We are not speaking of apparent directions. We are speaking of why it takes light longer to travel from point A to point B when viewed from a moving frame. TIME, DaleSpam, time.
Let's try ping-pong balls bouncing between two paddles instead of light bouncing between two mirrors for a minute first. We can keep velocities low so that we don't have to worry about relativity. To further simplify things let's have this ping-pong game in free space far away from anything else. Lets consider two frames that are initially together with the paddles located on the y-y' axis. The paddles move at a velocity v in the positive x' direction but are stationary in the unprimed frame. Now, lets look at the path of the ping-pong ball. Do you agree that the direction (angle with the x-x' axis) is different in the two frames? Do you agree that the distance is different in the two frames? -Dale
You don't seem to understand, DaleSpam. I figured out long ago, when I first started reading STR gedankins, how they used the observer's rest frame to confuse the issue of time dilation. Do your gedankin as Pete did his last one, place the observer in the moving frame and the ping pong ball/paddles in the stationary frame. All STR's claim there is no difference in which frame is illustrated as the moving frame. When illustrating STR's supposed 'time dilation' based on an invarient 'c', there is a difference in the way the illustration is portrayed. Place the observer in the moving frame, with the path of light between two points in the rest frame, then you have to state light is slowing in the rest frame. I know you know this, so are you trying to confuse the issue to support Special Theory?
I am not trying to confuse the issue, I am trying to find out what the issue is. Is it that you don't understand the idea of transforms between frames of reference? Is it that you don't believe that all frames of reference are equivalent? Or is it that you don't believe that c is invariant? It has to be one of the three and, based on all of the comments about directions, I thought the first was most likely. That is why I asked the two questions (about direction and distance in the two different frames). If you agree with those statements then the issue is one of the two other points. I am not trying to be a pain, I just don't want to argue about something that isn't the key point of disagreement. -Dale
I don't believe all frames of reference are equivalent when the speed of light is calculated based on 'apparent' paths to give 'real' time dilation. I also believe the speed of light is invariant in local co-moving inertial frames. It is not 'really' invariant when comparing one frame with another moving frame. 'c' is defined as invariant by making time and length variable based on an invariant 'c'. Do you believe that 'c' is invariant when a frame is moving in varing gravitational potential? I realize that isn't a 'STR' inertial frame, but gravitational freefall IS a GR inertial frame. An observer on board a craft couldn't tell the difference as no gravitational effects are felt. But it is easy to reference showing that a clock beats faster in lower potential (further from the gravitating object). The Shapiro effect has been confirmed, light moves more slowly through a gravitational field than expected (slows down). Do you believe a meter expands in all directions as the craft moves away from a gravity source? Do you believe there are frames in which a craft is NOT in gravitational freefall, even in deep space?
No, there is no medium here. We're talking about light travelling through vacuum, which is why I mentioned particularly that I wasn't talking about light. I don't intend to bring in any 'ether'-concepts.
2inquisitive, Is this really true? Do you know of a good link to this subject or can you expand on it a little more? I though light was invariint Thanks
Even Special Theory only states 'c' is invariant in an inertial frame in vacuum. It is no secret that 'c' is not invariant in General Relativity due to gravitational fields. Clocks slow in increasing gravitational potential. There is no 'Lorentz contraction in direction of motion only' to offset the time dilation in General Relativity. Slow time without contracting the meter and 'c' MUST decrease. http://en.wikipedia.org/wiki/Shapiro_effect (added link) Here are some quick references from Wikipedia. Read some of the associated papers with the RACE, PARCS and SUMO links I already gave to find out more. Read some papers on the Linear Ion Trap Atomic Clock (LITS) and the linear ion trap frequency standard for more. I don't remember always remember at exactly where and which papers, sometimes many, many pages long, to look for specific cut & paste excerpts. From a short source, Wikipedia: "The time delay effect was first noticed in 1964, by Irwin I. Shapiro. Shapiro proposed an observational test of his prediction: bounce radar beams off the surface of Venus and Mercury, and measure the round trip travel time. When the Earth, Sun, and Venus are most favorably aligned, Shapiro showed that the expected time delay, due to the presence of the Sun, of a radar signal traveling from the Earth to Venus and back, would be about 200 milliseconds, well within the limitations of 1960s era technology. The first test, using the MIT Haystack radar antenna, was successful, matching the predicted amount of time delay. The experiments have been repeated many times since, with increasing accuracy. .... Quote by Einstein "In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlinlited domain of validity ; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)." - Albert Einstein (The General Theory of Relativity: Chapter 22 - A Few Inferences from the General Principle of Relativity)
Stange. I could swear I read "medium" some where in your post at 59 past hour of yesterday, which excepted light. The part I commented on was:
Obviously if you do not believe that the distance measured in one frame is as valid as the distance measured in another then there is no reason to suggest time dilation. If you don't accept the postulates of SR then you have no reason to accept time dilation, length contraction, and the other conclusions of SR. One of your statements confused me. What is a "local co-moving inertial frame"? A co-moving frame sounds identical (except possibly for a spatial rotation) to the original frame. So I don't understand your "speed of light invariance" statement. Did you intend to mean that the speed of light is the same in every direction in the local prefered frame or am I misunderstanding? -Dale
Sorry DaleSpam, I wasn't trying to ignore your question, but it is difficult to answer clearly. I started a post earlier, saw that it was confusing, so I didn't submit it. Then, I had other things to do in my real world. Let me start by stating there are two ways to look at reference frames. First, the coordinate system way such as a Minkowski diagram and Lorentz transforms. Second, a physical way that conforms to 'the real world'. A 'co-moving inertial frame' is the coordinate system method. In that method, you can 'instantly' transform into the co-moving frame, no signal propogation delays between frames and no Sagnac effects or any other discrepancies as when 'physically' comparing one frame with the other. For instance, in the co-moving coordinate method, the Sagnac delay of EM propagation does not come into play, but it does in the 'physical' method. In STR inertial frames using co-moving frames, gravitational time discrepancies between the two frames do not show up because you instantly 'jump' into the other frame on a sheet of paper. I added the 'local' to my 'co-moving frames' because when considering a distant frame near the edge of the universe, the wavelength of the EM radiation has also been 'stretched' about a thousand times its length when it left its source by universal expansion. That would have to be taken into consideration also. I know this is still not clear, but it is the best I can do with my limited physics knowledge. Hope it does help, though.
That's OK, after all, I didn't answer any of your GR questions either Please Register or Log in to view the hidden image!. It's taken me about 7 or 8 years to get to my current understanding of SR, so I anticipate another 14+ years before I can talk reasonably about GR (and just forget ever asking me about QM). I guess I look at different frames just as different viewpoints, different ways of telling the story. Like in our previous discussion about rotating reference frames, sometimes it's easier to describe things from a different viewpoint. But even if it is more difficult a different way you should be able to get the same answers any way you approach it. In any case, I have no real justification about why different frames are equally valid. Basically it is just easier to work physics problems if you don't have to worry about drawing the lines exactly right to match some prefered local frame, but there is no real reason that the universe should make my physics problems easier. On the other hand, I think that the fact that the testable conclusions seem correct at least makes it reasonable to accept the postulate. -Dale
I guess you haven't been following the entire discussion. The example I gave was applicable to waves which travel through media. But this <I>need not be true</I> in the case of light. Therefore, I can't assert that this exactly could be the reason for invariance of light-speed. When I said that the discussion did not involve a medium, I meant the original discussion, in which light was going about in vacuum.
