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I'm sure this has been asked a million times but,

  1. Jan 25, 2004 #1
    If your in a ship traveling at the speed of light towards a planet and you are standing in the middle of the ship, would you be unable to walk forward in relation to the planet? Within the ship, would you be completely unable to move toward the the planet because the ship is already traveling at the speed of light towards the planet and thus you can not move faster? If you can move forward, how is this explained by Relativity?
  2. jcsd
  3. Jan 26, 2004 #2


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    Yes it has, and the answer today is the same as the last million times. You are starting your argument with a false premis. A ship can not travel at c speeds in special relativistic physics.
  4. Jan 26, 2004 #3


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    If your ship were moving at "speed of light minus 2 feet per second" which IS possible, would you be able to run forward at 5 feet per second? Yes, of course. Because you are moving relative to the ship. You can move at any speed, up to c, relative to the ship.

    "Would you be able to move forward in relation to the planet?" Again, the answer is yes. Relative to the planet your speed would be (u+v)/(1+ uv/c2) where u is the speed of the ship(relative to the planet) and v is your speed (relative to the ship).

    In the example I gave above, with the ship moving at "speed of light minus 2 feet per second"= 0.99988c and you moving at 5 feet per second= 0.0002838c, your speed relative to the planet would be
    (0.99988c+ 0.0002838c)/(1+(0.99988)(0.0002828))= 1.00017c/1.0001704= 0.9999996c, Slightly less than the speed of light.
  5. Jan 26, 2004 #4
    You're doing the transformation of velocity wrong. Are you familar with the velocity transformation rules? If not then see

  6. Jan 26, 2004 #5


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    Setting aside for a moment the impossibility of your ship travelling at c, the answer is no, you cannot walk forward. In fact, you cannot do anything at all. As the ship approaches c, time dilates. That is, it slows down. If you could reach c, time would be infinately dilated (IOW; still). So you could only do the amount of walking that can be done in "0" amount of time, which is none.

    To an observer on the planet, your ship would appear to approach at lightspeed and, if they could see in the window, you would appear frozen in place within the ship (That's if they could see you at all, which they couldn't). From your perspective inside the ship, you would arrive at the planet instantly, with the trip taking absolutely no time at all form the moment you reached lightspeed.
  7. Jan 26, 2004 #6
    Doesn't time dialation only effect an observer, as in at something traveling at the speed of light obseverers see a diluted time of the object moving at the speed of C yet both the observer and the object experience time relative to themselves identicly?
  8. Jan 27, 2004 #7


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    No he isn't.
  9. Jan 27, 2004 #8
    Indeed- but when moving at c the mover's time is dilated completely. The mover doesn't experience being frozen, but rather experiences getting in the ship, flooring the pedal, and arriving at the destination (the point where the ship left light speed).

    The faster you go relative to other objects, the slower time moves for you relative to them. If you are moving at the ultimate velocity, c, time moves ultimately slowly, zero (relative to whatever you're moving at c to).

    Your situation is impossible to ever occur then. Conceptually the answer would probably be "yes you could move towards the planet" (conceptually = ignoring the fact that time would be dilated so much that you would not be experiencing the movement). If your ship is moving towards the planet at c, your ship as observed from something moving AWAY from the planet (along your ship's previous course in the opposite direction) would appear to be moving faster than c. To use another example (All velocities are given in relation to something at the midpoint of East and West, a rock or something):

    You are moving east at c. Mister Jones is moving west at 5 MPH (miles per hour). Relative to Mister Jones, then, you are moving at c plus 5 MPH. Of course any light coming from your ship to Mister Jones would only move at c relative to Mister Jones, so how he would percieve this is um... interesting. Heh.
  10. Jan 28, 2004 #9


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    This response is not helpful, DW, and neither was the previous one from you that was deleted. These forums are open to discussions from beginners to experts. If someone asks an honest question, it is hoped that the knowledgeable members such as yourself can provide a helpful explanation without putting anyone down or trying to start sidebar arguments.

  11. Jan 28, 2004 #10


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    I am not putting anyone down here. I had sort of put Archon down in the last one because he was downtalking to him which was the only valid reason for you to delete it. What I posted here should be helpful because it corrects Archon's statement that he was not doing the transformation right and reafirms to him that what he was doing was fine. Where it comes to doing physics such things as whose frames you call primed or even if you denote them some other way or what directions axis are oriented or what sign convention one chooses really doesn't matter as long as it is done self consistently. He didn't choose the primes the way most people would have, but aside from calculational errors and lack of significance did it correctly anyway and would have had the right answer and so it Archon's claim that his method was wrong and his downtalking to him implying that he wasn't familiar with the transformation that was not helpful.
    Last edited: Jan 28, 2004
  12. Jan 29, 2004 #11
    I'm not going to pretend all that made perfect sense to me, but I'll just move on. I do have another what-if question though if yall wouldn't mind..

    Lets say I had a pole made from an amazing material, a material impervious to compression or deformation of any kind. This pole was quite long, 1 light year long to be exact. I Tie one end of this pole to a Rocket that that rapidly accerates, how long would the other end of the pole take to start moving, once again assuming there is no compression or stretching of the pole.
  13. Jan 29, 2004 #12
    One year.

    We were debating a similar question in my physics class a couple of weeks ago. The difference was we were thinking that if you had a rigid pole in the pole/barn paradox, you'd be both outside the barn and inside at the same time. We argued about it for a very, very long time until our teacher told us that this wasn't allowed in relativity.

    He sent us a link to a discussion about something similar online: http://www.madsci.org/posts/archives/jun2001/991868698.Ph.r.html
  14. Jan 30, 2004 #13


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    There you go starting with a false premis again. If I start with a false premis I can even come up with a logical arguement concluding that I am the pope. Fortunately there are no real matterials that are perfectly rigid just as there are no masses that travel at the speed of light. So, arguments concerning relativity that encorporate such things as premisis are known apriori to be unsound.
  15. Jan 30, 2004 #14
    DW it is not a "false premise" it is a "theoretical situation" if you do not have the imagination to grasp the hypothetical situation, then you input is really not needed. There have been many others here at the forums that do indeed know the flaws in my understanding and yet have taken the time and care to explain it to me(which is why I ask these questions in the first place, to further my understanding) and I have learned from them.

    Despite the concise and logical nature of science, and especially physics,the only thing we know for certain is that we do not understand much of it at all.

    You however, have added nothing to this thread and I have to wonder why you bother posting in the first place.
  16. Jan 30, 2004 #15


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    "Theoretical situation" is an ok way to describe it, but DW is also quite right when he says its a false premise. DW may be being a little curt with you, but the main problem here is you appear to have a fundamental misunderstanding of the way science itself works: you're asking the wrong questions and I think you know it, you just don't understand why its not a good idea. The answer to your question (the answer by my interpretation is "instantly", btw) does nothing at all to increase your understanding of physics because the premise of the question is a direct violation of the laws of physics - ie, a false premise.

    Besides the fact that most physicists would disagree, starting with the position that 'all we know is that we don't know much' won't get you very far in figuring out what we do know.
    Last edited: Jan 30, 2004
  17. Jan 30, 2004 #16


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    To put it another way, the other end of the pole would indeed start moving instantaneously, if it were infinitely rigid. But if it were infinitely rigid, then it could not be in this universe. You see, space is not infinitely rigid, nor can it accomodate an infinitely rigid substance. In real space, the maximum speed at which a signal can propogate is lightspeed. So, as soon as you refined your "amazing material" to the point where it really was "impervious to compression or deformation of any kind", it would cease to occupy space in this cosmos. As one famous author put it, your pole "vanishes in a puff of logic".
  18. Jan 30, 2004 #17


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    Not only do I have the imagination, but I had the insite to know where you are going with this from the first post. The answer to why you do not have a contradiction implied by relativity is exactly what I told you. You have started with a false premis. There are no perfectly rigid matterials in relativistic physics.
  19. Jan 30, 2004 #18
    russ_watters, I agree with what the basics of what you and most the rest of you are saying, but I must disagree with your statement that I learn nothing from asking these questions. These are question my (flawed?) intuition is asking, and I must not only understand the correct answer but I must understand the path to the correct answer. So yes, I am asking the "wrong" questions but I am doing so because I want to understand why they are wrong and in what way my view of the universe is biased.

    For example this is the path my mind takes on my problem.

    you have a pole a light-year long, the space tug grabs one side and starts to pull and lets say it accelerates to nearly the speed of light.

    I can see a couple of different things happening:

    1. If it didn't stretch, the end would move instantly. OR it would take an infinite amount of energy to move it, thus relativity is preserved. Relativity has already been proven wrong under certain circumstances(on the quantum scale) so how would I know this would not be another case of the breaking down? I not arguing I'm searching for understanding.

    2. It would have to stretch, and if you figure that the space tug accelerated to nearly the speed of light, by the time the other end moves, which would be about a year, the pole's length would be nearly twice as long. I guess this would be what would happen but it just doesn't jive with my intuition(damn my intuition). Is this stretching the same idea that is frequently associated with releativistic movement, the warping of physical dimension caused by relativity?

    3. It would break, this isn't very interesting although it is extremely likely.

    I guess I just want to understand what exactely the speed of light is. I know it has something to do with the mass of a body in motion approaching infinity, but perhaps my real question is why does the mass increase. Is this effect like gravity before relativity in which we have laws to describe it but are completely unable to say what what the effect is caused by. Or have I simply been protected from the truth in my education because the answer is thought to be too complex for the average joe?

    And on a completely different note,

    As far as what Russ was saying:
    The "all we know is that we don't know much" doesn't have to "get very far in figuring out what we do know," since all one has to do to find out what we do know is simple: be an effective gatherer of information, read up, keep up with current events. Basically it doesn't take much effort to do, althought understanding it perfectly may.
    I'm not saying that the science we have now is invalid or not useful, it goes without saying that is extremely successful in explaining a wide range of phenomenon. I'm just saying its useful to take a step back sometimes, to exit the current framework and see whats out there(ie. string theory).

    A quick look back in humanitity's past can lend many examples of the usefulness of this approach. For thousands of years man thought the earth was the center of the universe, only through the effort of a few that were willing to accept the fact that their vison of the world was incomplete were able to pur forth the correct theory.

    I could go on but I'm sure you could get the gist of where I'm comming from on this.
  20. Jan 31, 2004 #19


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    Relativity has issues on the very small scale only. Not on the very large scale.
    Huh? Well, I guess your intuition is doing ok - the reason it doesn't jive with your intuition is its meaningless babble. You realllllly need to get away from these convoluted, contrived, false premise hypotheticals. They are utterly meaningless. You need to learn how the box looks from the inside before you can speculate about what's outside of it.
    That's a cop out - if its so easy, why haven't you done it yet? If you did make the effort to learn these things (relativity itself doesn't take much effort to understand in a basic way), then these questions you are asking would never come up. You'd understand why they are meaningless.
    Nope. Virtually nothing was known about science until the scientific method was invented in the 1500s. What happened before that is largely irrelevant because they didn't even have a process by which to figure anything out. That's the way you're approaching this: blindly. You won't learn much for the same reason they didn't learn much.
    Yes, we understand perfectly well where you are coming from - and we're trying to help you fix it.
  21. Jan 31, 2004 #20


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    Who says relativity is false in quantum theory? All the advanced quantum theories including string theory and the standard model are explicitly (special) relativistic. The only quantum theory that isn't is the original, pre-Dirac version. That is still useful where slow speeds can be assumed, but it isn't accepted as the last word on describing nature.

    If you mean the inability of GR and quantum theories to be joined, the jury is still out. Maybe it's quantum that will have to give?

    If you mean the Planck scale, nobody has the faintest idea what happens at the Planck scale.

    Bottom line, there is nothing definite, nothing but predjudice, that says relativity has failed at all.
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