On Tue, Feb 18, 2014 at 6:21 PM, John Berry <[email protected]> wrote:
Let's take a pulse of light, some observers on the ground measuring the > time this pulse takes to traverse 2 detectors 1 meter apart, they get a > speed of 299,292.458 meters a second. > I don't think I've ever seen such an experiment. Do you have a reference? I've only seen experiments that use much more indirect means to measure the speed of light. > Then we have have this pulse run along train tracks past a train, they > detect this light pulse which they are moving with, and they are meant to > detect the same speed. > This is an impossibility, except for length contraction and time dilation, > only it is still impossible! > No. Special relativity talks about what instruments will tell us about the speed of light, independent of internal frame. Whatever experiment is used to measure the speed of light provides the same result, within a small error, no matter in what inertial frame the experiment is tried out. Length contraction and time dilation are ideas that follow from this assumption rather than being ideas that are used to explain it. Special relativity says that when you measure the speed of light using instruments from an inertial frame, you will get c, the speed of light. You've introduced time dilation and length contraction to explain this assumption, and in so doing you've gotten things backwards. It is the Doppler effect, consider that if I was shooting at you and moving > towards you, each bullet would have less travel time causing an increase in > the rate at which I seem to be firing bullets, this is the same effect as > pitch changes in horns as cars go by. > In the context of relativity, the "doppler effect" usually refers to the observation of redshift or blueshift, but I don't think that's what you have in mind here. Can you please clarify? Next let's go back to our train and light pulses, if the train is seen to > shrink from the earth frame, then the distance of the meter shrinks so even > though they are moving with the light pulse the stationary observer could > expect their speed of light measure to agree. > But now what if we send another pulse in the other direction??? > I think you're mixing velocity with proper velocity [1], but I can't tell for sure from your description. The observer on the train, which in order to be in an inertial frame must be moving at constant velocity, measures the speed of light to be the same whether the beam is aimed in the direction of the train or against it. This is about measurements. The observer on earth measures light coming from the train to be at the speed of light. All light, measured at the speed of light, *c*. > Now the earth measures the expected rate, sure. > But the train is travelling against the direction, this would cause them > to expect to find the light to be, ahem, superluminal. > No. We would not expect to find the light to be superluminal. We would expect to find it to be at the speed of light, c, because this is the fundamental assumption of special relativity. All of the other stuff, regarding time dilation, length contraction, and so on, flow from this assumption rather than being used to explain it. Eric
