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Light and Time

  1. Jan 15, 2014 #1
    Legend: A is the source
    B is the photons/light
    C is the destination (which isn't fixed)
    c is obviously the speed of light

    Yesterday I was sat thinking about how visible light can be older than the universe itself. I came to the conclusion that the expansion of space happens before the light has traveled through it, so the light has further to travel than it did when it set off, from point A to get to point C (which changes due to the stretching/expansion of space).
    So, the universe is only 12-14 Billion years old and the visible universe is estimated to be 90 Billion light years across. For simplicity and an example I'll say the light in question (B) was created very shortly after the big bang (call it 14 billion years as in this example it was created too soon after the big bang to write a simple/comprehensive number). So light is constant, the light is traveling through space at c but at the same time (for want of a better word) the space is expanding/stretching before the light has got to it's destination, so again it has further to travel, it has to catch up with it's destination whilst it's still essentially moving further away. I've read that light doesn't move through time but how can this be? How old is the light? If its traveled at c from A to C (C can be 45 Billion light years away from the original source for this example), the light would be 45 billion years old to us, but it would only be 14 billion years old to itself but if light doesn't travel through time then the journey would be instant from the Point of view of the photons so it wouldn't be 14 billion years old at all, it'd be the same age as it was when created. This gives light 3 different ages dependent on your view point.
    1. 14 billion years
    2. 0 years, it's as old as an instant
    3. 45 billion years
    Which would be correct? How old is it? We already know that light travels at c so it doesn't take an instant to travel the distances mentioned. So either light does travel through time or it doesn't. Does time stand still for the photons when they travel at c? If they do then they travel through 3D space outside the constraints of time, so is c = to the speed of time? If it is then in effect we're seeing the light being piggybacked on time through 3D space to it's destination (it's inevitable demise, which presume is absorption). Rather than the light moving through time it moves by time. So we experience light as taking 45 billion years to traverse the distance because we have mass so can't move by time so move with it, which obviously takes longer than an instant. I suppose you could say we see the instant of light in slow motion, spread out over 45 billion years. So our perception of this instant takes 45 billion years but it takes literally no time at all for B, the instant is an instant.

    To started wanted an answer but whilst writing you can see I've come up with my own. So can someone tell me:
    1. If I'm wrong then what is correct?
    2. Is there merit in my own answer, is it a possibility?
    3. If this is already a theory where can I read up on it for a better explanation?
    4. Am I thinking too much into it? lol

    Thanks :)
  2. jcsd
  3. Jan 15, 2014 #2


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    Einstein said, "Time is what a clock measures". Since a clock requires massive particles and massive particles cannot travel at c, then there can be no clock that could measure the aging of light. Or to put it another way, you cannot build a clock out of just photons. Therefore, there is no meaningful definition of time that applies to light so your question is meaningless. It's not that the answer is any of the choices that you provided or any that you didn't provide, it's that the question as applied to light is meaningless.
  4. Jan 15, 2014 #3
    What's said there does make sense for sure but it's just shying away from the question. Light and time can be measured together, that's why some light appears to be older than the universe itself. So in response I disagree that the question is meaningless. With your answer comes more questions, why for example. Why is it meaningless? A question must have an answer. I suppose the answer you've given is light is unaffected by time, but it doesn't answer why.
  5. Jan 15, 2014 #4
    Also, that wasn't meant to sound snotty if it did. I appreciate all replies.
  6. Jan 15, 2014 #5


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    I was not answering your question. I was pointing out that your question is meaningless. The definition that we use of time does not apply to light.

    What is it about light that you think we measure?
  7. Jan 15, 2014 #6
    I meant reply, not answer, my bad. Measurements of light include it's speed, distance travelled, brightness. But also, why does does time not apply to light? I'm no physicist so maybe I don't understand, I just look for answers myself and this is where I'm at, at the minute.
  8. Jan 15, 2014 #7
    Hi JiBo....
    Of course it is not, so I read no further in real detail. But it is useful and fun to puzzle through such questions. I still do it.

    Cosmic microwave background radiation [CMBR] or 'light' now reaching us was emitted about 380,000 after the big bang..BB...so it is just a bit 'younger' than the universe as measured from the big bang. The light I am typing this by was just emitted from my bulb moments ago. You can think of that light as wavelike and spread out or locally like photons, massless particles.

    Inflationary expansion, expansion of the entire universe faster than the speed of light, took place well before this, from about 10-37 sec to about 10-32 second after the BB. Then for about 4B years the expansion slowed somewhat and then began to speed up again and continues to do so now.

    Right now, most of the galaxies of the universe, many of which are not detectible, are receding way from us faster than 'c'. Some them emitted light long ago which is reaching us now and those also be moving away faster than 'c'.

    For a rough illustration of the chronology of the universe, try here: [first diagram]

    Check out these instructional posts fromMarcus for lots and lots of information:

    Effort to get us all on the same page (balloon analogy)

    Look 88 billion years into future with the A20 tabular calculator
  9. Jan 15, 2014 #8


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    No, we don't measure the speed of light, we define it to be 299792458 meters per second. We define a second (time) in terms of an atomic clock made from cesium. We define a meter (distance) as how far light travels in a particular amount of time. We can measure the brightness of light based on other definitions but that does not have any relevance to this discussion. So we don't measure the speed of light and we don't measure the distance it traveled. We can use a stationary (inertial) clock and our definition of the speed of light to measure distances. But we can't use that technique to measure the distance that light travels in a certain time to determine its speed.

    Keep in mind that your question is about the aging of light. You asked at least twice "How old is it (light)?". In Special Relativity, we use the term Proper Time to apply to the aging of a moving particle or object or clock with respect to a Coordinate System (which itself is defined in terms of the propagation of light). The faster this object moves in the Coordinate System, the longer it takes for time to progress compared to the Coordinate Time. We can talk about any speed up to, but not including, the speed of light because our definition of the Coordinate System doesn't allow it.

    So the term Proper Time (which is what aging is or how old something is) does not apply at the speed of light.

    We can talk about how long does it take for the light to get from one location in our Coordinate System to another location, but if you realize that the definitions of the Coordinates that we use to answer that question are itself defined in terms of the speed of light, then you will see that we are not measuring anything about time with regard to light. Instead, we are using our definitions of the speed of light and of time to define distances. So the answer to the question, how long does it take for light to traverse so many light-seconds is the same number of seconds.
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