I get that time can change depending on where you are in the world; however, I feel I have less of an understanding of time being relative after diving deeper into it.
In: Physics
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.
Don’t really have an answer for you. But if you look up “time dilation” you might have a better understanding of your question.
Or be far more confused then you were before, regardless its a fascinating subject.
If time were absolute, time would always go at 1 second per second everywhere under all circumstances and everyone would agree it does that.
This sounds reasonable, but science has shown this is not the case.
Say you look at someone traveling fast relative to you, both of you have something that is supposed to repeat ever 1 second. You know you both have the same thing. Yet when you measure the the other’s guys thing, you find it’s repeating every 2 seconds. Physics says that every possible experiment you could throw at that would show it’s taking 2 seconds. Every possible interaction would prove that it’s happening once every 2 seconds. So from your perspective, that guy’s time is only going at 50% of the speed.
But for the other guy, from his perspective the thing isn’t moving, for him it’s still repeating once every 1 second. Physics says that every possible experiment you could throw at that would show it’s taking 1 seconds. Every possible interaction would prove that it’s happening once every 1 seconds.
You and all of science says the guy’s thing repeats 2 seconds, that guy and all of science says the guy’s thing repeats every 1 second.
You are in disagreement because time isn’t an absolute, it’s relative.
And none of this is purely theoretical either.
GPS satellites have clocks accurate enough for time being relative to matter.
They have to do math to compensate.
Time is relative because the speed of light is constant in all reference frames.
Basically there is not a speed you can go that you will be able to observe the speed of light going any slower than 299,792,458 m / s (speed of light.)
Lets say you are in a car going 30mph.
Now lets say someone next to you is traveling 40 mph.
Relative to YOU, they are only traveling 10 mph. In reality they are only traveling 40 mph relative to the earth.
Even though that car in the first example is going 40 mph, it seems to get slower as you pull up to it. This is basic relativity.
Like the speed of sound, or the speed of anything, everyone just assumed that the faster you go to catch up to an object, the slower that very same object appears to get. However, it was different for the speed of light because if you were to travel along a light beam, you would never be able to catch up to the speed of light! The speed of light travels 299,792,458 m / s.
If I traveled at 299,792,45**7** m / s, 1 m/s slower than the speed of light, you would think that I would see light traveling at 1 m/s! But that doesn’t work! light would still appear to be traveling 299,792,458 m / s.
So if the speed of light is constant, then some other variable in the equation has to change.
speed=distance/time
Since distance and time relate to speed, they will always change the value of speed. So in order to keep the speed constant, you would have to warp both of those variables.
That’s where we get Time dilation and length contraction.
Its all due to the speed of light being finite and constant in a medium. Imagine being on a rocket ship traveling close to the speed of light and shining a pulsing light at an observer below. From your frame of reference the light goes directly down and reflects back traveling a distance that is double your altitude. The observer sees the light pulse travel a greater distance and the only way for this to be true is if it took longer. So obserbvers at rest experience time pass at a rate slower than an observer in motion; and this is all relative to the observers framer of reference.
Ill sketch a diagram and upload it
I dont know how to make this eli5 though
A shadow monster is also eating my phone.
This requires a fairly good understanding of physics for any detailed explanation. Time being relative is a statement that different observers don’t always agree on how much time passes. For any observer, time to them passes “normally” but the universe is such that there is no reason for any two observers to agree that the duration for any event takes the same amount of time. So you can sort of imagine two persons looking at each other for some period. One person might say, “I looked that person for 10 seconds”, while that other person might say, “That person looked at me for 9 seconds”. They are both telling the truth from their perspective because there is no “absolute” time.
This is not a psychological phenomena or any sort of “imagination” or trick. The reason is based on physics of relativity. This phenomena has been measured experimentally. For all “human” level purposes, the differences are so small as to be nearly unmeasurable. But in high gravity differentials or high velocity (very very very high velocities) this effect is measurable.
Youre not going to get a detailed answer here as it’s something that would require a deeper understanding of a bunch of other stuff about physics.
I’m not a physicist but this is how it was explained to me and it made sense.
Light doesn’t have mass, so it travels through a vacuum at the rate of causality (as things happen).
Imagine a person looking at a star. If you were a photon of light leaving the surface of that star then to you no time would pass between leaving the star and hitting the persons eye.
Star/eye in an instant.
Time is can only be measured in relation to which other things happen, with causality as the only constant of which things are “happening”.
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