How is it we know the speed of light?

  1. I don't see how we can say the speed of light is X value. Here's why:

    Imagine you are driving 50mph then a cop comes up driving beside you at 50 Mph too. If he aims his gun out his window and clocks you it would read 0 Mph right? So to get your real speed he would have to look at his own speedometer first to determine his speed in relation to the earth's surface beneath his vehicle no?

    So if the Earth is spinning around the axis at X, slinging around X miles per hour in our solar system, and our solar system is slung around our galaxy at X unknown miles per hour, and our galaxy shoots around the universe at X unknown miles more, even then we still have to ask, is our universe traveling at X speed through a Mega-verse, then how is it that we can say the speed of light is always X without comparing that in relation to something that isn't moving at all in the mega-universe stopped or at least start calculating the velocity of our speedometer in relation to such a non-moving point in space, if one even exists.

    We don't even know the size of the universe, nor where the center point is, so how in world can we say that light travels across the universe at X miles per hour, heck, we don't even really know how long an hour is since time is relative, from that vantage point isn't light different speeds at different locations all over the universe as is relative to the speed of the non-stationary locations of a speed detector? I mean sheesh, if I were a particle of light, I would think "I" light travel at 0 miles per hour, am the center of all things, and consider everything else around me here on earth to be moving at the speed of 'mass' or 299,792,458 m/s, the exact opposite, you know, like when your car is stopped at the light, then the car beside you moves so you think your are moving, but nope, they are. Maybe light is stationary, that's why it doesn't need energy to move through space, and lives forever, because it is sitting in one place until we matter slam into it at the speed of light?!

    It doesn't make a bit of sense at all when you think about it... Why do we always consider ourselves to be the stationary center that can judge the speed of all things when clearly it is we that are in a constant state of movement expending tons of energy that we can calculate to keep us in motion?!

    It makes more sense to say light moves at 0 mph, we are in fact the ones in motion!
  2. jcsd
  3. I really did not read your entire post, its more like a rant than science, as for measuring the speed of light, you could easily measure time time light takes to reflect from a mirror and get back to the source, and using the distance between the reflector and source, find the speed of light.

    Also there are many other relations from where c can be measured, you could measure the ratio of the magnetic and electric field amplitude of a EM wave, and the ratio and the value of permittivity or permeability of space would give you the speed of light.

    You could measure the energy released when mass defect takes place, and equate it to m*c^2

    Also measuring the wavelength and frequency of an EM wave in space.
  4. You really need to read a book on special relativity. This was an issue with Maxwell's equations in the late 19th century. People wondered, in what frame of reference are these equations correct. This led to false ideas about the ether.

    Einstein came along to solve the issue. He claimed that in all inertial reference frames (frames of reference moving with a uniform velocity where Newton's law of inertia is still valid) you will measure the same speed of light no matter what.

    So, if you are on the ground and throw a baseball at 50 mph, then make the same pitch while on a moving car going 50 mph, the baseball will be traveling at 100 mph. However, if you turn on a flash light standing still you will see it leave you at the speed of light. If you also watch someone turn on a light while they are moving 50 mph relative to you, it will still be going at the speed of light, not 50 mph greater.

    This creates some really strange physics. First of all, you and the guy driving in the car will effectively measure the speed of light to be the same. This seems impossible, but it isn't. There is so many more consequences of this fact. That I'm not going to say any more, you just need to read up on special relativity. Also, if you don't believe me and you claim it's impossible, tell that to the thousands of experimental physicists who show everyday that special relativity is scientific fact.

    I will throw in one more interesting thought experiment though:

    Imagine you are on the ground next to a train traveling at .75 times the speed of light. Also, imagine you can see inside the train. Now, on the train someone throws a baseball in the direction of the train's movement at .75 the speed of light. You can't just add the velocities to get 1.5 times the speed of light. Effectively what happens, is you observe the people on the train to be passing through time at a slower rate than you are. You will see them moving slower, you will see them throwing a baseball slower. No matter what, you can't violate the ultimate speed limit of light.
    Last edited: Jun 7, 2011
  5. singhvi - that doesn't make any sense because both mirrors are moving here on earth but we don't know in what direction in relation to a non-moving position in space time since we don't have anything to compare it to.

    silmaril89 - Oh, yeah, that makes sense, because time stops for the observer from their point of reference if they are light, and why light speed is a constant, no matter how you look at it, as zero or infinite, they both give the same answer when used in a calculation if time is relative to the observer and the calculations are done from an outside point of reference considered stationary, I just always see things from light's point of reference.

    I've never read anything on relativity, was just thinking, but I read up on special relativity just now, Galileo & Einstein clearly state the same as me in their definition: "there is no absolute and well-defined state of rest." Nope not for anything that exists in the universe there isn't it just simplifies things to me to think everything is infinitely fast except light which doesn't move, and gravity as like a kind of vacuum of nothingness that wants to be filled with whatever concentrated mass nearest by first-most.

    I always try to simply things to their simplest form so I don't have to remember crazy complicated formulas to understand the simple world around me.

  6. russ_watters

    Staff: Mentor

    The cool thing about the speed of light is that it is always the same, regardless of the speed of the observer. That's the primary point in Einstein's special relativity.
  7. No problem. The fact that the speed of light is the same for all observers makes it so there is not one "rest" frame. There is no better reference frame than anyone else.

    This was a good question for not having any knowledge of special relativity. It was a very puzzling questions for many physicists until Einstein came along. Before him, they had an idea called the Luminiferous aether. Check it out, it's very bizarre what they came up with to try and explain your question. Also check out this experiment.
  8. Special relativity brings up another question I've had on my mind, how is it that we know we are where we currently are in our galaxy/universe if our changing views of space-time as a travelling observer tell us were are absolutely positively not located right here right now where we perceive ourselves to be located?

    For example it appears we are currently close to crossing the axis of our galaxy, but it takes years for anything to reach us here where are, and we don't know our current velocity in the galaxy, so how is it we can make a map of the galaxy and label the planet earth as X "you are here" like a mall map or something. Can our computers really compensate for all the space-time curvatures between here and the other side of this galaxy so easily?

    I'm reading about the aether now. This should be fun.
  9. Drakkith

    Staff: Mentor

    Quick question. Say light is emitted from a stationary frame, and is detected later in another frame that is moving 75% the speed of light directly away from the original frame.The 2nd frame is 1 LY away when the light is first emitted. It will take much longer for light to reach the 2nd frame compared to if the frame was stationary at 1 LY, correct?
  10. Drakkith

    Staff: Mentor

    We do know our current velocity through the galaxy. While the distances are immense, the time it takes for light to cross back and forth is MUCH MUCH faster than speed of anything moving around the galaxy. Yes, the lag in position due to the speed of light is able to be compensated for, as it is not THAT much anyways compared to the scale of the galaxy.
  11. I'm not worried about the time it takes for light to travel here from other point, that is easily compensated for I'm sure, I'm worried about the effect space-time has on light and/or the radio-waves we use to estimate where things are actually located on the other side of the galaxy. The smallest planet between here and there, a large black hole, or medium sized star can and will cause space-time to curve depending on it's concentrated mass. Since we can't guess, see, nor detect and calculate all the masses that might be between here and the other side of galaxy it should be by all means impossible to actually know where things are located even more so on the exact opposite side of the center, much less, where we are located in relation to them. Calculations cannot even be close to correct, and when I say not even close I mean, at least 1-3% error, so we could actually be hundreds of light years away from where our computers calculate us to be. Our current location can only be a close estimate at best...
  12. Drakkith

    Staff: Mentor

    Sure, our position is most likely off by at least a few light years depending on where you are measuring from. Most of that is due to limitations in the way we determine distance on a galactic and universal scale, not on the light itself. Most of space is pretty empty. Almost all of the light from any visible stars we can see is taking an almost undisturbed trip straight to us. Even if there was a black hole between us and something else, they immense distances between everything means that unless the black hole is DIRECTLY between us and that star, the light is almost entirely unaffected.

    Also, overall the combined affects of gravity on light results in no net change, as on a galactic and universal scale things are mostly homogenous over very large distances. Everything "cancels" everything else in effect if that makes sense.
  13. WannabeNewton

    WannabeNewton 5,765
    Science Advisor

    You have to take time dilation into account since the frame is moving at 75% the speed of light with respect to the other frame.
  14. Drakkith

    Staff: Mentor

    I know. Maybe I forgot to say that I want to know how long it takes from the frame of reference of the stationary frame. Or am I misunderstanding something? Can you determine "when" the light is detected at the moving frame and compare it to a non moving frame?
  15. Depends, from your point of view, for the light, it would arrive at the moving frame the same time it left, from the moving frames point of view would arrive 1 LY later no matter how far it travels since times slows for it as it get faster, but from our point of view, as the stationary frame it would take longer than 1 LY to reach the moving frame.

    The way I see it though, we are wrong, it takes 1 LY no matter what to get there, since light is constant, we just perceive that it took longer to get there because we are moving away from both the frames at a relative velocity, if that makes any sense.
  16. Drakkith

    Staff: Mentor

    A lightyear is a unit of distance, not time. But I think I undestand what you are saying.
  17. BruceW

    BruceW 3,532
    Homework Helper

    You say the second frame is 1LY away when the light is first emitted. But you haven't said which frame this distance is measured in. The distance is different when measured by the 1st and 2nd frames.
  18. BruceW

    BruceW 3,532
    Homework Helper

    To answer the question, yes it takes longer for light to go between two points that are moving away from each other than for two points that are stationary with respect to each other
  19. BruceW

    BruceW 3,532
    Homework Helper

    (Think about it) the speed of light is finite, and the separation between the points is increasing with time, so light takes longer to travel between the two
  20. I like Tesla's aether better...

    And as far as our location using light as a point of reference, we know something really huge, most probably a black hole or bigger is at the center of every galaxy, so the sling-shot effect of the light we do receive here on earth is most likely huge. The stars we see bundled up in the middle could be practically anywhere until we know exactly what IS at the center and can enter some real formula to compensate for it.

    It would take 1 & 1/3 LY for the moving object to reach the other stationary frame but only 1 year would pass for the moving object, to put it simple, I assume that's how they measure light years, but I would think the moving object would only see 1/3 of a year pass, which is right?
  21. Drakkith

    Staff: Mentor

    That's what I was thinking. I had just never seen anyone ask it before and wanted to make sure.

    We aren't using light as a point of reference, we are using ourselves and measuring everything else in relation to that. And no, the slingshot effect from the black hole at the center of the galaxy is EXTREMELY small. If it was strong enough to greatly affect light all the way out where we are at, the entire galaxy would be pulled into the black hole, which is not happening.

    Edit: Also, we do have a very good measurement for the black hole in the middle of the galaxy. We have measured the velocity of the nearby stars orbiting it and have been able to determine a good estimate of its mass. See here:*
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