# Eli5 Whats special about lighspeed?

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What about lightspeed makes it related to time. Why would really dense things distort time?

In: Physics

The speed of light in a vacuum is special because nothing can be faster. It’s basically the speed at wich “now” propagates through space.

We don’t really know the “why” and physics doesn’t really try to answer that question, it’s more philosophical.

We only know that if it was different the laws of electromagnetism we observed would be inconsistent.

The speed of light is the fastest speed that anything can traverse space time. You could also call it the speed of information.

The sun disappears in a puff of smoke, we are still affected by its gravity for 8 minutes (we are 8 light minutes from the sun)

As for time dilation caused by dense things, it’s probably more simple to think of space and time as one thing. Spacetime.

When describing gravity, a lot of time you will see a flat plane with something large (large mass, therefore large gravity) creating a depression in the flat plane. Like putting a bowling ball on a spandex blanket.

Imagine that blanket can stretch almost infinitely from the weight of the bowling ball (the heavier the ball, the more it stretches).

If this spandex blanket were flat (nothing big is stretching it), it would take a certain amount of time for a moving object to go from one end to the other.

Now the blanket is severely stretched out by a very heavy object sitting on it. If we (on the undisturbed flat area where we are) we’re to observe that same object move across the stretched out area, it would seem to us that it takes longer to do so.

If we were on the moving object, we wouldn’t notice anything. We always experience one second per second. That’s why the term relativity is used. Appearing to move faster and slower through spacetime is always relative to some other observer. Not ourselves.

To your first point, light speed affecting time is similar to strong gravity forces. With spacetime there is a “speed limit” to how an observer would see you travel through space and time. Let’s say it’s “100”

If I’m moving 0 through space, I’m moving 100 through time. If I’m moving 25 through space, I’m moving 75 through time. So if you (the observer) are moving 100 through time and see me moving 75 through time because of how fast I am moving through space, you would see me moving slower through time. If you could see me on my spaceship moving fast through space, I would appear to you to be walking around the ship slower. The clock on my wall would tick slower.

Because we live in a 4-dimensional universe. There are 3 dimensions in space. These are Up/Down, Forwards/Backwards, Left/Right. We can move through these three dimensions at will.

But there is a 4th dimension we move through as well. This dimension is time, and we only ever move in one direction in time, and that is towards the future.

Together these make up space-time.

Now we need to talk about motion.

Imagine you are standing on a train that is movinf at 10m/s. And you roll a ball forward on that train, and you meassure the ball is rolling with a velocity of 5m/s.

But someone who is standing on the outside of the train, next to the tracks, would meassure the ball rolling with a velocity of 15m/s. This because they would meassure the speed of the train as well. This shows us motion is relative.

So we could assume that is the case with everything, correct? If instead of a ball, i take a flashlight and shine it infront of me on the train, i would meassure the speed of light. And the person on the side of the track would then meassure the speed of light + the speed of the train right?

But they dont. They also meassure the speed of light. The speed of light is constant.

So that means that for both of us to meassure the same velocity, when we are moving at different velocities, we have to experience time differently. Time is relative. The faster you move through space, the slower you move through time.

The speed of light is not special because light moves at that speed, it is special because we all move at that speed. If you add up the speed that you are travelling in through both space and time, it will add up to the speed of light, and in order to maintain that balance, the faster you move through space, the slower you move through time.

The reason why lightspeed is the maximum velocity, is because if you move at lightspeed through space you will no longer experience time at all. The entire existance of the universe will happen in an instant.

Theoretically, we could move faster than light, but we would need an infinite amount of energy to do so. Anything with a mass would need infinite energy to reach lightspeed, so therefore we cant. Light can because it is mass-less.

E = mc²

E is energy in Joules. m is mass in kilograms. c is the speed of light.

A rest mass has energy proportional to the speed of light squared.

In other words, if you create a nuclear reaction the amount of energy released would equal the amount of mass lost times speed of light squared.

its not just lightspeed that is related to time. every speed is. I have heard that you always go the speed of light but in time and space, ex if i go lightspeed in space I stay still in time but if i stay still in space i go lightspeed in time and everyting in between but im not exactly sure about this.

Well the special thing about light speed is that if you got an object with mass abow 0 and accelerate it you add kinetic energy to it. Einstein said that E = mc^2 + pc which means that energy is rest mass + momentum. So your total mass is your rest energy + kinetic energy. If you got rest mass as you accelerate you get more massive and require more energy to accelerate further. So you look at how much energy you need to accelerate a 1kg (or any) of mass to v speed. This function approaches a v value, you pick larger and larger energy values and the function gets closer and closer to a v value. At infinity it reaches c, the speed of light. So to accelerate anything to c you need an infinite amount of energy.