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

Really confused - basic special relativity

  1. Sep 2, 2011 #1
    Hey all,

    this may sound dumb, but this question has been bothering me for a while now.
    So we know that moving objects (or people!) measure time moving slowly compared to objects in rest (to them). We also know that distances appear to shrink.

    Now, first this question: if we send a person to the nearest star, say, 5LY away, and he'll travel very rapidly (0.999c), which correlates to gamma ~22, he'd experience (theoretically) both time dilation and space contraction, right? So for us on earth it would take t = (5/0.999) Y years to get there, right?
    I'm having trouble calculating how long it would take him, in his system, because of these two effects. I couldn't understand if they cancel out, or add up, or are equivalent (that is, if he'll get there, according to his clock, after the same amount of time, t, or after t/22, or after t/22^2). I know well the lorentz transformations but I can't really define the event here. Is it "the person arrived the star"? If it is, when I'm trying to calculate it like I understand it, I get this strange result:
    Earth system: x = 5LY, v =0.999c => t = 5/0.999 Y
    But then:
    x' = gamma * (x-vt) = ~22 * (5 -0.999 * (5/0.999) ) = 0 LY!

    I don't get it at all :-)

    My second question is: How does it feel like to be a photon? If space shrinks asymptotically to zero, does it "feel" like it's everywhere all the time? Another extraordinarily posed question, I know :-\

    I hope I made some sense - I've had a little relativity when studying mechanics a few years back, I believe I could at least answer the first question alone back then (although probably just by using formulas like a robot), but I'm apparently very rusty :-)

    Thanks a lot!
    Tomer.
     
  2. jcsd
  3. Sep 2, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    OK.

    Sure.

    Right.
    The two events are: (1) spaceship leaves earth & (2) spaceship arrives at star.

    In the earth frame, the distance traveled is 5 LY. But in the frame of the spaceship, the spaceship doesn't move at all. So Δx' = 0 makes perfects sense! The two events take place at the same location according to the spaceship.

    You want to solve for Δt', the time between the two events according to the spaceship frame. So use a different Lorentz transformation.

    You can also use the 'time dilation' and 'length contraction' short cuts, if you like. From the ship's viewpoint, the star is only 5/22 LY away and is traveling towards him at 0.999 c. Use that to figure out the time.

    Alternatively, you realize that from the Earth's view the spaceship's clock is observed to run slow. Thus Δt = gamma*Δt'. The time measured on the spaceship clock is the time interval between those same two events.

    You'll get the same answer any way you slice it.

    You might want to read our FAQ: https://www.physicsforums.com/showthread.php?t=511170"
     
    Last edited by a moderator: Apr 26, 2017
  4. Sep 2, 2011 #3
    Thanks a lot, it's much clearer now. And this "funny" result I got was clear to Galileo 700 years ago and has nothing to do with relativity. *sigh*... :-)

    I'll read the link - thanks again!

    Edit: I just read the not very lengthy link. So, nothing makes sense. But the photon is still there! So (forgive me for the new-age) if the photon had emotions he would still experience something, even though our transformations crash, wouldn't it?! Or do we conclude that emotions cannot move in the speed of light?? :p
    This is hardly satisfactory :-(
     
  5. Sep 2, 2011 #4
    From the link:

    That's what the mathematics of science tells us.

    Einstein did not develop mathematics, nor has anyone else, to deal with hypothetical emotions of inanimate objects. So far we have no experimental evidence that energy has emotions.

    You might as well ask "What does the number 24 feel?"
     
  6. Sep 2, 2011 #5
    I'm of course aware of that, and that's why the word feel was surrounded with quotation marks. However, a number is a purely hypothetical concept whereas emotions, whatever they may be, are there.
    By asking how does a "photon" feel I meant to ask how would a person feel going at the speed of light. This is simply not possible because a person has a mass, you'd say. But then I could talk about a "person made of photons", a weight-less person, an anorectic model maybe :-). If photons have energy without having any mass, why shouldn't "thoughts" or "emotions", as a possible form of energy, be able to arrive to the speed of light?

    I realize this is mambo-jumbo, but I just wanted to give some meaning to the question. There's a non-hypothetical particle moving at the speed of light as far as we know it. It is therefore very hard not to ask, "how does the particle see the world", Unlike asking "how does it feel to be 24".
     
  7. Sep 2, 2011 #6

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Last edited by a moderator: Apr 26, 2017
  8. Sep 2, 2011 #7
    Last edited by a moderator: Apr 26, 2017
  9. Sep 2, 2011 #8

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    You can find lots more by searching for "meaningless" on this forum.
     
  10. Sep 2, 2011 #9

    Doc Al

    User Avatar

    Staff: Mentor

    :rofl:
     
  11. Sep 2, 2011 #10
    I have read it. I wouldn't want to start a new discussion if everyone here is so upset about it. It seems rather obvious that the matter is settled. I agree that with the current model it makes no sense to talk about these questions, just like it makes no sense to ask mathematically how much 1/0 is (being restricted to the classical real line).
    However extensions to the real line have been made, and one can talk about the set which includes infinity. There 1/0 is defined. So instead of resolving it by saying "1/0" makes no sense, they altered the model.

    Therefore, I don't think the question itself should be banned or laughed about. The question raises the possibility that the model might not be perfect, that there is something missing, that it doesn't cover everything. It could sharpen out things.
    I think many of the posts there were a little hard. I count myself to be a follower of pure logic and wouldn't want to mix philosophy and physics. This is an obvious philosophical question. And still - the two feed one another. Isn't it legitimate to test physical theories with abstract notions, like suggested by others? They might not have the power to contradict or verify them physically, but they could raise new ideas regarding possible extensions of the theory or experiments. Philosophical ideas were and are the core of physical theories. Thought experiences were always an important tool in Physics.

    I understand why it upsets some physicists to see a mixture of the two - and indeed, when one goes to speak about the "feelings" of a photon one should perhaps mention that he's diverging from classical physical models. But the question itself remains meaningful in our world, even if certain transformations of a certain model aren't currently defined under the restrictions of the question.
     
    Last edited: Sep 2, 2011
  12. Sep 2, 2011 #11

    WannabeNewton

    User Avatar
    Science Advisor

    Well if you are going to use semi - riemannian manifolds to model space - time then you cannot avoid the issue. Not to mention , regardless of null geodesics having zero distance intervals on semi - riemannian manifolds, you still cannot lorentz boost to the frame of a photon to know what it "feels".
     
  13. Sep 2, 2011 #12

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Tomer, this forum is devoted to helping people understand relativity. People who understand relativity understand why the question of what a photon experiences is meaningless. Are you interested in learning relativity?
     
  14. Sep 2, 2011 #13
    I have seen the problem with time and length contractions may times including with myself. Despite what the experts tell us the numbers dont make sense. To maintain the speed of light constant relatively then common sense tells me that if you are moving at 0.5 c then relative to an observer at rest, time and distance should both reduce by 50% but lorenz contractions as far as I can make out says that this does not happen till you reach near 90% c.
    Also if time and distance both = 0 at c this means that distance(miles)= 0, time(per hour) =0 then at c speed(miles per hour) must = 0 not 186000 miles per second, which also makes no sense, at least with current interpretations.
     
  15. Sep 2, 2011 #14

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    There's no "problem with time and length contractions" but there is a problem with your common sense. This forum does not tolerate relativity bashers. Wouldn't you rather learn relativity so that it will make sense to you?
     
  16. Sep 2, 2011 #15
    I have seen where the real numbers have been extended to include a maximal and minimal element, but I don't know which of these should be 1/0. Is this value positive?

    If we have a field (the normal properties of addition and multiplication, without bothering to tie these concepts to a specific set of numbers) then defining a value to inf=1/0 can be shown to lead to contradictions.

    0*2=0
    inf*0*2=inf*0
    1*2=1
    2=1

    1/0 is a mathematical contradiction, unlike the concept of a supremum you likely intended to represent by this. The ordinal w is a much better representation of the concept of this concept.

    One can even take such an element and extend the algebra by the field properties and create the hyper real numbers.
     
  17. Sep 2, 2011 #16

    Dale

    Staff: Mentor

    The question isn't banned or laughed about, it is given the respect and attention that it deserves: a FAQ entry. The problem is that everyone who asks this frequently asked question ignores the answer and then tries to repeat the exact same discussion that occurs twice a week. There has been plenty of discussion on the topic: read it.
     
  18. Sep 2, 2011 #17

    Dale

    Staff: Mentor

    We should put that advice into the FAQ.
     
  19. Sep 2, 2011 #18
    This is about the third time I've noticed a comment like this. The first time, WannabeNewton noticed the confusion and clarified it.

    Again: Moving objects or people do not measure time moving slowly. So, I can't tell whether you are just being a little careless with the way you state this or whether you don't have a fundamental understanding of time dilation in special relativity.

    You might have said something to the effect that an observer at rest in his own reference frame observes that a clock moving relative to his rest frame appears to him (the rest frame observer) to be running slower than the clock at rest in his frame.

    More generally, given obervers A and B in motion with respect to each other, each carrying a clock: A observes B's clock to be running slower than his (A's) own, and B observes A's clock to be running slower than his (B's) own.

    Note that B does not observe that his own clock is running slow just because he is in motion relative to A (I thought that was what your original wording was implying).
     
    Last edited: Sep 2, 2011
  20. Sep 2, 2011 #19

    Dale

    Staff: Mentor

    What is important is not what experts say nor what makes sense to you, but what experiments prove. Both you and the experts have to change your mind if experiments contradict you.
     
  21. Sep 2, 2011 #20

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    I don't understand why you posted this. Are you trying to point out a flaw in Tomer's thinking or a flaw in the FAQ?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Really confused - basic special relativity
Loading...