Eli5: How does gravity affect time, dumb it down for me as much as possible
In: 286
Via space.
You are familiar with the gravity model that is visualised as a rubber sheet with weights on it? It shows us that objects in orbit are actually travelling in straight lines as far as they are concerned and that space itself curves around a planet.
Well our current understanding doesn’t really differentiate space from time. That’s where ‘space-time’ comes from. So anything that impacts one, impacts the other. Mass curves time via gravity and space
Massive objects curve space. (Edit… It curves spacetime, I know… But for this analogy let’s say it curves space) The closer you are to one, the more the curvature.(Edit: think of a heavy ball on a flexible rubber sheet) Two points on a flat surface are closer (straight line…closest distance between points) than the same two points when you curve the surface. So if you travel between the same two points when you’re near a massive object, the curvature of space leads to you taking more time. So, time moves slower closer to massive objects.
This is a direct result of General Relativity and has been proven experimentally.
It just does. The thing we describe as gravity has that effect, and it seems to be a fundamental property of objects with mass/energy.
“Why does gravity affect my position in space?” is the same question, but it doesn’t feel wrong, because we see that effect every day, while gravity’s effect on time requires high precision atomic clocks to even be measured.
Hi /u/JustTransportation51!
While we generally think of gravity as a force pulling us down, this is not how General Relativity (GR) – the bet theory of gravity we currently have – sees it.
According to GR, gravity is the curvature of spacetime. So, in order to understand what this means, we have to back up a little and ask “what is even spacetime”?
Newtonian physics saw space and time separate and unchangeable entities in though physical objects could move. In particular, Newtonian mechanics thought, that all objects moved through time at the same rate.
The theory of relativity, however, discovered that space and time are neither separate nor unchangeable. Rather, phenomena such as [time dilation](https://en.wikipedia.org/wiki/Time_dilation) or [Lorentz contraction](https://en.wikipedia.org/wiki/Lorentz_contraction) meant, that spatial and temporal coordinates could be transformed into one another and depend on the state of the observer. Thus, spacetime – i.e. the unification of space and time into one single object – was required to fully describe these phenomena.
Furthermore, General Relativity discovered, that the geometry of this spacetime was not fixed. Rather, the presence of mass and energy were able to distort spacetime. This distortion of spacetime caused by mass and energy is what we perceive as gravity ¹.
Now we can come back to your question: As gravity is the curvature of spacetime, space and time are affected by gravity. And in particular, strong gravitational fields curve spacetime in such a way, that time is slowed down.
Does this answer your question?
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#Appendix:
¹: To go into some more detail how a curvature of spacetime leads to the phenomenon we perceive as gravity:
To understand how a curvature of spacetime can lead to the effects we observe around us, we have to understand how curved surfaces change the behaviour of straight lines.
First things first: an object that has no force acting on it is force-free. Force-free objects do not accelerate and, therefore, move along straight lines.
In a flat geometry, two straight lines which are parallel at one point will remain parallel for all times. That is, two parallel straight lines will never cross on a flat surface.
So far so intuitive, right?
But what happens, if those straight lines do not move across a flat surface, but instead along a curved surface? We call such straight lines on curved surfaces [geodesics](https://en.wikipedia.org/wiki/Geodesic).
Imagine a [sphere](http://pi.math.cornell.edu/%7Edwh/books/eg99/Ch06/3776c40d.jpg) with two lines perpendicular to the equator. As they are both perpendicular to the same line, they are parallel at that altitude.
Imagine two objects that are moving along the lines perpendicular to the equator. They start out parallel, and move in a straight line upwards. Despite the fact that neither of them is turning, the two objects that started out moving along parallel lines will meet at the north pole. Hence, despite the fact that both objects are force-free at all times, they experience relative acceleration.
Such trajectories, that lead across curved surfaces without turning are called geodesics and they can be thought of as straight lines on curved surfaces. Objects under the influence of gravity follow [geodesics](https://en.wikipedia.org/wiki/Geodesic).
As gravity curves spacetime, geodesics can experience relative acceleration despite the fact, that both objects following said geodesics are force-free. And this relative acceleration of force-free bodies is what Newton mistook for the gravitational force. According to GR, though, there is no force, only curvature which causes force-free objects to move along paths that seem accelerated to outside observers.
This is why gravity is a fictitious force: The reason why two objects in a gravitational field may experience relative acceleration is not a force between them, but geodesic deviation between two force-free objects.
If you have any more specific questions, feel free to ask.
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For a great video on the basics of GR, check out [this](https://www.youtube.com/watch?v=NblR01hHK6U) video by PBS Spacetime.
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.
It’s actually the other way around. Time gradients which form around masses cause different parts of other nearby masses to follow slightly different paths through space-time. The net result is than the masses’ trajectories deflect toward each other, giving the appearance of attraction which we call gravity.
More mass-density means a steeper time gradient, which results in stronger gravity.
[This video explains it nicely.](https://youtu.be/F5PfjsPdBzg)
You and a friend are on an extremely fast rocket ship that is accelerating VERY fast. Your friend turns on a flashlight and shines it at the wall. If you are watching you would expect it to go horizontal to the wall. But if you’re going fast enough, it would actually take a curve downward (because the wall of the ship is moving up while the light is moving toward it, making it appear to bend downward). Now the curved path is longer than a straight path, right? So the acceleration of the ship caused the light to make a longer path. BUT…light always moves at the same speed, no matter what. How can light move at the same speed but go different distances (horizontal if still, curved if moving)? Well, speed = distance / time. If the speed is the same and the distance is longer, then time must be longer as well to keep the ratio the same.
This thought experiment demonstrates that fast enough acceleration can actually slow the rate of time that the path of light takes.
Okay so on more step: gravity is really nothing but an acceleration…we are all being accelerated to the center of gravity. So therefor, gravity as well can “bend” time.
Energy (mass is energy too) is what creates space-time, so when you have a more dense conglomerate of energy there is “more amount”of space-time so it is more difficult to move within it. Far form there where there is not such amount of energy concentrated space-time gets “weaker” so is more easy to “move” within it.
At the same time space-time dictate how energy (mass too) moves in spaces and displace in time.
So it’s a mutual feedback.
In relativity there is a very important equation that relates both by using math variables called tensors. In the equation energy-tensor says how the metric-tensor **is** and the metric-tensor says how the energy-tensor **moves**.
(Note:actually it’s energy-momentum what creates gravity not only energy)
This is about as “simple” as such a complicated topic gets:
You move through the universe at a fixed rate: *c*. You are always moving at this velocity, the only question is how much of that velocity is through time, and how much is through space?
If you’re moving at *c* through space, then time essentially stops for you, since space velocity + time velocity must equal *c*, thus if your space velocity is c, then your time velocity *must* be zero. Likewise, if you stop moving completely, you are moving at maximum velocity through time, and zero velocity through space. This is why people often say that a photon is emitted at the exact moment it strikes an object, from its own frame of reference. Photons have no mass, and thus travel at maximum velocity through space (*c*, aka “the speed of light in a vacuum).
Gravity dicks this up because when spacetime is curved, you are effectively accelerating through time even then you appear stationary. Earth isn’t just moving, it’s *accelerating* at a rate of 9.8m^2, and as long as you’re sitting on Earth, it’s pushing you along with it. If you jump our of an airplane, you temporarily remove yourself from Earth’s frame of reference – not completely, since Earth is still pushing air through spacetime and that air is pushing you – but you hitting the ground (hopefully after opening a parachute) is Earth “catching up” to you since you stopped accelerating when you jumped out of the plane but Earth never did.
So essentially, mass curves spacetime, which means objects with mass are always “accelerating” through time at some rate or another, meaning their movement through time is always going to vary based on the intensity of the gravity well they’re residing in.
I want to reiterate: This is an ***extremely simplified explanation*** of a phenomenon humans still aren’t super clear on.
So… space and time are two different ways of measuring the exact same thing. Like, we perceive things mostly in three dimensions, so we measure length, width, and height to determine how ‘big’ it is, but something that perceives more accurately than us would also measure time.
So, because time is just another measure of space, when something effects space, it effects time too.
Gravity bends that fabric. It’s like a weight, set on a cloth that’s pulled tight, so it pulls on the fabric and stretches it. But, like I just said, that fabric stretches in four directions (at least) so it stretches the length, the width, the height, and the time, of that fabric.
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