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Why do we judge time by light?

  1. Nov 3, 2005 #1
    I have been trying to figure this out in my head and I just can’t make sense of it. Why don’t we use matter, i.e. an oscillating spring, to keep track of the rate of time? I know light has a constant velocity in all reference frames, but does that make it a better source for an accurate rate of time? Any ways this was just buggin me and I thought I would hear what all you guys had to say. Thanks.
  2. jcsd
  3. Nov 3, 2005 #2
    Put your mind at ease. Matter, such as an oscillating spring, is used to keep track of the rate of time. Light is not used.
  4. Nov 3, 2005 #3
    Einstein uses light as a constant when he proves length contraction and time dilation doesn't he? I.e. if there is a light source L meters from a mirror it will take L / c seconds for the light to make the trip. Then from 2 reference frames d (distance the light traveled) and d' can be measured. If I’m not mistaken (which I am frequently) this setup constitutes a basic clock. If he would have used a spring when proving contraction and dilation there wouldn't be special relativity.
  5. Nov 3, 2005 #4
    He starts off saying the laws of physics are the same in all inertial reference frames. This leads to light being constant, which can then be used to derive the rest.
  6. Nov 3, 2005 #5

    This is the first postulate of special relativity, what leads to a Lorentz type transformation, not the actual Lorentz transformation.

    The speed of light being constant is an experimental fact, and is not explained by the first postulate. There are sevreral facts that could be taken as "second postulate", e.g. the famous E=mc^2. The laws of physics could be the same in all inertial reference frames and even speed of light different in any inertial frame, what would lead to a modification of _Maxwell laws, for example.
  7. Nov 3, 2005 #6
    How does that make a clock? Do you intend to stand there with a stop watch or something and time the light beam? The spring in the watch is what is measuring time, not the light beam.
  8. Nov 3, 2005 #7


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    The current acutal definition of time is a certain number of oscillations of a cesium atom. (This has been in use since 1967).

    see for instance

    This is not exactly a spring, but it is very close to a quantum equiavalent to it. A spring-mass clock will give very similar results, but with much less precision, since atoms are all identical and springs are not.

    For the units and defintions used when Einstein was formulating relativity take a look at


    Thus you will see that the second, in Einstein's time, the primary standard for time was based on astronomical measurements, and that the meter was based on a standard prototype bar in Paris.

    These were the defintions of units when relativity was formulated, and relativity applies to measurements in terms of these units, as well as more modern and accurate systems of units, which have changes as we have found more accurate ways to measure both time and distance (see the first wikipedia URL or the NIST site for current SI standards for time and distance).

    Spring clocks have absolutely nothing at all to do with the problem, I really have no idea why you think they would.
  9. Nov 3, 2005 #8
    This is true. I'm going to bed.

  10. Nov 4, 2005 #9


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    Einstein used the following reasoning:
    1) given the postulates, you can build a perfectly working clock with light rays
    2) we can calculate what this clock would show when it is moved
    3) since different types of clocks must change the same way with velocity, we can indirectly calculate what every type of clock would show - that is: time.

    of course you can use any clock you like, but afaik you can derive SR only from light clocks. That´s enough, because all sorts of clocks must agree.
  11. Nov 4, 2005 #10


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    You don't need a watch, you just count how many times the light has bounced back and forth compared to a pre-defined interval that that represents, a la pervect's statement that a second is defined as a specific number of "oscillations" of a cesium atom. You could do the same thing with a bouncing beam of light. You don't measure the oscillations, you just count 10 and define that 10 oscillations=1 second.
  12. Nov 4, 2005 #11
    What is wrong with this argument?

    Let a train be moving with velocity [tex]u[/tex] relative to a "stationary" person. Let one tick of a clock be when a ball, with a constant velocity [tex]v[/tex] (we ignore gravity) relative to the train, goes from the floor of the train to the ceiling which is [tex]d[/tex] meters. Relative to the train, its measurement of time [tex]t = d/v[/tex]. Relative to the stationary frame, the ball travels a distance of [tex]d'=\sqrt{u^2t'^2+d^2[/tex] with velocity [tex]v'=\sqrt{u^2+v^2}[/tex]. So the time [tex]t'[/tex] relative to the stationary frame is [tex]t'=d'/v'=\sqrt{\frac{u^2t'^2+d^2}{u^2+v^2}}=\sqrt{\frac{u^2t'^2+v^2t^2}{u^2+v^2}}[/tex]
    So [tex]t'^2=\frac{u^2t'^2+v^2t^2}{u^2+v^2} \rightarrow t'^2(u^2+v^2)-t'^2u^2=v^2t^2 \rightarrow t'=t [/tex].

    Therefore, since the laws of physics are the same in every reference frame, time is the same as well.
    Last edited: Nov 4, 2005
  13. Nov 4, 2005 #12


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    The problem with this argument is that the velocity of the ball is not v' = sqrt(u^2+v^2) at sufficiently high velocities.

    Note that the speed of light was observed to be constant before the idea of keeping time with a "light clock" was proposed. Until the speed of light was shown to be constant, there was no reason to propose using a light clock to measure time.
  14. Nov 4, 2005 #13


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    Just a quick note on timekeeping in Einstein's time. (No pun intended). While the primary standard for time in 1905 was astronomical time, chronometers were aparently used as a secondary standard.


    talks about how the US Naval observatory used a Frodsham clock (exact details of operation unspecified, but was probably a balance wheel) that was frequently reset based on astronomical events (the primary time standard).

    Frodsham has a website, but no information :-( - other sources indicate they were apparently highly respected clock-makers of the time, Frodsham being able to boast that their company made the clocks for the Queen of England.

    For the purposes of relativity, the primary astronomical time standard would not be particularly useful, as one cannot accelerate the entire solar system. Instead, one would have to rely on secondary time standards, like chronometers of the period, to measure the passage of time on a moving object.

    Quartz clocks apparently did not appear until the 1930's, according to
    http://inventors.about.com/library/weekly/aa072801a.htm, and from a later page in the same source, atomic clocks first appeared about 1949 with an amonia based clock from the National Institute of Standards.

    Pendulum clocks were also used in the period, and were among the most accurate clocks available in the 1900's according to this article, even as late as the 1920's. According to the first URL, however, the US Naval Observatory (USNO) did not use pendulum clocks on a routine basis in 1905, though apparently some pendulum clocks were in experimental use. Although accurate, pendulum clocks would probably not be suitable for use in a moving environement - the ship chronometer of the period would be best suited to any application that was not in a stationary, vibration-isolated room.

    That's as much as I could find out with a bit of googling - I'm not by any means an expert on history.
  15. Nov 11, 2005 #14
    What are the consequences in such a universe? Is it self consistent?


    I guess it would be a long way from our own....
    Last edited: Nov 11, 2005
  16. Nov 11, 2005 #15
    Isn't time just a measuring unit?! there is no real such thing as time, it is concentric/circular. Hence God saying he is alpha and omega. The reason we cannot fathom eternity is because of the way we've been taught to believe what time is. We always measure it in a straight line. Motion and energy. What happens if you have no motion or energy to measure? If light ceases to exist in the universe, does time? I don't know what physics says about this yet.. but it seems like quite the illusion to me at the moment.
  17. Nov 12, 2005 #16
    Your best taking this to the philosophy forums. Time is real, a measurement is real, therefore time is real. If time doesn't exist, then how can it be circular? Something isn't inheriently a circle if it doesn't exist. And when God said he was the alpha and the omega, it ment a beginning and an end, there is no beginning or end if time is a circle.

    What makes you think we can't fathom eternity? And we don't always measure time in a straight line, their are many theories that deal with circular time.

    Measuring what? Be more specific. Presumably if there is no energy or motion in a system, you will measure zero velocity and zero energy.

    Yeah, well stop being so philosophical for a second, look at the fact and you'll see the obvious conclusion.
  18. Nov 12, 2005 #17


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    No, seconds are a unit, time is a dimension like the three spatial dimensions, and it is, most certainly, real.
  19. Nov 12, 2005 #18


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    I agree, a question about "what is real" is definitely a philosophical question, not a scientific one.
  20. Nov 12, 2005 #19
    thanks for your guys feedback. You're right, but i don't get this last post... science is about what is real. We wouldn't even have science without questioning. We wouldn't even have physics, remember how Einstein came to his conclusions? Pure brainstoriming about what is real. Philosophy and science have to work together.
  21. Nov 13, 2005 #20
    No, he brainstormed about things that are real, he didn't brainstorm over whether or not they are real, like you are.
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