Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Who is the one moving?

  1. Jun 20, 2012 #1
    I had this thought for a couple of days now but couldnt solve it so I wonder if anyone could help.

    Imagine person A & B in space. Where person A is standing still in a certain point in space. Person B passes by person A, let's say he is just moving in a straight line (from our point of view as an outside observer).
    Is there any chance you can now say who is the person really moving (if we where either person A or B)? Some "absolute movement"?
    The problem for me is, let's say I'm person A, I measure the speed of person B and get the velocity, let's say, v1
    Now person B measures the speed of person A who will measure the speed (v2), which is equivalent to the speed person A is approaching then later on receding.

    v2=v1 in our case right?
    However, how can person B determine that he is moving from just measuring the speed against person A? My mind says, for person B it can look like just as person A is moving and person B is stationary, right? They are both seeing each other approaching and receding.
    Same thing if I drive next to a car at the same speed, it will look like the environment is moving while me and the other car is stationary (which isn't the case, since we know we are moving relative the "outside world").

    I know speed is relative, and I'm not familiar to any math that has to do with general or special relativity, yet.

    I hope you understand my question.

    Best Regards,
    Curious student.
  2. jcsd
  3. Jun 20, 2012 #2


    User Avatar
    Science Advisor

    Yes, that is the whole point of "relativity"- we can only talk about the speed of an object relative to some other object. Before you can talk about the speed of an object you must first establish a "frame of reference". When you talk about the speed of A, as seen by B, technically you are talking about the speed of B as measured in a frame of reference that is "stationary with respect to A"- one in which A is motionless and B has some speed v. But you could just as well establish a frame of reference that is "stationary with respect to B". In that frame of reference, B is motionless and A is moving at speed v.

    That actually goes back to Galileo. "Gallilean relativity" asserts that no mechanical experiment can determine a constant velocity. But then Maxwell's equations seemed to show that "absolute" speed could be determined by electro-magnetic experiments (of course, Gallileo knew nothing of electro-magnetism). After repeated experiments came up with null results, Einstein extended relativity to include electro-magnetic fields.
  4. Jun 20, 2012 #3
    Have you ever sat in a train and watched an adjacent train arrive or depart??.....if so you likely became momentarily confused about which train was moving...then you realized via some auxiliary factor, like 'I feel no virbation" or "I feel no acceleration" you had a basis for deciding which was moving....but even that was merely relative to you.

    The hot chick in the other train probably thought "I'm glad that creep watching me from the other train is going in the opposite direction. [And all along you were just doing physics!]

    You can infer a LOT of relativity from the idea that inertial motion, that is constant velocity motion, is all relative.

    For example, "How fast is that car going?"...that's an easy one, because everyone uses the local road as benchmark. But "How fast is that car going relative to the car ahead of her?" requires a different perspective...like speed relative to the first car. [You can solve equations BOTH ways.]

    What's the basis for 'speed' or observations in cosmology.... where maybe we want to look over the entire observable universe?? You already know there is no absolute rest: so people use a convenient 'rest' notion, an agreed upon framework from which to compare their observations. They choose to be at rest with the ancient relic radiation from the big bang! then everybody is using the same starting point. [Like agreeing to measure in meters versus feet.] That relic radiation is everywhere, so everybody has that frame from which to make observations. [That one gets complicated because in another galaxy their light is not the same age as ours...there is transit time between us.] vaguely similar to different times zones here on earth.

    Or "How fast is the earth moving?" this happens to involve acceleration, but the point is 'how fast relative to what"...the moon, the sun, our milky way galaxy, some distant star??? So people discussing things need to define to which reference they refer.
    Last edited: Jun 20, 2012
  5. Jun 20, 2012 #4
    Dear Curious student,

    You have had one of the crucial insights that underpins essentially all of physics. My understanding is that Galileo was one of the first people to state clearly what you've figured out here, and then a few years later Newton incorporated it into his laws of mechanics, which was more or less the beginning of physics. But for millennia before that this idea was not clearly understood.

    Indeed, when two people are moving relative to one another, either is perfectly entitled to regard themselves as being at rest, as long as neither is accelerating. Forget special and general relativity, this is true in plain old Newtonian physics. If either one of them does physics computations and takes measurements assuming he is at rest and the other guy is moving, he will get the right answer--and so will the other guy. The laws of physics don't care about your absolute speed; therefore it's impossible to determine whether anyone is "moving" or "still" in any absolute sense and the distinction becomes therefore somewhat meaningless.

    Special relativity is the framework that reconciles this fact with the apparently contradictory observation that everyone measures the speed of light to be the same, no matter how fast they are moving relative to one another. General relativity, among other things, updates "as long as neither of them is accelerating" to "as long as both of them are in gravitational freefall."

    Best regards,
  6. Jun 21, 2012 #5
    Wow that's cool to hear! Then at least my mind was working properly (yet), it just didn't feel right when I did not find an obvious solution. Relativity is interesting and I'm going to try to get a deeper insight in it, maybe not useful in any practical way but it's a tough job for the mind some times.

    Million thanks for the replies! I'm sure I'll be back with more questions.
  7. Jun 22, 2012 #6

    One suggestion: read some of the past discussions here in physics forums, anything that interests you, and pick what understanding you can...sometimes difficult at first, so read a little Wikipedia on a topic of interest to get some background....

    Here is one tidbit from which to start : so you now know speed is relative, right??!!.....so what happens when you want to measure kinetic energy [KE] of an object....KE is energy of motion, 1/2mv2 and so because velocity is relative, so is KE!!!! The amount of KE you observe depends on your velocity [motion] relative to the object, like a baseball.

    So if you 'ride a baseball', for example, meaning measuring from the reference of the baseball moving along, and measure it's KE of translational motion, there is none; but if you measure relative to ground,standing on the ground,you DO observe velocity and so you'll find the ball HAS KE. These are called different frames of reference. Which is correct?
    Newton taught us BOTH views are correct...and Einstein dramatically expanded that understanding!!!!!
    Last edited: Jun 22, 2012
  8. Jun 25, 2012 #7
    That must mean then that things like the force on a electron due to a magnetic field being present also depends on how you are moving relative to the electron that you are observing, right?
    Which maybe then makes it possible to say all physical parameters that depends on velocity are all relative? Or is this not true?

    But what about the electron, what does it feel?

    Best Regards,
  9. Jun 25, 2012 #8
    After some thinking: the electron feels the force QvB, where v is velocity of the "moving magnetic field lines"? Is this correct? For the electron it looks like it is at rest and the outside world is moving, which got me to my answer.
  10. Jul 1, 2012 #9
    None who have an answer to this?

  11. Jul 1, 2012 #10


    Staff: Mentor

    No, for one thing, there is no way to assign a velocity to a magnetic field, also the usual expressions of Maxwell's equations are already relativistic. So in all frames the force on an electron is given by the usual Lorentz force law: [itex]f=q(E+ v\times B)[/itex] where v is the velocity of the electron in that frame. Electric and magnetic fields are different in different frames so what is a magnetic force in one frame is an electric force in another frame, but the total EM force transforms correctly.
  12. Jul 4, 2012 #11
    I was thinking of something that was emitting magnetic field lines and that "thing" was moving, but maybe that won't change it either. For example you are moving a magnet around => a moving magnetic field with a velocity v?

    I had no idea, thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook