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Relativity And Light

  1. Sep 28, 2006 #1
    Why is it that relativity doesn't work with light? I'm trying to understand
    if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?
  2. jcsd
  3. Sep 28, 2006 #2


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    (I'm going to use 0.9c for your velocity because massive objects can't travel at c.) In relativity, the "velocity addition" formula is different from what you're probably accustomed to. Instead of u' = u + v = c + 0.9c = 1.9c you have to use

    [tex]\frac{u + v}{1 + uv/c^2} = \frac{0.9c + c}{1 + (0.9c)(c)/c^2} = c[/tex]
  4. Sep 28, 2006 #3
    Ok but it doesn't explain it.
  5. Sep 28, 2006 #4
    Well think about it, how do you know you are moving towards it with a certain speed? :wink:
    It is always relative to something else, and how do you know that it is you who is moving.
    When someone states that something is going at x% of the speed of light it is always relative to something else. Everything with mass is always going at 0% of the speed of light in an absolute sense.

    Only a change in movement, e.g. acceleration is absolute, but even there you can have the situation that some frames think you decelerate while others think you accelerate.
    Last edited: Sep 28, 2006
  6. Sep 28, 2006 #5


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    You can't move at the speed of light.

    As other posters have mentioned, for any velocity lower than 'c', if you move towards the light source (or if the light source is moving towards you) you will still measure the speed of light as 'c'.

    The later case has been measued in the laboratory - the speed of gamma rays emitted from VERY fast moving particles has been measured, and been found to be equal to 'c', regardless of the motion of the source.
  7. Sep 29, 2006 #6
    The speed of light is constant no matter how you are moving or how the light source is moving. This is fact and has been well proven.

    To answer your question then something has to give. It turns out that the things we expect to be always constant - time and distance - are not.

    You have to get over this fact before you can start dealing with all the apparent paradoxes in relativity.

  8. Sep 29, 2006 #7
    I believe Line is trying to ask, what is it that justified the idea that the speed of light is isotropic in the first place.

    After Einstein had considered simple emission theory (where speed of light is measured as C + V of the emitter), he couldn't really make it work and started to consider other options. Usually when we think about motion, we think about ordinary newtonian velocity addition (C+V), but Einstein started to ponder on the possibility, that different "inertial frames" are symmetrical also by the speed of light they measure.

    It is not immediately obvious how one could make that work. Like he put it himself, the "principle of relativity" (newtonian) and the "principle of the constancy of the velocity of light" are two "apparently incompatible" principles. But there was a way out if that mess.

    The key to relativity theory (and this is how relativity should be taught to mainstream), is to understand, that it is an arbitrary assumption about reality, that two events that happen in two different places "simultaneously", are really simultaneous in an absolute sense. When we receive information about event A and event B that happened in opposite directions, we cannot really say which one had happened first. So to replace newtonian addition of velocities, Einstein could attach a different notion of "simultaneity" to each inertial frame, in such way that when a beam of light does hit an observer, we can figure out just when the light started its journey by claiming it moved at the speed C. And the way to make this work as a coherent whole, is to perform Lorentz-transformation on spacetime between inertial frames. (hence the idea that reality is a spacetime)

    Note how the relativity is simultaneity is NOT about the order in which you observe events, but it is an assumption made about the real "now-moment" being relative, so to replace newtonian velocity addition.

    And when you consider motion this way, it just follows that by accelerating towards another object, your notion of simultaneity is also changing in such manner that you can never reach relative speeds more than C between each others. And what also follows is time dilation, and length contraction; The geometry of an object is defined by where each of its element is in space simultaneously, but as notion of simultaneity changes, the back and the front (to the direction of motion) of the object are found to exist closer to each others "simultaneously".

    Well, by invoking the idea of length contraction may be saying too much. Because after all is said and done, you should be pretty careful with the ontological idea that there really is a spacetime instead of motion.

    Here is a good longer read about how Einstein got to relativity:
  9. Sep 29, 2006 #8
    To measure the speed of any thing, you must register its position at one moment, and then again at another moment.

    If you allow for adjusting these "moments", you can always get to isotropic C for light (as long as its the absolutely fastest thing in the universe, for anything faster would be moving backwards in time in some inertial frames, and so it would break the whole logic of static spacetime).

    Well, you are talking about a model that is completely different from SR. I personally think it is very much possible, if not even likely, that the speed of light is not constant, but the path it takes is a result of the complex "web of matter" that is present in the situation. But that is completely different matter, what is important here is that SR is logically valid model as far as current observations and useful predictions go. Emission theories are not completely unproblematic either, albeit somewhat possible.

    Nevertheless, just saying "it is so" doesn't get us anywhere if we cannot show that it really is so.
  10. Sep 29, 2006 #9


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    Relativity does work for light - it is about light! That's the whole point (the reason it was developed) - to explain/deal with the observed behavior of light.

    What you are really asking is why does light not work the way particles work and the answer is (redundantly): because it is light! It has no (rest) mass so it travels at C.

    If, then, you are meaning to ask why light behaves like light -- well, why is anything the way it is? It just is. And that's not a question for science.

    AnssiH - neophysique deleted the post you responded to and it would probably be best if you deleted your response to avoid confusion...
    Last edited: Sep 29, 2006
  11. Sep 29, 2006 #10
    And why is it that light can't rest?
  12. Sep 29, 2006 #11

    No something goign X% of C is relative to the speed of light.And what's this that any mass is always movign at 0% of C. In that case they're not moving. WHat about cosmic rays, they move at near the speed of light.
  13. Sep 29, 2006 #12


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    No, MeJennifer is correct, you can only talk about about the speed of an object relative to another, or relative to a particular reference frame. When people use a phrase like "0.8c", what they mean is that in the reference frame they're using, the velocity of the object is 0.8 that of light in that particular reference frame, but in other reference frames it'd be a different fraction. Light travels at c in all reference frames, and relative to all observers--so if you are travelling at 0.8c in my reference frame, that doesn't mean you will only measure the light beam to be moving away from you at 0.2c, you will still measure the light beam to be moving away from you at exactly 1 c in your own reference frame.
  14. Sep 29, 2006 #13
    Can someone give me a down to Earth explanation why this works?
  15. Sep 30, 2006 #14


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    Newton's laws are valued for their simplicity, and they are reasonably accurate when relative speeds are low. An "inertial frame" is a coordinate system in which Newton's laws hold (e.g., we assume that Newton's laws are accurate, and we build a set of coordinate systems that make this so).

    Newton's laws become less and less accurate as relative speeds increase, and they become completely wrong as speeds approach c. This is because we measure time with clocks and distance with "rulers" (theodolites), and these measuring devices are physically affected by speed changes.

    Special relativity is a tool for describing mathematically how two or more inertial frames relate to one another. It is a mixture of mathematical and physical concepts.
  16. Sep 30, 2006 #15
    'Light' is a manifestation of disturbances in the electromagnetic field. Disturbances in the electromagnetic field propogate at a constantly-measured speed because they do. There isn't an explanation, that's just how fields behave. It's kind of like a first cause.
  17. Sep 30, 2006 #16


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    We can measure round-trip speeds in a coordinate-system independent way, and the round-trip speed of light is isotropic. We can not measure any one-way speed in a coordinate-system independent way (at least not so far).
  18. Sep 30, 2006 #17
    I'm not sure whether you're disagreeing with me or not, there. Am I being quoted because I said something inaccurate?
  19. Sep 30, 2006 #18


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    Your statement may be accurate if you are only talking about round-trip EM field propagation, but it is not accurate if you are also talking about one-way propagation. The difference is that we can actually "measure" round-trip speeds (isotropy) in a coordinate-system independent way, but not one-way speeds; at least not so far.
  20. Sep 30, 2006 #19
    It's difficult to get a handle on all this if you're lumbered with light travels at the speed of light. It's rather like talking about bullets travelling at the speed of bullets. In a weird kind of way light doesn't move at any speed. What relativity is all about is how we perceive things like time and speed via comparison of electromagnetic propagation within our atoms, bodies, and clocks, against electromagnetic propagation over some perceived distance. All this can get very deep very quickly, so to keep it simple for now: instead of thinking light travels at the speed of light, think light travels at the speed of time.
  21. Sep 30, 2006 #20

    Doc Al

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    Give us a break! :rolleyes:

    Much better would be to accept that fact that--per relativity--there is a "speed limit" built into the very structure of space-time itself. This speed limit, which happens to equal the speed of light in vacuum, affects the kinematic behavior of everything: light, bullets, you name it.

    The implication for light itself is that, as has been mentioned several times in this thread, light must travel at speed c as measured by any inertial observer.

    To really understand what this means you'll have to break down and learn some relativity. (I recommend N. David Mermin's latest pedagogical effort, "It's About Time".)
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