What is time??

Discussion in 'Science & Society' started by Shadow1, Feb 5, 2011.

  1. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    @river: time always has been a measurement of the movement of something.

    There is a problem with "the movement" of light, though. We can't see light "moving" through space, right? We can, however, measure the frequency of light, so if we use a fixed frequency as a reference we can also fix a distance spatially. There is nothing particularly difficult or weird to understand.

    What is weird, is that light has no 'time distance', and objects with mass do; light "travels" through space but has no time dimension in spacetime, because photons all lie on the (observer's) lightcone. So any light you see, like from distant stars, has an apparently infinite velocity, or is 'Newtonian' (i.e. instantaneous).
     
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. Pincho Paxton Banned Banned

    Messages:
    2,387
    What's this then?..... In a vacuum, light travels 299792459 meters in one second.

    Of course it has a time distance.
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. river Valued Senior Member

    Messages:
    9,528
    agreed

    true

    can't see the beam , as in a flash light beam



    light has infinite velocity ?
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    In flat spacetime, the worldlines of photons all lie on the future lightcone, and have zero Minkowskian 'distance', or zero degrees of rotation into the interior of the same lightcone. What you are referring to is measurement, relative to a predefined time interval: the second.
    Did you skip over the word "apparently" in the phrase: "apparently infinite velocity"?

    Can you tell by looking at a star how far away it is, or does the light appear to reach you "instantaneously"?
     
  8. river Valued Senior Member

    Messages:
    9,528
    I have been thinking this for several yrs now

    I would like an experiment where light is blocked from one point in space , a star , light yr away , then released , unblocked

    it would be interesting
     
  9. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    Would it matter if the light was blocked at the star (somehow), or blocked at the point of observation? What would doing either tell you about the light when it was 'unblocked'?

    That is, how would it be interesting? It's just that I can block light from distant or local objects, by closing my eyes, say. So are you interested in the difference between having your eyes open or closed? If you are, you might have missed out on some early childhood stuff.
     
  10. river Valued Senior Member

    Messages:
    9,528
    just at some point where it would take a yr to reach me , thats all
     
  11. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    "That's all"? How would you locate this point?
     
  12. river Valued Senior Member

    Messages:
    9,528

    a star one light yr away from us
     
  13. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    How do you measure the distance, is what I'm asking. How do you tell the distance to any star, how do astronomers do it?
     
  14. river Valued Senior Member

    Messages:
    9,528
    Proxima Centauri V645 Cen 4.2 11.05 (var.) 15.5 M5.5Vc

    so our nearist star is 4.2 light yrs away

    nevertheless block that light beam one light yr away from us

    then see what happens
     
  15. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    The very first thing that will need to happen, is locating the point 1 light year away between you and the star. How do you propose doing that? Where are you, relative to Proxima Centauri? Remember, the earth and the sun are moving through space.

    Again, why would this be "interesting"? What do you think it will tell you, and about what?
     
  16. river Valued Senior Member

    Messages:
    9,528



    by a space object that is launched from Earth and is directed to a certain point in space that blocks the light from a star , one light yr away



    it would tell me about the speed of light and about space
     
  17. Reiku Banned Banned

    Messages:
    11,238
    Nope, it don't. Arfa is right.

    In psuedo-vector space of relativity, distance equals zero when velocity equals "c" because of length contraction.

    \(0 = \sqrt{(1 - \frac{v^2}{c^2})\) when \(v=c\)

    So light actually does not take a year to travel a lightyear, it actually takes no time at all. It's birth is simultaneously it's death.
     
  18. wlminex Banned Banned

    Messages:
    1,587
    . . . if so . . . why does it take 4.2 years for light to 'reach' us (the observer) from Alpha Centauri . . . . and why do physicists say that the light presently reaching us from Alpha Centauri was initiated 4.2 years ago at Alpha Centauri? . . . if LF contraction reduces the distance traveled to zero
    (0)? . . . just curious
     
  19. river Valued Senior Member

    Messages:
    9,528
    Mister

    so my thinking is true ?

    so that if I block light a light yr from here , unblocked I would see the light instantaneously

    wow
     
  20. arfa brane call me arf Valued Senior Member

    Messages:
    5,332
    Well, that's "almost" correct. But light does have a 'time-distance' from an external frame of reference.

    What you mean to say is that light has no proper frame of reference, it can't be given a time coordinate in Minkowski spacetime because its worldline is congruent with the lightcone and can't be Lorentz-transformed (the transform is always 45[sup]o[/sup] = 45[sup]o[/sup], or 0 = 0, which is what you've really pointed out).
     
  21. river Valued Senior Member

    Messages:
    9,528


    we have come upon an interesting discussion here
     
  22. Reiku Banned Banned

    Messages:
    11,238
    Yes, moving in what is more collectively known as a null trajectory. But yes, that is true also, from a bradyon's point of view (we are bradyons, or tardyons, things that move sub-luminal speeds) experience light taking time to reach other places.

    A unique way to solve this paradox is by saying a photon does not have a frame of reference.
     
  23. OnlyMe Valued Senior Member

    Messages:
    3,914
    A distance in space does not become length contracted by virtue of an object moving through it.

    The object may become lenth contracted in the direction of its velocity.

    And an object/observer/clock may become time dilated by virtue of its velocity.

    But unless you can define space as an object and then give it a velocity.......

    Space does not become length contracted as any function of the velocity of objects moving through it.
     
    Last edited: Jan 12, 2012

Share This Page