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Light Speed Relativity

  1. May 5, 2007 #1

    I have made a discovery - for me that is; it may not be new - and I was wanting to have the knowledgeable soles here tell me if it is correct according to current theory if you could.

    I will submit it shortly but first off I just want to check one thing.

    Say you have two planets moving apart - planet A and planet B - and you consider planet A to be stationary. In this example planet A shoots a photon towards planet B and planet B shoots a photon towards planet A and (somehow) they do this simultaneously.

    Of course planet A sees planet B moving away and planet B sees instead that planet A is moving away. So they will see exactly the same as each other but opposite and will measure things oppositely but by a corresponding amount.

    Am I correct that according to special relativity both planets will receive the other's photon at the exact same simultaneous time if it was sent at the exact simultaneous time despite their moving apart from each other?

    Can we discuss the correctness or not of this before I go onto my actual realisation.

  2. jcsd
  3. May 5, 2007 #2


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    The pulses were 'shot' simultaneously according to whom?
  4. May 5, 2007 #3
    Yes 'simultaneity' of two events in SR is defined as 'having the same time with respect to an inertial coordinate system'. A coordinate system in SR is defined by grid of GAZILION of observers not moving with respect to each other having sinchronized clocks (there is specific procedure for sinchronization). The coordinates of an event are recorded by the local observer coinsiding with the event. The instantaneous speed of a body is recorded by the observer that coinsides with the body at that time.

    If you have such a grid of observers and planets have the speeds Va = - Vb with respect to that system, then they will emit and receive the signals at the same time. The situation is totally symmetric from point of view of that system of observers.
    Last edited: May 5, 2007
  5. May 5, 2007 #4
    Really, where?
    What would such definitions do except for confusing people?

    It is far better to accept the basic implication of relativity that the notion of two separate events being simultaneous is meaningless since each observer in the universe has his own measure of time.

    If you don't try to help people to understand that basic thing, how can you possibly expect them to understand relativity?
  6. May 5, 2007 #5
    I'm curious to know which part of smallphi's statement you find objectionable.
  7. May 5, 2007 #6


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    In every textbook on special relativity you can find some discussion of simultaneity in different inertial frames.
    Do you consider every coordinate-dependent quantity or concept (as opposed to physical quantities like proper time) to be "confusing" or "meaningless"? Maybe from a high-flown philosophical perspective, but from the practical perspective of actually trying to make numerical predictions about particular physical situations, I don't think you could get along without the use of coordinate systems (in both SR and GR).
  8. May 5, 2007 #7
    Coordinate charts are obviously a very helpful tool to make calculations but in relativity they very often obfuscate the physics. As long as you can separate coordinates from the physics there should be no problems. Even Einstein admitted that he was confused by coordinates in understanding gravitation.

    With regards to simultaneity, yes you can define it anyway you want, but physically it is meaningless to call two separate events simultaneous. Simultaneous means "at the same time", physically there is no global time so "at the same time" for two sparate events is meaningless. Each observer in the universe can have its own measure of time. This is a consequence of the principle of relativity.

    Wouldn't it make sense to teach such a concept to someone if they want to learn relativity, or do you think that if a person can simply apply the right formula he understands it? In that case we might as well give them a Java applet and tell them where they have to plug in the numbers and then claim that they are a master at understanding the principle of relativity.
    Last edited: May 5, 2007
  9. May 5, 2007 #8


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    I agree that it's important to keep in mind which quantities/concepts are genuinely physical ones, and which are merely coordinate-dependent. And I agree that "simultaneity" is merely coordinate-dependent. I just think your language is too strong when you say it's "meaningless", since coordinate systems have great practical value in making calculations, one needs to learn about them (especially inertial coordinate systems in SR) to have a good understanding of relativity.
  10. May 5, 2007 #9


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    In a possibly-ideal world, we could learn relativity from the top down, starting with a general 4-manifold with a Lorentzian signature metric and its metric-compatible derivative operator... then just declare various objects on which we "do physics".

    What is obviously lacking is a connection to our common-sense physical intuition, however imperfect it may be.

    Rather than simply banish classical ideas ["it is meaningless to call two separate events simultaneous"], it is probably more constructive to address these misconceptions by defining the concept operationally [and mathematically], showing its limitations [and revealing why we overlooked it with our Newtonian intuition], then refining the classical definition with more conditions.

    In addition, we often have to physically interpret the results of measurements. Measurements are, in some sense, dot products of tensors. For example, we might wish to give a name to a set of nearby events whose displacement 4-vectors from an event dotted with our unit 4-velocity are equal. Of course, that special set of events is associated with our unit 4-velocity. [...just as an "x-component" in Euclidean geometry is associated with a choice of axis.] (Of course, in a Newtonian world, the same set of events will be selected regardless of the choice of unit 4-velocity... so, we don't bother to associate that Newtonian concept with a unit 4-velocity.)

    We certainly should strive for "relativistic purity", but we must build on and refine our intuition and current understanding. And, of course, this differs from person to person. So, it's inevitable that there must be some mix of pure relativistic concepts and coordinate-based concepts, with various levels of sophistication.

    [Of course, we could probably replace "relativity" by "quantum mechanics" and argue analogously.]
  11. May 5, 2007 #10
    If we define the results of all possible experiments we can perform as 'reality', then that 'reality' is constantly expanding cause our instruments become more and more sensitive and sophisticated.

    Our theories are just approximations to that 'reality', they can never reach it since the reality is constantly expanding. Once we find an experimenal result inconsistent with current 'very best' theory we search for a new theory with bigger scope of validity.

    GR is the best theory we currently have about gravity but it certainly can't explain what happens at the center of a black hole or at the cosmological BigBang. In the future it will be replaced by theory that approximates a bigger 'reality', the Quantum Gravity that hopefully will resolve those questions.

    SR is the restriction of GR to flat Minkowski space with zero curvature. In the scope of that theory 'simultaneity' is perfectly well defined physically but is different for different sets of observers. The intial question of that thread was in the context of SR, so I gave the appropriate definition.

    Newtonian mechanics is approximation to SR for small relative speeds. Its 'absolute' time is perfectly well defined physically if you remain in the scope of that theory i.e. your instruments are not sensitive enough to detect SR or GR corrections. An obvious illustration of that is that here on Earth we use clocks to synchronize with each other. Of course you can always say 'I'm late cause according to GR there isn't absolute time' but nobody will let it slide LOL

    GR is the best we currently have but it is not the ultimate 'truth' and the above 'nesting' of a more approximate theory into a more exact theory (Newton nested in SR nested in GR) suggests there will never be the 'ultimate truth' theory. Just because we have GR, it doesn't meant he have to throw SR and Newtonian mechanics to the garbage. In cases where your instruments are not sensitive enough to detect GR corrections or you simply don't care about them, SR and Newtonian mechanics are WAY EASIER to calculate.
    Last edited: May 5, 2007
  12. May 5, 2007 #11
    Thank you for your replies to my initial question.

    The results unfortunately don't seem to be definite like I was hoping; some seem to say yes and some to say no. Some seem to say that time is different in different places so that it is difficult or impossible to pinpoint or even have simultanity. I was hoping for an answer like smallphi gave about the symmetry existing in an inertial frame. I think this would match the example I am using.

    Things can occur simultaneously even if we don't observe them doing so can't they? For example a butterfly may flap its wings in Brazil and a typhoon may begin in China simultaneously? If in my example one person walked from planet A towards planet B and a second person walked from planet B towards planet A with each measuring their pace from their own planet and each left at the same simultaneous moment then when they both met would they both say they have travelled the same distance as each other?

    Beginning from this shaky ground I will write about my 'discovery'. Please tell me if it is wrong and isn't current theory; or if it is actually correct by current theory. When I first found it I thought that it was objectionable to current theory but then I began to wonder if it actually was in agreement with current theory.

    I am writing here because maybe it does explain things correctly. Maybe I am worrying about nothing and it is afterall in accord with current theory and helps to show why.

    I will have to type up my 'discovery' well enough to hopefully explain what I found so I will submit that post soon. I have drawn a diagram with it so I will attach that then as well. In the meantime perhaps someone here can help me with my question about the two people walking towards each other from their planets.
  13. May 5, 2007 #12
    This is important, it means that even if we wanted to give a completely "pure" teaching of physics (top down, starting at abstract fundamental axioms and deriving all of classical physics, with students blindly trusting the end to justify the unmotivated bulk) then we still still couldn't, at least while we still haven't figured out the final GUT (quantum gravity, all fundamental particles, etc).
  14. May 6, 2007 #13


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    It's not that some say yes and some say no, it's that your initial question is not well-defined, because you don't specify in which frame the photons were sent "simultaneously". Simultaneity is dependent on your choice of reference frame, there is no single "true" definition of simultaneity in relativity.
    It has nothing to do with whether you observe them or not, it's just that simultaneity depends on your choice of reference frame, just like velocity or x-coordinate. It's not an objective physical question whether two events happened simultaneously or not, any two events that are simultaneous in one observer's rest frame will be non-simultaneous in another's, just the same way that any object which is at rest in one observer's frame will have a non-zero velocity in another frame.
  15. May 6, 2007 #14
    Ok. I'll try to lead into my question a little better hopefully.

    Say we have three ropes: rope A, rope B, and rope G; and we have two vehicles: vehicle A and vehicle B.

    On rope G we measure the middle, two points from the middle and two measures to each side of this:
    ie ---------------------------+---|---o---|---+---------------------------
    This is our guide rope.

    Rope A is attached to vehicle A and rope B is attached to vehicle B.
    Rope A is fed through a feeder in vehicle B and rope B is attached through a feeder in vehicle A.
    Rope G attaches to feeders in both vehicle A and B and is the middle rope.

    The ropes are fed in tight so that the vehicles are the space of the |---o---| apart.
    The vehicles move apart at constant speed.
    Or vehicle A stands still and vehicle B moves away at that speed or vehicle B stands still and vehicle A moves away at that speed.
    It doesn't matter as is stated by relativity; in that much I am correct yes?

    Vehicle A holds tightly onto rope A and feeds rope B out at the constant speed. Vehicle A feeds rope G out at half the constant speed.
    Vehicla B holds tightly onto rope B and feeds rope A out at the constant speed. Vehicle B feeds rope G out at half the constant speed too.

    As they feed out the ropes - except their own - they watch for the + mark to come up.
    When it does a tight rope walker from vehicle A starts walking along rope A towards vehicle B and a tight rope walker from vehicle B starts walking along rope B towards vehicle A.

    Question 1. Can we say that they have both set off simultaneously in this frame and co-ordinate system?

    They both move at an agreed upon speed away from their vehicle towards the other measuring back to their vehicle.
    That speed is faster than the speed the main ropes are feeding out so that they make progess towards the other car.

    Question 2. Will they meet at the 'o' on the guide rope simultaneously?

    They then continue on without stopping.

    Question 3. Will they each arrive at the opposite vehicle both simultaneously?
  16. May 6, 2007 #15


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    If I'm understanding your example right, they both set off simultaneously in the rest frame of the "o" at the center of rope G, although they don't set off simultaneously in the rest frame of either of the two vehicles.
    yes, and since this happens at a single location in space and time, this will be true in all reference frames--disagreements about simultaneity only happen for events at different points in space.
    In the frame of the "o", yes. But not in the frames of either of the vehicles.
  17. May 6, 2007 #16
    Well okay. I'll leave off there then.

    I was hoping that current theory said that events could both begin separately but at the same instant on two objects that are moving apart at a constant speed whether considered from a stationary mid-point or from either object being stationary due to the symmetry and inertia of SR.

    As that is not the case then I can not examine my 'discovery' against current theory as its basis would be faulty. I was hoping that I had discovered how light can be considered to be travelling at a constant single speed using this discovery but it is based upon an unaccepted idea and misuses simultanity under current theory.

    All is not lost though. I have learnt more about current theory so thank you for that.
  18. May 18, 2007 #17
    Let A and B be at relative rest.
    Each sends a signal to each other at a predetermined time by their sychronized clocks.
    Each one receives a signal at the same time on each clock.
    Now if one moves away from the other, he will receive the signal later.
    Any motion removes the symmetry they have.
  19. Jun 7, 2007 #18
    No necessarily, if both accelerate the same way but in opposite directions the situation is still completely symmetrical.
    Last edited: Jun 7, 2007
  20. Jun 7, 2007 #19
    That is why it states:
    "Now if one moves away"
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