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Time dilation and the perception of light

  1. Jun 13, 2012 #1
    Hi All. I have a little bit of a strange question I'm trying to figure out.

    If a human body was undergoing time dilation relative to another object, we believe that their brain, and therefore mind would slow down along with the rest of their body, correct?

    If this is so, and light continues to hit a time-dilated person's eyes at a constant rate (because c is constant), would light accumulate on their retina and cause their vision to become blurred or brighter than usual because it's hitting their eyes faster than their brain can process it?

    I suppose the same question applies to sound as well, though the speed of sound is not constant.

    I may be off base here and have some incorrect assumptions. Thanks for humoring me!
  2. jcsd
  3. Jun 13, 2012 #2


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    How can the light be hitting their eyes both at a constant rate and faster?

    And why are you concerned that a human body is undergoing time dilation relative to just one other object--there are billions of objects out there, which one is going to make a difference to the human body?
  4. Jun 13, 2012 #3
    Sorry, I know my question wasn't very clear.

    Let me try again.

    Say there is a light source, and I am far away from it. I begin moving towards it, and as I do so, time dilates for me (though it seems constant to me) and my body slows down relative to the light source. Even though my body has slowed down, the light hitting my eyes remains at a constant speed from my perspective due to the time dilation I'm undergoing. However, my brain is moving more slowly and taking longer to process the light hitting my eyes.

    Would there be some sort of "build up" of light caused by my eyes not being able to process the light quickly enough?

    Basically what I'm wondering is if things get brighter as you move towards a light source, and therefore more slowly when moving away. I know that they red/blue shift, but I'm wondering about brightness or some blurring due to not being able to process the light as quickly as it's coming in due to your body being slowed down.

    I'm just using a single light source/object in this example to make it as simple as possible. I know that at the speeds we travel at in our human experiences that time dilation is largely negative, so I'm guessing this affect would be as well, but I'm curious about the principal of it.

    I'm pretty sure the answer is "no," or, "this is an invalid question," but I don't understand why.

  5. Jun 13, 2012 #4


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    Your brain doesn't slow down due to time dilation nor do your bodily processes take any longer to process light coming into your eye. Time dilation is something OTHER people see you experiencing. You don't experience it at all. You WILL see light coming into your eyes differently than if you were not traveling a substantial fraction of c, but that's not due to any change in your mental processes.

    Although I don't have a link handy, there are "movies" on the internet that show what incoming light would look like as you increase in speed, and similarly as you go into a black hole and "experience" time dilation due to gravity.
  6. Jun 13, 2012 #5
    It's my understanding that time dilation is an objective phenomenon, but that the subject(s) of the time dilation wouldn't know it because their brain is slowed down along with their body, so they wouldn't see themselves as moving in slow motion, but they would see other people moving quickly, relative to them.

    Given this, and that the stimuli hitting their senses (e.g. Light) is NOT slowed down, wouldn't their brains "fall behind" and not be able to process the sensations as quickly as they would be coming in?

    Maybe I'm just reasking the same question again...
  7. Jun 14, 2012 #6
    I think I know what you are getting at and I think you are on the right tracks once we unravel which reference frames you are talking about. It is a good idea with relativity subjects to always be clear about which reference frame you are talking about.

    OK, I will use the example of a camera rather than a human brain and eye, because it is simpler to analyse, but the principles are the same.

    A laser light source that emits 10 photons per second (in its rest frame).
    A camera that has a fixed shutter speed of one second (in its rest frame).
    The camera and laser source are moving towards each other at 0.8c as measured in the rest frame of either.

    As seen in the camera reference frame:
    Taking only the simple classical Doppler effect into account, about 50 photons would arrive at the camera film in the the one second the shutter is open, due to the light source going towards the camera. On top of this the light source is subject to time dilation, so it actually only emits 60 photons per second in this reference frame and the nett result is that about 30 photons per second arrive at the camera film in the one second the shutter is open. All this produces a blue shift and a brighter image than if the source was stationary with respect to the camera.
    As seen in the laser source reference frame:
    In this reference frame the camera appears to be moving towards the laser source. The laser is now not subject to time dilation and emits its regular 10 photons per second. The fact that the camera is going towards the source means that it would receive 18 photons per second if we consider only simple classic Doppler shift. However, due to time dilation the camera shutter is actually open for 1.6666 seconds in this reference frame and the end result is that 30 photons arrive at the film in the time the shutter is open. (Note that this is the same result as for the camera reference frame but the explanation is different.)

    Now if you liken your eye and brain to the camera and its control circuits, then yes, in the reference frame of another observer moving relative to you, he explains why you see the blue shift and brighter image, partly in terms of your slowing brain processes just as if your brain was a mechanical device. However, in your own rest frame you do not experience any brain slow down and you explain everything in terms of Doppler shift and the slowing down of the emission rate of the light source.
  8. Jun 14, 2012 #7
    Time dilation would be applied for other FoR, not your.

    Brain processing speed has nothing to do with light speed. Brain processing speed is too slow than light speed.

    How brain works: Light comes into eye with some image. The image is created to retina. After clearing the image completely by lens and iris, the brain start processing the image. This process has nothing to do with light speed.
  9. Jun 14, 2012 #8
    Thank you all.

    If I'm understanding this correctly, my brain would slow down from the FoR of the light source, and the light source would slow down from my FoR. Therefor, the speed of light hitting my eye remains constant from my FoR, and I would notice no difference in the light that I see other than the red/blue shift.

    Because of this, my vision (or hearing or other senses) would not become "over-exposed," like a camera, but remain "normal" to me.

  10. Jun 14, 2012 #9
  11. Jun 14, 2012 #10
    I don't think the overlap is as strong as you think. In the other thread you seem to be mainly concerned with how the speed of light is measured by an observer with motion relative to a given reference frame, while this thread seems to be mainly concerned with the frequency of photons arriving at the observer. The two concepts are different and you seem to be getting them mixed up.

    You need to clearer in your own mind. You say "the light source would slow down" which most people would assume to mean the velocity of the light source would slow down (which is incorrect) but I think you actually meant the frequency of the light source would slow down in your FOR which is correct, but the frequency of the light source has nothing to do with the speed of light that you perceive. You also say "the speed of light hitting my eye" but that makes no sense because your eye does not measure the speed of light. It only measures the wavelength (colour) and the frequency of arrival (photons per second) which gives a measure of intensity or brightness but not speed.

    A simple model of an eye would be a grid of pixels (actually neurons) which fire and send a signal to the brain each time a photon hits the neuron. If the firing rate is high from a particular neuron, then the brain interprets that as bright pixel. Because firing rate involves a notion of time, the speed of the brain has an impact on perceived brightness. Everything that relativity affects with mechanical measuring apparatus equally affects biological measuring apparatus.
    Last edited: Jun 14, 2012
  12. Jun 14, 2012 #11
    I think I see what you're saying. The situations presented in both are similar, but asked them from different frames of reference (i.e. the observer vs an observer observing the observer). Correct?
  13. Jun 14, 2012 #12
    That is one important part. The explanations for a given situation depend on the frame of reference which is why you always have to be clear about which FOR you are talking about. I think you get that aspect now. However, I think you are still missing the difference between measuring the frequency of light and measuring the speed of light. Also when you say an object "slows down" you should be clear for your own sake as well as the understanding of others whether you mean the the velocity of the object slows down or the frequency of the object's clock rate slows down.
  14. Jun 14, 2012 #13
    Lets say there in a given reference frame you are moving to the right at 0.8 c and there is a light wave coming towards you. An observer in at rest in that given FOR might naively expect that you would perceive the light to be moving at greater than c relative to you. If he assumes you are subject to time dilation and your clocks (and brain) are running slow he would expect you to perceive that the light is moving even faster. It is length contraction and relativistic velocity addition and the different way you perceive what is simultaneous on top of time dilation that makes you perceive the speed of light to be same as what he measures (c).
  15. Jun 14, 2012 #14
    It's difficult to wrap my head around the details, but I understand what you're saying. I was thinking just about the time dilation aspect of this and leaving out the length contraction, relativistic velocity addition, and relativity of simultaneity.

    Also, thanks for the tips on how to write about this stuff. I realize now I was being unclear with my language.
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