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A photonic clock - absolute time

  1. Sep 21, 2004 #1
    Having been reading a little about the concept of absolute time and how Einstiens theories ruled it out but at the same time established light as invariant to all observers as absolute.

    If we built a clock that ticked at a rate determined by a beam of light would not that tick rate be an example of absolute time. As 'c' is invariant therefore absolute would not a clock governed by light also be invariant there fore absolute regardless of observer frame.?
    Say a clock with a light cell that afforded a time reference of 1 second for every 299792kms,
    Last edited: Sep 21, 2004
  2. jcsd
  3. Sep 21, 2004 #2


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    Hi asprin!
    The only problem with that clock is the universe would come to a crunch [or attenuated sigh] before it made a single tick.
  4. Sep 21, 2004 #3
    Absolute time is, as you said, ruled out. The only absolute is lightspeed, c. Speed has no less than TWO components: time and distance. But absolute distance is ALSO ruled out by Relativity, so no such clock could be devised. If you could pin down the distance then Ok; but you cannot.
  5. Sep 21, 2004 #4


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    There is of course an apparent contradiction in GR cosmology. The Principles of Relativity (SR) and Equivalence (GR) rule out any preferred frame, including a preferred or 'absolute time'.

    However, the isotropy and homogeneity of the universe, first assumed in GR in order to obtain its cosmological solution, and now apparently confirmed by the extreme isotropy of the Cosmic Microwave Background (CMB) (to one part in 10^5), allows such a 'preferred frame' to be identified. It is that cosmologically co-moving frame in which the CMB is globally isotropic, and in that frame an 'absolute' - or preferred - time can be defined. Such time is the age of the universe measurable by a function of the inverse of the Black Body temperature of the CMB.

    In such a frame of reference an 'absolute' clock can be defined. We may use the 'tick' of an atom, an atomic clock. Alternatively the vibration of a photon, sampled from the maximum intensity peak of the CMB BB spectrum, could be theoretically used to make a photonic clock. Define c to be constant and you have Asprin's photonic clock!

    The question is which of these times do you choose - they do not remain synchronised.


    [P.S. in case anybody is wondering the two clocks tell the time in the Einstein frame and the Jordan frame of SCC respectively.]
    Last edited: Sep 21, 2004
  6. Sep 21, 2004 #5
    if youhave a clock that has a frame reference of 1 meter say a ridgid alloy frame which means a light ray travels 1 meter and it's time is recorded in milliseconds then extrapolated into seconds by math. then regardless of where that meter long alloy frame is the lights velocity shoould be identicle.

    A thought experiment, say we take our 1 meter long device (clock) and send it into space at 0.5'c' what would the velocity of that light within the clock be given that the frame that is measuring it is dilated do you think?

    According to relativity the velocity should be identicle regardless of dilation.

    So therfore no matter where our clock is, it will always measure seconds by the velocity of light.

    If I am not mistaken relativity demands this result.

    But the interesting thing about all this is that absolute time can exist according to relativity if the clock is photonic, and that I find intriguing.

    no matter how many photonic clocks we build they will always tick at the same rate any where any time.....
  7. Sep 21, 2004 #6
    I know you probably didn't mean to say this but "defining 'c' as constant" is exactly what relativity is about.....which is why a photonic clock would be absolute time.
  8. Sep 21, 2004 #7


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    A photonic (or light) clock is one of the thought experiments that Einstein used to rule out absolute time.

    Imagine that you have such a clock that measures the time it take for light to cross 1 meter. This clock is traveling at a right angle to the light at some velocity wrt another observer. An observer sitting next to the clock would see the light travel straight accross and take 1/300,000,000 of a second to do so.

    For the observer to which the clock is moving, the following happens: Since the clock is moving, in order for the light to hit the same spot on the clock, the light has to travel at an angle from his perspective. (the traget spot moves after the light has been emitted. Thus for this observer, the light must travel a longer distance than 1 meter to cross. Since light is invarient, he must measure the SoL wrt to himself as the same as does the observer sitting next to the clock. This means that he measures the time it take to cross as more than 1/300,000,000 of a second.

    Ergo, tow observers measure the time between two events differently, and time is not absolute.
  9. Sep 21, 2004 #8
    Janus, as usual your response is ...well a correct assesment of a clock that uses an external light source ( i think)....and I thank you for that.

    But I am wondering if my idea needs clarification: So I'll try to explain it a bit better.

    We have a solid enclosed box of a ridgid alloy.
    Inside this box we have a light emitting diode or laser that transmitts light to a destination 1 meter away inside our enclosed box, for all intents and purposes no other light can enter and the pressure inside the box is allowed to be equal to the outside.

    The light emited by the diode is measured and it's velocity is determined as 'c'. the device then uses this light to calibrate it's clock which is located outside the box ( digital L.E.D. clock ) The second tick rate is determined soley by the light inside the box. ( not atomically but only by light)

    Now because the light ray is invariant regardless of the velocity of the box the tick rate would be constant.....or maybe that is what has to be agreed upon?
    So we have this 1 meter cube with a clock visable on the outside.

    The question I guess is would this clock indicate absolute time regardless of where it is in the universe ( space ) and if we had a million of the same clocks would they all tick at the same rate?
  10. Sep 21, 2004 #9
    the clock is a independent and closed time system based only on the velocity of light inside that system..
  11. Sep 21, 2004 #10


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    1 meter according to whom? Two separate observers will not necessarily agree that the box is 1 meter on a side, nor will they agree how far light in a moving box has traveled (as Janus said).
    Ie, the box is at rest with respect to itself. Ok, fine....but what if I am moving with repect to the clock? I won't agree with the time shown on the box's clock even if I carry with me my own identical light clock. What makes your light clock better than my identical light clock?
    That may be "preferred" in the sense that its a nice arbitrary choice, but that does not make it an "absolute" frame. An absolute frame is one where the laws of the universe work different than in the other frames. IE, in ether theory, C would only be C in the universal rest frame. The CMB may be a "universal" reference frame in the sense that it permeates the universe, but thats not the same as a "universal" or "absolute" frame in Relativity.

    Really, the best reference frame (the easiest to deal with) is the one in which I am always at rest. That way, everyone else has to transform their velocities, distances, times, etc to conform with my meterstick and clock.
  12. Sep 21, 2004 #11
    maybe a simpler aproach would be beneficial.
    I was thinking that what if our clocks just simply displayed the velocity of light inside the box and just maintained that reading.

    If one clock box was sent flying of at 0.5 c it would still show 299792 kms I would think. another box traveling at 5000kph would show the same velocity. So no matter how fast or where they are or what acceleration etc they endure, the figure being displayed would be a constant 299792kps on all our clocks.

    Is this a valid contention?
  13. Sep 21, 2004 #12


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    - Agreed, unwise choice of terminology! However such a universal frame may have more significance than SR and GR are prepared to acknowledge as it is the Machian frame of reference defined by the presence of matter in the universe - Garth

    Asprin - Your 'metre-stick' would appear shorter to different observers, therefore because the speed of light is invariant to all observers, different observers would measure shorter times taken for light to traverse the 'metre-stick'.
    I hope this helps, Garth
    Last edited: Sep 21, 2004
  14. Sep 21, 2004 #13
    I guess you are all correct, i have since posting done a little research on the "Light clock" and have found quite a bit of info. but the problem I have is that if length chnages then so to does time, and I keep asking myself "doesn't this distance time relationship still provide the same velocity result and if this is the case then the calculated tick rate would work out the same."....but obviously I must be mistaken.....
  15. Sep 21, 2004 #14
    is it true that only length changes at velocities and not width?
    and if so wouldn't havng the light ray perpendicular to direction solve this distance problem?
  16. Sep 21, 2004 #15


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    Certainly, but how can you use it to tell time?
    The length and time do change in such a way as to always arrive at C for the speed, but again, C is a speed, not a time. Your clock would always read C - it wouldn't ever tick!
    Ahh, but in which direction is perpendicular? If you're on a moving train and you throw a baseball out the window, what looks to you like perpendicular will look to someone on the ground like you threw it forward. That's (sorta) the basis for the Michelson Morley experiment.
  17. Sep 21, 2004 #16
    now I am on shakey ground here,

    If our beam of light has it's source and destination perpendicular to the boxes velocity and the source and destination can determin the time by the calculation using 'c' as constant then a second rate could be calculated.

    and yes it would always be determined by the light therefore 'c'.

    say a pulse is sent every second and the distance of one meter vs 299792kms is claculated then the second rate would reflect teh velocity of light i guess.

    once we have established a fixed distance regardless of perpendicular velocity we use the standard of 'c' and calculate our tick rate.

    I hope that makes sense......( it's 4 am here so I'm off)
  18. Sep 21, 2004 #17
    This light clock thought experiment is really just a way for me to test my understanding of relativity, in a public forum. Hopefully I and others can clarify our thoughts about time.

    Lets approach this clock with a fixed width as a presumption

    If we take just one clock as an example.

    WE have a oblong box that measures 1m*1m*2m. on the inside.

    Inside this box we have a light source with destination sensor set to perform perpendicular to the direction of velocity of our box. The box travels lengthwise in space.

    Now we know that as the boxs speed increases it's time dilates and it's length changes due to contractions. It gets shorter or longer , I'm not sure which. .....not important.....however our width stays the same 1 meter as it did when built here on earth.
    The beam of light has to travel a fixed distance across the width of the box even though the time is dilating.

    Now on the outside of the box we have three separate devices.
    the first one is an atomic clock that is subject to dilation,
    the second one is a velocity monitor for the light inside the box,
    the third one is a light speed determined clock.

    Now at rest on planet earth, Atomic clock reads a tick rate as normal earth time rate.
    The velocity monitor shows the velocity of the inner light at 299792kps.
    the light clock show a tick rate, the same as our atomic clock, because the meter is the same.

    Now as the distance of light travel is always an earth meter we can simply calculate our tick rate based on the velocity of light in the box. light travels 1 meter then 299792kms/1 meter = X amount of seconds. Time is calculated by distance light travels. The light is stobed so that a pulse is sent at intervals and the time it hits our sensor will vary as time dilates. but the moment of hitting our sensor indicates the velocity of light over distance not time so the light clock will only measure light speed inside the box.

    So the measurement will always be 'c' based on an earth meter.

    But as our box travells our atomic clock will slow down but our light clock will stay constant from earths perpective. But to an observer traveling on our clock the tick rate will appear to be quickening because the atomic clock is showing as constant ( actually slowing acording to our earth meter.)

    Now because our light clock will always measure the same tick rate relative to earths surface time and distances, 1 meter remains 1 meter. Then if we build 100 clocks and sent them out at differing velocities, then all atomic clocks will show dilated time rates and will all be variable were as all the light clocks will show earth time ( 1 meter) rates and be constant and show the same rate for all 100 clocks.

    This light clock is premised on the following presumptions:

    1) The distance our inner light beam has to travel is a constant 1 meter determined on earths surface as 1 meter.
    2) That width of an object does not suffer length contraction.
    3) That the velocity of light is absolutely invariant.
    4) That an atomic clock when suffering time dilation will slow it's rate according to that dilation.
    5) Atomic clock is defined as a clock that generates it's tick rate by the use of mechanical or atomic means.

    That an appropriate light sensor is available to detect the exact moment of being touched by light and that this moment is indicitive of part of a time period.

    Ok I think I said it all

    what do you think?
    Last edited: Sep 21, 2004
  19. Sep 22, 2004 #18
    An observer on Earth doesn't see the light beam moving perpendicular to the velocity of the box (velocity addition still applies), so the light clock doesn't work. But since he still measures the speed of the light beam to be c, and the light beam travels a diagonal path in his view, length contraction / time dilation come into play for his measurements to be consistent.

    FYI, any clock at all - light, atomic, biological or otherwise, is affected in the same way by SR's time dilation effects.
  20. Sep 22, 2004 #19
    I am sorry I don't understand what length contraction has to do with observers on earth? the observer on our clock is all that matters.....??

    I was under the impression that length contraction affected our object regardless of direction relative to Earth?
    Is this an incorrect assesment of contraction effects?
    Last edited: Sep 22, 2004
  21. Sep 22, 2004 #20
    Am I right in saying that
    The light tavels an Earth meter if the meter is perpendicular to velocity regardless of that velocity?
    we are measuring the dilated time for light to travel that Earth meter regardless of how dilated?
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