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Speed of light and the universe

  1. Nov 22, 2015 #1
    I have been thinking about light and i have read that the universe expanded faster than speed of light. So , then why people say that the speed of light is the fastest thing known if people know that the universe expanded faster than speed of light . Also , if the universe could have expanded faster than the speed of light , can we make something that can travel faster than speed of light using expansion ?
     
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  3. Nov 22, 2015 #2
    Well, let me correct you on one thing- we know almost nothing about how the universe expanded, or even about how it expands now. It is all theoretical. If it is true, however, the speed of light still would be the fastest speed, because the speed of the universe really doesn't count. We will never achieve a speed greater than the speed of light. It is against the laws of physics. I am not sure, but I believe it is addressed in the FAQs why that is. I can kind of explain why, but I am going to let a more experienced person give you a more detailed explanation.
     
  4. Nov 22, 2015 #3

    davenn

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  5. Nov 22, 2015 #4
    I just want to add, I checked, and there is nothing on the FAQs. (Mentors, I would suggest adding an FAQ about the speed of light, I have noticed a lot of similar questions on here).
     
  6. Nov 22, 2015 #5

    davenn

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    this thread isn't to do with the speed of light as such, rather the expansion speed of the universe
    which is what the link I provided give lots of info on

    there is a FAQ thread on the speed of light .....

    https://www.physicsforums.com/threads/why-is-the-speed-of-light-the-same-in-all-frames-of-reference.534862/ [Broken]

    Dave
     
    Last edited by a moderator: May 7, 2017
  7. Nov 22, 2015 #6

    phinds

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    Nonsense. We know quite a lot about the current expansion. For example, we know that the recession of galaxies at the edge of the observable universe is about 3c. There is nothing theoretical about our knowledge of the current expansion even to the point of knowing that the rate of acceleration is decreasing slightly over time. Things that are totally supported by experiments/empirical evidence are way beyond just theoretical.

    True. It is not proper motion, just recession.

    also true although I hesitate to use the word "never" in such contexts even though I think it applies in this case.
     
  8. Nov 22, 2015 #7
    I don't agree , it should count as motion because we are moving space while it expands because when the universe expand it can't be filled with nothing . It must be filled with dark matter or something so it should count as motion and then there is a speed faster than light even though it does not count as motion and we may be able to make some kind of expansion with 3c speed or more . Also , who wrote the rules of the universe? Why c is the fastest speed? Why it should be? It has no Mass right? it should also not count as motion because it is just a wave or nothing as the space expansion. Why space expansion does not count and light does? Light is not motion because nothing at all is moving because light has no mass.
     
  9. Nov 22, 2015 #8

    davenn

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    So you have a couple of PHD's hanging on your wall that you can make such unsubstantiated claims ?
    it doesn't really matter if your agree or not, studies have been done on the subject by people well above your and my pay-rates and intelligence

    Did you even bother to read through the link I gave ? did you try and understand any of it ?

    I suggest strongly that you take a few steps back and do some learning before arguing against some well established theories

    Dave
     
  10. Nov 22, 2015 #9

    phinds

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    Irrelevant. It's not up for a vote. As Dave said, you should learn some basics before spouting nonsense. This stuff is not intuitively obvious but thinking that your opinion trumps facts is not science it's voodoo.
     
  11. Nov 23, 2015 #10
    Yes, I shouldn't have added the part about the current expansion. I was just trying to make the point that what happened in the past is impossible to know. I have read many theories about what could have happened. We don't know anything about it for sure.
     
  12. Nov 23, 2015 #11
    Ok, then if you want to count it as faster than the speed of light, try recreating the Big Bang. As @phinds said, it is just recession, not proper motion, and light is motion. It just is. It is fact. Many, many expariments have tried to pass the speed of light. None have succeeded. One claimed to have succeeded but it was a bad expariment, and they retracted their results.
     
  13. Nov 23, 2015 #12

    Orodruin

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    If you are thinking about the OPERA experiment it was not (and is not) their physics goal to pass the speed of light. Their neutrino speed measurement was a byproduct, something they could do in addition to their main physics goal (which is measuring neutrino oscillations into tau neutrinos). It is not a bad experiment in itself and it has been successful in its main physics goal. They simply suffered from being a bit too eager to announce their FTL measurement when they probably should have taken a step back and triple-checked everything.
     
  14. Nov 23, 2015 #13

    bapowell

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    Yes, it is impossible to know what happened in the past with certainty, but that doesn't mean that all ideas are equally relevant. This is why humans have developed science: we can make observations, develop models, and make inferences about the universe -- both past and present -- that can then be tested.

    Many historical observations can be well-understood in the context of modern science: if you came across a crater, how much would you bet it was due to a meteorite and not an abandoned excavation project by ancient earth-visiting aliens? Neither can be verified with certainty, but I suspect I know which explanation you're betting on.
     
  15. Nov 23, 2015 #14

    bapowell

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    The universe does not expand at a given speed. That's misconception number 1. The speed at which objects recede from one another, v, is a function of the expansion rate, H, and their distance apart, r, via Hubble's Law, v = Hr. The more distant an object, the faster it appears to recede from us. This is true no matter what kind of expansion we're talking about. Notice that as long as H is positive, there is always a distance at which objects will recede with a speed greater than light, r = c/H. There is nothing wrong with this since the objects are at rest locally, their relative velocity due solely to the expansion of space. So misconception number 2, which is regrettably pervasive in the popular science arena, is that the universe underwent some kind of superluminal expansion early on. These are references to primordial inflation, which indeed was a unique phase of expansion in the early universe, notable because length scales increased exponentially.

    So, no, space does not -- ever -- expand faster than the speed of light because expansion does not occur at a given speed. And, while objects do indeed recede from one another at certain speeds, and if they are far enough apart they will recede form one another at speeds surpassing that of light, this is true always and is not a unique property of inflation or any other type of expansion.
     
  16. Nov 23, 2015 #15
    That's interesting. I was not aware of that, all I knew was the false neutrino speed part.
     
  17. Nov 23, 2015 #16
    The OPERA result turned out to have a very mundane explanation eventually.
    A fiber optic cable was wrongly connected.
     
  18. Nov 25, 2015 #17
    wait , i am not sure of what i am going to write but , galaxies are going apart from each other faster every time right? so you say that they receed at 3c which means that they are moving at 3c further away?
     
  19. Nov 25, 2015 #18
    Also , we know almost nothing of dark energy ( i think) what if dark matter and energy can travel faster than C? did Einstein knew of this in his relativity theory?
     
  20. Nov 25, 2015 #19

    phinds

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    That is a very garbled sentence so I'm not sure what you are asking. There are objects at the edge of the observable universe that are receding from us at 3c. There are objects that are closer to use that are receding at 2c. There are objects that are closer still that are receding at 1c. Pick a number. You can find an object at the appropriate distance that is receding at the number times c.

    This is NOT the "rate of expansion of the universe", its just the recession of objects compared to Earth.
     
  21. Nov 25, 2015 #20

    phinds

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    That makes no sense at all.

    Dark energy is not a thing that can have a speed and dark energy and dark matter are totally unrelated to each other. Einstein did not know of either one, and that makes no difference to anything at all.
     
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