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Velocity physics question

  1. Jul 6, 2007 #1
    This is driving me crazy!

    When you increase your speed in a direction that counters the direction the earth is rotating or moving, or that the entire solar system is moving wouldn't you be slowing down.

    With so many things moving in so many directions at once how is it that we do not run into all kinds of anomalies when doing experiments related to speed.

    For instance you could actually lose mass when increasing your speed because you happen to be traveling in a direction opposite the solar system or earth rotation or earth movement through the solar system.... :bugeye:
  2. jcsd
  3. Jul 6, 2007 #2
    Relativity my friend.
    From any observer, you can state that you are moving at 0m/s and everything else is measured with respect to you.
    It all depends on WHO is observing, and how they are moving.

    Experiments are all about observers and observables. When you're on a train, and you're moving at 60mph, but you have the windows shut you don't notice it. For YOU you're not moving. If you roll a ball on the ground at 5mph, you can say that ball is moving at 5mph with respect to your reference frame.
    To someone outside the train the ball might look to move either 65 or 55mph depending on which way it was rolled.
    Velocities are all relative.
  4. Jul 6, 2007 #3
    It depends what initial reference frame you are moving relative to. If you are moving oposite of the direction of earth is turning relative to the earth you are still moving with your initial velocity plus the the velocity of the point where you left is moving away from you. So in reference to that point you are actually moving at a greater velocity than if the reference point were say the sun.
  5. Jul 6, 2007 #4
    I can't accept that though, I have read and read and read about general relativity and much of it makes sense but when I think about the distrance the photons that communicate between the atoms in a system, like the human body, must travel based on speed I need a constant or an origin of this speed.

    The faster something moves the larger the distance photons and other elementary particles must travel as they communicate back and forth causing "time" to slow down, IE humans to live longer etc etc...

    This is all based on a speed relative to some constant..

    Therfore if a human body speeds up in the opposite direction of some other speed happening in the universe you could be shortening the distance the photons must travel...

    Does this make sense to anyone..
  6. Jul 6, 2007 #5

    I don't understand what "communication" you are speaking of. I don't think your interpreting relativity correctly. How is speading up and away from some other object that is also moving causing the photons (why they come into play here I don't know) to have to travel a shorter distance.
  7. Jul 6, 2007 #6
    Thats what happens in relativity, when you speed up time slows down because it takes longer for the elementary particles to play out their roles because the distances traveled are greater.. This is what generates what we see as time and is the reason why if you go very very fast you will live for so long.

    But since we are already traveling fast in the solar system couldnt we slow down by traveling at a higher rate of speed in the opposite direction... IE shorten the length of elementary particles..

    I am refering to the light clock experiement when I talk of photon travel time. And If I understand chemistry right, photons are the communicator of the electromagnetic force that pretty much governs how everything interacts beyond nuclear forces and gravity... so increasing photon travel distance makes interactions take longer... and vice versa
  8. Jul 6, 2007 #7
    No, you won't live any longer. You will be percieved to be older by anyone who was outside of your frame. Relativity has nothing to do with slowing the process of aging. You will die the exact same time you were meant to die anyway.

    In what oposite direction? Relative to what? Your speed isn't changing the speed of the subatomic particles in your body.
    Last edited: Jul 6, 2007
  9. Jul 6, 2007 #8


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    No. Thinking about it this way is what is causing you these problems. According to you, your rate of passage of time never changes. Time dilation only occurs between two objects moving with respect to each other.

    If you are in a rocket with no windows and do some physics experiments inside, then fire your engines for a while, then repeat the same experiments, you will find no differences. Why? Because your experimental apparatus is stationary with respect to you.

    Have you heard of the playing ping-pong on a train analogy? You can play ping-pong on a train moving at constant speed. If you didn't feel the bumps, there'd be no way to tell you are moving and the game works just as it does on solid ground.
  10. Jul 6, 2007 #9
    But when you are on the train you are still moving at a speed relative to everything else in the universe right? Everything is an observer, I think I am trying to think about too many things at once.. :confused:
  11. Jul 6, 2007 #10


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    Exactly, and they will all observe something different. Someone on the top of a mountain won't observe you the same way that someone standing beside you will (because his relative time is faster). Someone on the moon will see you differently than either of the others, both because of the time differential and because you are moving relative to each other.
    To get anything meaningful accomplished, you have to pick a reference frame to observe from and stick with it.

    edit: I should point out that the gravitational time discrepancy is so incredibly tiny that even an atomic clock has trouble detecting it.
    Last edited: Jul 6, 2007
  12. Jul 6, 2007 #11
    I think you're misinterpreting relativity.

    You're right, everything is an observer. What an observer sees or experiences, though, has nothing to do with what you see or experience.

    The fact that there are infinite observers does not mean that you should experience infinite different scenarios. Instead, every observer, whether it be a human or an atom, sees its own scenario. For instance, if you are in a spaceship flying away from the Earth at half the speed of light, people back on Earth will have aged much quicker than you when you return. According to them, time slowed down for you. According to you, however, time went by at its normal pace. Your copilot will have aged exactly as fast as you have, because he had no velocity relative to you. This is at least how I interpret it.

    I do have a question, however. In the above scenario, can't a passenger on a space ship traveling at 1/2c relative to the Earth say that the Earth was simply traveling at 1/2c away from him? Wouldn't that mean that the passenger should perceive time as slowing down for those on Earth? How, then, can this passenger have aged more slowly than those on Earth, if he perceived those on Earth as aging more slowly than him?
    Last edited: Jul 6, 2007
  13. Jul 6, 2007 #12


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    If you use "invariant mass" rather than "relativistic mass" you won't lose any mass as a result of your velocity, and you won't lose any sleep worrying about losing mass either :-)

    This is one of many reasons why I recommend using invariant mass, which is a property of the object itself and does not depend on the velocity of the object or the particular coordinates used.

    As other posters have mentioned, the main idea of relativity is that you don't even need to worry about your velocity - no experiment you can do will detect your absolute velocity.
  14. Jul 6, 2007 #13


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    Sure, but you can still play ping-pong, can't you?
  15. Jul 7, 2007 #14
    And that's all that matters :tongue2:
  16. Jul 7, 2007 #15


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    What counts is the person / object that is being accelerated. The acceleration is "real" in the sense that it can be felt as a force.
  17. Jul 7, 2007 #16


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    It is! :biggrin:
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