Eli5: How does gravity affect time, dumb it down for me as much as possible
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Space and time, they are interlinked.
Light travels at a constant speed, for all observers. Me standing here on earth watching a photon shoot across the sky or you in a spaceship traveling at 99% the speed of light will see that photon moving at the same speed “C”. This is unlike what we witness with everyday events.
If I am traveling down the road at 60 MPH and you pass me going 65, from my frame of reference you are traveling at 5 MPH. From your frame of reference you would be stationary and the ground would be moving away from you at 65 MPH I would be traveling 5 MPH away from you. Now the state trooper sitting on the side of the rode sees us traveling in the same direction, me traveling at 60 and you at 65. Different observers see different things this is why its often important to specify frame of reference.
I mentioned before that light travels at the same speed for all observers. For this to be true space and time have to combine into a single framework, they have to be linked, its why we call its Space-time.
I will now follow with an analogy. Note that this analogy is not perfect and breaks down if you get into the weeds of it but here we go.
Imagine a bowling ball on a mattress. The ball is a massive celestial body like the Sun and the mattress represents space-time. When you put the bowling ball on the mattress it deforms the surface. If you drew straight lines on the mattress you would see those line deform so the straight lines were no longer straight. The same is true for a object sitting in space-time, the object deforms space-time around it. The more massive the object the more it deforms space time. Now imagine a marble, this will represent our photon. If you rolled the marble in a straight line on the mattress passed the bowling ball, the marble will curve because the surface it’s traveling on is deformed and curves around the bowling ball. This is what happens to light traveling through space. When it comes too close to a massive object, it encounters “warped” space-time and curves. This is not because it’s being pulled by gravity but because the space-time it’s traveling through is curved, so its otherwise “straight” path becomes curved.
Now that you understand how and why spacetime are linked and how objects in space affect spacetime, on to your question. Why does gravity affect time. Well we know that a massive object in space warps space time, we know space and time are linked so if we warp space we also warp time. We call this Gravitational time dilation. The lower the gravitational potential, or closer the clock is to the source of gravitation, the slower time passes. The clock will speed up as gravitational potential increases, or the clock getting away from the source of gravity.
If you remember from middle school math the formula for time is Time = Distance ÷ Speed. If we go back to the mattress analogy: A straight line is the shortest distance between 2 points. If that line is now curved it MUST be longer because it is no longer straight and there for no longer the shortest distance. So if our speed “C”, the speed of light must remain the same for all observers then the only other variables we can change in our Time = Distance ÷ Speed equation is distance. Because our distance is now longer our time will be “slower”
In simple terms the math looks like this:
I can run 1 mile in 8 minutes. So my speed is .125 miles per minute. Now lets say that no matter what I CANNOT run faster or slower that .125 miles per minute. So our equation would look like this Time = Distance ÷ Speed or 8min=1mile/.125miles per minute
Now lets say that 1 mile track has now be “warped by gravity” into a 1.25 mile track. so our new equation would look like Time = 1.25miles ÷ .125miles per minute, meaning it would now take 10 minutes to complete that track, it is a “slower” time. This is the gravitational time dilation.
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