest of his post, so I'm willing to bet that it's a slip up, or a bit more vague. Sure, particles can't go "c" in the medium (but close of course), but they are still only bound by that limit, not the new, slower, speed of light. I think that's what he meant. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

yevaud · Mar 16, 2005

Understood. It seemed to imply that a particle will be moving at the speed of light (C) in the medium. That's why I beefed it. If he had said "photon" instead, then np, but photons don't cause Cerenkov radiation...charged particles do. Etc... <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

heyscottie · Mar 16, 2005

Yevaud, Saiph: I did not mean to say that particles can still travel "at", but rather "nearly up to" the speed of light in a vacuum. It was indeed a slip-up. The point was that they are travelling faster than light is travelling in some particular medium, and to draw a parallel that something similar to Cherenkov radiation would happen with a light emitter closing on my superluminally. Thank you both for your comments! Yevaud: As far as galaxies or objects moving together goes, I do not intend to imply that this does not happen -- only that it does not happen at superluminal speeds. Scott

yevaud · Mar 16, 2005

S'ok. Sorry I seemed to jump on you. Didn't mean to...I think I've (lately) been dealing with the likes of Extrasense and such, waaaayyyyy too much. It's making me prickly. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

heyscottie · Mar 16, 2005

Not a problem! You are a good addition to this board, by the way. It's good to have yet another person who generally knows what he is talking about, is willing to learn new things, and can refute some of the wackiness around here. Scott

kabtn05 · Mar 16, 2005

Thank You All. We learn a lot from you. What got me interested is this webpage: http://www.kettering.edu/~drussell/Demos/doppler/mach1.html and the rare experience of actually seeing sound shockwaves. So I thought if the universe expands at speeds greater than light, and if it curves back on itself, we'll never see it coming then. Will we? It seems like it would affect time too. Thanks for answering these posts!

newtonian · Mar 17, 2005

kabtn05 - Why would you think our universe curves back on itself? The balloon model? Current evidence of expansion fits better the model at Isaiah 40:22 - a stretching fabric of space similar to a stretching fine gauze. Many astronomers have switched to similar flat models for our universe - i.e. not curving back on itself. However, String theory does posit dimensions and membranes with various shapes including curving back on themselves. There are also recent models that explain how weak gravity is with these dimensions, gravity leaking into these dimensions. Alternately, there is a recent model indicating gravity is coming from other dimensions, and that this interaction causes the acceleration of our universe when great distances, and hence other dimensions, are involved. I have also considered, as astronomer Loeb has also, that we may interact with another universe that is beyond our visibility horizon. Your question has to do with light cones. If we are expanding towards another universe faster than light, or if another universe is approaching or even already interacting with our universe at faster than light, then we would not see it yet. In short, light cones do not always overlap at a specific reference point, e.g. earth as a reference point. Therefore we would not see another universe approaching us FTL until the light cone of the other universe reaches our reference point. I am not aware of any reason why another universe could not approach ours at FTL speed. We simply do not now the speed limits in a much larger universe which may contain not only our universe but also many other universes, as may be alluded to by the phrase "heaven of the heavens."

exevien · Nov 6, 2007

http://archives.cnn.com/2000/TECH/space/07/20/speed.of.light.ap/ I found this using google. Seems relative to the question.

usn_skwerl · Nov 6, 2007

and to go the other way..... http://www.hno.harvard.edu/gazette/1999/02.18/light.html <div class="Discussion_UserSignature"> </div>

origin · Nov 6, 2007

Hmmm, sounds like a way to earn a drink in a 'bar bet'. "I bet you I can throw a baseball faster than the speed of light!" <div class="Discussion_UserSignature"> </div>

vandivx · Nov 6, 2007

"Seems relative to the question." its 'relevant' to the question <img src="/images/icons/smile.gif" /> and this is very old thread vanDivX <div class="Discussion_UserSignature"> </div>

exevien · Nov 6, 2007

Does that mean if we slow the light down, that we would actually see things at a slower speed than that is really happening? If so, does this mean the ever so popular cloaking ability via so many fancy pancy space tv series?

vastbluesky92 · Nov 7, 2007

If you slowed light down you it would take longer to get to you, so you wouldn't see an event until longer after it happened than normally. You wouldn't see it in slow motion, if that's what you mean. <div class="Discussion_UserSignature"> --____________________________________________-- Don't be too hard on me...I'm only in PHY 1010

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