Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rest Frames and Photons and c

  1. Aug 5, 2011 #1
    This has been discussed in a number of threads and in FAQs.
    I was wondering what constituted a rest frame as nothing in the universe is at rest, if you work the figures out we are moving at around 2.5% c.
    In another thread I found out that at rest simply meant not accelerating.
    This means that despite what has been said before photons moving at c are the only things permanently at rest, they cannot go faster than c, they always move at c and can only decelerate which means their frame of reference can only go in one direction. All other things have the choice of going faster or slower and so moving their frame of reference in either direction, this is obviously why c is so important in relativity, it provides a floor that all else is built on.
    Therefore photons are not moving at c from our frame of reference but we are moving at c from a photons frame of reference. So by going faster we are actually slowing down from a photons frame of reference, so is everything we think we know back to front.
     
  2. jcsd
  3. Aug 5, 2011 #2

    Dale

    Staff: Mentor

  4. Aug 7, 2011 #3
    Noticed your reply in the other thread. Please define at rest. We cannot use the earth as we have no idea what its actual velocity or acceleration are on account that we are unable to leave the solar system and compare it with other bodies. There is as far as I know nothing in the universe whose velocity is 0.
    Relativity does not differentiate between frames of reference only that c is constant.
     
  5. Aug 7, 2011 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I hope this doesn't turn into an anti-relativity thread. Please reread the FAQ.

    And in relativity, there is no such thing as "really at rest".
     
  6. Aug 7, 2011 #5

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    "At rest" only makes sense in some reference frame. An accelerating object can be "at rest" in its own (accelerating) reference frame, there's no problem with that.

    Accelerating reference frames have the sometimes undesirable feature of fictitious forces. This is why we often choose to work in non-accelerating (inertial) reference frames. Sometimes, though, we actually do want to work in an accelerating reference frame (most often to describe the motion of objects near the Earth's surface). There's nothing wrong with using an accelerating reference frame.
     
  7. Aug 7, 2011 #6

    DrGreg

    User Avatar
    Science Advisor
    Gold Member

    John15, I think you are confused by what is meant by a "rest frame" or being "at rest".

    In relativity it doesn't make sense to say "X is at rest", but it does make sense to say "X is at rest relative to Y". This means the velocity of X is zero as measured by Y, or equivalently that X and Y have the same velocity (relative to anyone).

    The "rest frame of Y" is a frame in which the velocity of Y is zero. Again it doesn't make sense to talk about a "rest frame" unless you specify the object whose rest frame it is e.g. "the rest frame of Y" = "Y's rest frame".
     
  8. Aug 8, 2011 #7
    This is not meant to be an anti-relativity thread, just interpretation of it regarding c as constant.
    X is at rest relative to Y, or is Y at rest relative to X unless you have an outside reference frame you cannot know the difference.
    So is the earth at rest relative to c or is c at rest relative to the earth.
    What frame of reference can you use to prove your answer, dont forget if its the earth moving at c then all experimental equipment will also be moving at c within the earths ref frame.
    It is possible that our view of the universe is back to front. Is c constant because it cant go faster or because it cant go slower.
    I know it sounds implausible but over the last year I have come to the conclusion that there is something screwy about the way the universe appears to work which is why we need to invent stuff like extra dimensions to explain what we can see.
     
  9. Aug 8, 2011 #8
    "It is possible that our view of the universe is back to front." What's that mean?

    "Is c constant because it cant go faster or because it cant go slower." I have a new favorite quote!
     
  10. Aug 8, 2011 #9

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    You get to pick any arbitrary inertial Frame of Reference as long as all objects with mass are traveling at less than c in it. You can choose the surface of the earth wherever you are located at this current time. You can choose the sun at this current time. You can choose the center of our galaxy at this current time. Or you can just imagine one in which you define an object called X that is stationary or you can imagine one in which you define an object called Y to be at rest. Or you could image one in which X is traveling to the left at 80%c and Y is traveling to the right at 90%c. You could chose one that is traveling at 99% the speed of light in the direction of the moon from where you are right now. It doesn't matter which one you choose but which ever one you do choose, light travels in all directions at c with respect to that Frame of Reference. Why are you having such a hard time with such a simple concept?
     
  11. Aug 8, 2011 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    where on earth did you hear something so silly?

    I there probably ARE objects in the observable universe which have a velocity of 2.5%c relative to the earth, but so what? As is pointed out in other posts in this thread, it is nonsensical to say "we are moving at around 2.5% c" unless you add "relative to object X", and again, so what. What object X did you have in mind?
     
  12. Aug 9, 2011 #11
    Try a different angle. Remove all mass so that all that is left is photons moving at c. In this case relative speed = 0 as everything is moving at the same speed. If we then add a star is its speed relative to c=0 or c=300000kps which is what we measure it as (approx)
    To get speed we are moving just add all the numbers from earths rotational speed through to the speed galaxy is moving through space the numbers are available on the net although I assume some are approximate. I would think they are relative to everything else moving through space.
    My readings have lead me to believe that it may be time for those capable of doing it take stock of all we know and check that all interpretations of that data are correct. I say this because we appear to know about most of what there is to know yet are still unable to put it together without using exotic things like extra dimensions which suggests that something has been missed or misinterpreted. The obvious place to start is in working out why physical constants are constant, I would think this is probably the most important thing towards a theory of everything.
    Another thing I find strange is a neutron decays into a proton and an electron. You would think that as opposites attract being neutral it would be stable, I dont understand how the strong force works in this case either as I thought it held these things together as well.
     
  13. Aug 9, 2011 #12

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    But at different directions so their relative speeds are NOT 0.

    No, speed is measured relative to an object or frame of reference, not a speed. Saying "relative to c" is meaningless.

    Rotation, since it involves acceleration, can be calculated "absolutely"

    "The speed the galaxy is moving through space" relative to what? You still seem to think that things, like earth or our galaxy, has some "absolute" speed. That is not true.

    The fact that you do not understand something does not mean it is not true. As far as rechecking what we know is concerned, people are doing that all the time.
     
  14. Aug 9, 2011 #13
    It seems that people are dealing appropriately with the OP's questions; I'd just like to point out that the thread title really ought to have ", oh my!" appended to it.
     
  15. Aug 9, 2011 #14

    phinds

    User Avatar
    Gold Member
    2016 Award

    :rofl:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Rest Frames and Photons and c
  1. Rest frames (Replies: 2)

  2. Photon at rest (Replies: 5)

  3. Rest frame of the photon (Replies: 16)

Loading...