On Wed, Feb 19, 2014 at 9:25 PM, Eric Walker <eric.wal...@gmail.com> wrote:
> On Tue, Feb 18, 2014 at 6:21 PM, John Berry <berry.joh...@gmail.com>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. > Are you arguing that this is impossible? This is a thought experiment so it only has to be theoretically possible to make such a measuring instrument. I would consider it obvious that such an instrument is possible. > > >> 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. > No to what? > Special relativity talks about what instruments will tell us about the > speed of light, independent of internal frame. > Which are? What is wrong with the instrument I suggested, how could you discount them as impossible? > Whatever experiment is used to measure the speed of light provides the > same result > So why ask about the instrument if all are the same? > , within a small error, no matter in what inertial frame the experiment is > tried out. > So? You see you say 'what ever frame' but can you point out an experiment that measures 2 different inertial frames for the same light at the same time? The fact is that small differences are observed and small differences are all that would occur with an aether that is entrained. But we do not need to bother ourselves with what has and has not been observed. We can concern ourselves with what is possible and what is impossible due to being logicically consistent or inconsistent. Length contraction and time dilation are ideas that follow from this > assumption rather than being ideas that are used to explain it. > It is used to explain it in every book I have read on Special Relativity. It would be odd to say that the speed of light is always the same from all reference frames despite it being illogical and impossible. If you are saying that this in not explained by length contraction and or time dilation then you are saying it happens without any established explanation. Would an explanation that it isn't the same speed if a detector does not drag the reference frame make more sense than it is the same with no possible way of reconciling this with logic? Fact is that Special Relativity is based entirely on the idea that the speed of light is constant based on distortions of space and time. That was it's selling point, it explained the consistency of the measured speed of light. > 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. > No, I do not! First the speed of light was measured more constant than expected in the earth were moving through the aether. Then length contraction and time dilation were considered in these specific experiments due to motion through the aether. Finally Einstein proposed that length contraction and time dilation as a transformation of space and time in a given reference frame could explain the difference, and it does for half the problem, just not the other half. > 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? > It is the same thing, it does not matter if the shift is the shift or light colour, or the shift for frequency of sound, the frequency of firing a bullet or many other examples. In all cases it has to do with the distance from a source and a receiver increasing or decreasing causing an apparent change in the rate of that thing being recieved. There is however a big difference in that the slower the object (bullets, sound, light) travels the greater the shift with a given velocity. Lets say I throw slow balls at you as I move towards you, I throw one a second, but it takes several seconds for each ball to get to you, if I am half a second closer to you each ball I throw, then you receive one ball every half second despite that I am only throwing one every second, this can't continue of course because I will eventually pass you. If I now throw balls at you as I move away I am still throwing one a second but each ball is a half second further away, so you get one every 1.5 seconds. The same is true for light or radio communication, if you are observing the rate of time on a spaceship leaving earth you see it slow down as it moves away at high speed since each second the time it takes light to get to earth increases by one second. This is the same Doppler type effect but with an apparent distortion of time, those on the space ship look at earth and see the same effect, time on earth seems to be running slower. This is the argument that SR uses to say that time can be seen to slow down in other reference frames, that both reference frames seem somehow equal, that both are somehow experiencing time faster than any other frame. > 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. > You can't just say it is so. We all understand that SR claims the speed of light is constant. What I am asking is how can it be measured to be constant? One possibility that I have never heard anyone propose is that light travels at different rates in different reference frames, but in such a manner that it is consistent from that frame, this has the curious effect that the same light could be observed to pass twice if the observer changes reference frame once the light event passes in their initial frame. Or miss it all together by moving to a reference frame it would have passed in before it hits in your initial frame. The other thing I am arguing is that time dilation being somehow observed to be equal between 2 reference frames is not possible when communication between the frames is perpendicular to motion and therefore without the effects of Doppler effect on observed time rate. 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. > According to SR, no we wouldn't. But if what was moving was anything else including a particle moving at almost the speed of light. If it was anything that can possibly be understood we would. Right now you are using circular reasoning that seems very much like arguments for belief in God. You can't just say the speed of light is always c because you say so and that is that. Thought: The speed of light is seldom C in practice and can be lowered to a snails pace, would you still argue that all observers would measure it the same once slowed significantly? Imagine 3 identical light speed measuring devices, each much as explained previously, 2 light sensors and a clock, this could be easily rigged, making it work well at easily obtained speeds might require both a highly accurate clocks and distances exceeded 1 meter between the 2 sensors, but this is a thought experiment so none of that really matters. Now one is stationary relative to earth, one is moving in one direction and one in the other, but all 3 measuring devices are aligned the same along a track. The sensor designated A that light hits first are all set to be lined up at the moment that light hits, so the clock on each reference frame starts at the same instant from the same light event. Now there are 3 B sensors, one is moving toward the light source, when the light hits this sensor will be before the it hits the others (and all observers would agree with that) since the B sensor on that one is closer (less than 1 meter), the light does not have as far to go to hit it. Next the stationary sensor B that is still is next as sensor be is still 1 meter away. And finally the B sensor on the frame moving away will be hit last as the sensor if simply further away. Now you can find books of SR that will argue that contracting of space and dilation of time can make the frame moving away read the same as the stationary one, this is easy to see. but these effects would also apply to the one moving closer, indeed there could be a second pulse going from B to A also. Consider also that if we wished, the 2 moving reference frames could be stationary relative to the earth frame when censor A is triggered, they could move and stop again before censor B is triggered, here you can appreciate that the B sensors are closer and further away to the light pulse than the one that didn't move. This removes and possibility that length contraction could explain anything, now we are left with some censors closer and some further, all should measure the same according to SR but this is an impossibility unless you arbitrarily speed time on one reference frame and slowed it in the other, but is there were light pulses also going from B to A also, then even this degree of selective thinking could not get things to line up. You can't just say that the speed of light is the same, you must explain how that can possibly be true. If no explanation exists then it isn't, and the experiments are merely saying that light travels with an entrained medium that the experiments are all entraining. 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. > Light travels at a constant velocity isn't a theory! It is a counter-intuitive assumption or experimental result. SR is the theory that the (flawed) observation can be explained by transformations in space and time (length contraction and time dilation). Without an explanation of how it can be, then it is simply an absurd impossibility that indicates that experiments to detect drift have been insufficient to do so. John
