Hammar Posted March 8, 2009 Report Posted March 8, 2009 You are traveling away from a clock at the speed of light. While continuing to travel at the speed of light you look back at the clock. What do you see? Quote
InfiniteNow Posted March 8, 2009 Report Posted March 8, 2009 Hello Hammar, The short answer is that your question is meaningless. Nothing with non-zero mass can travel at the speed of light, so any answers will be based on speculation alone. The only thing which can move at the speed of light is a massless photon. The photon can never stop moving, it is (by definition) ALWAYS moving at the speed of light, and for this reason it does not have a valid rest frame (it is never at rest) by which to measure other objects. If we speculated that a photon DID have a valid rest frame (which it does not, but let's just pretend), then it might be that a photon is everywhere and at everytime all at once. This is because of the infinite time and length dilation it would undergo relative to the other objects moving at sub-luminal speeds. Again, though, this is merely a thought experiment, as a photon can never be at rest and the "reference frame" of a photon is undefined (much like dividing zero into some other number). The thing is, when you suspend the laws of physics (like your question implicityly requires), absolutely anything becomes possible... you would now be dealing with fiction instead of reality. Anyway, good luck in your studies. Ask more questions if you have them, and hopefully you will find compelling answers to sate your curiousity, as well as new questions to feed it. Quote
CraigD Posted March 8, 2009 Report Posted March 8, 2009 You are traveling away from a clock at the speed of light. While continuing to travel at the speed of light you look back at the clock. What do you see?InfiniteNow’s response,The short answer is that your question is meaningless. Nothing with non-zero mass can travel at the speed of light, so any answers will be based on speculation alone.is correct. Despite being about the physically impossible, however, answering this and similar questions through a straightforward application of the theory of special relativity can be educational, and fun, even if what it describes is impossible. So ...What do you see? You see that the clock C1 is “frozen”, showing zero change while a clock C0 moving with you changes at its usual rate. This is due to time dilation. If you are “seeing” C1 with light, the frequency of that light is zero, having been redshifted infinitely. As that light has zero energy, you can’t possibly detect it, so can’t really “see” C1 at all. You also see the distance between you and the clock as zero, due to length contraction. The entire universe is contracted to a zero length behind and in front of you in your direction of travel. The universe is effectively no longer very 3 dimensional, but nearly reduced to 2 dimensions. The mass of the clock is infinite, due to mass dilation The force you experience due to the gravitational interaction between it and you, or anything directly in front or behind you, is infinite. Because only an infinitesimal part of C1 and the universe directly behind and in front of you is directly behind and in front of you, these effects apply only to that infinitesimal part. Other nearby parts are nearly infinitely time, length, and mass dilated/contracted. An observer moving with C1 sees exactly the same thing looking at C0. In short, were you able to see these impossible things, they would look very, very weird! Quote
Hammar Posted March 8, 2009 Author Report Posted March 8, 2009 The short answer is that your question is meaningless. Nothing with non-zero mass can travel at the speed of light, so any answers will be based on speculation alone. Forgive my ignorance but I was unaware that it has been proven that travel at the speed of light and beyond cannot be achieved. I was under the impression that all we have are theories which claim that to be the case. Are you not speculating yourself? You see that the clock C1 is “frozen”, showing zero change while a clock C0 moving with you changes at its usual rate. This is due to time dilation. How could you see anything behind you? If you are traveling at the speed of light is it not logical to assume that none of the light from behind you could ever catch up with you. Based on available observations that would seem to be the case. For example, I'm traveling in a car at 60 mph and another car is traveling a short distance behind me at 60 mph. The car behind me won't catch me until either it speeds up or I slow down. Quote
sanctus Posted March 8, 2009 Report Posted March 8, 2009 Forgive my ignorance but I was unaware that it has been proven that travel at the speed of light and beyond cannot be achieved. I was under the impression that all we have are theories which claim that to be the case. Are you not speculating yourself? How can you prove something you cannot achieve? No more seriously there were experiments (see for example: this site) which measursed the increase in mass. Time dilatation which is also predicted by SR I am even more sure that it has been verified but experiments many,many times; every particle accelerator uses it for example... So we have more than only theories I am pretty sure that it is no speculation. But then yeah, maybe one day someone comes up with a new theory where things change and relativity was just a good approximation to it in its applicability field (just like Newtonian mechanics works perfectly fine for slow velocities) Quote
Hammar Posted March 8, 2009 Author Report Posted March 8, 2009 How can you prove something you cannot achieve? How do you know you cannot achieve it? Time dilatation which is also predicted by SR I am even more sure that it has been verified but experiments many,many times; every particle accelerator uses it for example... I've seen the experiments using Cesium clocks taken on flights and am aware of the corrections needed for GPS satellites. In both cases I believe that the prediction of relativity is always incorrect. In those experiments the traveling clock runs faster than the stationary clock. The explanation is that the gravitational effect on time is greater than the effect the increase in speed has on time which is why the net effect is that the traveling clock runs faster. The problem I have with these experiments is that I don't believe they are extensive enough. Take for example the airplane experiment, there's no clock that was simply taken to a stationary altitude equivalent to the altitude the aircraft was flying at. If a cesium clock at an equivalent stationary altitude shows the same error as the clock in the airplane then what? If the high altitude stationary clock shows the same time as the sea level clock then what? Could altitude itself cause some of the errors? How about radiation? How about temperature, was the clock in the airplane held at the same temperature and humidity as the clock at sea level? What abount interference from systems onboard the aircraft? The experiments I have seen so far aren't very scientific. Can you point me to articles on the experiments involving particle accelerators? Quote
sanctus Posted March 9, 2009 Report Posted March 9, 2009 Not really, because: take the muon particle it has a (proper) life time of a fraction of a second, but in accelerators we can measure them because the time it takes them to decay seen from our reference frame is longer. It is just the fact that we can measure them that makes the predicted time dilatation right.Most likely there are some experiments on this but I do not know any. Quote
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.