What is Gravity, Anyway?


Welcome to my second “Science Answers” post! About a month ago, I sent out a post requesting science questions from all of you; you can find it here. This post addresses the second of the questions I was asked. If you have a question, you can ask it in the comments here or on that post, or ask it in an email. Or find me on Facebook!

Q: What is gravity? (asked by Simon)

Wow…great question. This is a question the greatest scientific minds have asked and tried to answer for centuries. It’s a question not even Stephen Hawking, the scientific genius of the century, has fully answered.

There are a few parts to the gravity question, and they have each been addressed one by one over time:

  • How does gravity work?
  • What is gravity?
  • Why does gravity work?

Isaac Newton stood on the shoulders of the giants before him—Aristotle, Ptolemy, Copernicus, and Kepler—and figured out how gravity works. But he was at a loss to explain what exactly this mysterious force was.

Einstein built on Newton’s work and came up with a theory for what gravity is—that is, distortions in space-time.

spacetime

We have yet to understand why gravity works. Why is space-time warped? Why do objects distort it as if it were the material of a trampoline? What exactly is the nature of space?

But, lucky for me, the question above specifically asks what gravity is. And that, I can explain.

The best way to do that is to turn one of gravity’s oldest tricks, one that has perplexed scientists and philosophers for thousands of years: What makes the planets move?

Gravity in a Geocentric Universe

People started wondering how the world goes round thousands of years ago. Our story begins in the year 150 CE, five centuries after Aristotle’s time, when the Greeks were firmly entrenched in the geocentric universe.

aristotle

The idea behind the geocentric universe was that the Earth was at the center of the universe, and everything revolved around us. It’s kind of like when toddlers believe they’re the most important beings in the universe—humanity had just begun to walk, and couldn’t imagine anything different.

Now, with our modern understanding, that sounds just a little bit crazy. I mean…what the geocentric universe basically suggests is that Earth’s gravity is crazy strong.

Seriously. For the universe to revolve around the Earth, Earth’s gravitational field would have to be strong enough to reach across an estimated 46.6 billion light years and hold somewhere around 3 x 1055 grams of matter.

That’s 30,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 grams of matter. That’s a lot of matter.

But…wait a second. Isn’t the gravitational force of any one object infinite? According to the law of universal mutual gravitation, every object in the universe tugs a little bit on every other object. Gravity extends infinitely throughout the universe.

That’s true. But gravity also follows the inverse square law, which states that the greater the distance from the gravity source, the weaker the gravitational pull.

inverse-square-law-sphere

Meaning, even though Earth’s gravity technically influences every object in the universe, by the time it touches the nearest interstellar object, it’s way too weak to hold an object even as small as the moon in orbit.

So the geocentric universe doesn’t work at all.

How Gravity Really Works

I’m not going to spend a whole lot of time on our basic understanding of gravity. I’ve done that in previous posts, and I want to devote the rest of this post to Einstein’s theory of space time. But before we move on, it’s necessary for us all to be on the same page.

First of all, planetary orbits are not circular. Johannes Kepler discovered that the planets, including the Earth, orbit the sun on ellipses, with the sun at one focus.

ellipse_stringKepler1

Planets also speed up through the part of their orbit that brings them closest to the sun, known as perihelion. (Similarly, the opposite end of their orbit is called aphelion.) And a planet’s distance from its star determines how fast it moves in general.

Now, Kepler may have figured all that out, but he never could figure out why a planet’s speed is determined by its orbital distance. Isaac Newton clarified that with the inverse square law, which I mentioned earlier.

Basically, gravity gets stronger the closer you get to an object, and any object in orbit has to move faster to avoid a collision.

But…at the risk of repeating myself…why?

Because all objects caught within a gravitational field are essentially in free fall. Do you know Newton’s first law of motion? Any object will stay moving exactly how it’s moving, or stay still, unless acted upon by a force.

Hey, we’ve got a force. That’s gravity.

So, left alone to gravity’s devices, Earth would hurtle toward the sun and plunge to its inevitable death. Except it doesn’t, because it’s in orbit. It’s moving fast enough that by the time it falls all the way to the sun, it has moved PAST the sun.

Okay, that’s great. But it still doesn’t answer the million-dollar question: What the heck is gravity?

This is the fun part.

Albert Einstein: What Gravity Is

spacetime

Gravity is distortions in space-time.

What?

Yeah, I know. That’s what I thought, too. What the heck even is space-time?

Well, to put it simply, it’s a four-dimensional universe. There are three dimensions of space, and one of time.

Basically…see that trampoline grid up there? That’s space-time. It’s like Earth is sitting on a trampoline, making its material sink down a bit. But it’s not just distorting space (all three dimensions of it, too). It’s distorting time.

Yeah, I hear ya. Crazy, huh?

Think of it this way:

We live in a world full of stuff. You don’t spend a minute of your life not touching something. The air is full of atoms and molecules. When you breathe, you take in oxygen—more stuff. You can’t escape stuff.

Now, have you ever tried putting two things in the same place?

I don’t mean putting two pencils in the same pencil holder, or two pairs of pants in the same drawer. I mean…have you ever tried getting two pencils to occupy the exact same space? As if there was only one of them?

You can’t do it. It doesn’t work.

Mass will resist occupying the same space as other mass. Even the molecules in the air can’t occupy the same space as your pencils. So it doesn’t seem like too great a leap to assume that mass can’t exist in the same place as a vacuum, right?

By a vacuum, I don’t mean your household vacuum cleaner. I mean the vacuum of space. I mean a place where there is no mass—a place that sucks matter in because it needs some to fill it up. (That’s how vacuum cleaners got their name, by the way.)

A space can’t be a vacuum and have mass at the same time because it contradicts the very definition of being a vacuum, or of having mass.

So…what does this have to do with space-time?

Well, imagine that you have a vacuum. That’s space. And you put matter in it. That’s a star, or a planet, or a moon, or maybe something as massive as a galaxy.

One fundamental rule of vacuums is that they suck matter in. They essentially try to contract. So isn’t it reasonable to suggest that this vacuum contracts around the matter we put inside it?

Now remember, our vacuum in this case is space. So here’s what happens when space sucks itself in around a mass, such as the Earth.

3d space time

If I’m interpreting this visual correctly, space quite literally takes up less space closer to the mass. It’s been squeezed in, compressed.

So isn’t it reasonable to conclude that if an object crosses a certain distance in a certain time farther away from the mass, it will cross that same distance in less time closer to the mass, because space itself has shrunk down?

The same would go for time, but perhaps to a lesser extent. After all, we clearly see the effects of space-time on an object like Mercury that whips around the sun at higher speeds than the Earth, but time doesn’t seem to run too much differently.

Unless we simply perceive time as passing by at the same pace, when really it has dilated.

Now, just to be clear, this is all just my own attempt to explain why mass could possibly distort space-time, and why that would make objects travel at different speeds.

Maybe I’m wrong. I’m probably wrong. But I hope I’ve at least offered some food for thought. I’ll definitely come back to this another time, once I’ve managed to wrap my head around gravity a bit more.

Questions? Or just want to talk?

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