# Does time dilation affect the relative motion that causes it?

Does the relative motion of a thing slow down the relative motion of a thing? Hope that makes sense or you can work out what I mean.

Time dilation won't cause any acceleration, if that's what you're asking.

Time dilation won't cause any acceleration, if that's what you're asking.
I guess it is essentially what I’m asking. Specifically relative acceleration. Can you give a non-mathematical, simple as possible explanation why it doesn’t?

Drakkith
Staff Emeritus
I'm not sure how to answer you question. It doesn't cause an acceleration because that isn't how it works. Time dilation isn't a force, and you need a force to cause acceleration. Time dilation is merely a consequence of moving.

If time dilation is a consequence of moving and time dilation is essentially “moving slower” then why doesn’t time dilation affect the motion that causes it? How does the time difference of time dilation occur without acceleration?

it's a matter of observed time. When I observe a spaceship flying past at some speed close to c, the spaceship and its occupants appear to be traveling through time very sloooowly, relative to how "fast" I am traveling through time. That's just how I *see* it, not necessarily how it actually is, because how it "is" is entirely relative and dependent on how you are observing what you are observing. If I am zooming along next to the spaceship, it and its occupants and myself all appear to travel through time at the same speed.

In the original case, the spaceship is traveling at some speed close to c, this speed never changes, at least relative to the observer. It's just that, relative to the observer, the spaceship and its occupants age slowly.

Time dilation is *caused* by observing an object travel at a speed close to c relative to the observer.

Here I think this picture might help: both frames are in the same coordinate system (forgive the crappy editing). On the left we have the red observer watching the blue dot fly past at some speed close to c, the observer measures this speed based upon how fast he observes the blue dot to travel through his established coordinate system.

From the perspective of the observer, the rate at which the blue dot experiences time is slower than the rate at which the observer observes himself to experience time. But: the blue dot is always traveling at that speed v which is close to c, in this case it is invariant, relative to the coordinate plane which I have established.

Now, in the other case on the right, they are both moving with the same velocity close to c through the same coordinate system. They measure their speed relative to the same coordinate plane as before. It's the same speed, yet they observe themselves to be experiencing time at the same rate as each other.

There is no effect of time dilation upon your speed, because time dilation, I think, is *not* essentially "moving slower" it is, I think (again), "experiencing time more slowly," the difference is in how it affects observations.

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Yup, there it is, nothing new under the sun. What if time dilation isnt a perceived effect, but an actual change in the plane of time?

Yup, there it is, nothing new under the sun. What if time dilation isnt a perceived effect, but an actual change in the plane of time?

What do you mean by an "actual change" ? Time "really is" dependent on your coordinate system. And what do you mean by the "plane of time" ?

What do you mean by an "actual change" ? Time "really is" dependent on your coordinate system. And what do you mean by the "plane of time" ?

What do you mean by an "actual change" ? Time "really is" dependent on your coordinate system. Cool, wasnt sure if it actually worked that way.

And what do you mean by the "plane of time" ?
Thats the best description I could give for what I see in my head. I guess the correct term would be world line? Look Im not trying to act smart here, Im just trying to understand by asking questions, weird though they may be. Basically what I meant was as two different objects progress through time, their world lines would hardly ever be parallel but would change vectors continuously relative to one another, this implying that we each have our own worldline?

ghwellsjr
Gold Member
If time dilation is a consequence of moving and time dilation is essentially “moving slower” then why doesn’t time dilation affect the motion that causes it? How does the time difference of time dilation occur without acceleration?
Time dilation does not cause the moving thing to move slower. Its speed is the same relative to some other thing as that other thing's speed is to it because there is also length contraction. The two act in concert to yield the same relative speed between the two things whether you consider the first to be moving and the second stationary or the other way around.

If we know that a clock hand goes around a clock face at speed X, and the clock is accelerated to relativistic velocity Y relative to us, then the Einstein velocity addition formula tells us the velocity of the clock hand relative to us at any moment.

When Y increases the velocity difference decreases.
(velocity difference = clock hand velocity relative to us - clock face velocity relative to us)

So it seems:

Relativistic velocity addition is the reason why clocks run as they do.
Or: Time dilation is the reason for relativistic velocity addition being such as it is.

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