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Infinity for a Photon

  1. Jun 27, 2015 #1
    According to relativity, from a photon's frame of reference time is instantaneous, correct? So in an instant a photon would, to its frame of reference, experience being absorbed immediately after its creation, as well as hundreds of years of travel through space in the same instant.

    Say hypothetically one specific photon would never be absorbed. Perhaps most mass in the universe is converted to energy and there is nothing that would absorb it. If it kept on going forever (whether or not it would in reality) wouldn't it experience infinite amount of time instantaneously? How would that even be possible, since an instance has an end but infinity doesn't?

    I apologize if this question's premises are too out of line to answer.
  2. jcsd
  3. Jun 28, 2015 #2


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    Staff: Mentor

    According to relativity theory, there is no "photon frame of reference". Therefore, asking what time (or anything else) looks like from the "point of view" of a photon is a meaningless question, in the context of relativity theory.

    To elaborate on this a bit, an "object's frame of reference" means a reference frame in which the object itself is at rest. A photon always travels at speed c, in any inertial reference frame. Therefore, there is no inertial reference frame in which that photon is at rest.

  4. Jun 28, 2015 #3


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    Assigning a frame to a photon is like dividing by zero. You're just gonna have a bad time if you try.
  5. Jun 28, 2015 #4
    Does something have to be aware to have a frame of reference? Wouldn't it have a frame of reference by the mere fact it exists?
  6. Jun 28, 2015 #5


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    From wiki: In physics, a frame of reference (or reference frame) consists of an abstract coordinate system and the set of physical reference points that uniquely fix (locate and orient) the coordinate system and standardize measurements.

    A reference frame is something we 'assign' to an object in order to talk about its position relative to other objects (among other things). Typically, or at least here on the forums, the object whose reference frame we are 'observing from' occupies the origin of a coordinate system. In other words, we place the object in the center of a 3-D graph (our coordinate system) so that its position is (0,0,0) and treat it as stationary with respect to our coordinate system. We then state the distance to other objects relative to this point. This allows us to simplify calculations and concepts and bring up things which aren't so obvious if our object is moving relative to our coordinate system.

    We can them 'transform' our coordinate system from our original object to another, placing this new object at (0,0,0) and making it stationary with respect to this coordinate system. When we do this, all of our measurements have to change too. For example, if Object B is located at (10,0,0) and traveling at 10 m/s with respect to Object A's coordinate system, when we transform our frame to that of B, it is now object A that is moving at 10 m/s and B is now stationary at (0,0,0).

    Now, the reason I bring up all that, is because one of the postulates of special relativity says that light travels at c regardless of the motion of the observer or the light source (commonly stated as light travels at c as measured by any inertial, or non-accelerating, reference frame). What this means is that no matter how our system of objects are positioned or how fast they are traveling with respect to their different frames of reference, they will always measure light as traveling at c according to their coordinate system. So if object A shoots out a beam of light, it will measure the speed of the light as c. If I then transform my reference frame to that of object B's, which is moving away from A with a velocity of 0.5c (50% the speed of light), you might expect B to measure the light as traveling at 0.5c. But it doesn't! It measures the light as moving at 1c!

    This fact, that light travels at c as measured by ALL inertial reference frames, means that you cannot assign a reference frame to light at all! Remember that if we assign a reference frame to something what we are really saying is that we are centering a coordinate system on the object and treating it as stationary. But light cannot be stationary, so we cannot assign it a reference frame!
  7. Jun 28, 2015 #6


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    When we speak of something's reference frame, that's a convenient shorthand for a reference frame in which that something is at rest. Light moves at speed c in all reference frames, so there are no reference frames in which light is at rest.

    Jtbell has already linked to the FAQ on the subject.
  8. Jun 28, 2015 #7


    Staff: Mentor

    Awareness is completely irrelevant in this context. A rock can have a valid frame of reference. Please read the FAQ that has already been referenced multiple times.

    Thread closed.
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