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Age of the universe from a photons point of view

  1. May 14, 2010 #1
    if time has no meaning for a photon, do you think its correct to say that from its point of view that age of the universe is 0 seconds?
     
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  3. May 14, 2010 #2

    Astronuc

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    No, not anymore than the age of the universe is 0 seconds to us (the observers), or our own lifetimes. The age of the universe is what it is - regardless of any single photon, lepton, baryon, or event.

    This would seem more of a philosophical question than a physics question.
     
  4. May 14, 2010 #3
    If the "material" universe is fundamentally vibrations void of a core substance that vibrates, can the universe be said to exist at all? Which begs the question: What or who am "I"?

    What is there that really, really exists? Is this not the same fiery question that both physicists and philosophers dance around?

    I feel your question is a meaningful one. Especially if we direct such inquiries toward our own sense of being, rather than to a photon or thing that is seemingly external.
     
  5. May 14, 2010 #4
    thnka you for saying my question is meaningful, Im really sorry that i dont feel i can return the favour. Supposing string theory is true , a big suppose, and everythign is made of vibrating strings , why would that imply the universe doesnt exist?
     
  6. May 15, 2010 #5

    Chalnoth

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    Yeah, no.
     
  7. May 15, 2010 #6

    Chalnoth

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    Well, yes, in a way. In the limit of a transformation to the coordinate system of a photon, all distances are crossed in zero time. But this isn't a terribly useful transformation to do, because it effectively destroys the information about two dimensions (time and space in the direction of motion), leaving only the two dimensions transverse to the motion.
     
  8. May 15, 2010 #7

    Chalnoth

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    The age of the universe is entirely dependent upon what coordinate system you use. The age that is generally quoted uses the proper time of an observer that is stationary with respect to the CMB as its time coordinate.
     
  9. May 17, 2010 #8
    "Strings"? would these be God's guitar strings? Vibrating strings is only a term. There is energy, which we interpret as moment or waves or vibrating, but what's vibrating? If there is some "thing" vibrating, then what is it made out of? etc, etc, etc...... Vibrating strings, is the same trap we get into by saying God created the universe (then where did God come from? etc, etc, etc...).

    It is said that the grand total of all positive and negative forms of energy are equally balanced within the universe so that the sum = 0. This is why the universe can arise from nothing. It is a perfect child that needs no other, no parents. The phenomenal universe doesn't exist; there is no solid out there, out there; it only appears that there is; and that's good enough.
     
  10. May 17, 2010 #9
    You could think of it that way. The space time distance between two points connected by a photon is zero.
     
  11. May 17, 2010 #10
    You tell me. Define "exist" and I'll tell you if something "exists" or not.

    Ummmm..... No.

    What physicists generally do is to come up with some agreement about what certain terms mean and that allows you to make some falsifiable statements about the universe. Rather than debate what "existence" really means, you come up with a definition, and then run with it. If you define "exists" as meaning "eats spinach and likes the color blue", then the universe does not exist. If you don't like that definition of "exists" then come up with one.

    Personally, I think the OP was just using a poetic metaphor to ask a question.
     
  12. May 17, 2010 #11

    apeiron

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    Which brings up the further point that a photon travelling "a long time" between emission and absorption would be red-shifted by the expansion of the metric?

    And how is this to be thought about? Did the photon grow colder on its journey or was it always (taking the timeless view) that cold?
     
  13. May 17, 2010 #12

    Chalnoth

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    One way of looking at this is that the photon hasn't changed, but space has stretched.

    This picture is relatively obvious if you consider the simpler case of just the doppler effect, where it's not as if the photon wavelength has actually changed, just that the motion of the source makes it so that each subsequent peak of the wave is closer or further apart than you would normally have.
     
  14. May 18, 2010 #13

    apeiron

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    That was actually my starting point - we can see that the universe expands and so a travelling photon gets red-shifted. Something is in fact changing about the "timeless" photon.

    So given that, do we want to say that the photon started out its journey blue and gradually turned red (a wavicle view)? Or do we want to say that the photon exists over the whole of the path in timeless fashion and so was in fact the same colour all along?

    The first view says there is change actually happening along the way, the second that the photon always had one temperature.

    I ask really because I can't decide how much to take photons as "real" particle exchanges rather than just quantum sum over histories that equilibrate between two locations at different spacetimes. In one view, something travels and is changed. In the other, there is just a path that crystalises to connect locations.
     
  15. May 18, 2010 #14

    Chronos

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    I really dont see the point. Time is meaningless from a photon's perspective.
     
  16. May 18, 2010 #15

    apeiron

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    So then this is about what we can see from our perspective (the red-shifting) that the photon can't.

    Why does the photon not see it? Because it was always red? Or just did not notice itself turning red?
     
  17. May 18, 2010 #16

    Chalnoth

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    The photon can't 'see' anything, because as Chronos correctly points out, time is meaningless to the photon. As far as the photon is concerned, it's exactly as you point out: an exchange of a particle between emitter and absorber. That is to say, there isn't any clear distinction between a virtual particle and a real particle. A real particle is generally considered to be a particle that is on its "mass shell". Well, when you consider an exchange of photons between two charged particles, the mass of the virtual particle that is exchanged asymptotically approaches zero as the distance between the two charged particles becomes large.
     
  18. May 19, 2010 #17
    I would define "exist" as that which is unquestionably real and genuine.
     
  19. May 19, 2010 #18

    Chalnoth

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    That doesn't help (it's a circular definition).
     
  20. May 20, 2010 #19

    Ich

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    You know, a photon has no "proper properties". Contrary to a massive particle, which has a well-defined, coordinate independent rest mass, a photon has only energy. And the energ is completely dependent on the reference frame you use.
    Let's think about an empty universe (so that SR applies, too), then you can describe the same thing differently:
    In the Emitter's frame, the photon has energy 2 eV all the time. When it hits the observer, she will only measure that part of the energy (say, 1 eV) that doesn't contribute to her kinetic energy.
    In the Observer's frame, the photon has energy 1 eV all the time. The emitter had to invest more energy (2 eV) because he also gained kinetic energy by the recoil.
    In the Emitter's frame, the photon leaves with 2 eV, in the Observer's frame it is measured with 1 eV. The explanation is doppler shift.
    In cosmological coordinates it is said that the photon's wavelength stretches due to "stretching of space". What this means operationally is that you use at every point a "comoving" inertial system to measure the energy. "Comoving" means at rest wrt the emitter at the emitter's position, at rest wrt the observer at the observer's position, and smoothly interpolating between. By this definition, you'd say that the photon gradually loses energy.

    I think it's clear that neither physics nor the photon care about which picture you use to describe the process.
    Replace "photon energy" with "baseball speed" when you throw it to someone who is running away. You get exactly the same existential questions - is the speed really changing, or not?
     
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