I’ve seen a lot of physics videos and a friend tried to explain to my brain can’t understand why time is just a perception.
In: Physics
This is around the point where physics goes beyond our intuition, so it’s where a lot of us get tripped up.
So it is tied up with relativity and the speed of light.
The speed of light in a vacuum is a constant, c, somewhere around like 300 ~~trillion~~ million ~~miles~~ meters per ~~hour~~ second, ~~give or take a few orders of magnitude~~ (the original thing I remembered was off by about 6 orders of magnitude, which is generally considered more than “a few”). Either way, this is a *constant*, and that makes light behave *strangely* compared to other things we’re used to.
To look at this, take the example of shooting a nerf dart out of a moving vehicle. If you’re driving forward at, say, 25mph, and you have a supercharged nerf gun that is not at all safe that fires nerf darts at 25 mph, and you fire the nerf dart out of the car in the same direction as the car is moving, the nerf dart will be moving at 50mph (barring air resistance, etc). Conversely, if you shoot the same nerf gun facing the back/behind the car, the dart will have a net speed of 0 mph.
This is how most objects operate. This is not how light works.
If we take a similar example and (ignore a few laws of physics for a bit): if we have a spaceship that can travel at exactly the speed of light (impossible in reality, but we can pretend for this example), and we take a flashlight and turn it on facing the direction of travel, given the example above we might expect that the speeds would combine, and the light from your flashlight would move at 2 times c. But it won’t. It will move at c.
Similarly, if you point it backwards, the velocity of the light won’t be canceled by your exactly identical velocity in the opposite direction, the light will still travel at c in that direction.
Further, unlike other velocities that are all relative to where we’re observing them from (ex: if you throw a beach ball vertically in that spaceship (or out of it) it will be moving at 0mph to you, but to others watching from a relatively stationary viewpoint, it will be moving quite a bit faster), the speed of light is the same *no matter your frame of reference*.
What this necessarily means is that as you go fast enough to be a significant fraction of the speed of light, time will slow down for you compared to where you’re being observed from.
Again: This is not how we generally experience reality, but we’ve done experiments with atomic clocks that have confirmed Einstein’s findings.
EDIT: unit confusion in the speed of light because I’m old and American. Thank you for the correction.
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