I just can’t put my head around the fact that gravity isn’t just a force
EDIT : I now get how it gets stretched and how it’s comparable to putting a ball on a stretchy piece of fabric and everything but why is *gravity* comparable to that. I guess my new question is what is gravity ? 🙂 and how can weight affect it ?
In: Physics
We don’t really have a “Why” answer for you.
Keep asking ‘why’ about any topic, and you eventually end up at physics.
Asking ‘why’ about physics, you won’t really get a satisfying answer.
To try to explain gravity for you:
Time and space are connected, and collectively referred to as ‘Spacetime’.
Think of space like it’s a big 3-dimensional square grid that exists everywhere in the universe.
Time is harder to picture, but time is just another dimension, another coordinate, exactly the same as up/down, left/right, forward/back.
In old, less-accurate physics, ‘Gravity’ is a force that pulls massive objects (objects that have mass) closer together. The notion of spacetime doesn’t exist here, time and space are thought of as separate.
This is Newtonian gravity.
In modern, more accurate physics, gravity refers to the idea that massive objects distort spacetime.
This is General Relativity.
In exactly the same way that gravity affects how you move through space, gravity also affects how you move through time.
We don’t know “Why”, but in the early 1900s, Albert Einstein did explain ‘How’ gravity works.
By thinking of gravity as “Mass Distorts Spacetime”, rather than “Masses get pulled closer, but time isn’t involved at all”, Einstein solved all the little issues that Newtonian gravoty couldn’t quite answer.
It was surprising to many, but the idea is deceptively simple if you can accept that time and space are sort of the same thing.
It’s the fact it’s such a simple idea, and that it accounted for every small discrepancy found between Newtonian gravity and reality.
It’s the fact that we keep finding or inventing new ways to test General Relativity, and every time it’s been able to accurately predict reality.
All of these things together tell us that no matter why, this is HOW gravity works.
Cause space and time are related concepts. Say it takes you 10 seconds to walk from point A to B. Now imagine the space between point A and B is stretched like a rubber band, now it will take you longer than 10 seconds.
Strength of gravity determines space distortion and hence time dilation.
I know right! It’s super cool. You ever watch that Star Trek Enterprise episode where they were encountering “spacial distortions” that warped everything they touched? Well instead of thinking about gravity as a force think about it a bit like that, a distortion in the fabric of the universe. It stretches everything – including time.
Interestingly, we know for a fact it causes time to distort because of things like GPS. The satellites are far away from the Earth’s surface, and time flows differently for them than it does for us, down here time is a little slower (because we’re closer to a large mass, Earth).
The difference is very tiny, but it’s big enough that it adds up over time and causes the GPS accuracy to go down, as it uses accurate time measurements to determine position, speed, etc. [Another experiment involved two aircraft](https://en.wikipedia.org/wiki/Hafele–Keating_experiment) with clocks on board flying in opposite directions, also causing time to slow down for the aircraft flying east (because when you go faster your mass goes up and more mass means more time distortion).
Again, these differences are imperceptible to us humans, but that they exist is very spooky, to me at least.
>I guess my new question is what is gravity ? 🙂 and how can weight affect it ?
I’ll be honest here. While it’s possible to explain how gravity is a curve in spacetime by illustrating it as a ball on elastic fabric, you’re venturing into stuff that *can’t* be explained like you’re five because even modern day scientists aren’t sure of this stuff.
I am not a scientist or a physicist by any means, but I have a very (*very*) basic understanding of some of this stuff, so I will try to explain it in as simplified terms as I can, understanding that some of this may be over-simplified or factually inaccurate as a result. Some stuff can only be simplified so much.
Gravity is a curve in spacetime caused by the presence of mass. Objects with mass are essentially “accelerating” through spacetime. That’s why you’re pinned to Earth right now. Earth is accelerating through spacetime at a steady rate of 9.8m/s^2 and it’s essentially ‘pushing’ you along with it.
If you jump off of a building, you leave Earth’s frame of reference. You’re no longer accelerating through spacetime, but Earth still is, so you’re going to be there when it catches up and slams into you. Now, you’ll eventually reach terminal velocity if the building is tall enough, because as Earth accelerates through spacetime it’s pushing a big buffer of air in front of it, which is going to accelerate you in the same direction as Earth a little bit, though not fast enough to prevent the eventual impact.
We perceive and categorize that acceleration through spacetime as *gravity.*
To try and explain time dilation…spacetime is just that. It is space *and* time. When you move through one, you move through the other. Velocity (V) is a function of Distance (D) over Time (T). It can be written as V = D/T. For example, if D = 1 mile and T = 1 hour, then V = 1 mile per hour.
If you’re moving quickly relative to another person, then any action you take is going to be spread out across more spacetime relative to the other person. Since (D)istance in this case is static (the distance you moved relative to the other person), then when you alter (V)elocity, (T)ime *has* to change in order to keep the equation balanced. In essence, because you’re traversing spacetime faster than the other person, any action you take will be spread across more spacetime, which is perceived as time “passing more slowly” for you.
Taking what I said earlier about gravity being mass accelerating through spacetime, the reason gravity affects time is because an object in a gravity well is *traversing spacetime* at a different rate than objects outside of a gravity well.
TL;DR: [https://xkcd.com/895/](https://xkcd.com/895/)
We’ve [observed that it does](https://en.wikipedia.org/wiki/Tests_of_general_relativity), very smart people have figured out equations to describe and predict this (although not cecessarily in this order) and you get relatively nice equations if you assume spacetime is stretchy. It might be possible to accurately describe this without dilating tome, contracting lenths and having to add velocities in a strange manner, but the math probably gets messy very quickly.
Same thing as with motion of planets, for example. Before we considered nice, easy eliptical orbits centred on the sun, people used a [bunch of circles](https://en.wikipedia.org/wiki/Deferent_and_epicycle) centred on earth to describe these. It worked (somewhat) but was (needlessly, as it turned out) complicated.
With gravity doing weird stuff besides just pulling on objects with mass, it is similar. The easiest way of explaining everything we see is accpeting that it just does all those weird an wonderfull things.
I admit that it does not really answer your question, but I have found that some physics concepts, especially general relativity (which holds the answer to your question) and everything quantum, is so far removed from our everyday life that trying to gain an intuitive understanding of it is somewhere between difficult and impossible. Just accepting it, doing the appropriate math and trying not to think about the “how”s and “why”s too much is the way I found most helpful.
Similar to how you can do loads of usefull stuff by solving problems in mathematiucal spaces with lots and lots of dimensions. The math is relatively easy, as is imagining a 1D, 2D or 3D object, but go highrt than that? I can’t . . .
Picture this: the space around you is a massive piece of fabric and that gravity are balls of different weights placed in random places of that fabric. By putting a heavy ball on the fabric, it creates a little “crater” and if you put another lighter ball in this crater, it basically rolls toward the heavier ball. This rolling of the lighter ball to the heavier ball is gravitational pull.
Then comes light. Imagine light to travel like a drop of water along the fabric at a constant speed that does not change. What you perceive as time is essentially the duration it takes for a drop of water (i.e. light) to reach your eyes from an object. Now, if you picture a large heavy ball in the middle of a big sheet of fabric versus a small light ball in placed in the the same position of this sheet of fabric – the distance of the balls to your eyes will seem the same. However, due to the weight of the heavier ball creating a larger “crater”, the actual amount of fabric between you an the heavier ball is actually larger. Going back to the water droplet that is light, light will take a longer time to travel from the larger heavier object to you because it needs to traverse a greater amount of fabric (and it’s speed remains constant). This longer time it takes is a simplified explanation how gravity warps time
It comes from Einstein’s happiest thought:
Cut the cable and a man falling in an elevator, if he can’t see the world outside, would not be able to tell whether he was actually falling, or simply floating in empty space.
So if ‘free fall’ *in a* gravitational field **is the same as** sitting in empty space *with no* gravitational field, then why do we smack back into earth?
Einstein says, instead of gravity being a force, it’s a warping of space AND time, so that even though your space coordinates are stationary in space-time, **standing still** in space, the TIME coordinate brings you to the earth’s surface.
It’s not only that space is warped, it’s that time gets warped, too, so that as you progress into the future, your future trajectory in spacetime intercepts the surface of the earth.
The TLDR; version is space and time are actually just parts of the same thing. Gravity bends space, therefore it also bends time, too.
Imagine a coin being bent by a hydraulic press. You can’t bend heads without bending tails.
Not my area of expertise but I understand where your hangup is so I’m going to try and build off the examples already present.
Time, as a concept, is just a point of relativity. A second is the period of time it takes for the second hand to get from 1 tick to the next, for example. Everything that we experience as “time” is really just stuff in existence moving at whatever rate it does, and us creating some concept that we can understand it by. A “day” is a day because its how long it takes for the earth to spin in a complete circle and we’ve all agreed that we’re going use that as a standard unit of measurement.
More importantly for this, time is how long it takes for certain processes to occur. I.e; human’s bodies doing what they do.
Gravity is a “force” that is relative to the mass. So heavier objects exert more force against the universe around them. At the local scale, this force is so minute, so minor, so insignificant that it’s effectively not there, so we don’t see time dilation in our regular day.
But at a universal scale, there are countless objects in universe that are large enough to exert this force in a measurable way (black holes, planets, stars, galaxies themselves, etc). And when we start to escape and enter these forces, time gets dilated pretty dramatically.
So using the fabric example, as you move closer to a source of gravity, that “force” gets exerted more. The gravity ball is whereever it is and we’ve put a clock in it. Now, for that clock hand to tick that 1 second amount of distance, it has to have more power to it. So even though it’s only ticked “1 second” worth of distance, it took “2 seconds” worth of time.
On a larger scale, like space travel. As you move away from celestial bodies, time keeps going “normally” for you. You hang out in space, then you swing back to earth where everyone has been under the influence of earth and the moon and the sun and so forth. When you get there, for the 3 years you were out, for everyone else to hit that 3 year point, it took them 6 years worth of time to travel there because all the gravity was slowing them down. So you land back and whamobamo, your body has only done 3 years of cellular processes and their bodies did 6 years worth of cellular processes, and now you’re younger.
Gravity doesn’t impact *time* because time is just a concept we’ve all agreed upon. But it makes it so stuff has to do more work to get from point a to point b, and we interpret that “work” at the passage o time.
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