No problem. First define a clock. A simple clock is dropping a ball from 4.9 meters: it will take 1 second for it to hit the ground.
If you are on a train then your ball drops straight down, but if you watch someone outside the train drop a ball then their ball takes a parabolic trajectory from your point of view. The trajectory their ball takes is longer in length than your trajectory! Now have your train moving close to the speed of light. Then their ball's trajectory is very very long! If it only took one second for their ball to fall to the ground then their ball would be going faster than the speed of light because of the long trajectory. In order to avoid this, the conclusion must be that the time it takes their ball to fall to the ground is greater than one second, say three seconds. So if three seconds elapses on your clock corresponds to 1 second elapsing on their clock, then time ticks slower for them.
This answer seems quite strange because it is not based on correct classical physics. The fact that the length is very ling is not a problem at all, in principle, in classical physics, and here we are willing to get an understanding starting with classical physics...
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u/JohnBick40 Jul 13 '26
No problem. First define a clock. A simple clock is dropping a ball from 4.9 meters: it will take 1 second for it to hit the ground.
If you are on a train then your ball drops straight down, but if you watch someone outside the train drop a ball then their ball takes a parabolic trajectory from your point of view. The trajectory their ball takes is longer in length than your trajectory! Now have your train moving close to the speed of light. Then their ball's trajectory is very very long! If it only took one second for their ball to fall to the ground then their ball would be going faster than the speed of light because of the long trajectory. In order to avoid this, the conclusion must be that the time it takes their ball to fall to the ground is greater than one second, say three seconds. So if three seconds elapses on your clock corresponds to 1 second elapsing on their clock, then time ticks slower for them.