How Does Bent Time Make Gravity?

If you have an object initially at rest and then let go, you can be sure that it's going to fall. Isaac Newton told me that this is due to the force of gravity. And I understood this, but then

gravity. And I understood this, but then Einstein came along and ruined everything. He told me that gravity

everything. He told me that gravity isn't a force. It's the bending of spacetime. And I didn't understand.

spacetime. And I didn't understand.

People tried to help me by coming up with demonstrations like this where they take a fabric that represents the fabric of spacetime and then they put a heavy mass on it that bends the spacetime and

sure enough another object is attracted to it. But this made me even more

to it. But this made me even more confused. If this is spaceime then

confused. If this is spaceime then what's the spaceime bending into? What's

all this space above and below it? And

the most obvious problem gravity is the thing pulling the heavy ball down stretching the fabric. You can't use gravity to explain why gravity works.

So, in the end, I was left wondering, if gravity isn't a force, then what's actually making the object start moving in the first place? Why do I feel like I'm accelerating when I'm standing on the Earth, not accelerating, but then

when I'm falling toward the Earth and actually accelerating, I'm not? This is

a mess. But last night, I woke up in the middle of the night and finally understood I understand Einstein's field equations. To understand this, we're

equations. To understand this, we're going to keep it simple. Let's pretend

that there are only two dimensions, one space dimension and one time dimension.

So instead of X, Y, and Z axes, we just have a time axis and a space axis. When

the axes are straight lines like this, it represents flat spacetime. If a

person is just standing still with no velocity, then they'll move up in a straight line on this graph here. This

is because they're only moving through time and not space. If they have a velocity, they'll be moving in a tilted straight line because they're always moving forward through time, but also through space. And then if they're

through space. And then if they're accelerating, it'll look like a constant curvature. Now, let me show you what

curvature. Now, let me show you what happens to these lines when something with mass is introduced. Now, let's say I have something with a lot of mass, like the Earth. Well, in this case, it's a one-dimensional Earth, and it's

stationary. So, from its world view,

stationary. So, from its world view, it's just moving in a straight line through time. But because it has a lot

through time. But because it has a lot of mass, it actually does something to the graph itself. It bends it. Here's a

view of the bending on both ends of the one-dimensional Earth. Now, notice

one-dimensional Earth. Now, notice something about this bending. It's only

bending the time axis, not the space axis. Notice how all the time lines have

axis. Notice how all the time lines have curvature to them, but all the space lines are completely straight. The

physical meaning of this is that clocks that are closer to something with a lot of mass tick slower compared to clocks further away. But rulers near something

further away. But rulers near something with mass don't shrink nearly as much.

The geometry of time is about a billion times more distorted than the geometry of space near Earth. So that popular picture of gravity as warping this fabric of spaceime is wrong. Well, not

wrong, but the original depiction was meant to represent the fabric of space at one specific moment in time. It

wasn't meant to show space and time because time bends much more than space.

And the bending of space has nothing to do with why something falls. Which means

that spandex sheet demo I did at the beginning is pure baloney. So how does the bending of time make something move toward Earth? You can see this by taking

toward Earth? You can see this by taking our stationary object that's just sitting in space only moving through time. Remember stationary objects just

time. Remember stationary objects just move straight vertically through time.

You can almost think of it like it has this innate momentum carrying it straight and it won't deviate from that course unless it hits something. But now

let's put it near the Earth and watch what happens. Because of the skewed

what happens. Because of the skewed graph, the object inevitably moves through space as well as time. So

there's no sudden momentum gained or anything, but the skewed coordinates made it start moving through space while moving through time less. But that's not how we perceive it because we perceive

flat spacetime. So then let's see what

flat spacetime. So then let's see what this black straight line looks like in flat spacetime, which is how we perceive things. It's curved exactly in the same

things. It's curved exactly in the same shape you would get when something's accelerating. So that's why we perceive

accelerating. So that's why we perceive something as accelerating towards Earth when it's falling. Now before we continue, I want to thank the sponsor for this video, BetterHelp. BetterHelp

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description. And thanks to BetterHelp for sponsoring this video. Now, let's

get back to our discussion. This flat

spac-time view isn't correct, but it is intuitive. Because on our small scale,

intuitive. Because on our small scale, spacetime is essentially flat. It takes

a lot of mass to noticeably bend time.

Other than the Earth, nothing in our life is big enough to bend our local time. So, we think in flat spacetime,

time. So, we think in flat spacetime, and that's okay for almost everything that we do in physics. So, it's okay if you don't want to call gravity bent time. Just call it a force and it still

time. Just call it a force and it still works out just fine most the time. But

with this view of the bent axis of time, it will help us finally understand why my accelerometer says I'm accelerating when I'm standing on the ground, but not when I'm in freef fall. You can try this

yourself on your phone. Get any app that shows you the accelerometer data from your phone. So, this is telling me that

your phone. So, this is telling me that when my phone's just sitting on the ground, it's accelerating. And sure

enough, if we look at the graph of something sitting on the surface of the Earth, it has a constant curvature. But

notice that while sitting on the surface of the earth, the phone isn't moving through space. As you can see on the

through space. As you can see on the graph, it doesn't cross any space lines, but it is accelerating. So acceleration

is defined as anytime we don't move in a straight line in spaceime, no matter whether or not the spacetime is bent or not where we are. So in general relativity terms, this rubber chicken is

not accelerating, but the drone that dropped it is. So the ground is dragging you through spaceime right now deviating you from your natural straight line course. That's why you feel an

course. That's why you feel an acceleration or in other words that's why you feel a force against the ground or feel the weight of your body on the ground. But as soon as we don't let it

ground. But as soon as we don't let it touch anything like when we toss it into the air as soon as it leaves my hands the acceleration drops to zero because the earth isn't accelerating it anymore and it's just moving through time.

Accelerometers don't have incorrect intuition. They just spit the truth. So

intuition. They just spit the truth. So

if I were in true flat spacetime and started accelerating in a rocket ship, it would look like this. And when I'm just standing on Earth, it looks like this. They're the same path. The same

this. They're the same path. The same

thing is happening. This is called the equivalence principle. There's

equivalence principle. There's absolutely no way to distinguish acceleration in flat spacetime in the middle of space from the pull of gravity while standing on Earth. We can't

distinguish between them because they are the same thing. But wait, if changing and bending time makes things feel like a force, does that mean that all the forces are just different

versions of bending some coordinates?

All mass bends spacetime for every other mass. But what if some other properties

mass. But what if some other properties bend some coordinates only for that property? Could all the forces just be

property? Could all the forces just be bending of the coordinates of that property? Well, that's actually what

property? Well, that's actually what Einstein thought. He said mass bends

Einstein thought. He said mass bends spacetime, which is the fabric that contains everything. But

contains everything. But electromagnetism, the weak force and the strong force each bend their own distinct geometries. This is now called

distinct geometries. This is now called gauge theory. They treat the forces as

gauge theory. They treat the forces as curvature of internal geometries that they call U1, SU2, and SU3. This bending

of spaceime and geometry works better than treating things as forces. But why?

It's as if the universe isn't a stage where particles act out their roles, but a living mathematical fabric that reshapes itself with every bit of matter and energy it contains. Reality itself

seems to be a geometry. And whenever a particle exists, it doesn't just move through that geometry, it changes what geometry means. And thanks for watching

geometry means. And thanks for watching another episode of the Action Lab. I

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