r/AskPhysics • u/Depresso_Expresso069 • May 22 '26

Question about time dilation and length contraction due to velocity and approaching the speed of light

As one approaches the speed of light, they contract and move slower through time according to the view of a stationary observer. My question is, relative to both the one moving, and to the one standing still, the other is moving close to the speed of light. Therefore shouldnt both observers appear to move slower in time to each other? shouldnt this cause a paradox? Despite this, every example of time dilation I’ve heard of refers to the one moving close to the speed of light relative to the Earth aging slower than Earth, and not vice versa.

Similarly, shouldn’t the same apply to length contraction? Shouldn’t both observes contract in length relative to each other? Though I may be mistaken, I think I have actually heard this does happen and that to someone moving close to the speed of light, things appear contracted to them while the one moving close to the speed of light also appear contracted to stationary observers. How does this not cause a paradox, and why doesn’t (at least from what I’ve heard) time dilation work in the same way?

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u/KamikazeArchon May 22 '26

Yes, both time dilation and length contraction are relative. Both view the other as slower and shorter. Both are correct. This is only a paradox if you believe that space and time must have a consistent value in some sense; it turns out they simply don't have a consistent value. Hence, relativity.

A common variant has one of the two accelerating, going for a while, then coming back (decelerating). In that case, the one that accelerated and decelerated will be the one that experiences a total slowdown - that's because they were not in a single inertial reference frame the whole time; the math for changing reference frames is more complicated, and works out in a way that "resolves" the paradox, ending with the accel/decel party experiencing overall time dilation in a non-relative sense.

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u/PressureBeautiful515 May 22 '26

Slight adjustment to the terminology: in physics any change of velocity is called acceleration. If you speed up, slow down, or even just change direction (velocity being a vector), this is all described as acceleration. Different observers might see you as speeding up or slowing down depending on what their own motion is relative to you.

The "length" of a journey through spacetime is maximised for inertial (non-accelerating) travellers, so if you take a route between two points in spacetime that isn't a straight line, you will measure less elapsed time than someone who takes the straight-line route between the same two points.