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I How do we know we are looking back in time in space?

  1. Apr 1, 2017 #1

    DWT

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    Sorry new here trying this question. From the perspective of a light particle leaving its star that particle would arrive everywhere in the universe instantly since it is traveling at the speed of light. But we trust our perspective of time and distance in light years to describe what we are seeing as being old light.

    How can light have age if from its point of view it got here immediately?
     
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  3. Apr 1, 2017 #2
    The answer is that both are correct. To us the photon is "old", while to the photon its entire existence is timeless. This is one of the many un-intuitive but true principles of relativity. In our normal lives we don't perceive this happening because we never really experience relative velocities approaching the speed of light, so it seems strange. But in reality, there is no "preferred reference frame". There is no single correct and objective point of view to look at things from. To us, the photon has existed for months or years of millennia, but to the photon it has existed for no time at all.

    If this seems wrong, consider some other principles of physics that might be more familiar that also deal with this. An electron moving through a magnetic field experiences a force, while an electron at rest in a magnetic field doesn't. The moving one says, "There is some force-field here which is pushing me because I have charge, even though I don't think I'm moving." The other one says, "No, I'm not moving and there is no force pushing on me." Neither is incorrect; they are in different reference frames and so they experience different things.

    If this is hard to wrap your mind around, you're in good company.
     
  4. Apr 1, 2017 #3

    DWT

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    This makes sense to me.
    For me it just gets a little hard for me to understand attributing age to a photon from our prospective, when the photon itself transcends time. I can completely understand how approaching light speed would slow down time for the traveler but I am wondering if something different happens when light speed is reached.

    Is it possible that when we see light that has traveled to us from 2.5 million light years away, the image we see is how the object is at the current time? Not 2.5 million year ago?
     
  5. Apr 1, 2017 #4
    If the object being viewed doesn't change for 2.5 million years, then yes :smile:
     
  6. Apr 1, 2017 #5
    I think you're getting confused about what is meant by time dilation or length contraction. The rules governing this are the rules of special relativity. Any observer, in any non-accelerating reference frame, has to see the speed of light as constant, right? So an observer going any speed, even if he's nearly at the speed of light, still sees light traveling at the speed of light. This means that a person going very fast will see distances contract and other people's time be stretched out so that light can still travel at a constant rate.

    Any traveler doesn't see himself as moving near the speed of light, right? For him, it's the whole rest of the world that's moving really fast in the other direction. So he doesn't "feel" any time dilation or length contraction, but he sees other people experience these effects. If an observer was moving at the speed of light, he would see the entire universe contract down to zero length in the direction that he was moving; essentially, he would see the universe as some massive flat pancake that he was moving into, through the thin dimension. That means that it would take zero time for him to pass through the entire universe, not because time is changing for him but because he's moving through a very short distance.

    We would see him as not experiencing any time, being frozen, while moving through the universe at the speed of light. Neither frame of reference is more correct.
     
  7. Apr 1, 2017 #6

    phinds

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    there is no such thing. This is one of the most common mistakes in cosmology, propagated, as they all are, by ignorant pop-sci presentations. To have a "perspective" (even forgetting for a moment that this is anthropomorphizing light) there has to be a rest frame and there is no rest frame for light.
     
  8. Apr 1, 2017 #7
    Perspective is just another way of saying reference frame, it's not really anthropomorphizing anything. I mean, is it anthropomorphization to calculate the earth's reference frame?

    Also I don't understand why we can't define a rest frame for light. It's not accelerating (at least as long as it travels through flat minkowskian space). Is it not allowed because it would see massive objects moving at the speed of light?
     
  9. Apr 2, 2017 #8

    Bandersnatch

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    You can't have a rest frame for light in SR, because it'd contradict the postulate you used in deriving SR - namely that all reference frames see light as travelling at c.
    In other words, in no FoR can light both travel at c and be at rest, can it?
     
  10. Apr 2, 2017 #9
    A simple way to see that the speed of light is finite is to bounce light off a distant mirror and time the delay - for a mirror on the moon the round trip for the light takes about 2.5 seconds: https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment
     
  11. Apr 2, 2017 #10

    PeroK

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    A lot of what you have written is not correct (underlined). In particular, massive particles cannot move at the speed of light, so speculating on what it would be like to travel at the speed of light is invalid. There is no reference frame for light and, in particular, the equations for time dilation and length contraction do not apply to light.
     
  12. Apr 2, 2017 #11

    PeroK

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    The answer to your question does not depend on relativity, only on the finiteness of the speed of light, which has been known since around 1676. The simple fact is that if a signal takes a finite time to reach you, then the information is "old" when it reaches you.

    Imagine someone on Mars streams a video of a rock (which takes 20 minutes to reach Earth, say). After the video has been streaming for a few minutes they smash the rock. When the video first reaches Earth, how could it possibly show a smashed rock? It would show the rock as it was when the video was started. The video received on Earth would simply be 20 minutes behind what happens on Mars.
     
    Last edited: Apr 2, 2017
  13. Apr 2, 2017 #12

    DWT

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    I do understand that all speed
    Thanks I do understand what you are saying, my main concern is that we are doing these experiments and making calculations bound by time within our 4-dimensions.
    Analogy: It would be like living in a two dimensional world and trying to understand a sphere by describing its infinite number of flat sides. You will always come up with something that looks like a circle.

    Particles moving at light speed are no longer bound by 4-dimensions. If there is intelligence in the "5th Dimension" then everything we observe could be a construct. But I guess it is as good as its going to get so we might as well continue to draw our circles.
     
  14. Apr 2, 2017 #13

    Grinkle

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    You are looking for more answers than theory can provide, and imo you are substituting nonsense where theory stops providing any explanation.

    We do not derive from fundamental principles that c is constant for all reference frames / observers. We confirm it experimentally, consider it axiomatic and then derive the consequences.

    In looking for 'why is it so' answers amongst the consequences one must acknowledge 'we don't know' when getting back to 'it is so because c is constant for all observers'.

    There can be no reference frame for a photon because c is constant for all observers. We don't know why c is this way, we observe it to be so.
     
  15. Apr 2, 2017 #14
    That is a very big assumption.
     
  16. Apr 2, 2017 #15

    DWT

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    I agree.
     
  17. Apr 2, 2017 #16

    phinds

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    It is more than just a big assumption, it is a violation of the forum rules to posit things outside mainstream science with no proof. @DWT I see you're new to the forum so I'd advise you to read the rules. Speculation such as your CAN be enjoyable to discuss, but this is not the forum in which to do it as you'll see when you read the rules.
     
  18. Apr 20, 2017 #17
    Good morning.
    New here and I have a question similar to that of the original poster in terms of speed of light and an observer.
    I am hoping there is a simple reasonable answer for my question:
    Provided we all come from one point in space and then expanded almost instantaneously outwards, as suggested by the Big Bang Theory, for us to "look back" our point of origin and just now see light that was emitted tens of billions of years ago (near Big Bang) seems to suggest that we (observers) have moved to our position in space-time faster than the speed of light. I know this suggestion is wrong but I am not understanding how a photon in space travels at the speed of light and we are now looking at it hit our telescopes lenses (when that photon originated very close to the age of the Big Bang)?
    In hopes I make my question clear, point A is where we observed a photon from 12 billion years is coming from and point B is us. How did we get to point B (assuming we come from the same general spot in space before the Big Bang) before the photon?

    Thank you.
     
  19. Apr 20, 2017 #18

    phinds

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    That is emphatically NOT what the theory says. It is probably the most widespread misconception in cosmology, propagated by pop-sci venues.
    There is no such thing

    The recession velocity of object in space is not proper motion so no speeding tickets are issues. For example, the objects at the edge of the observable universe are now receding from us at about 3c.

    I suggest the link in my signature
     
  20. Apr 20, 2017 #19
    Check this out too
    https://en.wikipedia.org/wiki/Metric_expansion_of_space
    The Universe itself at the largest scale is what is expanding,
    (although gravitationally bound objects like galaxies are not expanding, gravity dominates there so holds them together.)
    Everywhere locally though, the speed of light is constant.
    Metric expansion of this sort is not constrained by the speed of light.
    It is not a situation of objects moving through space, space itself is getting bigger
     
  21. Apr 20, 2017 #20
    Hi Phinds,

    thank you. Saying that some objects at the edge of the universe are moving away from us at 3c is mind blowing and didn't realize it was that fast. I understand that the objects itself aren't travelling that fast, but rather space expands at a rate between us that if we where to "keep up" with the object we would have to travel at 3c (and exponentially after that.

    Why are experts referring to is as "looking back in time"? The depictions of the earlierst universe observable light always get my head scratching as to why are those photons arriving now. Given your explanation above, and given we are receiving these photons now, suggest that space has expanded at a faster rate than c between us and the observable light that is reaching us now. How is then light, bound by its own speed, able to overcome that rate of expansion.
    Meaning, those objects at the other edge of the universe will emit light that will never reach us because space time is expanding faster between us than the light could possibly travel?
     
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