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

I Is motion through space or spacetime?

  1. May 9, 2016 #1
    I was chatting to someone recently about the motion of objects and whether they propagate through spacetime or not. They were/are convinced that motion through spacetime is simply not possible arguing something along the lines of the following:

    "Objects move through space. If you depict an object in spacetime, you have a world-line. The world-line doesn't move through spacetime, it simply extends across spacetime.

    Physicist's portrayal of this seems to come from their feeling that because the magnitude of a massive particle's velocity four-vector is traditionally normalized to have magnitude ##c##, it makes sense to describe the particle, to a nonmathematical audience, as "moving through spacetime" at ##c##. This is simply inaccurate. A good way to see that it's inaccurate is to note that a ray of light doesn't even have a four-vector that can be normalized in this way. Any tangent vector to the world-line of a ray of light has a magnitude of zero, so you can't scale it up or down to make it have a magnitude of ##c##. For consistency, Greene would presumably have to say that a ray of light "moves through spacetime" at a speed of zero, which is obviously pretty silly.

    The reason we normalize velocity four-vectors for massive particles is that the length of a tangent vector has no compelling physical interpretation. Any two tangent vectors that are parallel represent a particle moving through space with the same velocity. Since the length doesn't matter, we might as well arbitrarily set it to some value. We might was well set it to 1, which is of course the value of ##c## in relativistic units. But this normalization is optional in all cases, and impossible for massless particles."

    They also referred me to this blog post: http://scienceblogs.com/goodmath/2008/01/17/the-nasty-little-truth-about-i/ which even after reading the first line immediately set off my "crackpot alarm" , and continuing to read the post further confirmed this.

    My opinion is that objects do propagate through spacetime. My reason being that, although an objects worldline is a fixed trajectory through space, it is not remaining at a single point on that trajectory, it is moving along it. To move along its worldline the object must have some velocity associated with it - even if it remain stationary in space it will still be moving in time.
    I think the problem in understanding is related to self-referential definitions of time that crop up, indeed time is a very difficult quantity to define without any self reference, but it doesn't mean that it isn't a physical quantity.

    I have to admit, I'm starting to doubt myself a bit. Is my understanding correct here, or is this other person correct?

    If someone could clarify this and also provide a convincing argument I'd much appreciate it.
     
    Last edited: May 9, 2016
  2. jcsd
  3. May 9, 2016 #2

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    We describe motion using the notion of velocity, which is the time rate of change of position. In other words, you need both space and time to describe it. I see the rest of the discussion as a matter of semantics. A worldline is a path through spacetime. A trajectory is a path through space.
     
  4. May 9, 2016 #3

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    I mostly agree with your initial quoted text. A world line represents motion - the motion different in different coordinate systems. The analogy I would make is to a curve on on piece of graph paper. The curve doesn't move, but there is change of one coordinate with respect to the other all along it. In the case of spacetime, we call change of position coordinates with respect to time (coordinate) velocity, and we can say the world line represents motion with respect a given space-time foliation. But the curve itself doesn't move, and no point (event) on the world line moves (with respect to what would it move? we would need to add another dimension for that).
     
  5. May 9, 2016 #4

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    I think it's just a matter of how you prefer to look at it. The worldline of a point-particle can be described by a parametrized path, [itex]x^\mu(s)[/itex], where [itex]s[/itex] is proper time. So you can view that as a point particle "moving" through 4-D spacetime as a function of proper time [itex]s[/itex].
     
  6. May 9, 2016 #5

    PeterDonis

    Staff: Mentor

    I would not agree with this particular part of the quoted text. The length of the tangent vector for massive particles does have an obvious physical interpretation: the rest mass of the particle. Knowing that a particle has a particular worldline in spacetime does not tell us all there is to know about the particle; it has other properties, of which rest mass is one, and the length of the tangent vector models that property.
     
  7. May 9, 2016 #6

    robphy

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It seems to me that the reason we normalize velocity 4-vectors for massive particles is that we can think of it as ##\hat t##, which can be interpreted as "the displacement one-tick later along an inertial worldline" and which can be dotted with a 4-vector to extract the time-component of that 4-vector according to that observer. In addition, the set of all of unit 4-velocities traces out the future unit-hyperbola (which Is a visualization of the metric tensor and whose asymptotes are along the light-cone).
     
  8. May 10, 2016 #7

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Hmm. That's the length of the 4-momentum which is not the tangent vector. The tangent vector (in general) is just derivative of curve coordinates with respect to some parameter. Its norm is arbitrary, and can be varied by chaning parameter. Thus, we make a 4-velocity by making proper time the parameter.
     
  9. May 10, 2016 #8
    Apologies about this part. In retrospect I realise I've been a little rude with this statement.

    I agree that the curve itself doesn't move, and that the events themselves don't move. This is fixed. But doesn't a particle "move" from event to event along its worldline and so moves through spacetime? I get that (neglecting GR and the expansion of the universe, that spacetime is essentially a static, absolute background, but what I don't see is why objects can't move through this background? The worldline of a particle still has a tangent vector to it at each spacetime point, so wouldn't this be describing the rate of change in the objects motion through spacetime (with respect to proper time)?!
     
  10. May 10, 2016 #9
    This is how I was thinking of it, but would you say that I'm technically incorrect to say that objects move through spacetime? It seems to me that it would be possible, since even when an object is at rest in space, time is still increasing, but maybe I'm misunderstanding something here?
     
  11. May 10, 2016 #10

    atyy

    User Avatar
    Science Advisor

    I think stevendaryl gave the mathematics in post #4 that corresponds to this.
     
  12. May 10, 2016 #11

    A.T.

    User Avatar
    Science Advisor

    To avoid confusion, I prefer to use another term than "move" here, like "advance in space-time".
     
  13. May 10, 2016 #12
    So what is correct then? Do particles advance in spacetime but propagate through space? I am left feeling confused now as to what is the correct understanding/interpretation?!
     
  14. May 10, 2016 #13

    A.T.

    User Avatar
    Science Advisor

    It's semantics.
     
  15. May 10, 2016 #14
    But is it correct at all to think of a particle propagating along its worldline in spacetime? Following from what Stevendaryl wrote, if one parametrises a particles worldline by its proper time then it has a well-defined four velocity, so isn't it in this sense propagating in spacetime? Otherwise the whole point of introducing four velocity etc. seems pointless if what is actually physically correct is that the particle is propagating through space with time labelling each point along its trajectory such that it maps out a path in spacetime?!
     
  16. May 10, 2016 #15

    martinbn

    User Avatar
    Science Advisor

    Velocity in this context means tangent vector. Even in (pure)geometry people say velocity of the curve, when they mean the tangent vector, and there are no particles and no motion along the curve. It brings intuition from one area to help with the abstraction in another.
     
  17. May 10, 2016 #16
    I understand that, but the tangent vector to the curve at any particular point is still quantifying the rate of change in the curve at that point (with respect to its parametrisation), right?
     
  18. May 10, 2016 #17

    martinbn

    User Avatar
    Science Advisor

  19. May 10, 2016 #18
    But this is my point. Given that the curve has a velocity vector associated with it at each point, doesn't this mean that an object travelling along this curve will travel through spacetime as it advances along the curve?
     
  20. May 10, 2016 #19

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Ultimately, as you've seen a variety of answers, this is a question of philosophy. The physics is in the math and how you relate observables to mathematical quantities. The intent of my post was to assert that most of the initial quote was not wrong and is similar to how I think about it, not that it the only correct way to think about it.
     
  21. May 10, 2016 #20
    Fair enough.

    Out of interest, what are your thoughts on the blog post that I linked in my first post? Is any of it correct or is the author misunderstanding things?
     
  22. May 10, 2016 #21

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    Another comment on this topic. Ultimately, it's a matter of taste whether you think of particles as moving through space as a function of time, or moving through spacetime as a function of proper time (or some other affine parameter). However, I think that the latter interpretation has certain advantages in helping to understand intuitively some of the features of Special and General Relativity.

    Time dilation: Since velocity through space is a frame-dependent quantity, velocity-dependent time dilation is similarly frame-dependent. That makes it seem like it's not real. But it is really the case (in the twin paradox) that the traveling twin will end up younger than the stay-at-home twin. So it's a little complicated to intuitively understand how something that is frame-dependent (time dilation) can end up making a frame-independent difference (the traveling twin is older). Of course, it all works out, but it's a little complicated to see that it's all consistent, and respects the equivalence of all inertial frames.

    In contrast, if you view the traveling twin as traveling through spacetime as a function of proper time, then it's clear that the amount of proper time to reach a particular destination in spacetime depends on the traveling twin's 4-velocity, in exactly the same way that the amount of coordinate time required to reach a particular destination in space depends on the 3-velocity.

    Gravitation: Popularizers explain that gravity in GR is just curvature. It's easy to understand how curved space can end up deflecting the path of a particle in motion. However, if you think of velocity as moving through space as a function of time, it's not at all clear how curvature leads to an object falling when it is initially at rest. Curvature deflects paths, but if an object is at rest, it doesn't have a path to deflect. This puzzle is resolved if you think in terms of 4-velocities. What you think of as an object at "rest" is actually an object with a sizable 4-velocity in the t-direction. Curvature deflects the object's path, so that the 4-velocity is no longer purely in the t-direction, but has a component in the z-direction (towards the Earth).
     
  23. May 10, 2016 #22
    This makes a lot of sense and is how I've thought of it up until now, but at the moment I feel confused more than anything. At first I thought that the blog post that I linked into my first post was complete nonsense, but now I'm not so sure. Is what the author wrote correct at all?

    I thought the whole point of introducing a 4-dimensional spacetime was exactly because there is no observer independent way to separate space and time coordinates and hence they must be considered as coordinates of a single four dimensional geometry? Given this, it seems to me that one should be able to have motion through time and motion through space (since an object at rest, i.e. only "moving" in time with respect to one frame will be observed to be moving through both space and time with respect to another). However, according to the person I spoke to, to move through spacetime would require introducing a "meta-time"?! In the introduction to special relativity that I've had, the four-velocity of a particle was introduced by noting that if we wanted an observer independent notion of velocity, we most compute the rate of change of an objects four-position with respect to a frame-independent quantity, this being the proper time. This made sense to me at the time, since the proper-time of a particle is a parameter, since it is absolute (i.e. frame-independent), and hence parametrises both spatial position and coordinate time.

    Sorry if I'm rambling a bit here, but I just feel a little bit lost and trying to make sense of something I thought I understood, but know I'm not so sure.
     
  24. May 10, 2016 #23

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    I don't understand that complaint, since "proper time" already is a kind of "meta-time".
     
  25. May 10, 2016 #24
    That's what I thought, since proper-time is an additional independent parameter, so it can be used to parametrise time as well, right?

    What are your thoughts on the blog post that I linked?
     
  26. May 10, 2016 #25
    As a personal preference, while some may think of a world-line (-surface, -tube) as a mere path or trajectory of an object through space-time I like to think of it as being the object itself, a 4D object. What we perceive as the "object" is then just a slice of the actual 4D object intersected with an arbitrary 3D "now" surface. The 4D object itself is fixed and unchanging in this view.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted