I need someone to make sense of it for me. I appreciate the clock scenario where it stays at 12 o’clock if you move away from it at the speed of lìght, but regardless of how fast someone travels, their body will still age just as fast as anyone else (roughly). I don’t understand how just putting distance between someone’s self and the rest of Earth would somehow make them age at a slower rate? You’re aging, just further away..
Hope this makes sense!
In: Physics
TL;DR:
In short, the direct answer to your question as to why different people travelling at different speeds age differently is because the speed of light must remain constant for all these people. The only way this can happen is if time and space are allowed to vary so that their ratio, c (the speed of light) remains the same.
The key idea is that the universe conspires so as to keep the speed of light the same for ALL observers, regardless of their frame of reference. This axiom of the constancy of the speed of light is directly responsible for time passing at different rates for different observers. Let’s see how.
Suppose that you have a friend who is stationary (with respect to, say, the Earth). Suppose also that you’re in a spaceship travelling at, say, 0.5c with respect to your friend’s frame of reference. In other words, if your friend measures your speed, they will see that you’re moving at 0.5c. (c = speed of light, so 0.5c means “half the speed of light”).
Now, let’s perform a physics experiment. Actually, let’s perform two experiments — you perform one experiment, and your friend performs the other experiment.
Inside your spaceship, you try to measure the speed of light. How do you do that? Well, c = d/t and so you measure the distance that light travels in a certain time period. Suppose that you measure how long it takes light to reach from one end of your spaceship to the other end. You know what d is because you can easily measure the length of your spaceship. It is important to note that your clock and your measuring stick retain their length. 1 meter is exactly equal to 1 meter, and 1 second is exactly equal to 1 second in your frame of reference. Keep this in mind. So, you measure what t must be. Then, when you perform the calculations, you get that c = 299,792,458 m/s.
Likewise, your friend, who is not in your frame of reference, also performs the same experiment. He also notes that 1 meter is exactly equal to 1 meter, and that 1 second is exactly equal to 1 second in HIS frame of reference. He measures the speed of light by measuring how long it takes light to reach from one end of your spaceship to the other end. When he does the calculations, he too gets that c = 299,792,458 m/s.
How is this possible?
It’s because when your friend measures distances, he finds that your spaceship is actually SHORTER than what YOU measured. Even though 1 meter = 1 meter for him in HIS reference frame, and 1 meter = 1 meter for you in YOUR reference frame, when you compare the length of a meter from one reference frame to another,
1 meter in one frame of reference is no longer equal to 1 meter in the other frame of reference
Your friend has just discovered the phenomenon of [length contraction] (https://en.wikipedia.org/wiki/Length_contraction).
Now, c = d/t, and your friend measured d to be shorter than what YOU measured it to be. Yet, c must always equal 299,792,458 m/s for both you and your friend. How is this possible? Well, if d is different for your friend, then t must ALSO be different. However, the RATIO, d/t MUST equal the same: c. Hence, if d is smaller, then t must be bigger so as to keep the ratio, the speed of light, the same. Your friend has just discovered [time dilation] (https://en.wikipedia.org/wiki/Time_dilation).
This makes sense — the word “contraction” in “length contraction” means to shorten. The word “dilation” in “time dilation” means to lengthen. So, if length contracts (i.e. d is shorter) then time must dilate (i.e. t is bigger) so as to exactly compensate.
Now I hope you can appreciate “relativ”ity. In your reference frame, time and space act the same — 1 meter = 1 meter, and 1 second = 1 second. Likewise, in your friend’s frame of reference, 1 meter = 1 meter and 1 second = 1 second. However, 1 meter in your friend’s frame of reference, WITH RESPECT TO (i.e. RELATIVE TO) your frame of reference is no longer 1 meter. Similarly, 1 second in your friend’s frame of reference RELATIVE TO your frame of reference is no longer 1 second.
Weird stuff starts happening only when we start measuring things RELATIVE TO other frames of references. Otherwise, in their own individual frames of references, everything appears to be normal.
Once you have understood the above, then the next natural question to ask is “why does the universe force the speed of light to remain constant for all observers?” And unfortunately, physics doesn’t have the answer to this question. It’s just how the universe seems to work. Perhaps a deeper theory will answer this question.
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