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Light to Time, Time to Light

  1. Feb 4, 2008 #1
    Intuitively, I want the speed of time to be isotropic. But there are strong indications that it is not!

    Intuitively, I don't want the speed of light to be isotropic. But there is excellent evidence confirming that it is.

    What effect does it have, if I just swop the names?

    So now my clock ticks away a set distance, over a variable length of time!

    Does doing this rescue my poor intuition?
  2. jcsd
  3. Feb 4, 2008 #2
    IMO, the speed of time *is* isotropic - expanding outward from every point in space all at once.

    The only thing I know of that propogates at the speed of light isotropically is a wave; a particle, on the other hand, might only travel at speeds approaching that of light in one direction only.

    Then waves propogate at the speed of time in all directions, and particles might approach travel at the speed of time in one direction only.

    If you say so. :smile:

    I have no idea. :frown:


  4. Feb 4, 2008 #3

    could you guys explain to me what you mean by ' the speed of time ' ?


  5. Feb 4, 2008 #4
    In short: the rate at which chage propogates.

    i.e.If someone were watching you at this instant from five lightyears away, they would see you as you were five years ago - not as you are now.


  6. Feb 5, 2008 #5
    I agree with Antenna Guy, and perhaps go a little further. Our physics suggests that time can be compressed or stretched, effectively changing the distance between two points of time.

    I can't think of another way of talking about covering, travelling across, a distance of time other than with reference to speed. It seems nonsensical to say it takes a certain amount of time to cover given distance of time.

    My question was a philosophical one though, can we reasign the position of meanings in a concept or theory, in order to avoid the counterintuitive?
  7. Feb 5, 2008 #6
    John, your solution sounds wildly counterintuitive itself! Call light "time" and time "light"? Are you serious? They are very different features of our experience!


    Regarding the speed of light being isotropic - there is an intriguing claim (made by Einstein) that this is conventional. All evidence suggests the two-way speed (average speed to get somewhere and come back) of light is c, but it is only a convention to say that the one-way speed (speed in a single direction) of light is c.

    Say we send a signal out from earth to the starship Enterprise that is five light years away, and we get a message back. Conventionally we assume the light took five (of our) years to get there and five to get back. But maybe it took two to get there and eight to get back. How would we ever find out?

    First thought: send a message saying "Come back Captain Kirk!" and when he gets back ask him whether the outward journey was quicker than the inward journey.

    Problem: if the one-way speed of light is different on the outward and inward journeys, the Lorentz transformations will be drastically different too, so Kirk will always think the two journeys took the same amount of time, and we will learn nothing about the one-way speed of light.

    Second thought: hang on, can't we just measure the one-way speed on Earth with some fancy detector?

    Problem: this requires synchrony of clocks between the point where the light is emitted and the point where it's detected. So we presume a convention of simultaneity.

    I'm told this all works out. It's an amazing claim. It will really make you think twice before saying "light from the sun takes 8 minutes to reach the earth" or "those supernovas we see today occurred millions of years ago". Yes, by convention!
    Last edited: Feb 5, 2008
  8. Feb 5, 2008 #7
    Hi Lord Ping,
    As always a realy good reply.

    No I'm not serious I think the light to time thing was a bad example.

    I was toying with the idea that intuitive sense is somehow flexible! Can't find a good example where this works though and that maybe makes my inquiry mute.

    The best I can come up with is this as a question:

    1*1=1, so 1^2=1 so, 1Km^2=1Km but, 1000m^2=1,000,000m which as any one knows is equal to 1000Km. But 1Km=1000m There is an apparent counter intuitive result from the maths.

    What is it that I do to resolve this? Do I brush over it, turn a blind eye? Or do I establish a rule for the application of maths that states that I can't use maths in this way?

    Neither of these options seem to to resolve the issue for my intuition, it continues to tell me that there is a breakdown of equivalence. Yet I continue using maths and just avoid the contradiction. Is this not the same as saying that my intuition is somehow invalidated?
  9. Feb 5, 2008 #8
    1^2 = 1

    It doesn't follow that: km^2 = km

    I don't think this observation is counterintuitive. "km" represents a length in space, so if you square it, the result must have dimensions of area and not of length.
  10. Feb 5, 2008 #9
    O Crap I did it again.

    Let me die quietly in a corner somewhere.
  11. Feb 5, 2008 #10
    Let me pose a problem that might shed light on the relationship between c and t:

    Let A, B and C be three mass-less, stationary observers located on the corners of an equilateral triangle in otherwise empty space. This triangle is exactly five light-years along each edge. Observer A has two fancy massles clocks that show the exact same date and time, and decides to send one off at the speed of light to his friend B.

    Somewhere close to five years down the road, B and C notice simultaneously that A is futzing with one of his fancy, synchronized clocks. Then, at the five year mark, B suddenly finds himself in possesion of one of the clocks - and it appears to show the exact same time as the clock A still has!

    Meanwhile, C writes down the time he observed one of A's clocks vanish.

    After another five years have passed, C notices that B now has a clock that looks just like the one A has, but it's five years behind A's clock. C check his notes, and sure enough, B's "new" clock shows the exact same time as the clock that "vanished" five years ago.

    A has been getting a little anxious because he has been waiting for ten years to see if B received his gift - so you can imagine his disappointment to see that B's new clock is now ten years behind his.


  12. Feb 5, 2008 #11
    Hello Antenna Guy,

    interesting nick btw, so I guess that the fellows may be on the verge of some discovery...

    But what attracted me was the mention of ' stationary massless observers...'

    My question is ; in outer space, ( or anyware outside from a confined labroom ), can something massless be stationary ?


    Last edited: Feb 5, 2008
  13. Feb 5, 2008 #12
    Why not?

    An arbitrary point in space is [arguably] massless - and is stationary by definition.


  14. Feb 5, 2008 #13
    Hello again,

    well, sorry, for some reason I can accept a stationary point in space but not a stationary massless observer.

    In fact, imo, the observers don't need to be masless for the proposed scenario to take place, they are only observers. I would agree of massless observers if they would all be in the same reference frame, itself moving about the universe

  15. Feb 6, 2008 #14
    Now that we agree that "they are only observers", we can get back to relating c to t. :smile:

    What reference frame do you figure all three [massless] observers are in?


  16. Feb 7, 2008 #15
    Hello Antenna Guy,

    First off, the massless observers… I wanted to put that one on the back burner for a while, but I must address it on one issue.

    The observers cannot be massless, not even in their own reference frame, as I wrongly stated before. The fact that massless objects or particles move at c makes it impossible for A to send the clock over to B. It would have to travel at supra c speed… not allowed.

    Now, a question about the given scenario…

    How does A calculate a 10 year lag on B’s new clock?

    - When he sent it off, both were reading the same time.
    - When B receives it 5 years later, it still shows the same time as A’s clock.

    Let’s say that upon receiving it, he shows it to A. Then, wouldn’t it be that, five years later, A sees that the clock in B’s possession… is lagging only by 5 years and not 10?


  17. Feb 7, 2008 #16
    Where does that "fact" come from?

    From the instant B receives the clock, it would still take another five years before A can read it.


  18. Feb 7, 2008 #17
    Hello AGuy,

    I read that in different places over the years. Don't have the references but you can find it in Wikipedia if you type in “speed of massless particle ” (sorry couldn't copypaste), choose the article on Chirality.

    Also came up in responses from the general physics forum, where I opened a thread called 'Can something massless be stationary', since this got my attention...

    Sure, but at the time B receives it, BOTH clocks have 5 years to the counter, so the diffference will be 5 years when A reads it, not 10.

  19. Feb 7, 2008 #18
    I don't think the premise that "only [invariant] massless particles can travel at c" implies that "all massless particles travel at c".

    Unless someone can offer proof to the contrary, I'd prefer to move on.

    Nope. The clock does not experience "time" as it travels from A to B. When A observes B receiving the clock, the distant clock still shows the time it left A. However, A cannot "see" B receive the clock (or the time it shows) until five years after the event (for a total of 10 years after it left A).

    I'd like to delve into Euclidean 4-space to separate the distance and time components of c - but, at this rate, I'm not sure I'll get there...


  20. Feb 7, 2008 #19
    Antenna Guy,

    no problem for you moving on..., don't hold up for me. I guess this whole massless thing got most of my attention.

    i said only 5 years based on the fact (EDIT: not really the fact but what I thought was happening) that since everything in the scenario is massless, then time would be the same for all, including both clocks, in movement or not.

    I really tried to avoid the massless component but, frankly, I was reluctant to reply. Since massless particles do not experience time, how could a massless clock ever be able to indicate it (nevermind even being built)?

    Anyway, sorry for the interruption...


    Last edited: Feb 8, 2008
  21. Feb 8, 2008 #20
    The conversation has moved on but I feel compelled to point out that I don't think a point in space can be described as stationary. A location is described relative to objects in a common reference frame but that isn't a property of space itself. Oddly enough it seems like the concept of "a point in space" is a surd given relativity.
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