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

Relativity by Albert Einstein

  1. Jun 15, 2003 #1
    I've been reading the book Relativity by Einstein, but need some things cleared up. I have only studied physics in college (took both mechanics and topics pertaining to more smaller scale phenomenas) and have a vague familiarity with concepts of physics but would like to expand on this a bit more.

    So here are my questions so far:

    -I am assuming that every object has an absolute velocity; am I right?

    -We measure kinetic energy with respect to a rigid body such as that of the earth. Do I possess kinetic energy at this moment in which the earth is moving? That is I am moving at a certain velocity even if I am at rest with respect to the earth. How can the measurement of kinetic energy be accurate then?

    -I am not quite sure what Einstein was intending to explain near the beginning of the book with Gallaien coordinates.
  2. jcsd
  3. Jun 15, 2003 #2
    Insofar as Special Relativity is concerned

    objects do not have an absolute velocity. Because kinetic energy depends on velocity the kinetic energy of other bodies will change when you change your velocity.

    I don't rember the book well enough to know why he used Galilean coordinates, but I do know that Galileo had earlier stated a version of relativity that did not depend on the speed of C being constant.
    Last edited by a moderator: Jun 15, 2003
  4. Jun 16, 2003 #3


    User Avatar
    Science Advisor

    Greetings !
    If the reference body is the Earth then you have no
    kinetic energy.
    He was describing the evolution of coordinate systems.

    Live long and prosper.
  5. Jun 16, 2003 #4
    What I meant in absolute velocity is absolute speed and I am pretty sure that every object has to possess absolute speed; that is the reality of how fast the rigid body is traveling, or resting. What I still don't understand is how we can ascertain the speed of an object into the delta kinetic energy equation by measuring its speed relative to the earth which may be moving along the same dimensional line as the object. The equation for the change in kinetic energy deltaKE=m(vf^2- vi^2) is only valid for one relativistic subtraction of velocity (or speed since the velocity is squared). An object may start from rest, as seen by an observer, and move to six meters per, lets say, an hour. If the object was moving at a certain absolute speed (integrating the absolute speed of the earth, along the direction of the rigid body's travel) it would have a different change in kinetic energy even if it increased its velocity by six meters per hour. Forgive my ignorance, as I know I am missing something here; either that or I have disproved Einstein himself, which I know is not happening.
  6. Jun 17, 2003 #5


    User Avatar
    Science Advisor

    Greetings !

    Nope, no absolute speed and accordingly no absolute
    particuilar energy, though the total energy of a system
    in the same reference frame remains the same. (If
    Universal expansion and space itself are left out for
    the moment so we have a classical reference frame.)

    Live long and prosper.
  7. Jun 18, 2003 #6
    Take this example to understand why there is no absolute speed nor absolute velocity.
    Now, suppose there is a train, you are standing on the ground and the train is moving, you see it moving at 100 km/h (east let's say).
    Then you drive your car, and go in the speed of 100 km/h along with the train, now you see that the speed of the train is 0 km/h.
    On earth, this is easy to explain, although you see the speed of the train as 0 km/h , but you know that your velocity comparing to earth is 100 km/h east, so you can figure out that the velocity of the train next to you is 100 km/h east comparing to earth.

    But now, suppose that this train, and your car are in the middle of space, and there is nothing round you, it is like if ALL The universe is only the train, your car and yourself.
    You look from the window, you see that the train next to you is not moving comparing to you.
    Now, how can you know what is the absolute speed of you or the train ?
    You see, if it was 10 km/h for both of you, you would experience that same as if it was 100 km/h or 200 km/h or 0 km/h, so it is imposible to know the absolute speed of neither you nor the train (i think now you have a good idea about why there is no absolute speed).

    Now, let's come back to earth to give another example that might be even better !
    You are on the train (on the roof of the train), the train is moving 100 km/h east, you shoot a ball from the train, from what you see (or what in physics we call 'from your frame of refference'), the ball is moving 10 km/h east.
    Your friend on the ground is watching the train and you while you are kicking the ball, he sees that the train's velocity is 100 km/h east, and that the ball's velocity is 110 km/h east.
    Remember that your friend on ground is only comparing the velocities to his velocity (or to the ground).
    Now, you can take into account that the earth itself is moving, so the velocity of the train as a person on the sun sees is not the way your friend sees it, he will have to add the vector of velocity of earth to the vector of velocity of the train (as your friend sees it) to get what the guy on the sun is seeing !
    Let's go a little further, how do we know that our solar system is not moving too ! so the guy in the middle of the milkyway will see the velocity of the train different from the guy on the sun, different from the guy on the earth, and different from the guy on the train !
    How do we know that that even the milkyway is not moving ? it might be moving and another person will see the train's velocity even different.
    Now, who is right ?
    If there was a point in space that is known to be the center of the universe (a point that does not move at all), then we could say that all speed comparing to that point are right and absolute, but there is no such point !
    So, there is no absolute velocity, all you can do is compare velocities to your own velocity (and same applies for speeds).
    Last edited: Jun 18, 2003
  8. Jun 18, 2003 #7


    User Avatar
    Science Advisor

    It's hard to believe you have been studying the book Relativity by Einstein when you say things like:
    The fact that there is no such thing as an absolute velocity (or speed) is pretty much covered in the first chapter- that is, after all, the reason for the name "relativity". ALL motion is "relative" to some reference frame.

    You also say
    You just said yourself that kinetic energy must be measured "with respect to a rigid" body, yet you say "Do I possess kinetic energy at this moment?" The question makes no sense until you specify "with respect to" a specific rigid body. If you mean "kinetic energy with respect to the earth", then, because you are not moving with respect to the earth, the answer is "no". If you mean "kinetic energy with respect to the sun", then the answer is "yes" because the earth (and you ) is moving with respect to the sun. However, that doesn't really mean anything because the question of "kinetic energy" only becomes important when it changes (and converts into some other kind of energy).
  9. Jun 20, 2003 #8
    Thanks for all of your explanations. I think that all of you have misinterpreted by what I meant by absolute speed.

    HallsofIvy, I have only read the first few pages of the book and I don't recall Einstein ever saying anything about non-existence of an absolute speed for an object. He did say that, according to the theory of relativity that the speed of light in reference to any object at any reference frame, is the same. The examples that you guys are using seems to contradict what he elucidates later.

    Here is what I thought a while ago; the thought that provoked my question in this forum. The fact that there are magnitudes of speed is self-evident. Since there are magnitudes of speed, and there is no such concept as what we can call a negative speed (since speed is magnitude) and that an object with a constant velocity is not varying in its speed (speed does not "happen" to the object, even in a relativistic point of view, an object has to possess its own speed even to be compared to other objects around it), I find it reasonable that an object moving in constant translatory motion at least possesses an absolute, inherent speed; although this perspective may change in respect to a system.
  10. Jun 21, 2003 #9
    Galilien coordinate is from Newton's view to explain the space-time
    and Einstein want to explain why the Galilien coordinate was wrong
    and need to replace with Lorentz Transformation
  11. Jun 21, 2003 #10
    Sorry, i don't understand the logic in your conclusion, can you make it a clearer please ?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook