Vilas Tamhane

Yes the detector is in the spaceship.

I did go through your earlier post and I tend to agree. I could not access the paper you have mentioned above. But your views are unorthodox and surprisingly I don’t see opposition to your views. Length contraction is a contraction of spatial coordinates and so I take it that length contraction is space contraction. Even if length contraction is assumed to take place, world of SR becomes quite weird. In fact to prove SR we should have an experiment in which MUTUAL time dilation is observed and this is impossible because in any such experiment we will have to bring moving clock at rest to compare results. I think there is something wrong with this mutual changes taking place. In experiments not only we seek one way change but we know in which frame the real change happened. We tag the frame that is accelerated. I do not find anything wrong with this. On the contrary I suppose we always have history of acceleration in mind when we talk of motion. Disturbing idea is that SR results are considered real. If the spaceship A moves with a velocity v1 and another spaceship B with v2, then their views about the universe are different and even if we take these as measurements, these are not of much use unless they really take place. Obviously, in reality, space cannot shrink differently when observers move with different speeds and in fact it cannot get shrunk at all, not only for it being empty space but also because a moving object cannot influence it. And if in reality space does not shrink, time cannot dilate. In spite of the correctness of Galilean relativity and the equivalence of inertial frames and in spite of the fact about relative nature of velocity, in real world we always seek to know past acceleration. And to my mind there is deterministic value to the velocity of the object and there is a definite arrow in the direction from smaller to the bigger objects. For example we do not say that (though we see platform moving backward), the train is stationary and the earth is moving. Because smaller object cannot contain bigger object and so it is always the smaller object that is accelerated to move in the frame of bigger object. So if the arrow starts with a spaceship, it directs to earth, then to solar system, then to Milky Way and then to universe. Possibly this points to the absolute nature of the universe which you mentioned in your post. A miniscule spaceship will observe (assuming correctness of SR) that the whole universe is shrunk in the direction of its velocity and it contracts and expands with change in the direction of the spaceship. This is not possible in reality and this certainly does not happen and if space does not actually shrink then time cannot flow at a different rate. So the basic assumption that moving clock runs slow could be wrong. I believe that there must be some other reason, other than SR, that slows down decay rate of unstable particles.

If #3 is correct then in the spaceship light pulse will fall back and path of light will be diagonal backward. Interestingly, for the observer in the ship as well as for the observer outside the ship (at rest), path of light will be same. It will be diagonal. This will make everything topsy-turvy. I am not questioning experimental data. It can fit to some other theory not yet known. Even if a particular apparently erroneous theory is used to explain experiments then I do agree that the theory should be used. At the same time I would insist that inconsistencies in the present theory must be examined. This is the only way which gives birth to new theories, more correct than the existing ones. And in SR there are many inconsistencies.

Postulate can never be sacrosanct. I think the 3rd proposition is true, since all observed orbits of binary stars follow Kepler’s law. Thus once light is emitted in particular direction, it will not have any relationship with the velocity of the source. I do not know of any fourth observed possibility.

We don’t go anywhere and remain in the spaceship. We of course don’t know relativity yet because we are stuck up at the basic postulate.

Your reply is not clear to me. All inertial frames are equivalent for mechanical laws because all material bodies acquire velocity of the object in which they are contained. If this is not the property of light then it is bound to behave in a different way in a moving system. One and only one criteria for light velocity to be same in the moving system is that it should behave the way all other material particles behave. I am not conversant with lasers. In any case, based on any observation, light can be proved to possess only three types of behavior. 1. Light behaves like material objects. In this case its velocity cannot be constant ‘c’. 2. Velocity vector of light is constant ‘c’ but it moves along with the source. 3. Once a pulse is propagated in a particular direction, it keeps the direction and moves with velocity ‘c’ in that direction. Any of the three propositions go against Einstein’s postulates.

Yes. I am considering pre-relativistic validity of the postulate. In any case, there cannot be time dilation for the observer in the ship. Once more note that I am not considering any frame other than the ship.

No. Time dilation and length contractions are the effects which are based on the postulate and I am considering validity of the postulate based on the nature of light velocity

I am still in doubt about the velocity calculations, the traveler who goes to Alpha c at a speed of 0.866c would make. His clock, after reaching the destination reads half of the time in clock at rest. Using his clock and distance of rest frame, he will conclude that he was traveling at the speed of 1.732c. This cannot be because he has traveled less distance, which is recorded on some instrument. However this instrument cannot directly measure the distance. It will be dependent on time and velocity. Velocity cannot be directly measured. In other words, there is no instrument which can directly measure the distance traveled. In addition we know that there in so experiment that has proved length contraction. Therefore the only instrument he has is clock (and also the clock at rest) and the known distance in the rest frame. Earlier I am told that this is frame jumping. This may be true while the frames are different. Here we are facing some real situation at hand.

To my previous question, whether light pulse acquires transverse velocity of the source, there was no answer. Therefore in this post I will assume both the possibilities. Consider a spaceship moving with some velocity, which we will consider as zero. Spaceship is in the frame O1. Observer in the spaceship arranges a source that can direct a light pulse in the direction perpendicular to the length of the spaceship. He arranges a detector D at the top to receive the pulse. Spaceship is now accelerated lengthwise to the new velocity v, which the observer detects on his instrument. Now he is in frame O2. He again sends the pulse. Will the pulse miss the detector? In this thought experiment only the view of the observer in the ship is considered. Case 1: Light pulse does not move along the source. In this case the light pulse is bound to miss the target. It will fall back and its path will be in the diagonal direction, inclined backwards. So in order to hit the target, the observer will have to shift the target to left and by measuring the shift, he can measure his own velocity (w.r.t. frame O1). Case 2: Light pulse moves along with the source: If the pulse is ballistic, then like any other material object, it will have velocity greater than ‘c’. Since this is not possible, we assume that it is ‘c’. In this case the pulse will move diagonally forward and its velocity will be ‘c’ in the diagonal direction. Therefore its vertical component will be lesser than ‘c’ and it will go on reducing with the increasing speed of the spaceship. Although observer in the spaceship will not notice horizontal component of the light pulse, as he too is moving along with it, he will find that vertical velocity of the light pulse is reduced and with this light clock he will be able to measure his own velocity. According to Einstein’s postulate, whatever might be the velocity of the spaceship, light pulse will always hit the detector with a velocity ‘c’. However this postulate can be assumed to be correct only if we disregard cases 1 and 2. However these cases are most fundamental and are based on the observation of velocity of light. So should we assume that Einstein’s basic postulate is wrong?

Yes, I can see that I goofed up. If I draw the diagrams which I have suggested then, S1 and S2 will arrive simultaneously but would age differently and so the time dilation equation holds good.

I have gone through this link earlier and was not satisfied. There is a following paragraph in it. “In contrast, stellar aberration is independent of the distance of a celestial object from the observer, and depends only on the observer's instantaneous transverse velocity with respect to the incoming light beam, at the moment of observation. The light beam from a distant object cannot itself have any transverse velocity component, or it could not (by definition) be seen by the observer, since it would miss the observer. Thus, any transverse velocity of the emitting source plays no part in aberration. Another way to state this is that the emitting object may have a transverse velocity with respect to the observer, but any light beam emitted from it which reaches the observer, cannot, for it must have been previously emitted in such a direction that its transverse component has been "corrected" for. Such a beam must come "straight" to the observer along a line which connects the observer with the position of the object when it emitted the light.” In the above, it is not clear what the author has to say about emitted light. At some other place I got the explanation that the photon emitted in the direction of line of sight are not received by earth as they have component of velocity of the star. They will of course miss the observer. However rays emitted at backward angle will reach the us. If the second point is true, we have some problem. As the star goes on acquiring horizontal component of velocity w.r.t. line of sight, then we will receive the light rays at an angle (in contrast to what the author of the link says), in that case aberration seen by us throughout the year, will not be uniform. If we are to receive light in vertical direction (neglecting aberration) then our problem becomes bigger. Velocity of this component of the light will be less than ‘c’. In this case, taking time that takes for light to reach us, we will not be able to see uniform motion of star or in other words, motion of star will appear not to follow Kepler’s law. In the experiment with aberration of light it is interesting to observe that all the way we assume that light received is vertical. Aberration then occurs due to velocity of earth with respect to vertical direction. Since aberration angle changes uniformly, it is safe to assume that we do receive light vertically. No where it is made clear if the direction or velocity of light changes when motion of the star is horizontal to line of sight. Therefore it all points out to the fact that in whatever direction light is emitted by the star, it will not acquire any component of the velocity of the star. It is rather unacceptable that light will not acquire longitudinal velocity of the star but it will acquire transverse velocity. Star propagates light in all directions. But if we have a well collimated light pulse then we can easily test, even in the laboratory, if the emitted pulse does move along the source or not. Your first sentence is very important but what does it convey? Suppose light acquires velocity of the source, then transverse radiation will move along the source. Thus if light is directed upwards by a source moving horizontally and if light moves along with the source, then actual path of light will be diagonal and along this diagonal path, velocity of light will be c. This does not violate principle of isotropy of light but it makes light change its direction of propagation. I am considering light not as a wave but as a photon packet. It is but in the link there is a mention of velocity of light when star is in transverse motion to the line of sight. To me, author of the link appears confused. He does not state in very clear words, direction and speed of photon emitted by the star. That velocity of light emitted by source is always ‘c’, is known to every body. What is not clear to me is, when light is emitted perpendicular to the motion of the source, if light changes direction.

There are many experiments that prove that light emitted by the moving source does not acquire velocity of the source. Only one I know is that of binary star. However in this it is proved that when the star moves in the direction of our sight or away from it, velocity of light is not c+v or c-v. What is not given is that when the star moves across the line of sight, still it will not acquire velocity of the source. In short photons emitted by the star in the direction perpendicular to the motion of the star will not move along with the star. If this was not the case then we would miss the photons and we will not be able to see the star when it is moving perpendicular to the line of our sight. Am I correct? Are there links which bring out experimental observations?

Thanks for your efforts, but my diagram suggested was different. I don’t know yet how to transfer images. S1 and S2 travel in the same direction. Turnaround points P1 and P2 are on the same horizontal line and so their return world lines meet at the same point P on the time axis of M. I have drawn the picture but I don’t know how to transfer it.

I think there is a contradiction. You say that measurement of the velocity of the traveler is valid and there is no limitation to the velocity of light. What it means is that the traveler was cruising with a speed more than that of light. In any case, in relativity what is common and same is the measurement of velocity v of the traveler. Both the observers agree that velocity ‘v’ of the traveler is same. If traveler uses his clock, then not only that he finds his velocity to be more than light but there is disagreement about the velocity v.

Interpretation of the time dilation equation is not same as that of length contraction. dT’ is the proper time in the moving frame and dT is not the time duration in moving frame as measured by the stationary observer, but it is the time that is accumulated by the stationary observer. This creates problems. Recall twin paradox. If dT’ is 2 years and if Gamma=2, then on arrival, S1 finds that stationary clock has ticked 4 years. Remember that each observer finds clock in the other frame as running slow. So on completing the voyage the twins should not find any time difference. This difference is explained by the turnaround of the twin. If you take a look at the space-time diagram then you will find that 3 years of Dave are 3 years for Bob but when Bob turns around there is a jump in lines of simultaneity. According to traveling Bob, clock of Dave suddenly jumps to higher value of time. Thus bob’s clock doesn’t run slow, it is Dave’s clock which runs fast. And this interpretation creates problem. Draw vertical line from O as time axis of a stationary frame and mark an event P on it. At the mid point M, draw horizontal line and mark two points P1 and P2 on it. O-P1-P is the world line of S1 and O-P2-P is the world line of S2. Note that both S1 and S2 meet stationary observer at P and at the same time. However if you draw lines of simultaneity for S1 and S2, you will find that there is a small jump for S1 (1 year) and larger jump for S2 (2 years). Thus according to the present convention, age of the both of the twins will advance by 2 years but S1 will find stationary clock advanced by 1 year and S2 will find that it is advanced by 2 years. Though, according to the diagram they meet at a single point P, indicating that the clock of stationary observer has ticked same for both. Clearly this cannot be the case and clearly age of S1 and S2 cannot be same. Is there something wrong with the equation? Should it be just like that of length contraction, as, dT=dT’/Gamma ……….2 In the equation 2, dT is not the time ticked by the clock in stationary frame. It is the time duration of the moving observer as measured by the stationary observer.

Equation for time dilation is, dT= dT’ *Gamma From a space station M, two spaceships depart for a space voyage and they turn around to return at their mid periods. Gamma for spaceship S1 is 1.5 and it sets for a total time of 2 years as measured on its own clock. Gamma for spaceship S2 is 2 and it sets for a total time of 2 years as per its clock. On their return S1 should find that M-clock shows 3 years and S2 should find that it reads 4 years. How can it be?

I don’t get you. Can you elaborate your statement? I thought case is very simple because now earth and the traveler are in the same frame.

Yes, you made a very good point. But now both the clocks are in the same frame. Distance is back to normal and so according to earth’s clock speed of the traveler was 0.866c and according to traveler’s clock it was 1.7c

I don’t think that the question raised by tar is replied satisfactorily. Traveler on reaching Alpha C stops and finds that original distance in earth’s frame is restored. According to tar traveler is fully justified in using his time and the distance to find out the velocity with which he traveled. It is about 1.7c. You may say that once the traveler is in the earth’s frame, he should use coordinates of that frame. But it may also mean that calculated time in the moving frame is wrong and bears no real significance. There is one more thing I wish to know. When traveler stops at Alpha C, what happens to the space-time diagram? Coordinates of the traveler’s frame coincide with those of earth’s. Distance that was half for traveler goes on increasing till it is 4.5 light years. This is definitely the case as the contracted distance cannot remain hanging somewhere. After all it is the same space for both the frames. If the space axis rotates till it coincides with the space axis of the earth then time axis should also rotate and coincide with the orthogonal time axis of earth. As traveler starts decelerating to stop at Alpha C, he will find that the distance goes on expanding till it becomes 4.5Ly. Similarly his clock should go on running fast till the traveler’s clock and stationary clock show the same time. What is applicable to distance should also be applicable to clock.

This is true and for this reason a scientist should be an inquisitor and not a worshiper. I get feeling that something is wrong at the higher level. Not being a scientist myself, I am not aware if there is any rot at the top and extend of its spread. To give one single example, popular journal ‘American Journal of Physics’ clearly mentions that any paper that goes against established theories will not be accepted as this can be reviewed only by experts. This is wrong. A paper should be valued on its merit and in fact critical papers should be valued with greater respect, as it is far easier to add to knowledge which is already known.

I am afraid it doesn’t help. I call it motivated advocacy. You can always find such descriptions in the books on occult science. Things come first. Measurements later. Bertrand Russell would hate such a paragraphs. It conveys nothing meaningful.

That is what I said in my previous post. Nevertheless many thanks for your elaborate explanation.

It is difficult to agree with you. I believe that if a theory in physics is correct then it should be possible to explain its basic verbose part to a layman of average intelligence. Mathematics is just a tool for making calculations and predictions. Therefore common notion that one can understand a theory only if one understands mathematics is wrong. Any way, SR also deals with space-time. Question is, ‘Does space contract’. Answer that this is only a measurement and only a perspective of an observer in a different inertial frame’ is not acceptable. Time is found to ‘really’ dilate. Therefore with or without experiment we can state that length contraction is also not apparent. You cannot say that it is just space-time geometry. Geometry is just visualization of mathematics. This geometry is applied to real world and when we do so we find that space contracts. This is not just semantics. So the question ‘how space can contract’ remains.

Yes and this is automatically included when we use Lorentz equations and not length and time segments in which simultaneity is eliminated. Thanks.