I have played a bit with spacetime diagrams and Lorentz transformation and can see time dilation, length contraction and the relativity of "at the same time".

Classical: http://www.netestate.de/demo/classical_0.5.gif

Relativistic: http://www.netestate.de/demo/relativistic_0.5.gif


The red lines are longer on the t-axis (time dilation) and closer spaced on the x-axis (length contraction).

But I can also see that events are not always pushed further apart in time by the transformation. This is certainly true for events at the same place but the situation for events separated by time and space on the axis of movement seems to be different.

If a bullet with 0.5c is fired at x=0.1 t=0.1 in the reference frame where the 11 objects are not moving, it will become stationary in the other reference frame. Due to length contraction, it will hit things earlier in this reference frame (time contraction) while the things hit will seem to disintegrate slower (time dilation). The generated diagram shows this but I can only post 2 links. Replace relativistic_0.5.gif with relativistic_0.5_2.gif in the link above.

Is this correct? Are things along the axis of movement destroyed in fast motion while they disintegrate in slow motion from the reference frame of the bullet?

  • $\begingroup$ Just to complete Sure's answer: > Are things along the axis of movement destroyed in fast motion while they disintegrate in slow motion from the reference frame of the bullet? yes, they are destroyed in faster motion than in the bullet's reference frame (but we would call it "normal" motion rather than fast motion). $\endgroup$
    – user65081
    Commented Dec 17, 2014 at 15:40

2 Answers 2


You can't speak about "time contraction" if events are not occurring at the same space position. What you noticed is the relativity of simultaneity, which basically, indeed, depends both on space and time. You can even show that two events that happens at the same time at two different places in an inertial frame are such that they will happen one before the other (and reciprocally) in another inertial frame which moves with respect to the first one.

Simultaneity is not preserved, such that objects can even appear to be "rotated" in some frame. (Consider, for example, an horizontal circle moving vertically in some frame. Now, approach this circle from the left, and you'll notice that it is rotated. More precisely, the further points on its right are "in advance" compared to the closest ones on its left, and the circle is no longer horizontal)

  • $\begingroup$ Let me try to wrap my head around this: It is a fact that the time distance between events can get smaller or bigger with Lorentz-transformation. Only events that are not occurring at the same place can have a smaller time distance but one cannot speak of time contraction here because time and space are entangled - the time distance of such events does not have meaning. Only when events happen at the same place or at the same time in one reference frame do time and space become disentangled such that I can speak of a change of time or space intervals. Is this correct? $\endgroup$
    – brunni
    Commented Dec 19, 2014 at 11:53
  • $\begingroup$ Exactly. Even worse : only if you're moreover in an inertial frame. There is no unique way to measure time or space interval in an accelerated frame, such that it becomes even more blurry. $\endgroup$
    – sure
    Commented Dec 19, 2014 at 15:50

we don't have time contraction, it's the length contraction. and we have just one of them in any reference frame but not the 2 of them together.


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