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Does light recalibrate its speed?

  1. Feb 14, 2012 #1
    Hello, I'm looking to understand the concept of the constancy of the speed of light. In particular, I think I need to understand what is meant by the speed being constant for all observers. Even more particular, I'm looking for experimental evidence that I can understand that will show me that light does/does not change speed by the action of being observed.

    Suppose I have a system for measuring the speed of light involving beam splitters, mirrors, gears etc. I become satisfied that the speed of light within that system is constant throughout. I realize that all of the devices in the system are moving at the same relative speed as each other, so it doesn't tell me much about the speed of light relative to a moving object. I consider letting light from a nearby planet enter my system for measurement. I realize that since my device is on the moon, say, the light will be traveling in a vacuum all the way up until it interacts with the first atoms of the measuring device.
    I become disappointed, because I realize that the device will have tampered with the light that I wished to have measured. For all I know, from the perspective of this device, the speed of light changed from c+v to c upon physical interaction at the boundary of my device--a sort of recalibration. I seem to only be able to measure light that I've interfered with.

    What experiments have been done regarding the speed of light where light is closing in on a moving observer and where the speed measurement is done in such a way as to clearly not interfere with the speed? I'm thinking that such a measuring technique must enclose the starting and ending points, but I know there are some pretty clever scientists too.
  2. jcsd
  3. Feb 14, 2012 #2


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    As long as your measuring apparatus is also in vacuum and doesn't interfere with all the light coming from the planet, why would you expect the light to be effected by your measurement? I'm thinking, for example, of a shuttering device that blocks the light and then unblocks it to let some light through at a measured time. Why would you think that as soon as shutter opened, it would have any more effect on the remaining light? Then you let the light progress to a sensor some distance away that measures when the light reaches it. If the difference in the two times is always the same, no matter where the light comes from, wouldn't you have confidence that the light was propagating at the same speed in every case?
    Those experiments have been done, essentially as I have just described it. And they have been done simultaneously on the light coming from two different stars that are moving with respect to each other (binary stars in orbit around each other). All these experiments show that the speed of light is independent of the motion of the source.
  4. Feb 15, 2012 #3
    Thanks for your reply.
    I wouldn’t expect a non-interfering apparatus to affect the measurement by definition :smile:. My understanding is that vacuums don’t contain mirrors. My original device contained a mirror/splitter which interrupted the vacuum.
    I wouldn’t initially think that it would have an effect, but now that I’m asked, I have two thoughts on the topic: 1) I’ve read of double slit experiments that weren’t supposed to interfere with the light in a given slit, but somehow it seemed that the intent of measuring the flow through a slit was sufficient to affect the outcome of the experiment drastically. 2) I’ve opened a shutter (comprised of my hand and a copper pipe) and allowed a magnet to fall vertically through. Even though the magnet did not contact the shutter physically, the magnet's speed was regulated by the fields that existed at the time the magnet was passing through the shutter. It doesn't initially intuitively seem like copper and a magnet would have this relationship, so I'm open to understand boundary conditions of other experiments.
    Yes, I would have confidence in the speed being the same in those cases, at least between the front edge and back edge of the device. I would like to understand the boundary conditions of the shutter to see about extending that confidence to the region at and in front of the shutter.
    I think I’m already satisfied with speed being independent of the source, since even sound and water waves offer that independence, so I’m really more interested in understanding the case where the observer is closing in on the light which seems like a different case. The binary star experiments that I know of were prior to the age of space exploration. Although they speak to the sameness of the speed of the twin lights, those older experiments would have been measuring light that spent the last leg of its journey through the earth’s atmosphere which, at least in my imagination, could serve to regulate the twins to a new speed relative to the atmosphere (argh—now that I think about it, even the earth’s magnetic field).

    I wish I knew of a specific canonical experiment that gave me confidence about the boundary conditions. The one atmosphere-free measurement that I have recently read about involved the GPS satellites, but that experiment claimed to have resulted in a measurement of v+c, so I’m now disoriented.
  5. Feb 15, 2012 #4


    Staff: Mentor

    The phenomenon you are talking about is known as optical extinction and is a well-known criticism of many speed of light measurements. However, because it is a well-known criticism there are also many experiments that are not subject to it. This is a good place to start:
  6. Feb 15, 2012 #5


    Staff: Mentor

    Please post your source for this. I suspect it is probably an unreliable source.
  7. Feb 15, 2012 #6
    Last edited by a moderator: May 5, 2017
  8. Feb 16, 2012 #7
    That is a very unreliable site, for sure much less reliable than this one. The claim is probably due to a simple - but frequent - misunderstanding that also appears to be yours: the mere equation "v+c" is in itself not in contradiction with SR. As a matter of fact, the similar (c-v) is even part of Einstein's 1905 derivation of the Lorentz transformations. And sure, GPS also uses it.
    In jargon it is called "closing velocity". For example, assume that a radio wave moves at c in one direction, and a GPS receiver in opposite direction. The relative speed (or closing speed) is then c+v by definition, because the speed of light is c and that of the receiver is v as measured in that reference system.
    Last edited: Feb 16, 2012
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