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Newb question

  1. Aug 30, 2011 #1
    Hi yall, I know FTL is impossible so please dont flame,but if you were on a rocket that just so happened to be travelling at or faster than 3 x 10^8 m/s relative to say Earth, and they shined a powerful laser parallel to your path, would the beam of light still zoom past you at 300,000 km/s? Or does SR(c being constant) only hold true up until right below the speed of light?

    My physics teacher said the light would still pass you at c when asked this question...

    Seeing as how if you go .9c and light still passes at at 3E8 m/s then I would assume it would still act the same at 1c and above, is this not correct?
     
    Last edited: Aug 30, 2011
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  3. Aug 30, 2011 #2

    PAllen

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    To answer a question about FTL, you need a theory that admits it. Otherwise the only answer is: the question is completely meaningless. At least as meaningless as: what would pi be if 1+1=3 ?
     
  4. Aug 30, 2011 #3
    Time for you would change if you are on said ship so when you see light passing you by that light will still be travelling at the same speed of 300,000 km/s. Regardless of what time frame your in light will always be travelling at this velocity in a vacuum.
     
  5. Aug 30, 2011 #4

    DrGreg

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  6. Aug 30, 2011 #5

    ghwellsjr

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    Instead of asking about a rocket which cannot travel at c, ask yourself and your teacher this question:

    Suppose you have two powerful lasers pointing in the same direction and you turn the first one on and off, sending a beam away at the speed of light and then you do the same thing with the second one, will the second beam zoom past the first one at c?

    Please post your answer and then ask your teacher and post his answer.
     
  7. Aug 30, 2011 #6
    Thank you for your patience and replies, and if its not too much to ask I have another very highly hypothetical/skeptical question. I know that these are pretty much scifi world crap, but would yall say that c would still hold constant in comparison to tachyons? I understand theres not much info out there about them much less if theyre even real. You dont have to answer if you dont want but I just wanted to see what the smarter people out there thought about how tachyons would act in comparison to light. Like one of the relativity vids I watched in which Einstein said if you were going the speed of light and held a mirror in front of your face, you would still see the reflection. Surely this would still hold true for tachyons, correct?
     
  8. Aug 30, 2011 #7

    PAllen

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    I do not believe Einstein said such a thing, and would reject as unreliable a source that said as much unless it provided a reliable reference.

    The tachyon question is really interesting. Tachyons as particles can be treated in a consistent way in relativity (and also in potential new physical theories like string theory). Note that, at present, despite considerable effort, there is not a shred of evidence they exist (and personally, I doubt they do). However, I have never seen an analysis of what a 'tachyon frame of reference' would be like (or why it should be conceptually rejected). I hope someone else here can answer this.
     
  9. Aug 30, 2011 #8
    He very well may not have said it, it was a relatively new video doc with actors and also had one on Faraday in the same series. I cant rememberthe name of it. I guess whoever made it was just trying to simplify the explanation greatly.
    If Einstein was correct, and Im pretty sure he was, wouldnt that mean that even though the tachyons are going 3E8 m/s(+), light would still pass them at c? Or would it not work like that? I just cant see(no pun intended) that going FTL would make it so that it completely outruns it and therefore it would see nothing/have no interaction?? It just seems like if something going .99c still gets passed by light at 3E8 then so would something going +/= to c.
    Then again tachyons have no interation with matter......but neither do neutrinos, correct? But yeah they probably dont even exist, and if they do well most likely never know.

    On that note, would neutrinos have the same characteristics as photons?
     
  10. Aug 30, 2011 #9

    PAllen

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    Einstein assumed nothing goes faster than c; the idea of tachyons came after his death.

    Since a tachyon will arrive at a target ahead of light emitted at the same time, I do not think it would see light moving at c.

    Tachyons are proposed to interact with matter - that's how experiments tried to detect them.

    Neutrinos definitely interact with matter, as do photons. If neutrinos were massless, they would travel at c, just like photons; however, they are not massless and travel very slightly slower than c.
     
  11. Aug 30, 2011 #10
    Okay, thank you thats what I wanted to know. I didnt know they interacted with matter, or atleast are supposed to. How could you possibly detect them? Also, can they transfer energy to something?

    I read that they go through tachyon condensation which sends it to a stable state where no physical tachyons exist. So, its kind of like when they appear, they are unstable and will always reach a point where they are turned into "regular" particles, how long do they last in their superluminal state?

    What is the current popular status on tachyons in the particle physics/quantum/string theory world?
     
    Last edited: Aug 30, 2011
  12. Aug 31, 2011 #11

    PAllen

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    In classical SR, tachyons have imaginary mass but their energy is real, so they can exchange energy in perfectly normal fashion with regular particles.

    In QFT, tachyons are treated a bit different, and condensation is relevant. However, that is not an area I know anything about. Someone else can hopefully answer about QFT treatment of tachyons.

    I suspect that a 'tachyon frame of reference' is not considered valid. Again, someone more expert in this area should comment.

    In general, normal treatments of SR and GR do not consider tachyons at all, especially since the searches for them all came up negative.
     
  13. Aug 31, 2011 #12
    If tachyons exist they are said to move backward in time, This however violates causality in special relativity.

    Wiki has an interesting slide show of what a tachyon would look like were you able to view it.

    http://en.wikipedia.org/wiki/Tachyon the slide show is on the right of the page. Hope this helps
     
  14. Aug 31, 2011 #13
    So they do violate causality? I had heard otherwise.

    Does anyoneknow what most high level scientists out there think of them today?

    Do open strings REQUIRE tachyons? What do yall think of that?
     
  15. Aug 31, 2011 #14

    ghwellsjr

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    Did you miss my question for you and your teacher from post #5?
     
  16. Aug 31, 2011 #15
    Sorry, Idk how I missed your post. But no the second wont pass the first. The first one was sent before the second and therefore will always stay ahead of it. I dont have his class anymore but that would be my answer. I guess I was just thinking of tachyons as matter and in turn thought SR would still apply to them. Im not what one would consider an expert.

    Anyway, I dont quite understand the concept, do open strings always have tachyons? And what is the current popular view on tachyons and string theory itself?

    Obviously no tachyons have been discovered and you would think they would pop up somewhere and give off cherenkov radiation in cosmic rays and high energy events like that that can generate large amounts of particles, unless theyre very weakly interacting? I read that since we dont detect them, that they may have no electrical charge, is this correct?
     
  17. Aug 31, 2011 #16

    ghwellsjr

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    Well, if you think that the first beam of light would always stay ahead of the second beam of light, why would you make these statements?
     
  18. Aug 31, 2011 #17
    I was going off what my physics teacher said, and I quote "the way I understand it is, in your realm no matter how fast youre going light will always be perceived to go 300,000 km/s." That was his answer when I specifically said "if youre travelling at the speed of light, will light still travel at c relative to you?"

    Sorry about the mixup, I was just under the impression that matter would behave that way, for example, I hear some astronomers say theres places in the universe that appear to be going FTL...

    Obviously I was under the wrong impression.
     
  19. Aug 31, 2011 #18

    ghwellsjr

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    Your teacher's answer, "no matter how fast youre going light will always be perceived to go 300,000 km/s" is correct but he probably should have pointed out that you can't travel at the speed of light, let alone any faster.

    However, this answer is a whole lot different than the phrase you used in your opening post where you said "zoom past you". Do you understand the difference?
     
    Last edited: Aug 31, 2011
  20. Aug 31, 2011 #19
    It does seem that some areas of the observable universe are traveling FTL, but in all actuality, the space in between us and them is increasing (cosmic inflation). The objects themselves are moving less than the speed of light locally, but due to inflation, they are receding FTL. So if that matter receding FTL emitted light, it would never reach us. It is still moving at speed c, but we are outpacing it.

    Think of this as the treadmill effect (where this "treadmill" represents space). A runner (light) can move at a high rate of speed, but the treadmill is moving faster. So the runner never reaches the end of the treadmill.
     
  21. Sep 1, 2011 #20
    How do we see it if its expanding faster? Or do we not see it?
     
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