1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about SR

  1. Aug 20, 2003 #1
    Suppose we have 2 inertial systems. We know exactly, that atomic clocks of equal type run at 2 times the different rate on them. Without asking how we know that, assume that this is true.

    What can we say about these systems without any more data?
    Seems we can say they have relativistic velocity relative to each other. Seems we can say about their size dilation. We can find mass ratio.

    If we knew time it takes by either system's clock to transmit and receive signal from other, we could tell their distance. If we knew how time for signal travel changes, we could deduce if they approach, depart, and approximate angles of motion.

    Suppose, it take 2 years to transmit and receive signal measured by clock of 'faster' system. What else can be said without resorting to direct 'visual' observation?
    Last edited: Aug 20, 2003
  2. jcsd
  3. Aug 20, 2003 #2
    I've been dabbling with the same question for a long time. If clock rates can be transmitted, then it's possible to compare clock rates between two frames. Using the relativistic transformations, it's possible to calculate the relative speeds between the two frames - the values will differ! The postulate that all motion is relative is relative is violated, even in SR!

    I could never get any serious physicist to consider this. They all say "SR works, and that's the end of it!". But of course SR works; the earth is moving very slowly! Jump into a very fast rocket and SR no longer works the same way.

    On the other hand there are the crackpots, who believe SR is completely flawed. There doesn't seem to be any room inbetween...
  4. Aug 20, 2003 #3
    Wait. Relative speeds measured by which standard of distance? The very standards of distance differ too! If speeds are measured in either systems standards of distance and time, and then values compared, they should match? What should differ, is value of time to travel before collision, and distance to travel. Either system would measure these values different.
  5. Aug 21, 2003 #4
    That's exactly the problem! Since SR assumes all inertial frames are equal, there are no criteria to choose one standard. One result is as right as the other even if they differ. That is known as a paradox.

    That's correct, but the fact that they get different measurements necessarily implies that not all inertial frames are equal. There's nothing new about this anyway, this imbroglio is well known as the Twin Paradox. What's new, in my opinion, is that Einstein did not live in a time when space travel and electronic clocks were commonplace. He couldn't think of a scenario where the two inertial frames would have a chance to communicate their measurements, so he naturally dismissed the problem as a pseudo-problem.

    These days, though, as far as I can tell, people are too afraid to think Einstein might have overlooked something.
  6. Aug 21, 2003 #5

    Ok, let's go.

    You cannot compare clock rates in the fashion you require. That would require instaneous transmission of information, which is prohibited. If you transmit a signal to another object, and if that object is moving away at a substantial fraction of c, then it will take time for the signal to catch up with it, and indeed this extra time will allow the fellow on the receding ship to observe the other person as traveling slower. Similarly, if the ships are apporaching, it will take less time allowing for the same effect.

    No. While observers will disagree on the simultaniety of events, the spacetime location of said event ( I assume you know how to calculate that. the sum of the squares of spatial distance and temporal distance) will be the same for both. In other words they will have different parameters for space and time input, but the output, which is all that matters, will be the same. No paradoxes whatsoever.

    Again, I stated above that can't happen (the instaneous exchange of info). The twin paradox is only a paradox because you violate the requirements for SR. You have one twin accelerate to get back to the other twin. This breaks the symmetry of SR in constant velocity, and as such the traveling twin experiences a force and knows he is in motion.
  7. Aug 21, 2003 #6
    I won't force this on anybody as I don't have the patience to explain the whole thing in details, with all the math. Most people's attention span on the internet is not long enough for that kind of thing anyway - the boss is always lurking behind the cubicle walls.

    I can give you the key if anyone's interested though. Don't be fooled by the notion that the twin paradox doesn't happen in SR because one twin has to accelerate to turn back. I can't believe intelligent people can be that naive. The naivete lies in the erroneous notion that the traveling twin must physically return so that a comparison can be made. In an age where electronic communication can send clear data and voice across astronomical distances, I can't understand why people still resort to that explanation.

    You can have your twin leave Earth on a very fast rocket, and send you a message like this after some time:

    "Hi, it's me. From my frame of reference, today is July the 4th, and my clock reads 12:30PM. I'm now going to send you a signal of exactly 10 KHz... bzzzzzzzz"

    Once you receive that, a few minutes later, you can reply:

    "Hi, I got your message. Here on Earth it's already July the 21st, London time is 5:43PM. The signal you sent me came at 12.8 KHz. I will send you what measures to me as a 10KHz signal... bzzzzzzzzzz"

    That is information enough to apply the relativistic transformations and figure out who's moving and who's not. The rocket is moving, period, and that has nothing to do with acceleration. It has to do with the fact that light waves move at constant speed - they provide the preferred inertial frame that is missing in SR.
  8. Aug 21, 2003 #7
    Thats one other question I wanted to ask: what constitutes "equal frames"? Seems to me, that besides simple idea that laws of nature "work" similarily in "all inertial frames are equal", that there is also hidden assumption that some standard must be assumed as universal or brought to common value to make comparison possible. Seems also that atomic clocks are good pick, as with time we measure energy flow in a frame. We can't really compare 2 frames until we can establish their time rate difference. Two frames are truely indistinguishable if they share common time rate. To me, this seems a criteria to compare frames as "equal". Any difference there forces us to use SR transforms.

    If we know velocity between frames, we can compute time dilation. But as well we could compute velocity if we knew time dilation. C is like a glue between frames. And distance we can compute only when we measure time of a signal travel. Its obvious that distance would measure different value for frames that have different time rate.

    This makes me think that difference in time rate always means relative velocity, which means momentum energy.

    I suppose you don't mean this gives preferred inertial frame in absolute sense. It only gives reason to declare "home" as preferred frame for both twins. If man would somehow accelerate earth to 0.99c relative to spot where it was before, this would change our preferred frame, and next twins would make experiment that wouldn't show any different results, althought their result should differ wildly from frames of first twins.
  9. Aug 21, 2003 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The reason that the acceleration explanation is still given is that it is correct. The only way to compare the actual elasped time for either clock is to have them both in the same frame of reference. The only way this can happen is if one of the twins undergoes the proper acceleration.

    The above situation can never happen. You could not measure the incoming signal as having a higher frequency than it was sent at (after you account for Doppler shift) it will always be lower as long as the relative velocity between you remains constant. You might receive his 10khz signal at 8khz , and if you reply with a 10khz signal yourself, he will receive a 8khz signal in turn.

    The only time someone would measure a higher frequency than that transmitted is if the receiver is undergoing an acceleration at the time. This is another reason why acceleration is needed to resolve the twin paradox.
    Yes light moves at a constant speed relative to any observer.

    Thus the same light signal transmitted at c relative to the Earth will also be measured by the twin in the rocket as traveling at c relative to himself even though the rocket and Earth are moving relative to each other. This is the whole basis of Relativity.
  10. Aug 21, 2003 #9
    Thank you Janus.

    Seriously people, please please please actually learn SR and master it. Once you do, you will stop saying "Einstein was wrong!" His relativities work very extremely well in their intended areas. It does seem counter-intuitive at times yes, but that is merely because we do not live our everyday lives in a relativistic world.
  11. Aug 22, 2003 #10
    It does sound like you understand what I'm trying to say. Obviously there's no preferred frame in an absolute sense, but at the same time if there isn't some agreement between different observers then comparison becomes impossible. The situation is no different from currency exchange; no currency has an absolute value, each currency must be defined in terms of some other currency. It happens that the US dollar has been chosen as the "absolute" standard. That means all currencies may change, but the US dollar always has the same value: exactly US$1! The US dollar is the speed of light of the international money market.

    I have no problem with that kind of reasoning as long as people don't lose sight of what they are doing, but economists (or at least the ones who write for newspapers) seem to be as confused as physicists. Whenever the American economy gets weaker or stronger, most newspapers all over the world report that their local exchange rate has changed. Seldom do you see in newspapers that the cause of the fluctuation in the local exchange rate can be attributed to the American economy. And of course American newspapers for the most part don't make any news about fluctuations in their currency, as it's fixed by definition!

    The concept of the American currency changing while the rest of the world remains the same is too complicated to think about, even if it makes more sense. Likewise, the concept of your own frame of reference moving while the rest of the universe remains in place may be a bit hard to express formally, but it does make a lot more sense.

    Can you imagine, if the Japanese yen and the German mark are defined solely in terms of each other, it becomes absolutely impossible to know, say, the exchange rate between the yen and the sterling pound. Economists have no option but to take the whole system into account and arbitrarily choose one frame of reference as their standard. Physicists have it easy, they can look at two moving bodies, define their movement solely in terms of each other, and blissfully ignore the rest of the universe. Not only can they do that and mess up everyone's understanding of the simplest things, but they can also say that anyone who complains about it doesn't understand physics. Which would be okay if they didn't equate that with a failure to understand reality.
  12. Aug 22, 2003 #11
    Actually, that is not true. The traveling twin will have experienced less time even after you account for the asymmetrical GR effects. Do the math and you'll see.

    Besides, the idea that you know that you are moving because you experience acceleration is phony. You can only measure your own acceleration if the accelerating force is not evenly distributed. A body freely falling towards a massive object cannot measure its own acceleration - which is why GR must attribute time dilation effects to gravity as well. One patch upon the other upon the other...

    For most velocities, the doppler effect is greater than the time dilation, which is why I gave a number higher than 10KHz as an example. If I gave a number less than 10, someone would say I forgot the doppler effect. No matter what I say, people like you will always assume I don't know what I'm talking about...

    The first statement is not consistent with fact. And the second statement is false; GR doesn't resolve the twin paradox, it only says it can't happen in reality. Which is quite a moot point as no paradox can possibly happen in reality. Only ideas can be paradoxical, and SR is an idea.

    The above is not strictly correct. It's impossible to measure the speed of an incoming light signal sent by a moving sender, as you won't be able to establish the distance the signal traveled nor the time it took to arrive. The best you can do is send a signal and have it bounce back from a wall, which is exactly what the Michelson-Morley experiment was about. And there's nothing particularly impressive about the speed of light not being affected by the speed of the sender - all waves work the same way, their speed being defined solely by the medium in which they propagate.

    The only thing that is different about light is that you can't measure you own velocity compared to it. If you are traveling alongside a flat wall, you will notice that the faster you go the longer it takes for the echo to reach you back. Not so with light. You can go as fast as you want and a light pulse bouncing back from the wall will always take the same amount of time. But the explanation is very simple and, contrary to what many people like to say, totally intuitive: whatever affects the speed of light rays also affects the rate of clocks, because both involve the same phenomenon of electromagnetic waves traveling through space. But any stationary observer will see what the moving observer can't: that the faster they go, the longer it takes for the light pulse to bounce back.

    Now of course Einstein denies us the ability to use the expression "stationary observer", and as a result we've had some 80 years of disagreement between people who think relativity is right but doesn't make sense, and people who think relativity is wrong because it doesn't make sense. A totally senseless debate if ever there was one!
  13. Aug 22, 2003 #12

    This is going to be fun!

    1) The twin who experiences acceleration should have experienced less time. And me, along with a great many other people have done the math and it works out perfect.

    2) Free fall. This is a special case of acceleration. If I am in a rocket and I fire it and accelerate, I know I am accelerating. Same holds true if the twin has to reverse his direction. With free fall however, you are within a gravitational field and are falling at the maximum rate you can. As such, you won't feel any resistance. GR will still give you accurate results. And why would gravity (which is a warping of spacetime anyways) NOT have any effect on time? It has effect on warping space, and definitly has an effect on time.

    3) It is no wonder people think that. You seem to lack even a basic knowledge of the principles involved. Please, take time and actually learn the material. There is no way to make an incoming signal in the fashion you described get a higher frequency (other wise it will have had to gain energy). It will always be lower.

    4) How is the statement about acceleration not consistent with fact? If I undergo an acceleration as a beam is coming in, then my frame of reference might be such that I move towards the beam and cause it to blueshift. And GR resolves it because of the simple fact the twin paradox is outside the field of SR. SR deals ONLY with constant velocity. That is why it is called Special relativity. In order for the twins to have any chance of meeting up again, one of them must undergo an acceleration to change his or her velocity vector and hence break the symmetry. Again, see reply 1 in my list to see that GR calculations do work.

    5)It is possible. Merely set up a timer. For example in your moving aparatus, watch the time it takes the signal to move across two points, be it moving or not moving with you. Due to lorentz contractions and time dialation, you will measure the light to travel at c. And the MM experiment was an L shaped infermometer if I recall. Beam is sent, and split perpindicularly, and reflected back. If any direction of the beam was affected by the earth's movement in the supposed ether, it would cause interference. And the wave thing is not so. If I am in a high speed boat, and launch a water wave forward with some force, it will travel at the impulse from the force and the boat's speed. Same with sound waves.

    6) And rightly so. What is the observer stationary to, and on what grounds? Also, you left out one important group of people in your debate. Those who know SR is right, and that it makes sense. It is perfectly logically sound. The only problem is people don't want to swallow their common sense and accept that the universe will not always follow our particular instincts, but that is simply because we do not live in a world where relativistic effects are noticable, nor in one where quantum effects are either.
  14. Aug 22, 2003 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The doppler effect causes a redshift when two observers are moving away from each other.

    Why not? All it requires is a network of observers synchronized to return measurements according to some particular coordinate chart.

    You can bounce it in a triangle too, or any number of complicated shapes.

    That's not strictly correct; the speed of a wave in a medium as measured in the rest frame of the medium is what is determined.

    Don't forget the group that thinks relativity does make sense.

    This is also not strictly correct. SR is capable of dealing with acceleration; the catch is (just like in Newtonian mechanics) that the laws of physics take different forms in accelerated reference frames.... however one doesn't even need to appeal to accelerated reference frames, because you can do all of the necessary computation using the coordinates of any inertial reference frame.
  15. Aug 22, 2003 #14
    Ah, yes you are correct there Hurkyl. SR can deal with acceleration.
  16. Aug 22, 2003 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I've done the math. And in order to reconcile what each twin measures, you have to take in to account the acceleration felt by one of them.

    That's not what anyone that understands Relativity says. It say that you can never tell who is "really" moving. You can only say what relative velocity they have with respect to each other.

    Calling complimentary effects "patches" just reveals further misunderstanding on your part.

    As someone has already pointed out, the doppler effect would have worked to decrease the frequency received even further. So saying the frequency received would be higher, is wrong whether you take Doppler into effect or not. So no assumption need be involved.
    Both statements are consistant with fact and true.

    You don't need to invoke GR to take the acceleration into account, SR is perfectly capable of handling acceleration. and acceleration in SR breaks the symmetry that makes the Twin paradox a paradox.

    Let's see, I put a sensor on the end of the ship towards the sender and another at the end away from the sender, I then measure the time between the triggering of each of the sensors, and "voila", youv'e measured the speed of the incoming signal with respect to yourself.

    The rest of your post is just the same old diatribe by someone who can't come to terms with the fact that there is no prefered frame of reference.
  17. Aug 23, 2003 #16


    User Avatar

    Staff: Mentor

    Just an editorial comment: I'm someone who has a rough idea of what relativity says and means. I'm not an expert. Because I am not an expert, I trust the people who ARE experts.

    I remember in math class in high school something (can't remember what) didn't make sense to me. I tried for a long time to prove my teacher wrong. But my teacher was RIGHT. From that, the most important thing I learned is that when I have a problem with the accepted explanation of something, instead of trying to figure out how the accepted explanation is wrong, I try to figure out how I am wrong. Certain people in here should learn the same thing.
  18. Aug 23, 2003 #17


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  19. Aug 23, 2003 #18
    How about posting only when you have some constructive input?

    Within framework of SR, please help me answer my initial questions.
  20. Aug 25, 2003 #19
    Actually, there are many experts on relativity who dispute several aspects of the theory. But of course those don't count, right?

    I know it's not politically correct to affirm that Einstein's theory is not absolutely, perfectly, thoroughly self-consistent. People thought the same of Newton for 300 years, why should it be different this time around? Yet the flaws in Newton's thinking were as obvious to his contemporaries as they are now. Science moves so slowly because it has a lot of inertial mass, it takes too long to convince people of some obvious truths.
  21. Aug 31, 2003 #20
    Having been the rounds on this before - I will again point out a couple of facts - first - Einstein's postulate regarding the absolute velocity of light was a direct consequence of his belief that there was no preferred reference frame. In other words, he was strongly biased by the notion that it is impossible to measure our velocity with respect to space) We now have the CBR dipole anisotropy - moreover there is a yearly second order effect that marks our changing velocity with respect to space (the CBR). And since the 1700's we have been able to detect aberration which is a measure of our local motion with respect to the path of photons in free space) So the reason for the postulate of one way anisotropy may be incorrect. Secondly, there are many authorities on SR that analysis the twin problem - totally disputing the need for acceleration to arrive at the age diffeence (Take a look at Resnick's book - Introduction to SR published by Wiley & Sons) - I will concur with amadeas - to the extent that one questions SR, he is branded as a crackpot - but there are a lot of really bright crackpots out there. Nonetheless, SR seems to survive every test - but the tests cannot distinquish between apparent time dilation and actual clock slowing for the high speed traveler.
  22. Aug 31, 2003 #21
    My question assumes that there is actual clock slowing. Now, without assuming that there must be spatial distance between objects and relative velocity, I want to arrive to these measures from given clock difference alone.

    1) Suppose two rockets are sent and setup to relative motion predicted by SR such that their relative clock rate differs 2 times. Astronauts have been sleeping and now need to tell where they are, given fact that they have exactly same type of atomic clock onboard as other ship, and they can receive signal generated by specific frequency on other ship.

    2) Suppose two ships are never sent anywhere, resting next to each other, but having their actual clocks differing 2 times by some other means (hyper-duper-technology). Astronauts are cheated into thinking they travel at relativistic speeds.

    Is there any definite difference between those two cases, if they can't directly measure their relative distance by some radar ranging?

    What I want to arrive at, is understanding, why we consider space and motion in it primary, and time dilation secondary, while it seems that changes in timeflow seems to lead equaly to concepts of distance and velocity.
  23. Aug 31, 2003 #22


    User Avatar

    And later awarded the Nobel prize when he fails to disprove SR.

    It is untrue that people believe SR and GR to be 100% correct - obviously untrue, because the major work in theoretical physics at this time is specifically to undermine the absolute truthfulness of GR and SR by conciling it with QM in some sort of unified theory. But the note is that all of the predictions made by relativity so far have been shown repeatedly to be correct, and relativity is clearly the best theory for the limited case of scenarios we are around so far. Whether this is a special, limited case is another question.

    CBR doesn't say anything about spacetime and privledged frames of reference in the relativistic sense. The velocity of the photons we measure from it is always c, and the anistrophy is about their energies.

    These are by relativistic definition, the same thing.
  24. Aug 31, 2003 #23


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    For this time dilation, their relative velocity would be .866c.

    If each astronaut slept for 10 hrs by their own clock, they will each figure that they are 8.66 light-hours apart when they wake, and that according to the signal received from the other ship, only 5 hrs has passed for the other astronaut (after correcting for transmission delay and Doppler effect)

    Both astronauts will measure that while 10 hrs have passed for them, only 5 have passed for the other.

    In the second case, I assume that you are talking about putting one ship inot some "time retardation field".

    In this case, the one in the field would measure 10 hrs as having passed for him, and 20 hrs for the other astronaut.

    For the astronaut outside the field, if he measured 10 hrs for him, then he would measure only 5 hrs as passing for the astronaut in the field.

    You wouldn't have the same situation/measurements as you did with the actual relative motion.

    If you are talkng about some Super-duper technology that mimics relativistic effects perfectly while the astronauts sit next to each other, then you are talking about somethign along the lines of "If Pigs had wings".

    Such technology would in all likelyhood violate Relativity and causality, and if such technology could exist, then the rules of Relativity don't apply. Therefore invoking such technology does nothing towards increasing understanding of Relativity,

    Both time and space are relative. Whenever you deal with relativity, you must consider both time dilation and length contraction together.

    Relative velocity between two frames as meaured from those frames will always be the same.

    Relative velocity between two frames as measured from a third frame moving wrt to the both of the first two, will not be that same as the relative velocity of the two as measure from either of the two.

    This is a direct result of applying the time and length transformations.
  25. Aug 31, 2003 #24
    SR is counterintuitive - that is why so many tests have been performed to validate or invalidate it - but I will bet that the last word is yet to come - recall Einstiens confessions of doubt near the end of his life - yet when any other inquirer suggests that SR may be flawed - he is chastised. How strange - I am forever reminded of Isaac Asimov evocative statement: "Every idea, no matter how foolish, will find its adherents who will defend it to the death."
  26. Sep 1, 2003 #25
    Since when did our intuition dictate the laws of physics? SR and GR are both very accurate, and the last word has come. What we are doing now in theoretical physics is to push the scope of relativity into the quantum realm. This will yeild a much more accurate picture, but much as Newton's laws are derived from GR, GR/SR will be derived from this. And Einstein did not have doubt in his Relativity theories, insomuch as he doubted very much so QM.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook