A physicist explains gravity in terms we regular folks can understand.

[DontTreadOnMike]

I DID NOT WRITE THIS. This is a comment left by a brilliant redditor by the name of RobotRollCall as an answer to the question, "What keeps the planets moving around the sun?"

People may tell you the answer is "gravity," but that's a bit unhelpful in my opinion. That's just giving a name to the phenomenon. Gravity is what we call the fact that objects move in orbits. And naming something doesn't help you understand it.
The truth is that there is no force keeping planets moving around the sun.
Consider a particle — a cricket ball, a planet, whatever — moving through an otherwise empty universe. The particle will continue moving at a constant speed and in a straight line, because there's nothing else around to interact with it. If something smacks into the particle, then the particle will undergo so kind of acceleration and change its motion, but until that happens, the particle will just keep doing what its doing without any change.

This is a fundamental property of matter. It's called inertia. Inertia is the Latin word for "laziness," and that's not by accident. Matter is lazy. It won't do anything unless it's forced to. Which is why we call the thing which creates a change in momentum force.
So it's reasonable to assume, then, that whenever you observe a change in momentum, there must be some force at work.

But this turns out not to be the case in all situations. Consider riding along as a passenger in a car. If the driver turns a corner to the right, you'll feel yourself pushed toward the door; if the driver makes the turn sufficiently quickly, the pushing sensation can be quite pronounced, or even uncomfortable. You experienced a change in your momentum — you went from sitting comfortably to smacking against the passenger-side door of the car — so there must have been some force acting on you, yeah?
Well, no, not actually. Yes, there was a force, but it didn't act on you at all. It acted on the car. The driver turned the wheel, the tyres changed their orientation with respect to the road, this put a torque on the car and caused it to smack up against you.
When we sit at our telescopes and watch the planets orbit the sun, it seems apparent to us that there must be some force acting on them. They're changing their direction — just as the car turning the corner did — so they must be undergoing a change in momentum, and a change in momentum is defined as a force, so there must be a force, yes?

Except we, here on Earth, perceive no force on ourselves. If the Earth is turning as it moves around the sun, then the ground should act, from our point of view, like the car door. People closer to the sun should feel the Earth pressing up against their feet harder than usual, because the Earth is turning into them; people on the other side of the planet should feel the Earth being pulled away from their feet slightly, because the Earth is turning away from them. But no such apparent force is perceived. Okay, so maybe it's just too small … but it turns out no such force can be measured at all, even using the most sensitive instruments.
This has always been a great puzzle. But Newton worked around it by postulating that there exists a mysterious, intangible, practically magical force that acts on everything exactly equally. We don't measure a force pressing up against our feet at noon because just as the Earth is turning toward us, we too are turning toward the sun. So the change in momentum cancels out. It's there. It's just not possible to measure because our measuring apparatus is itself being accelerated.

This explanation satisfied no one, least of all Newton himself. But it was the best he could do.
It took Einstein, two hundred years later, to finally figure it all out. It turns out that the elaborate maths of invisible forces canceling each other out exactly to keep us from measuring any acceleration are entirely unnecessary. The reason we don't measure any acceleration as the Earth turns the corner toward the sun is because there isn't any. At all. No acceleration exists in our reference frame as we fall around the sun.
The implications of this fact are wide-ranging, and very subtle. A full explanation of how gravity works is beyond the scope here, I think. But the short version is that the Earth — and all the other planets — behave just like particles in an otherwise empty universe: They move in a straight line at a constant speed, due to their inertia, their laziness. The reason the Earth's trajectory around the sun looks curved is because in the vicinity of the sun, straight lines are curves.

Our intuition — there's that word again — tells us that straight lines are, well, straight. They're parallel to themselves everywhere. But it turns out this definition isn't really a very good one. A better definition of a straight line comes from the advanced study of differential geometry, in which a straight line is defined as a line without infinitesimal deflection. Say you're following a path. At every point, you're moving in some direction at some speed; we can represent this as what's called your tangent vector at that point. Consider your tangent vector at one point, and then the tangent vectors at very nearby points. If those tangent vectors are all exactly equal, then you experienced no deflection along that infinitesimal part of the path. If this is true everywhere along the path — that the tangent vector at one point is equal to the tangent vector at the points immediately surrounding that point — then you're moving along a straight line. This is true even if the tangent vector at one point is different from the tangent vector at a point far away along the path.

As the Earth orbits the sun, it experiences no infinitesimal deflection from its trajectory. It just continues "following its nose," as it were, as it moves through space. Along infinitesimal intervals of the Earth's trajectory along its orbit, its tangent four-vector in spacetime stays the same. But at distant points — say when the Earth is on this side of the sun, compared to when it's on that side of the sun — the tangent vectors can be quite different, or even exactly opposite.

So the Earth moves along a straight line, without deflection … and yet that straight line turns out to be an ellipse when looked at from a distance? How can this be?

The answer is that the geometry of the neighborhood of the sun is curved. In flat geometry, like the Euclidean geometry we all learned about in primary school, straight lines are straight. They're parallel to themselves everywhere. But this turns out to be a simplification of how geometry works in the real world. In the real world, geometry does not have to be — and in fact, rarely is — flat. But it's a property of our universe that geometry everywhere is what's called locally flat. That is, if you consider a small enough region of the universe, the geometry you find in that region will be flat.

This should not be a challenging idea. We deal with it every day. The surface of the Earth is curved. But if you consider a small enough region of it — the square mile surrounding your house, say — the curvature vanishes because it's too slight to care about, or even detect. In just the same way, the curvature of spacetime vanishes when you consider a small enough region of it. Which is how it can be that, as the Earth moves around the sun, it maintains a constant speed in a straight line along infinitesimal intervals of its trajectory, even though the whole trajectory forms a closed curve. If you zoom in close enough, spacetime is flat, and the Earth moves in a straight line. But zoom out, and the curvature of spacetime around the sun becomes apparent, which means the Earth's whole trajectory also curves, even though it's locally straight everywhere.

So things like planets move along curved paths because the geometry of the spacetime through which they move is curved. Which naturally enough brings us to the question of why spacetime is curved. The answer is that the presence of stress-energy creates spacetime curvature. Stress-energy is a technical term, but you can think of it as a sort of sum of energy density, momentum density, energy flux, sheer stress and pressure. All those things put together — along with at least one other thing that isn't important right now — affect the geometry of the universe around them. At everyday scales, most of the contribution to stress-energy comes from simple mass: the sun has mass, therefore it curves spacetime. But the sun also has pressure, internally, which means it curves spacetime slightly more than it would if the curvature were related to mass alone. But if you think of stress-energy as being mass-plus-a-tiny-bit-of-other-stuff, you will be mostly right most of the time.

Stress-energy creates curvature, and curvature affects trajectories. The physicist John Archibald Wheeler summed this up in a simple and beautiful sentence that's now facetiously known as Wheeler's Prayer: "Space tells matter how to move, and matter tells space how to curve."

So to sum up, gravity is the name we give to the fact that things fall. But the reason why things fall has to do with the relationship between spacetime geometry and stress-energy. Matter moves along trajectories that are locally straight because the geometry of our universe is locally flat, but in regions where the geometry of our universe is curved, those straight trajectories end up looking like things like ellipses and hyperbolas. What keeps the planets in orbit around the sun, then, is not a force at all, but rather the geometry of spacetime around them, which is curved by the stress-energy of the sun.

-RobotRollCall

 

[DontTreadOnMike]

Hm. Well I thought it was interesting anyway haha

 

[Tinker Grey]

It is good, Mike.

 

[DontTreadOnMike]

It is good, Mike.

Yeah RobotRollCall is awesome. I have his/her posts set up as an rss feed on my phone's home screen so I can learn about physics as soon as my alarm goes off in the morning haha.

 

[SithDoughnut]

I understood it perfectly up until the straight line bit. I think I get it. Interesting stuff, nonetheless.

What I do want to know though is if gravity is not a force, but the result of the bending(?) of spacetime, what's the force affecting spacetime?

 

[DontTreadOnMike]

I understood it perfectly up until the straight line bit. I think I get it. Interesting stuff, nonetheless.

What I do want to know though is if gravity is not a force, but the result of the bending(?) of spacetime, what's the force affecting spacetime?

He touches on it. It's stress energy. In other words "the presence of mass plus a little of some other stuff"

Matter and energy bend space. It's really as simple as that.

 

[vorbis5]

Great stuff. I enjoyed reading this

 

[Jazer]

So to sum up, gravity is the name we give to the fact that things fall. But the reason why things fall has to do with the relationship between spacetime geometry and stress-energy.

They fall because they are being pushed, not because of any sort of a pull or attraction.

 

[1611AV]

Except the universe is expanding at a exponential rate of speed. If this is to be explained by dark matter, then it should be losing its influence over regular matter as the galaxies move away from each other, yet it continues to accelerate the galaxies outward. It would seem logical that gravity would slow the expansion of the universe.

 

[DontTreadOnMike]

They fall because they are being pushed, not because of any sort of a pull or attraction.

Mmm nope gravity isn't a force. Everything moves in a straight line. When something gets pulled in and caught in earth orbit, it's still moving in a straight line. Draw a perfectly straight line on a piece of paper. Now bend the paper. That's what gravity is; bent space. So the planets are really traveling in a straight line through space that is curved by the sun

 

[Nabobalis]

Except the universe is expanding at a exponential rate of speed. If this is to be explained by dark matter, then it should be losing its influence over regular matter as the galaxies move away from each other, yet it continues to accelerate the galaxies outward. It would seem logical that gravity would slow the expansion of the universe.

You are right, you are talking about dark energy which is causing the expansion to accelerate.

 

[DontTreadOnMike]

Except the universe is expanding at a exponential rate of speed. If this is to be explained by dark matter, then it should be losing its influence over regular matter as the galaxies move away from each other, yet it continues to accelerate the galaxies outward. It would seem logical that gravity would slow the expansion of the universe.

Yep, the galaxies are speeding up instead of slowing down and that's exactly why they postulated dark energy (dark matter is something else).

They don't know what dark energy is. It's just that thing that is causing metric expansion.

Actually now that we've gotten in to metric expansion we should probably be more accurate and say that galaxies are not moving away from each other, the space between them is growing. All distances everywhere are increasing.

 

[1611AV]

Yep, the galaxies are speeding up instead of slowing down and that's exactly why they postulated dark energy (dark matter is something else).

They don't know what dark energy is. It's just that thing that is causing metric expansion.

Actually now that we've gotten in to metric expansion we should probably be more accurate and say that galaxies are not moving away from each other, the space between them is growing. All distances everywhere are increasing.

Thats an interesting concept.

 

[DontTreadOnMike]

Thats an interesting concept.

It get's even crazier. The universe is expanding but it's also already infinite!

How can that be? Here's how it was explained to me. Imagine the number line:

...,-5, -4,-3, -2, -1, 0, 1, 2, 3, 4, 5,...

It stretches on forever in both directions. It doesn't matter how high you count, you can always count one number higher. Now multiply every number on the timeline by 2.

...-10, -8, -6, -4, -2, 0, 2, 4, 6, 8, 10...

The number line is still just as infinite, but now all the numbers are twice as far apart.

The universe has always been infinite, it's just that in the past, all the "numbers" were closer together. There was no big bang because nothing went bang. It wasn't an explosion, it was a rapid expansion and it's still happening right now. The problem is, at some point in the distant past, everything was so close together that light couldn't travel very far before running in to something. So we have no way of knowing what the universe was doing before that point because the edge of the visible universe is like an opaque wall to us. But if you could travel to the edge of the visible universe and looked beyond it, you would just find more galaxies as far as you can look.

The universe is a far more interesting place than any of us can imagine.

 

[1611AV]

It get's even crazier. The universe is expanding but it's also already infinite!

How can that be? Here's how it was explained to me. Imagine the number line:

...,-5, -4,-3, -2, -1, 0, 1, 2, 3, 4, 5,...

It stretches on forever in both directions. It doesn't matter how high you count, you can always count one number higher. Now multiply every number on the timeline by 2.

...-10, -8, -6, -4, -2, 0, 2, 4, 6, 8, 10...

The number line is still just as infinite, but now all the numbers are twice as far apart.

The universe has always been infinite, it's just that in the past, all the "numbers" were closer together. There was no big bang because nothing went bang. It wasn't an explosion, it was a rapid expansion and it's still happening right now. The problem is, at some point in the distant past, everything was so close together that light couldn't travel very far before running in to something. So we have no way of knowing what the universe was doing before that point because the edge of the visible universe is like an opaque wall to us. But if you could travel to the edge of the visible universe and looked beyond it, you would just find more galaxies as far as you can look.

The universe is a far more interesting place than any of us can imagine.

I find some common ground in your post.

I to use the number line when trying to explain eternity/infinity to someone who cant rap their mind around the idea of this being possible. It is a great tool to use to show a limited mind something that goes beyond our ability to grasp to its full extent.

I also believe the universe to be without "a edge". I do not believe in absolute nothingness. There is definitely something(s) beyond the universe.

Isaiah 42:5 Thus saith God the LORD, he that created the heavens, and STRETCHED them out; he that spread forth the earth, and that which cometh out of it; he that giveth breath unto the people upon it, and spirit to them that walk therein:

Isaiah 45:12 I have made the earth, and created man upon it: I, even my hands, have STRETCHED out the heavens, and all their host have I commanded.

 

[DontTreadOnMike]

I find some common ground in your post.

I to use the number line when trying to explain eternity/infinity to someone who cant rap their mind around the idea of this being possible. It is a great tool to use to show a limited mind something that goes beyond our ability to grasp to its full extent.

I also believe the universe to be without "a edge". I do not believe in absolute nothingness. There is definitely something(s) beyond the universe.

Isaiah 42:5 Thus saith God the LORD, he that created the heavens, and STRETCHED them out; he that spread forth the earth, and that which cometh out of it; he that giveth breath unto the people upon it, and spirit to them that walk therein:

Isaiah 45:12 I have made the earth, and created man upon it: I, even my hands, have STRETCHED out the heavens, and all their host have I commanded.

Scientists have found no reason to believe that there even is a "beyond" the universe. If you don't believe there is an edge then where does "beyond" start? Also, there's no reason to believe that there isn't more matter beyond the "edge" of our visible universe. The visible universe is a spherical region with earth at the center and consists of the regions of space where light has had time to reach us. Our visible universe is growing each year because each year, light from one light year further is able to reach us.

Let's say you could magically teleport to the edge of our visible universe, what would you see? Well if you looked back, you would see that earth is now at the edge of YOUR visible universe. If you looked forward you would see more stars and galaxies that mankind has never seen. You would see new constellations, and older constellations from the other side, but beyond that, everything would look pretty much the same. There is no end to the universe and there's no reason to assume that there's an end to the matter in the universe either.

 

[1611AV]

Scientists have found no reason to believe that there even is a "beyond" the universe. If you don't believe there is an edge then where does "beyond" start? Also, there's no reason to believe that there isn't more matter beyond the "edge" of our visible universe. The visible universe is a spherical region with earth at the center and consists of the regions of space where light has had time to reach us. Our visible universe is growing each year because each year, light from one light year further is able to reach us.

Let's say you could magically teleport to the edge of our visible universe, what would you see? Well if you looked back, you would see that earth is now at the edge of YOUR visible universe. If you looked forward you would see more stars and galaxies that mankind has never seen. You would see new constellations, and older constellations from the other side, but beyond that, everything would look pretty much the same. There is no end to the universe and there's no reason to assume that there's an end to the matter in the universe either.

Would you consider dark energy "matter"? Also the beyond I describe is spiritual.

 

[DontTreadOnMike]

Would you consider dark energy "matter"? Also the beyond I describe is spiritual.

Oh gotcha.

And keep in mind that I'm a 3D animator, not a scientist. But I did stay at a holiday-......I mean. I do hang out on science forums. Ask Science Questions And Get Answers is a good one

The way I understand it, dark energy is not matter. Matter is just a type of energy and dark energy (they think) is a different type. They don't know what it is.

Dark MATTER is different. They know how it behaves, they know it's attributes, how much of it is in the universe, etc, but they have yet to find it. It's hard to find something that doesn't react electromagnetically with itself or with other matter. That means it doesn't run into things and it doesn't emit or reflect light. It passes through everything. How do you find something like that? Well give me 8 years to earn my phd and I'll let you know haha

But no, dark energy is not matter. They don't know what it is. It's just an easier term to remember than "whatever-is-causing-all-distances-to-increase." They don't know what it is yet but the game is afoot!

 

[1611AV]

Oh gotcha.

And keep in mind that I'm a 3D animator, not a scientist. But I did stay at a holiday-......I mean. I do hang out on science forums. Ask Science Questions And Get Answers is a good one

The way I understand it, dark energy is not matter. Matter is just a type of energy and dark energy (they think) is a different type. They don't know what it is.

Dark MATTER is different. They know how it behaves, they know it's attributes, how much of it is in the universe, etc, but they have yet to find it. It's hard to find something that doesn't react electromagnetically with itself or with other matter. That means it doesn't run into things and it doesn't emit or reflect light. It passes through everything. How do you find something like that? Well give me 8 years to earn my phd and I'll let you know haha

But no, dark energy is not matter. They don't know what it is. It's just an easier term to remember than "whatever-is-causing-all-distances-to-increase." They don't know what it is yet but the game is afoot!

Man, you did stay at a holiday... well you know. Not bad for a 3D animator.

 

[DontTreadOnMike]

Man, you did stay at a holiday... well you know. Not bad for a 3D animator.

Are you saying I did well or poorly? haha

Like I said, I can only go by what I think the real scientists are saying. I can't verify this stuff myself.