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What is speed?

  1. Mar 17, 2012 #1

    I have been studying a lot of physics on my own through the internet (I am 16 years old, I intend to keep studying physics when I get older), and I get the most of Einstein's theories and quite a lot of the quantum mechanics, but there is something I don't understand about special relativity;
    According to special relativity, a particle with mass can't reach the speed of light, c, because its mass would just keep increasing. Fine with me. BUT, the theory also states that speed is just something relative, and is different according to what you compare it to. But my question is then, if a particle is approaching c, let's say 99% of it, what is it traveling relative to? Space itself? Is there a theoretical "map" of space wich objects can travel across, or is speed only relative?

    Another example:
    How can a particle with mass be at rest, since everything in the universe is moving? Let's say we take a stone. We put the stone on a table, and let it lie there on the table. The particles would then be at rest, but not according to an observer standing outside of our solar system, or outside our galaxy. To that observer, the stone would be traveling really fast, and therefore not have a rest mass.

    Please help me out on this, I really don't get it.
  2. jcsd
  3. Mar 17, 2012 #2

    Doc Al

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    You tell us. To say that the speed of a particle is 0.99c without specifying with respect to what is meaningless.
    There is a frame moving with the particle in which the particle is at rest. Being 'at rest' isn't absolute, but is with respect to something, just like any other speed.
  4. Mar 17, 2012 #3
    But if nothing can have a speed without comparing it with something else, how does the effects of approaching c affect the object? For example, the room is contracted a lot when moving fast, but if you need something to move relative to, I could perhaps say that I am moving at 0.99 of c relative to a spaceship, wich in the spaceships point of view I am moving and it is at rest.
    But from my point of view, the room isn't contracted, or is it?

    Please correct me if I'm wrong on something, I'm just curious, I'm not saying I'm totally right.
  5. Mar 17, 2012 #4

    Doc Al

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    If you are moving at 0.99c with respect to a spaceship, then it is the spaceship which measures the length of your room to be contracted, not you. From your own point of view, your room doesn't contract.
  6. Mar 17, 2012 #5
    Oh then I get it. Thank you!
  7. Mar 18, 2012 #6
    Cibek allow me to enlighten you at the risk of getting these facts attacked by materialists. If you are learning from YouTube videos, that's great keep doing that!

    Ok in order for you to properly understand 'speed' you need to know a few more things.

    You see everything in the universe is relative to the person experiencing it. huh?
    If you could set up an experiment where "John' gets to go in a spaceship around the solar system very close to the speed of light. 'Jack' stays on earth to watch John in the spaceship with a very special telescope.

    From the point of view of Jack, Johns mass increased dramatically. Johns wrist watch seems to be ticking so slowly and it would appear that John and his spaceship seem to shrink in size.

    Now let's go into John's head (his experience). You see if John would weight himself(or the ship) nothing changed, the mass did not increase. When he looks at his wrist watch, tick tock tick tock everything seems normal, time does not slow down for him. He does not appear to shrink in size also. Where ever John goes (is moving) everything is fine and dandy.

    In pursuing your interest in studying Physics you need to always keep this in mind.

    I'll go even further, watch the double slit experiment: again you will notice some bizarre facts. You see most Harvard Physics teachers agree that all the physics books need to be re-written. They are based on a 100 year old model that does not include the 'experience' in this universe, very objectively. When everything you ever seen, read, experienced is subjective.

    So you are right, the whole universe is moving, nothing is sitting still, but how fast is it moving?? we can only measure speed from something else that is 'moving' at a different speed.
    Things are not as they seem and the new science is proving this yearly. And yes the vacuum of space is NOT empty but it seems as if this vacuum follows you where ever you go at any speed so you can't measure speed simply from the vacuum.

    So good luck in your studies and search for 'the right' stuff on youtube, do not keep a narrow materialist closed mind and who knows maybe you will be the one to discover the next big thing. =)
    Last edited by a moderator: Sep 25, 2014
  8. Mar 18, 2012 #7
    Am I right in saying that speed is in no way a method of prolonging your life? It's just a matter of perspectives right? Once you get rid of clocks and other peoples perspectives there is absolutely nothing I can do to live longer based on the speed I'm going. Biologically everything keeps ticking away as normal.
    If I'm in a spaceshop I will experience the same lifespan no matter how fast I'm going.
  9. Mar 18, 2012 #8


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    However, that does not prevent others from perceiving you as seeming to live a very long time. You could blast off from Earth in 2020 at 20 years old and come back in 2120 (Earth time) to celebrate your 21st birthday, with another 60 years of life ahead of you.

    By Earth calendars, you were born in the year 2000, but died in the year 2180 - a 180 year span.
  10. Mar 18, 2012 #9
    Think of a simple way to quantify your lifespan, eg how many heart beats or breaths you take in a lifetime or how many books you can read. No matter what your speed is relative to anything else that does not change. However if you go on a round trip from Earth when you return people that were younger than you when you left, may well have died of old age. Consider the classic twins paradox. Imagine they have a genetic condition that makes them die after one million heart beats. If one stays on the Earth and the other goes on the round trip in a fast rocket, the one that stays on the Earth dies before the other one, but they both die after their allotted million heart beats. Here is another curious fact. Imagine there is a star system 200 light years away. If we say your maximum lifespan is 100 years there seems little possibilty of getting to the star system if it takes fastest thing know (light) 200 years to get there. However with time dilation and a fast enough rocket it would be possible to get there in 50 of your own years. So even thought time dilation seems entirely subjective or hypothetical being only relative, it can make the difference between getting somewhere far in a single lifetime or not.
  11. Mar 18, 2012 #10
    OK that makes more sense. I also recall reading somewhere that time slows down a tiny bit if you're standing next to a large object. I guess this is all part of the relationship between time and gravity. Does that mean our perspective of time is influenced by the mass of the earth also? And does that mean a fruit fly or an ant's perspective of how long their lifespan's are could be completely differen't from how pathetically short we see it as being?
  12. Mar 18, 2012 #11


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  13. Mar 20, 2012 #12
    Wow, thanks for all the replies, it really helped me understand relativity better!

    I have another question.
    Let's imagine you could remove all matter and all things in the universe, so that only space itself would have been left. If only I, my body, existed in the universe, would speed exist? Since their would be nothing for me to move relative to, could I move at all? This is another thing I don't get. If I were to move from point A to point B (in space), I would have to move through something. But according to relativity, I could not have speed since there would be nothing to move relative to.

    Is this a wrong view of relativity, or am I right? This is what I meant that you perhaps could move relative to space itself.
    Last edited: Mar 20, 2012
  14. Mar 20, 2012 #13


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    Speed of the blood through your veins?

    But yes, speed is relative to a point of reference.

    This is true. Though note you would still experience the initial accelerative force, so you would know your speed has changed.

    There is no 'space itself'.
  15. Mar 20, 2012 #14
    What about spacetime? Isn't that "space itself"? Space can bend, so it has to be something?
  16. Mar 20, 2012 #15
    I don't think relativity means you don't have a speed. It means observed speed is relative. So in that sense any speed you do have, alone in the universe, would be absolute. Said different, the only measure of time / length would be proper.

    In that sense you could identify your motion as relative to "space". More specifically a position in space, not space itslef, like DaveC426913 had already mentioned.

    The issue being apparent in the scenario of you being the only one in space, coordinating a location to determine your speed. At any time you can re-cordinate your speed as it were and all physics will be the same, that is all observed physics tee hee hee.
    Last edited: Mar 20, 2012
  17. Mar 20, 2012 #16
    That is out of context. It is something, but not something you can coordinate relative motion to.

    In that your speed is X relative to space itself; is not true, there is no "space itself" in that sense. With gravity, yes "space itself" bends. (as you pointed out specifically spacetime)

    Slightly off topic, There is coordinate acceleration & proper acceleration. Is there also coordinate speed & proper speed?
  18. Mar 20, 2012 #17


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    This is an interesting situation. This thread is asking "What is speed?" while at the same time, another thread is asking what "at rest" in Einsteinian relativity means!!

    Well, they are really two aspects of the same question so I will quote my answer from the other thread:
    The only difference is that while "at rest" means the spatial coordinates are constant while the temporal coordinate varies, speed means the spatial coordinates are not constant while the temporal coordinate varies and average speed is the difference between the magnitude of the spatial coordinates for a pair of events divided by their temporal coordinates.

    The point for both of these threads is that in SR, you need to first establish a FoR before you can talk about the meaning of speed (or rest where speed equals zero).
  19. Mar 20, 2012 #18
    Actually there is a 'space' itself. Scientists created light from a vacuum in 2011, but we do not know enough to properly explain it.

    BTW when you said remove all matter in the universe, well matter doesn't really exist anyways. Every atom is about 99.99999% space. Imagine that, your entire life you never ever actually touched anything! Scientists have calculated that if you were to take all the 'real' matter or solid substance in the universe and pack it closely together then it would be the size of a 3 centimeter cube. =)
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