Okay so I know that this has been a prompt several times before on this sub, and this post is me giving one final shot at attempting to understand the intricacies.
My brain, due to lack of prerequisite/foundational knowledge or perhaps sheer stupidity, simply cannot comprehend the fact that a clock on Earth will tick faster than, say, a clock on Voyager II. I have two main questions, likely only surface level following your guys’ explanations:
1. What exactly is spacetime, and how do gravity and speed warp it? I’ve heard the ‘trampoline’ theory of time where objects with mass create a dip in spacetime that other objects of lesser mass ‘fall’ into. I’ve seen the visual, and I feel as though I’m missing something fundamental that explains exactly what it means to warp/create a dip in this ‘plane’ of space. How does that affect time?
2. What do you mean, “The speed of light is constant”? If you’re in a spaceship, hurtling through space at 50% the speed of light wouldn’t the light coming at you from the front of the spaceship be 50% faster than usual to you, and the light at the tail of the spaceship be 50% slower for you? And how the hell does this make it so you age slower than your twin brother on Earth? Why was one hour on planet Miller seven years on Earth in Interstellar? Help???
I don’t want to be stupid; being stupid really is my biggest fear in the universe. But how can I possibly wrap my brain around the concept of time, pretty much the only constant I thought there was in life, being not-so constant?
In: Physics
If you were standing in the bed of a pickup truck traveling 50mph, and you threw a ball forward at 30mph, you would measure the speed of the ball at 30mph. Me, standing at the side fo the road, I observe the ball traveling at 80mph, adding the speeds together. This makes sense for objects in motion.
The equation for speed is distance divided by time. For you, throwing the ball, it traveled 30ft in 1 second so that speed is 30ft/second. For me, it looked like it traveled 80ft, so I measured 80ft/second.
Light is measured the same way, distance divided by time. It’s been proven that if you’re traveling at 50% of light speed and you measure light’s speed as distance divided by time, you’re gonna get the same answer as me while standing still. How can this be? Well, the only answer is, your seconds must be slower than mine.
Carl Sagan actually did a [really great video](https://www.youtube.com/watch?v=PKE1WyEPdNA) explaining exactly why it has to be this way. Basically, for the universe to be logically consistent, there *has* to be a cosmic speed limit. And that limit is c.
1. Basically what it sounds like, all three spacial dimensions + time, represented just like another spacial dimension. These sorts of constructs are primarily *mathematical* models, but they happen to be *really* useful in describing reality, even to the point of predicting behaviour that we haven’t actually seen yet – this is how we discovered the black hole. When we treat time like another spacial dimension, it turns out to be a really useful way of thinking about the universe, so thats as good a reason as any to say that’s how it actually is.
If you’re having trouble simply *visualizing* “curved space”, check out [hyperbolica](https://youtu.be/yY9GAyJtuJ0?si=cuWGfHwIi1ZjWfUy), a game that is designed to give you an idea of what curved space is like. Mass and gravity curve spacetime because that’s what mass and gravity *are*.
2. No. That’s the whole thing! The speed of light is always moving at the speed of light *relative to the observer*. Weird right? Check out Einstein’s thought experiments to get some insight as to how/why time and length dilation are natural effects of a constant speed of light.
As a sidenote – the “speed of light” may make it seem like light is special, but actually its more like a “speed of causality”, and light (in a vacuum) moves as fast as possible.
There’s better explanations out there but if you have any Qs i can try
The speed of light is constant in all frames of reference. If you shine a light in a direction, and measure it, you’ll get 299,792,458 m/s. If you move in the direction of the beam of light, it’ll still move at that speed. It doesn’t matter how fast you move, or even which direction you go. It won’t change.
Now imagine you have a pulse of light bouncing between two mirrors. The time it takes for the light to travel from one mirror to the other is just the distance between the mirrors divided by the speed of light.
Now imagine that same setup, but it’s on a spaceship passing you to the side at a considerable fraction of the speed of light. The pulse of light isn’t just bouncing up and down. From your perspective, the light has to travel the distance between them while also traveling horizontally with the spaceship. You can use the Pythagorean Theorem to find out how far the light has to travel, with one leg being the distance between the mirrors, the other leg being the distance the spaceship travels in the time it takes the light to move between the mirrors, and the hypotenuse being the path the light takes. Of course, you don’t need to solve for anything to realize the light travels a greater distance than the distance between the mirrors.
Since the speed of light is the same in all reference frames, you will observe the light taking a longer time to move between the mirrors than someone on the ship watching the light pulse bounce up and down. You are measuring a longer amount of time between two events than an observer on the ship, so from your perspective, time is moving slower on the ship due to the ship’s motion.
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