When we look at a beautiful panorama such as this one, photographed at Ocean Beach in San Francisco, we probably don't immediately start thinking about science. The image is even less likely to inspire a thoughtful contemplation of astronomy. And yet, it is precisely the laws of physics that we have to thank for the existence of this natural beauty we take for granted on Earth. Every aspect of this picture from the sea, to the rain clouds, and even the mountains only exist because of gravity. In fact, the entire Universe as we know it owes its existence to gravity, a force that was created and perfected a few milliseconds after the Universe itself was born. It seems intuitive to us that gravity is the force that holds an entire planet together. What may be less intuitive is that of the four forces: Strong Nuclear Force, Weak Nuclear Force, Electromagnetism, and gravity, gravity is by far the weakest. The type of electromagnetic forces we're going to talk about in the next section Hydrogen are responsible for giving the mountain we see its definition. Without the electromagnetic force holding atoms together in shapes, gravity would flatten the surface of the Earth. Making the distinction between these forces is important because gravity is only the weakest force on small scales. When we talk about something as big as a planet, a star, or a galaxy, gravity suddenly becomes the strongest most influential force in the entire Universe!
Now, let's look at gravity on different scales. We'll begin here on Earth with the picture above, noting that gravity keeps the sea wrapped around the surface of the Earth, keeps the rain falling, and stops the clouds from simply flying off the Earth and out into space. On Earth, gravity is not strong enough to overcome the other forces and squeeze the Earth into a perfect sphere. In space, however, gravity dominates.
Now, let's look at gravity on different scales. We'll begin here on Earth with the picture above, noting that gravity keeps the sea wrapped around the surface of the Earth, keeps the rain falling, and stops the clouds from simply flying off the Earth and out into space. On Earth, gravity is not strong enough to overcome the other forces and squeeze the Earth into a perfect sphere. In space, however, gravity dominates.
Gravity in a Solar System: Orbits and the Distance Inverse Square Law.
Gravity also keeps Earth orbiting around the Sun. We'll talk more about orbits later in this section, but for now, you should remember what we talked about briefly in the section on accretion, namely, that the bigger and more dense an object, the more gravity is has. The Earth has enough gravity to keep our bodies, the atmosphere, and even our moon close to it. The Earth even has enough gravity to pull the Sun toward it, but the effect is immeasurably small.
The Sun, which if you remember is a million times as big as the Earth, has enough gravity to pull on all the planets and asteroids and comets in the entire solar system! We've already talked about how the Earth generates enough gravity to hold itself together. A good question to ask here is: "if the Sun is so massive and has so much gravity, why doesn't it suck us right off the Earth, or the Earth right out of its orbit?" The answer to this question is Distance. It is true that the Sun has much more gravity than the Earth, but we are closer to the Earth. Gravity follows what scientists call the Distance Inverse Square Law. Don't let the name scare you. It just means that if you move twice as far away from object, gravity will become four times weaker. If you're having trouble here, put very plainly, the strength of gravity diminishes extremely quickly when you move away from an object, and, conversely, the strength of gravity increases extremely quickly as you get closer to an object. This same law explains why the Earth can hold the moon against the pull of the Sun's gravity. The Earth and the Moon are closer together, and so their mutual gravity is stronger than the pull from the Sun.
As a result of the Distance Inverse Square Law, the Earth has a sphere around it inside of which the Earth's gravity is stronger than the gravity of the Sun. Inside that sphere objects such as satellites, the moon, and even asteroids can reach a stable orbit around the Earth. Every cosmic body has such a sphere around it. Our astronauts enjoyed less gravity when they walked on the surface of the moon, but they weren't ripped off the moon by Earth's gravity and pulled into free-fall toward Earth. We're also not ripped off of the Earth and pulled into the Sun. Gravity is a force that is intimately tied to the distance between objects. There is actually a point in space between the Earth and the moon where their respective gravitational pulls reach a stalemate. If you move any closer to the Earth, you will fall toward the Earth, and any closer to the moon, you will fall toward the moon. That exact point in space is called a Lagrangian Point, and its the point between two bodies where the gravity of each body effectively cancels the other out. If you could stay exactly at that point in space, which is intuitively closer to the moon because the moon is smaller and has less gravity, you could be pulled equally by both bodies and never fall toward either. This is a simplified explanation of Lagrangian Points, and there are actually 5 of them around the Earth. For more information, you can visit the wiki.
Distance and mass aren't the only factors that define an objects orbit, however. You also have to consider an objects velocity (speed). In order to illustrate how an objects speed plays a part in how it will orbit another object, lets imagine a hypothetical rocket-ship. This particular ship is unlike the ones you're familiar with. For this rocket ship, we're going to put it parallel to the ground! In other words, take a rocket ship, and lay it on its side just a few meters above Earth's surface. If we launched it, we'd expect it to go a distance and then arc downward and crash. If we launched it faster, we would expect it to go a little bit further before it crashed. Lets keep on launching faster and faster and faster rockets, watching them go further each time, until one rocket reaches a high enough speed wherein its arc toward the Earth actually matches the curve of Earth's surface. We've reached orbital speed. The rocket will fall downward toward the Earth, but the surface of the Earth is curving down away from the rocket at the same time! The object is still falling toward the Earth, but it is moving so fast that it will never crash into the surface.
Now lets put that same speedy rocket on top of Mount Everest. Remember that because of the Distance Inverse Square Law, the rocket on top of the mountain feels less gravity than the rocket closer to the surface. It won't be pulled down quite as fast. Thus, the speed needed to orbit the Earth is reduced because the rocket's downward arc is smaller.
If we put that rocket outside of Earth's atmosphere, it can go even slower and still orbit Earth, always falling but never crashing. Knowing all of this, you can even make some educated guesses about the speeds of all the planets as they orbit the Sun. In general, the further away a planet is, the slower it will orbit its star.
The Sun, which if you remember is a million times as big as the Earth, has enough gravity to pull on all the planets and asteroids and comets in the entire solar system! We've already talked about how the Earth generates enough gravity to hold itself together. A good question to ask here is: "if the Sun is so massive and has so much gravity, why doesn't it suck us right off the Earth, or the Earth right out of its orbit?" The answer to this question is Distance. It is true that the Sun has much more gravity than the Earth, but we are closer to the Earth. Gravity follows what scientists call the Distance Inverse Square Law. Don't let the name scare you. It just means that if you move twice as far away from object, gravity will become four times weaker. If you're having trouble here, put very plainly, the strength of gravity diminishes extremely quickly when you move away from an object, and, conversely, the strength of gravity increases extremely quickly as you get closer to an object. This same law explains why the Earth can hold the moon against the pull of the Sun's gravity. The Earth and the Moon are closer together, and so their mutual gravity is stronger than the pull from the Sun.
As a result of the Distance Inverse Square Law, the Earth has a sphere around it inside of which the Earth's gravity is stronger than the gravity of the Sun. Inside that sphere objects such as satellites, the moon, and even asteroids can reach a stable orbit around the Earth. Every cosmic body has such a sphere around it. Our astronauts enjoyed less gravity when they walked on the surface of the moon, but they weren't ripped off the moon by Earth's gravity and pulled into free-fall toward Earth. We're also not ripped off of the Earth and pulled into the Sun. Gravity is a force that is intimately tied to the distance between objects. There is actually a point in space between the Earth and the moon where their respective gravitational pulls reach a stalemate. If you move any closer to the Earth, you will fall toward the Earth, and any closer to the moon, you will fall toward the moon. That exact point in space is called a Lagrangian Point, and its the point between two bodies where the gravity of each body effectively cancels the other out. If you could stay exactly at that point in space, which is intuitively closer to the moon because the moon is smaller and has less gravity, you could be pulled equally by both bodies and never fall toward either. This is a simplified explanation of Lagrangian Points, and there are actually 5 of them around the Earth. For more information, you can visit the wiki.
Distance and mass aren't the only factors that define an objects orbit, however. You also have to consider an objects velocity (speed). In order to illustrate how an objects speed plays a part in how it will orbit another object, lets imagine a hypothetical rocket-ship. This particular ship is unlike the ones you're familiar with. For this rocket ship, we're going to put it parallel to the ground! In other words, take a rocket ship, and lay it on its side just a few meters above Earth's surface. If we launched it, we'd expect it to go a distance and then arc downward and crash. If we launched it faster, we would expect it to go a little bit further before it crashed. Lets keep on launching faster and faster and faster rockets, watching them go further each time, until one rocket reaches a high enough speed wherein its arc toward the Earth actually matches the curve of Earth's surface. We've reached orbital speed. The rocket will fall downward toward the Earth, but the surface of the Earth is curving down away from the rocket at the same time! The object is still falling toward the Earth, but it is moving so fast that it will never crash into the surface.
Now lets put that same speedy rocket on top of Mount Everest. Remember that because of the Distance Inverse Square Law, the rocket on top of the mountain feels less gravity than the rocket closer to the surface. It won't be pulled down quite as fast. Thus, the speed needed to orbit the Earth is reduced because the rocket's downward arc is smaller.
If we put that rocket outside of Earth's atmosphere, it can go even slower and still orbit Earth, always falling but never crashing. Knowing all of this, you can even make some educated guesses about the speeds of all the planets as they orbit the Sun. In general, the further away a planet is, the slower it will orbit its star.
Gravity in a Galaxy.
This picture is an artist rendition of our very own galaxy, the Milky Way. The picture should look a little familiar because it looks very similar to the pictures of accretion disks. Our solar system in its entirety does indeed orbit the center of the Milky Way, in the same way that Earth orbits the Sun, and the moon orbits Earth. We know now that this is possible because of the Distance Inverse Square Law. Our solar system is at the right distance and going the right speed to orbit the center of our galaxy.
You may be wondering what could possibly create enough gravity to cause something as big as the Sun to orbit around it. It's a good question. The answer is that at the center of our galaxy (in this photo located right in the center of the spiral disk where the light shines brightest) is a Super-massive Black Hole that weighs as much as 4.1 billion Suns. The object has so much gravity that, close to the center, stars are pulled around it and orbit at millions of miles per hour! Imagine an entire star whipping through space at that speed! As a result of the intense gravity of the Super-massive Black Hole at the center of our galaxy stars are pulled closer together, and thus the center of the milky way bulges and glows brighter than the rest of the galaxy. In this way, galaxies have a lot in common with a solar system, in that smaller objects orbit a larger object at the center.
Now lets take our exploration of gravity one step further and see how two or more galaxies interact.
You may be wondering what could possibly create enough gravity to cause something as big as the Sun to orbit around it. It's a good question. The answer is that at the center of our galaxy (in this photo located right in the center of the spiral disk where the light shines brightest) is a Super-massive Black Hole that weighs as much as 4.1 billion Suns. The object has so much gravity that, close to the center, stars are pulled around it and orbit at millions of miles per hour! Imagine an entire star whipping through space at that speed! As a result of the intense gravity of the Super-massive Black Hole at the center of our galaxy stars are pulled closer together, and thus the center of the milky way bulges and glows brighter than the rest of the galaxy. In this way, galaxies have a lot in common with a solar system, in that smaller objects orbit a larger object at the center.
Now lets take our exploration of gravity one step further and see how two or more galaxies interact.
Gravity on Multiple Galaxies: The Tidal Force.
In this picture we see the gravity of two entire galaxies causing them to attempt to orbit around each other. However, it is at this level that things begin to get a bit unfamiliar to us, because we are used to dealing with orbits on a relatively flat plane e.g the planets orbit along the Sun's equatorial plane (with a few degrees variance). Even our entire galaxy, if viewed from the side, takes the shape of a relatively thin disk. We might also think of the rings of Saturn, which are 170,000 miles across, but only about 30 feet thick in many places. Once we look at a scales bigger than one galaxy, things begin to get much more complicated. In fact, we will soon see that when you look at the structure of the Universe on the scale of many galaxies, you begin to see shapes that are no longer even remotely circular. It's important to note that an entire galaxy acts like the Sun or Earth in that the galaxy has its own weight - the sum of its parts - and thus its own gravitation pull. When two galaxies get close together, they can tug on each other, orbit each other, and even collide. In the picture above, two galaxies are pulling on each other and distorting each other's shapes. The stretching we see between these two galaxies is intuitive to us. Naturally these two massive objects pull on each other. However, many of us would not link the shapes of the galaxies above with, say, the tides on Earth. However, the Tidal Force, which is just a mechanical process that it built in to the laws of gravity, permeates every orbiting body in the Universe, and it is acting on these two galaxies.
The Tidal Force.
Think about the immense gravity each of those galaxies exert on each other. Now, note that the closer two objects are to each other, the stronger their gravitational attraction (Distance Inverse Square Law). Therefore, the portions of each galaxy closest to its neighboring galaxy feel a much stronger attraction than the backsides of the galaxies. This is called the Tidal Force, and this is why the outsides of both galaxies are not getting stretched as much as the insides. This is also the reason why the tide rises on the side of the Earth where the Moon is. The moon pulls on the surface of the Earth directly beneath it, essentially stretching the Earth (and the land, water, and air on it). As the moon orbits, the Earth beneath it bloats outward toward it. We can't feel this effect directly, but we can certainly observe it rather easily if we're standing on a beach when the tide comes in. The moon doesn't just bulge out the Earth beneath it, it squeezes the entire Earth into an oblong shape, raising the tides on two sides of the Earth at any given time. The Tidal Force is responsible for many nuances in our solar system. For instance, Jupiter has a small moon named Io (eye-oh) which has the most active volcanoes in the entire solar system (around 400 Volcanoes going off at the same time all the time). On Earth, volcanoes form when heated material rise up from the hot core of the Earth and erupt onto the surface. The heat of the Earth's core is generated by the Earth's rotation, and is also leftover from the formation of the Earth. On Io the volcanoes are caused by the tidal force. How? Jupiter's immense gravity, coupled with the Distance Inverse Square Law, tells us that the side of the moon Io facing Jupiter feels a stronger gravitational pull than the side that faces away from Jupiter. With Jupiter's gravity pulling with different strengths on different sides of the moon, and with it's naturally eccentric orbit, Io is stretched and kneaded by Jupiter's gravity. This stretching and crunching causes enough friction inside the moon to melt its interior, and results in Io's significant volcanic activity.
The Dark Side of the Moon: Tidal Lock
When we look up at the moon, we see the same side of it as we did the night before, and the night before that. In fact, we see the same side of the moon the earliest humans saw. Quite astonishingly, the Earth has looked up at the same side of the moon for over 4 billion years. It is easier for us to assume that the moon just doesn't spin, but that is not the case. Let's try another little experiment. You can use any two circular objects, but I prefer using a bottle cap and the bottle I got it from. Let's pretend the bottom of the bottle represents the Earth. Using a marker or pen, draw a little line on the top of one side of the bottle cap and rest it on the same surface as the bottle. The bottle cap now represents the Moon. The idea here is to try and slide the bottle cap around the base of the bottle in such a way that the line you drew on it always faces the bottle. You will quickly notice that in order to keep the line you drew on the bottle cap lined up with bottle, you have to spin the bottle cap as it moves around the base of the bottle. Thus, the moon must have a rotation; it must spin on its axis. But why does it spin in such a way that one side of it always faces the Earth? Is it some kind of cosmic coincidence, or are other forces at work?
Interestingly enough, the same force that causes the tides on Earth also caused the Moon to reach a synchronous rotation with the Earth - the tidal force. We say that the moon is Tidally Locked to the Earth. The process may be a little difficult to visualize, so lets take a step back and learn some interesting facts about the early Earth and Moon. For right now, let's not think at all about their orbits, and lets only concentrate on their rotations - their spins which were set in motion during their formation by the process of accretion. Both objects spin, and always have, but as a result of the tidal forces, their spins have been gradually slowing down. Essentially, the Moon has slowed down the spin of the Earth by dragging on it with the tides, and the Earth has done the same to the Moon. 4 billion years ago, a "day" on Earth was a lot shorter than it is now, because the Earth was spinning faster. There was even a time early in the formation of the Moon where it spun faster and thus showed its "dark side" to the Earth. However, because the tidal forces create a drag on rotation speed, the spin of the moon slowed down to the point where it spins in such a way where only one side of it ever faces the Earth. Once that happened, it became Tidally Locked to the Earth. As a result of the tidal force, given enough time, two orbiting bodies will always try to reach this synchronous rotation with each other, the smaller of the two bodies slowing down more quickly and thus showing only one side to the larger body. Io is another example of a moon that is Tidally Locked to it's planet, Jupiter.
There is one more thing to consider when we talk about the Tidal Force. The Tidal Force is also causing our Moon to get pushed further and further away from Earth. Conversely, the Tidal Force between Mars and its small moon Phoebe is causing that moon to get closer to Mars. The eventual fate of our Moon is to get slung out into space; this is already happening at a rate of about an inch per year. Phoebe on the other hand will eventually crash into Mars. What is the difference between these two moons that causes them to have such different destinies? Well, our Moon orbits the Earth in the same direction that the Earth spins. Phoebe orbits Mars in a different direction than Mars spins. In essence, the rotation of the Earth is whipping the Moon forward, causing the Moon to speed up slightly and thus reach a higher orbit. Mars' rotation is creating a drag on Phoebe, and slowing the Moon down, causing it to get closer and closer to the planet, until one day a long time from now, it will hit the planet's atmosphere and then crash into it's surface.
Interestingly enough, the same force that causes the tides on Earth also caused the Moon to reach a synchronous rotation with the Earth - the tidal force. We say that the moon is Tidally Locked to the Earth. The process may be a little difficult to visualize, so lets take a step back and learn some interesting facts about the early Earth and Moon. For right now, let's not think at all about their orbits, and lets only concentrate on their rotations - their spins which were set in motion during their formation by the process of accretion. Both objects spin, and always have, but as a result of the tidal forces, their spins have been gradually slowing down. Essentially, the Moon has slowed down the spin of the Earth by dragging on it with the tides, and the Earth has done the same to the Moon. 4 billion years ago, a "day" on Earth was a lot shorter than it is now, because the Earth was spinning faster. There was even a time early in the formation of the Moon where it spun faster and thus showed its "dark side" to the Earth. However, because the tidal forces create a drag on rotation speed, the spin of the moon slowed down to the point where it spins in such a way where only one side of it ever faces the Earth. Once that happened, it became Tidally Locked to the Earth. As a result of the tidal force, given enough time, two orbiting bodies will always try to reach this synchronous rotation with each other, the smaller of the two bodies slowing down more quickly and thus showing only one side to the larger body. Io is another example of a moon that is Tidally Locked to it's planet, Jupiter.
There is one more thing to consider when we talk about the Tidal Force. The Tidal Force is also causing our Moon to get pushed further and further away from Earth. Conversely, the Tidal Force between Mars and its small moon Phoebe is causing that moon to get closer to Mars. The eventual fate of our Moon is to get slung out into space; this is already happening at a rate of about an inch per year. Phoebe on the other hand will eventually crash into Mars. What is the difference between these two moons that causes them to have such different destinies? Well, our Moon orbits the Earth in the same direction that the Earth spins. Phoebe orbits Mars in a different direction than Mars spins. In essence, the rotation of the Earth is whipping the Moon forward, causing the Moon to speed up slightly and thus reach a higher orbit. Mars' rotation is creating a drag on Phoebe, and slowing the Moon down, causing it to get closer and closer to the planet, until one day a long time from now, it will hit the planet's atmosphere and then crash into it's surface.
Gravity in the Universe.
This gorgeous picture is a representation of the largest known structure in the Universe, namely, the structure of the Universe! In the picture above of our Milky Way galaxy, each individual point of light represents a star or cluster of stars. In this picture, every little point of light represents clusters of entire galaxies! The first thing you should notice is that we're no longer dealing with circles, spirals, ellipses, or anything similar to what we see on smaller scales. Rather, at this scale, the Universe more closely resembles threads in a jumbled up fabric. The same force that causes the Earth to orbit the Sun, the Sun to orbit the center of the Galaxy, and Galaxies to orbit each other has an even bigger design for the Universe. However, even if we have a good grasp on how accretion words, and how stars orbit in a galaxy, the structure we see in this picture is much less intuitive. How did the Universe end up looking like a stretched out fabric if everything in it seems to want to form somewhat circular orbits? The answer to this question is well beyond the scope of this website, but if you want to get a good head start, you can start with Dark Matter and Dark Energy. We will talk about both of these things at a later time, but what we know about them at present does little to form a coherent picture. When we talk about Dark Matter and Dark Energy, we're reaching the very limits of modern science.
What is Gravity?
No Straight Lines in Space?
One of the difficulties people have with understanding how gravity works in space is that there are really no straight lines because everything is in motion. Lets say we sat on the surface of Mars with a huge cannon and fired a cosmic cannonball straight at Earth. Well, by the time the ball covers the distance, the Earth will have moved out of the way. Our next try, we do some calculations, and we shoot the cannonball to where we figure earth is going to be at the time it takes the cannonball to reach it. In order to do those calculations correctly, we need to take into account the fact that the cannonball will bend toward the Sun, and we even need to take into account the fact that Earth's own gravity will disrupt the cannonball's trajectory. So in order to hit the Earth with a cannonball shot from Mars, we essentially need to send it on a curbed path that will intersect the Earth's obrital path at the precise time that the Earth is there to collide with it. For Clarity, the picture above represents a possible solution to a large asteroid that, much like the cannonball, is on a curbed path toward impacting the Earth. If we shoot a rocket - again along a curbed path - and hit the asteroid, we may be able to change its trajectory enough so that it doesn't hit us. Of course, in reality, we could also accidentally create a rain of smaller but equally deadly debris similar to the shrapnel of a grenade.