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Special relativity and the speed of light constant

  1. Jan 8, 2015 #1
    Hi guys. I know that light does not travel through time. Is that because its speed is constant for all observer?
  2. jcsd
  3. Jan 8, 2015 #2


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    I don't understand what you mean. Light travels across time. It takes time for light to get from one place to another.

    If you mean that in the frame of reference of light it sees no change in time then you have a fundamental misunderstanding (a common one) because there IS no "frame of reference of light" and so the thought that it does not experience time is a non-starter.
  4. Jan 8, 2015 #3


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    Light travels through space not through time. But no observer can determine the speed with which any particular beam or ray of light travels. However, all inertial observers can measure the round trip "average" speed of two beams or rays of light one going from a source to a mirror and one reflecting back from the mirror to the source. The observers have one clock or timer at the source to measure the time interval and a ruler to measure the distance between the source and the mirror. From that they can calculate the "average" speed of the two beams or rays and they will get the same value called "c". So what we do in Special Relativity is define Inertial Reference Frames (IRF's) in which all beams or rays of light travel at c no matter what their directions. This enables us to establish a consistent set of coordinates for any IRF.

    Does that make sense to you? Any questions?
  5. Jan 8, 2015 #4
    That's a perfect subsequent comment that helps clarify the first.
  6. Jan 8, 2015 #5
    How can anything travel through space without the passage of time? Any observer who can see a photon travel through space will always be able to also account for a passage of time. If you meant from the perspective of a photon, since it doesn't experience time it cannot experience space (separation), in that context they are non-existent and therefore such a concept (travel through space without time) is nonsensical.
  7. Jan 8, 2015 #6
    He didn't say there is not "passage of time"....whatever you intend to mean with "passage". He said light doesn't travel through time, which could mean any number of things.

    Wiki - In a light-like interval, the spatial distance between two events is exactly balanced by the time between the two events. The events define a squared spacetime interval of zero ([PLAIN]http://upload.wikimedia.org/math/6/6/0/6602be59bc8c0b4a73716ac3e62afa31.png)[/U]. [Broken] Light-like intervals are also known as "null" intervals.

    Consider 1. spacetime relation/mink-metric and 2. the spatial interval is exactly equal to the time interval.

    That's one of many possible answers for the second comment about the difficulty (impossibility to absolutely) measure the one-way speed of light.
    Last edited by a moderator: May 7, 2017
  8. Jan 8, 2015 #7
    Link, the definition of "through": http://www.merriam-webster.com/dictionary/through

    Anyway you look at it using the correct definition amounts to the same meaning of "Light travels through space not through time", that light can somehow experience movement in space without movement in time.

    The space time interval you brought up has nothing to do with it, if you believe otherwise please explain how a light-like interval is related to no experience (travel through) time? Because even that interval has both components of time and distance at its core.
  9. Jan 8, 2015 #8
    So we see light travelling through time due to our passage through time? And speed of light isnt c?
  10. Jan 8, 2015 #9


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    I can't imagine how you could come to that conclusion based on anything that has been said here.
  11. Jan 8, 2015 #10
    Think of it this way, space and time are "linked" together, they form space-time, if you are not moving through space(remaining completely still), you move through time. So now think of it this way, you are moving north in an empty road at a certain speed (say 100 kmph), your advancement toward the north would be equivalent to your speed (100 kmph). Now you take a road northwest at the same speed (100 kmph), this time however,your advancement towards the north in this road is slower than the north road. If you wanted to go north(say north was a flat line), going northwest would only slow you down, you would get there, but slower. In this case, north is time, and west is space, the faster you move in space, the steeper the angle of the "northwest", that is why objects at the speed of light would move slower than ordinary objects.
    There is a book about this called "The Fabric of the Cosmos", there are also documentaries based on the book with the author himself as the host, you can watch them online and they are very helpful.
  12. Jan 8, 2015 #11
    Don't think of time as a flowing river, it is more of a frozen one.
  13. Jan 9, 2015 #12
    Not surprising as i based it on another thread which gives this analogy:

    Pretend you are in a service elevator in a building under construction. You get in the elevator and ascend to the top (it is a one way elevator).

    As you go up through level after level of floors, you realize that there are loooong strands of yarn tied to pillars from floor to floor at all sorts of angles. The first piece is tied to the the 2nd floor pillar right next to the elevator. The other end of that same piece is tied waaaaay up on the 6th floor to a pillar out on the north east corner of the building.

    You can only experience one floor at a time, so you only see sections of yarn that are horizontally in your line of sight. What you see is that the line of yarn (that is, only the short section you can see on the floor you're on) starts very close to you (right by the elevator) and "moves" away from you, ending up, four floors later, way out at the north east corner. When you were on the first floor there was no yarn to be seen, and when you passed the sixth floor, there was no yarn to be seen.

    You have experienced this bit of yarn as an apparent movement through the building'sspace as a function of yourtravel through the floors (and, incidentally, through time). It was "emitted" on the second floor, and "absorbed" on the sixth floor.

    All the while, the string has had no experience of "moving" from floor to floor, no experience of emission or absorption - or of any "time" whatsoever.

    Not sure if this is a good one.
  14. Jan 9, 2015 #13
    For light, wont it have a graph that has the gradient of c? The graph will still show that light travel through time too.
  15. Jan 9, 2015 #14
    What do you mean by no frame of reference of light? Forgive me as I just started this.
  16. Jan 9, 2015 #15


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    You would be well served to read the entries in the Cosmology FAQ. It explains this and many other things.
  17. Jan 9, 2015 #16


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    Once again, I have no idea what you are talking about. I cannot imagine how this "analogy" has anything to do with whether or not the speed of light is c.
  18. Jan 9, 2015 #17
    Then, surely what you meant to say is that light does not experience time (if light could experience anything, which of course it can't anyway). According to SR, the closer anything goes to the speed of light, the slower its natural processes proceed according to a "stationary" reference system, and at the limit of the speed of light (which can never be fully reached), its proper time would stop - everything would appear to happen in a single instant (for that object, but not for us). Consequently, you could say that proper time of a light ray is "frozen", or that light does not "experience" time.

    That has been derived based on two assumptions, each of them based on observations, and which you mentioned in one phrase:
    1. The measured speed of light in vacuum is the same for all standard inertial reference systems (a consequence of the "relativity postulate").
    2. This speed of light is a constant, independent of the velocity of the light emitter (the "light postulate")

    Special relativity is based on those assumptions, as you can see in the introduction here:
    together with the simpler derivation here (equations 1 + 2 are what you refer to with "its speed is constant for all observer"):

    Does that help? :cool:

    We see light traveling through space as a function of time because we are not radiation! Thanks to that we have functional clocks and rulers with which we can set up reference systems to measure "time" and "space".

    The one-way speed of light and everything else are the speeds as determined with our measurement instruments, which have been adjusted to our reference system of choice according to convention. This is explained in the second section of the first reference ("Definition of simultaneity"). The subtle but important point is that in practice people could not - and according to the theory we cannot - establish a "true" one-way speed of light; and because of that, we simple "make" the one-way speed of light in vacuum relative to our measurement system equal to the measured two-way speed. And this speed is usually indicated as c. (if you now read post #3 again, you may understand it; I think that it did not exactly answer your question but it's important for understanding special relativity).

    PS as a matter of fact, the second reference http://www.bartleby.com/173/
    gives a neat introduction to special relativity by Einstein. :)
    Last edited: Jan 9, 2015
  19. Jan 9, 2015 #18
    The interval has everything to do with it and is how spacetime distance is modeled in simple mink spacetime.

    You are taking what ghwellsjr said to strictly, as I said he could mean any number of things with "light doesn't travel through time". Personally I don't like it (particularly because he said it does travel through space), but he did "qualify" it with the comment after; referring to the difficulty of pairing simultaneous timed clocks and then moving one of those clocks to the "finish line" while still maintaining the timing between the clocks & having an accurate measure of the length traveled, and issue of moving through spacetime, no need to separate the two dimensions as mutually exclusive and actually is wrong.

    you said
    For the underlined part, strictly speaking you are right, and it was it doesn't makes sense to say what you said, it makes perfect sense to say light (speed) is a null interval.

    yes that interval has both "components at its core", that is not the interesting part. The interesting part is they (time/space) are perfectly balanced and that it is invariant. That doesn't mean space and time are non existent, it means the interval is null. If only it would make physical sense to ask "What would the photon measure?" Since it doesn't we could just look at different boosts at increasing speeds, calculate the resulting interval and eventually see at c, [PLAIN]http://upload.wikimedia.org/math/6/6/0/6602be59bc8c0b4a73716ac3e62afa31.png[/I]. [Broken]

    It's fine to intuit what the photon may experience, that is no space, no time. It's fine to say if a photon could carry a clock it would read zero and that's because it's clock would have no length in direction of motion so the clock doesn't even exist in that direction and even if it did it wouldn't matter because the ruler that the photon has to measure length has no length in that direction of motion....or we could say it's a light like/null interval. 6602be59bc8c0b4a73716ac3e62afa31.png

    That could be said as independent of comparative measures of length and time every observer measures a photon to have a null interval. independent of the variance in measures of length and time. Seems it doesn't matter what one measures for the distance through space and through time that a photon travels we all calculate the same rate of c, note we live in a continuum with a maximum "rate" of c.
    Last edited by a moderator: May 7, 2017
  20. Jan 9, 2015 #19


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    The "reference frame of X" is a frame where X is at rest. There is no such frame for light.
  21. Jan 9, 2015 #20


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    I don't understand why you have taken offense at my post. If you had read the whole thing you would have seen that I specifically addressed your concern. I said that you have to measure the round trip distance that light takes going to and from a mirror and you have to measure the time interval that it takes for the light to travel that total distance and from that you can calculate an "average" speed.

    No observer can see a photon traveling through space and thereby account for the passage of time it takes for the photon to get from one point to another. As I said in my first post, we define the speed of the photon (a one-way propagation of light) to be the same as the measured two-way speed of light in any Inertial Reference Frame. This is a matter of definition, not a matter of observation.

    The OP said "light does not travel through time". I was agreeing with him.

    You should look up the definition of "travel". That's what I was addressing. Things that travel, change their location and if you want to know the "average" speed, then you have to measure both the total distance traveled and the total time taken. I already said all this. Please read my post thoroughly before you react out of context to one sentence.
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