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Nothing can exceed the speed of light

  1. Oct 12, 2005 #1
    Hi. I have a couple questions about relativity.

    I hear that nothing can exceed the speed of light, but then I hear that all motion is relative. When one beam of light goes past another beam of light going in the opposite direction, aren't they moving at 2c with respect to each other? And if I shine a flashlight and walk in the opposite direction, aren't I traveling over c relative to the light? And when any light occurs at all, isn't everything else travelling at the speed of light relative to it?

    If this is not correct, does relativity mean that nothing can travel faster than light from any one reference frame's point of view?

    How do you determine what an object's speed is? What is the ultimate reference frame for something's speed to be "relative to" because everything in the universe is travelling at c compared to something else???

    Thanks to anyone who can help!
  2. jcsd
  3. Oct 12, 2005 #2


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    Relativity works in weird ways. It's not as weird as quantum physics, but it leads to accurate predictions.

    Suppose your friend is on a train passing by you at v, shining a flash light at you. You will see the light coming at you at c!!! As nothing can go faster than c, even light itself cannot violate this. But your friend on the train will argue that he is also seeing the light going towards you at c! The consequence of this is time dilation. He thinks your clock is going a bit slow and you think his clock is going a bit slow. The dilation will perfectly make up so that everybody will measure light at c.
  4. Oct 12, 2005 #3

    Does that mean that no matter what you are doing or what is going on, you will see light travelling at c, and someone else who is travelling at a different speed in a different way will see the same beam of light going at the same speed in the same direction?
  5. Oct 12, 2005 #4


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    Indeed. As I've said. The rule that the speed of light is constant in all reference frames is so divine that even time will bend to serve this rule.
  6. Oct 12, 2005 #5

    I have to digest this...

    Thanks for the info... I'll be back on tomorrow if you or anyone else has any additional comments on any of my questions.

    Thanks Again!
  7. Oct 12, 2005 #6
    I'm not an expert or anything, but from what I understand:
    Time is distorted to keep light at its speed, so If you were traveling the speed of light.. or near it in the opposite direction of another beam of light ... or any EMR for that matter.. then time would almost stop for you so that the beam of light would still be traveling at c. .. and if you could go faster.. then please let me know that the answer from #4 on is A on the econ test I took a few years ago.
  8. Oct 12, 2005 #7
    Hummm, So what happens when I setting outside and I'm looking at the North star and I turn around as fast as can to look at a star in the south. Could I say that because everything is relative that I did not move but everything else did and if so would that movement not appare to FTL?
  9. Oct 12, 2005 #8
    (along with above here is a copy of another post I had made)

    You know I heard allot of this before and will I'm just not buying it. Why is everyone so quick to ftl is impossible. I see a one problem with Einstein theory. If mass changes with the speed of an object, then is mass a virable? With out know what interaction makes this happen I think it is a bit much to say FTL is not possible. If one day we find the cause of this change in mass it might be possible to change the mass of an object to nothing, Einstein own theory say mass is not a constain nor is time. Would this not mean once we have a better understand of both, that both time and mass could be changed to make FTL possible, or do we keep doing as we have been doing, and say everything impossible? Another good one, is the people who says there no way to get around gravity or there not antigravity, they are sure that it is impossible yet nobody know what cause gravity. How do you know it is impossible when you don't why is possible?
  10. Oct 12, 2005 #9


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    There are "things" that certainly do go faster than light. A shadow for example is not restricted by the speed of light. Or shining a laser onto the moon and then quickly changing the angle of the laser we can make the beam on the moon appear to be moving faster than light. At the end of the day however nothing is actually going faster than light. There are many more such phenomena. Search google for "faster than light". The conclusion is, we are unable to communicate anything meaningful faster than light. That's what it really means.
  11. Oct 12, 2005 #10


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    Relativity says that nothing can have a velocity greater than that of light in a given reference frame, and likewise that light always travels at c in a given reference frame. But it is quite possible that, in a given reference frame, the distance between two objects whose individual velocities are less than c will be increasing at a rate greater than c--for example, if one object is moving at 0.8c to your left, and the other is moving at 0.8c to your right, then in your frame the distance between them is increasing at a rate of 1.6 light years per year. But, if you transform into the rest frame of one of these objects, then in this object's frame the second object will not be moving at 1.6c in this frame--instead you must use the formula for addition of relativistic velocities, [tex](u + v)/(1 + uv/c^2)[/tex], to find that in this frame the second object will be moving at (0.8c + 0.8c)/(1 + 0.64) = about 0.9756c. So the light-speed limit is about the individual velocity of any object in a single reference frame, not about the rate that the distance between multiple objects is growing or shrinking in a single reference frame.
  12. Oct 12, 2005 #11


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    Special relativity does not say all motion is relative, only inertial motion (motion that doesn't involve acceleration) is. If we are moving apart at constant velocity v, then we can look at things either from a frame where I am at rest and you are moving at velocity v, or a frame where you are at rest and I am moving at velocity v. On the other hand, if you are orbiting around me and I am not accelerating, it is not equally valid in SR to say that you are at rest and I am orbiting around you--the question of who is accelerating and who is not is an objective one, because the person who accelerates will feel G-forces (the centrifugal force, in the case of circular motion) while the one who doesn't will not.
  13. Oct 13, 2005 #12
    We always need to have a constant don't we:wink:
  14. Oct 13, 2005 #13


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    The cause for this change of "mass"* is the input of energy needed to change the velocity of your object.
    A simplified way of thinking about it is like this:

    As you add energy to an object to accelerate it, that energy adds inertia of its own to the object, thus making it even harder to make further increases in the objects speed. This in turn increases the amount of energy you need to supply in order to make further increases in the objects speed (Just as if the object had gained mass). But this further increase of added energy itself adds inertia, etc. etc. The upshiot becomes that the total amount of energy needed to get an object up to a certain speed(relative to yourself) approaches infinity as the objects speed approaches the speed of light.

    * I put mass in quotes here because there is some debate in convention as to whether it should be strictly considered as mass
  15. Oct 13, 2005 #14
    Let me rephase what I was saying above. In our current understand in physics FTL travel is not possible, but our current understanding is limited due the fact we have very little understanding into origin of mass, gravity, and time, and until such time as we do have this understanding I think it is bold to make statements and call them law without knowing all the facts. I personal think we don't haft as much stuff as we think we do. I remember in school the model of the atom and sub atomic particals where not in the picture, now they have named them. I kind of like the Bertrand Russell saying above.
  16. Oct 13, 2005 #15
    So what happens if you are travelling at c/2 and you pass various space ships travelling at velocities of different parts of c, both travelling toward and away from you?

    Is your time different compared to all of these people depending on how you pass?

    Would you "see" some ships with people who are aging much faster than you, and others who may pass you that are not aging at all really?

    Doesn't it seem kind of strange that merely being in motion compared to something else affects the passage of time?

    Or does it have more to do with the amount of energy that object contains?

    I've heard it said that as you approach the speed of light (I still don't know in relation to what) you gain mass...??? what is this mass... is it hydrogen?plutonium?... probably einsteinium! :) seriously though, do you actually gain mass, and does it go away as you slow down again?

    Also how do you reach the speed of light, because no matter what speed you are going, there are going to be somethings that are coming toward you, and others that are going away from you, so how do you judge when you've reached the speed of light???
  17. Oct 13, 2005 #16


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    Yes, but you don't need Einstein's Relativity for that - the delay caused by the fact that th distance is changing affects your ability to watch what's going on on the other ship.
    The first time I heard it, yes. But the evidence is so overwealming that it really isn't up for question - you get used to it. And besides - a lot of principles in physics, technology, etc. seem like magic the first time a person hears about them. That's ok, as long as you keep an open mind about learning new things.
    You can use a beam of light to measure your speed relative to any object you choose. You can even measure your speed relative to the beam of light (it'll always be zero).
  18. Oct 13, 2005 #17


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    The mass isn't "matter" in the conventional sense. Its inertia thats given by the energy added to the object to make it speed up. You don't actually gain any more atoms to your body you just have more inertia and mass is really a measure of inertia
  19. Oct 14, 2005 #18
    Thanks Very Much but Still a lingering question

    Thanks to everyone for the responses, but regarding the quote above...

    What I meant was, if you can't go light speed, but you can get very close, then how do you determine which object "counts" as an object to measure your velocity to?

    For example, I am going light speed right now compared to the light that is coming past me from my monitor or lamp... in fact relative to those two things I am going light speed in two different directions! So what I am asking is, if I am going .9999c compared to the earth, and then a superfast rocket goes by me in the other direction at .9999c compared to earth, then I am going .9999c (compared to the earth) in one direction and the rocket is going .9999c (compared to the earth) in the other, so what happens? Aren't we both traveling nearly twice the speed of light in relation to each other since we are both seeing the earth disappear behind us at .9999c?
  20. Oct 14, 2005 #19


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    In your own reference frame, light always moves at the same speed. In other words, even if I observe you chasing a light beam at 0.8c in my frame, in your frame you will not observe that light beam moving away from you at only 0.2c, you'll still observe it moving at 1c in your own frame.
    Light actually doesn't have its own reference frame in relativity, because if it did, it would violate the rule that the laws of physics must work the same in every reference frame (so if there are some frames where light moves at c, it must move at c in all valid reference frames). So, there's no inertial frame where light is at rest and you are moving at c.
    Nope, even though in the earth frame you are both moving at 0.9999c in opposite directions, in your frame the earth is moving away from you at 0.9999c but the other ship is not moving at 2*0.9999c...again, you have to use the formula for addition of velocities in relativity, [tex](u + v)/(1 + uv/c^2)[/tex], which in this case tells you that in your frame, the other ship will be moving at (0.9999c + 0.9999c)/(1 + 0.9999^2) = 1.9998c/1.99980001 = 0.999999995c away from you, in the same direction the earth is going.
  21. Oct 15, 2005 #20
    Starting to get it...

    Thanks very much JesseM.:smile:
    But I may have more questions to come!
  22. Oct 16, 2005 #21
    The idea of mass increasing is not even in fashion in the physics education community anymore. The point is that it is completely unneccessary in the theory of relativity. Attacking the idea of relativistic mass is a completely in-effective way to attack relativity because most people teaching relativity these days don't like the idea of relativistic mass anyway.
    Most people who argue for ftl simply do not understand relativity. Did you know that relativity does not limit how fast you can travel? Did you know that according to relativity you could concievably get to the nearest star in just a few minutes?
    What relativity does say is that, even though you can travel as quickly as you like you cannot get back in time for dinner. The reason is that while you can get to the nearest star in a few minutes, for people back on earth watching it will still take over 4 years. This does make star empires somewhat impractical so I guess it is the human drive to dominate larger and larger portions of the universe that is so threatened by relativity. But maybe when we truly grow up and leave such childish ideas behind, then we will be ready to travel to other stars.
    I had not heard this one before. We certainly do not know any way of generating antigravity yet, but I had not heard that anyone has argued that it could never be possible.
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