# Invariant spacetime and time dilation

Discussion in 'Physics & Math' started by Confused2, Jun 11, 2020.

1. ### Confused2Registered Senior Member

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607
My thoughts (could well be wrong)..
In a frame moving relative to the observer..
From the emitter POV the receiver/mirror lags behind in time/space. When the beam gets to the receiver/mirror point in time/space the emitter lags behind and is in the right place for a bounced beam.
?

3. ### Confused2Registered Senior Member

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607
Event A,A': pulse fired
Event B: pulse detected
B 9' 8' 7' 6' 5' 4' 3' 2' 1' 0' A'
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0
A
Path of beam is not A->A' ?

Last edited: Jul 18, 2020

5. ### phytiRegistered Senior Member

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492
Can't take credit for the graphic. First saw it on Wiki. Just simplified it with one signal from origin to reversal. This separates signals to before and after reversal.

7. ### Neddy BateValued Senior Member

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2,121
Try this, it is only about two minutes long:

You can see the angled path with your own eyes. There is even a slow-motion version of it at the end.

--------

Notes:

In the reference frame of the cart, the steel ball drops straight down vertically in both cases. We know this because the ball always lands in the same place, at the base of the pole holding up the electromagnet.

In this video, the angled path that we see in the reference frame of the laboratory is actually a parabola, because the ball's downward speed increases as it drops, due to the acceleration of gravity. If this experiment were done with something dropping a constant speed, the path seen in the lab frame would just be a straight diagonal line.

Thus the angle that you asked about, which you called alpha, is caused by the constant relative velocity between the two reference frames. The angle is only non-zero in one frame, not the other, (assuming zero angle means perfectly vertical).

Last edited: Jul 18, 2020
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8. ### Ethernos 1997Registered Senior Member

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265
So, s^2 =0
X^2+y^2+z^2=c^2×t^2
S^2=1+c^2×t^2

9. ### phytiRegistered Senior Member

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492
Confused2;

The only answer I got when asking the same question was the momentum of a photon (p=hf), where h is the plank constant and f is the frequency, carries it forward. It doesn't seem correct, since blue light would have a greater angle than red light!
We can accept that the angle must change, even if we don't know how.
Once a light clock is made, it works all year. The earth is constantly changing direction relative to the sun. If the light didn't adjust, the clock would stop working.

10. ### phytiRegistered Senior Member

Messages:
492
Light speed is independent of its source.
Light has no rest mass.
Light does not acquire the speed of the source.
That is why there is time dilation.
The angle may be a result of the complexity of em radiation.

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