Speed of light becomes Infinity and Zero

  1. Speed of light becomes Infinity and Zero!!!

    Hi Guys, Im just new to SR & GR. I learned about Lorentz contraction and time dilation, but they dont make sense when i consider the following situation...

    Your(Moving non inertial frame) Time runs slow relative to a stationary observer when your speed increases and it will become infinity(as t = t0/√(1-v2/c2)) if speed v equals 'c' right?
    Since photons travel at speed c, their time relative to our world or the whole universe must be infinity.. Since it takes infinite time to move relative to us, we can safely assume that the photon doesnt move at all(relative to our frame).. i.e. speed of light = 0 :confused:

    Now consider Lorentz contraction... L = L0√(1-v2/c2))
    This means that as the object's speed increases, its length decreases(in the direction of motion)... So if the object's speed equals 'c' ,then its lenth would be 0 (in the direction of motion)... But we know there is no absolute frame(since einstein forbade the ether :wink:)
    So we can say that photon travels at c relative to universe, but if we consider universe moves at c relative to photon, then the length of universe would be 0 and hence photon can traverse the whole universe in no time.. which means Speed of light = Infinity :confused:


    I know im wrong but i dont know where i went wrong.. The above anomalies are the conclusions im arriving at whenever i think of relativity... Please someone clear that for me...
     
    Last edited: Jan 6, 2012
  2. jcsd
  3. Re: Speed of light becomes Infinity and Zero!!!

    how can time run slow and become infinity?time ceases at light speed. btw your equation is wrong
    https://www.physicsforums.com/showthread.php?t=511170
     
    Last edited: Jan 6, 2012
  4. DaveC426913

    DaveC426913 16,138
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    Yup.

    This is why photons (and anything else moving at c) do not have a valid reference frame.

    One definition of a valid reference frame is that it is a frame in which you are at rest. Since, by definition, photons always move at c in any reference frame, then you have a reference frame in which photons are both stationary and moving at c.

    So no, photons do not experience time. Photons do not experience anything at all (since experience requires the passage of time). It is no valid to try to think of a photon's frame of reference.
     
  5. Re: Speed of light becomes Infinity and Zero!!!

    Sorry small correction, i was gonna say Time runs slow for moving frame and becomes 0 when speed equals 'c'. Now the time becomes infinity for the stationary observer(relative to moving frame).. which means the observer cannot perceive the event at all... And again im getting that SPeed of light = Infinity :confused:

    And please tell me the correct equation for time dilation...
    thanks in advance...
     
  6. ghwellsjr

    ghwellsjr 5,062
    Science Advisor
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    Where did you get that equation from? Did it explain what t0 is or what t is?
     
    Last edited: Jan 6, 2012
  7. Re: Speed of light becomes Infinity and Zero!!!

    He did no such thing. :wink:
    Well that is quite correct, in a certain way: if an astronaut could travel near the speed of light, he/she would be able to the cross the universe in almost no time according to himself/herself. :tongue2:

    Einstein formulated it as follows in 1905:
    - see section 4 ("Physical Meaning of the Equations ..") of:
    http://www.fourmilab.ch/etexts/einstein/specrel/www/
     
  8. Re: Speed of light becomes Infinity and Zero!!!

    t = time duration for the desired frame of reference
    to = time duration for the relative frame of reference

    for example: if a rocket is moving with some speed, then 't' is the time duration for the rocket which we want to calculate and 'to' is the time duration for the stationary frame of reference from which we are observing the rocket....
     
  9. Re: Speed of light becomes Infinity and Zero!!!

    What i meant was that einstein eliminated the thought of ether by introducing relativity...

    Wow that gives me an idea.. photon can be converted to particle/anti-particle pair and vice versa right?.. So if we find a way to photonize(matter annihilation) ourselves then we can travel at light speed.. In the destination we again do the reverse process(pair production) to bring our former self :rofl:
    And hence the mystery of Nightcrawler's teleportation is solved. Case Closed...:cool:



    And btw thanks for the link.. that cleared lots of doubt.... thank you very much..
     
  10. ghwellsjr

    ghwellsjr 5,062
    Science Advisor
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    I asked you where you got the equation from and you didn't say.
    Are those explanations of t and to from the same place where you got the equation from or are you adding your interpretation?
    Please note that your example does not follow your previous explanations of the two times and in fact is wrong because it will give you more time for the rocket instead of less time. That's why I'm trying to find out if it is your interpretation of the equation that is wrong or if was stated incorrectly in wherever it was you got the equation.
     
  11. Re: Speed of light becomes Infinity and Zero!!!

    I got that equation from wikipedia.. See the Time dilation article in wikipedia...


    Im totally confused... i guess i will have to learn it from the begining once again..
    And thank you very much for your time...
     
  12. ghwellsjr

    ghwellsjr 5,062
    Science Advisor
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    Here's the link to the article I found:

    http://en.wikipedia.org/wiki/Time_dilation

    I don't see your equation in there. If that is the same article you referred to, could you point out the section, paragraph, etc where it is? If not, could you provide the link to the article you found.
     
  13. Re: Speed of light becomes Infinity and Zero!!!

    The equation is in section 2, Simple inference of time dilation due to relative velocity

    http://en.wikipedia.org/wiki/Time_d...nce_of_time_dilation_due_to_relative_velocity
     
  14. ghwellsjr

    ghwellsjr 5,062
    Science Advisor
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    Are you talking about this equation:

    [​IMG]

    It's not the same as t = t0/√(1-v2/c2) nor do I find these explanations in the text:
    RiddlerA, please provide the link to where you got the equation and its explanation.
     
  15. Re: Speed of light becomes Infinity and Zero!!!

    My first physics class in high school used Wilson/Buffa, and i remember them using those symbols in their time dilation equation as well. The way they had it, t0 represented a change in proper time for a "moving" observer (most of the problems involved a scenario where one person left earth on a spaceship and returned after some time) and t represented a change in time for the stationary observer.

    I also recall hating that textbook :tongue:.
     
  16. Re: Speed of light becomes Infinity and Zero!!!


    http://www.thebigview.com/spacetime/timedilation.html


    How do you say that the equation from wikipedia and what i mentioned earlier are not the same? it looks the same to me....
     
  17. Re: Speed of light becomes Infinity and Zero!!!

    by convention Δt is the measure of change in time and t is of time at an instant. when a moving clock S' reads t' and stationary S reads t then
    t'=t[itex]\sqrt{1-V^{2}/C^{2}}[/itex](of course FoR is S)
    If an event takes Δt in S it would appear to take longer when happens in S'(seen from S) hence if Δt' is the time taken in S'
    Δt'=[itex]\frac{Δt}{\sqrt{1-V^{2}/C^{2}}}[/itex]
     
  18. ghwellsjr

    ghwellsjr 5,062
    Science Advisor
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    Here's the equation from your link:
    t' = t/√(1-v2/c2)
    Well, all the equations have the form of a time on the left side of the equal sign and another form of a time on the right side of the equal sign divided by the square root of something that is less than one which means the time on the left side of the equal sign will be greater than the time on the right side of the equal sign. So the issue is: what do these two times represent?

    In your first post, you had t on the left side of the equal sign but in your link, t is on the right side of the equal sign. You don't see this as a difference? This is why I asked you what the references you were looking at were explaining the different forms of t represent.

    So now we have a big mess to clean up.

    Let's start with the article in your link. This has to be one of the worst explanations of time dilation that I have ever seen. It is flat out wrong. Note the paragraph heading Time expands, space contracts. In order to show that time gets larger, they use this form of the equation were t' on the left side of the equation is larger than the t on the right side of the equation and they even show a plot with t' getting larger with velocity. And to explain it they reference the twin paradox where one twin travels for one year at 99%c. His age increase is represented by t and his earth brother's age increase is calculated by t' as 7 years. What happened to the reciprocal nature of time dilation? Both brothers see the other one as aging less than them self but they don't mention that. What they do mention is:
    The truth is that in the space traveler's reference frame, one year is equivalent to 1/7th year on earth and in the earth reference frame, one year is 1/7th year for the traveler, or 7 years is 1 year for the traveler. Do you see the difference? They got it backwards. And yet they seem to realize that it's not quite right because just above the graph they say:
    OK, now let's look at the wikipedia article. Their equation is:
    Δt' = Δt/√(1-v2/c2)
    This is identical to the one in the link you provided except that there are a couple of deltas in front of the t's and their explanation is:
    This again is designed to show the dilation or expansion of time.

    But now let's look at how Einstein explains time dilation. We'll look at Einstein's 1905 paper that harrylin linked to in post #6:
    http://www.fourmilab.ch/etexts/einstein/specrel/www/
    Look down near the end of section 4 where you see this equation:
    τ = t√(1-v2/c2)
    He explains this as t being the time on a clock at rest in a frame and τ (tau) being the time on a clock moving in that frame. It's so simple, but don't overlook the fact that the square root factor is multiplied, not divided, meaning the time dilation results in the moving clock running slower than the stationary clock. That's all there is to it. What ever speed any clock is moving at in a frame determines how much time dilation there is for that clock. If you switch to a frame where the moving clock is now at rest, the time dilation switches to the other clock.

    So why is there so much confusion? It's because some people want to have the equation for time dilation be the reciprocal of the one of length contraction because time dilates and length contracts. But what they fail to realize is that it's the interval of time that gets longer resulting in the clock running slower which means it is displaying less time, not more and so we need to use the same form of the equation for both time dilation and length contraction. Look at your first post to see that you have the two equations different. Look at the Lorentz Transform and you will see that the form of the equation for time is identical to the form of the equation for distance with just the t and x interchanged.
     
  19. ghwellsjr

    ghwellsjr 5,062
    Science Advisor
    Gold Member

    Re: Speed of light becomes Infinity and Zero!!!

    That's right.
    But this is not right. You should have left it the same way with no division. So if something takes Δt in S, it will take less time in S' (moving with respect to S).

    Also, in Special Relativity, the term "event" has a specific meaning. It refers to an instant in time at a specific location. It shouldn't be used to refer to a duration or an interval of time. An event is the four coordinates making up a "point" in spacetime.
     
  20. Re: Speed of light becomes Infinity and Zero!!!

    why not?what i meant is this (an example) consider A moving at 0.86c and B is stationary. Now B has to wait for 2 years in order to have A 1 year old but he himself ages 1 year in 1 year (of course B is FoR)
    sorry about that!dint know
     
  21. Re: Speed of light becomes Infinity and Zero!!!

    Because from A's rest frame, B is the one who ages slower.
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?