ELIF: how is time relative?
In: Physics
Time is relative because the rate at which you see time pass for someone or something depends on how fast the are moving or accelerating, the faster they are moving, the slower time appears to go.
The short version is that light (and anything else with no mass) must be traveling at the speed of light relative to everything, all the time. So when two objects with mass are moving at different speeds something has to give to make this true. Since Velocity = Distance / Time, you can fix this discrepancy by changing how fast time passes, and this happens in reality. (Distances change some too but that’s actually much harder to understand and explain imo so I won’t)
My favorite way to visualize this is picture a photon of light (which you can imagine as looking and generally behaving like a tennis ball for the purposes of this thought experiment) bouncing between two mirrors in a spaceship. To an observer on the ship, it’s moving back and forth at the speed of light, no problem.
However let’s now say this ship is moving past the Earth at 90% the speed of light. From the perspective of someone on the earth this photon is now zigzagging back and forth because the mirrors are moving past you along with the ship. You don’t really need to even be able to do the math here to know that if the photon is bouncing up and down at the speed of light AND moving sideways at the same time, it would have a total velocity higher than the speed of light from your perspective while being still at the speed of light for the person in the ship. *Since this photon is still of course moving at the speed of light from your perspective you would find if you calculated it without time dilation it would be moving slower than the speed of light from the perspective of the person on the ship.* This obviously is a problem, the speed you see and would expect the other observer to see are different from the values they would observe.
Since the ship is moving past you at 90% the speed of light though, there is a significant amount of time dilation. Anyone on this ship would seem to be moving in slow motion to you so that if you did the math you would find from their perspective the photon is in fact bouncing up at down at exactly the speed of light.
Now since motion is totally relative, this dilation occurs at everyone’s perspective, including for the person sitting on the ship looking at Earth. Now this might be a little confusing, if they are moving slowly relative to you, and you are moving slowly relative to them, then time is being lost somewhere right? Well yes but also no, part of the solution is that since time and space are very much connected just the fact that they are moving affects this problem, but explaining that concisely is above my pay grade. The bigger thing that affects this is acceleration, acceleration is not actually relative, and any acceleration something undergoes will have a large, much more one sided effect on time dilation, and as I understand it will make up for this lost time if they decelerate back to your velocity.
Latest Answers