There are two levels I can use to explain this. First, I’ll use a more
Newtonian model of the universe, as it is the simplest way of explaining
it.
In a Newtonian universe the answer is pretty simple. To
start with, anything that wasn’t orbiting around the super-massive black
hole located in the center of the galaxy would simply fall in a
straight line to the black hole. The reason why is the exact same reason
why if you drop a pencil it will fall in a straight line to the Earth,
which is likely the most dominant gravitational force nearby.
By contrast anything that is orbiting the black hole has what is called centrifugal force. Centrifugal force is why if you hold a weight and swing your arm in a circle the weight feels like it is pulling on your hand even when it is upside down. There is of course a very long and perfectly well reasoned explanation for why centrifugal force appears to disregard gravity, but that’s a post for another time.
To go back to galaxy, all of the objects that are orbiting the galaxy have centrifugal force. That force is constantly pushing on them in the opposite direction as the gravity of the black hole, thus allowing a stable or semi-stable orbit.
Now Isaac Newton was a brilliant man and all, but he still spent most of his life in the seventeenth century. Since we are all in the twenty-first century, things have gotten a good bit more complex. This will get kind of technical, so read at your own risk.
Albert Einstein was the first physicist to suggest that space-time could be non-uniform in density. He basically took Minkowski’s idea of space-time and added the (very important) concept of non-uniformity. What that means is that if you have on cubic meter of empty space (a complete vacuum), it is possible for half of it to be denser than the other half. This probably doesn’t make much sense. It isn’t that the other half has more “space” in it, because it is still half a cubic meter. It doesn’t have more “nothing” either, because a vacuum is a vacuum, that’s about as empty as it gets (though how empty a vacuum is highly debatable. But that is a post for another time). The best way to explain it is with the analogy of a graph. It would be like if you taped to half pages of graph paper together, where one half has four squares to an inch and the other half has five squares to an inch. You still have one sheet of graph paper, but half of it is denser.
If that didn’t make much sense to you, it only gets worse. Because using Einstein’s understanding of the universe, the objects in a galaxy don’t orbit. For that matter, the Earth also does not orbit. The reason why is simple: An orbit is a circle. According to Einstein, gravity only causes objects to travel in straight lines. Never circles or arches. This is probably contrary to everything you know about how anything from a solar system to a galaxy works. But fear not, for an explanation is ahead.
The concept of non-uniform density in space is the reason why something can look like it is traveling in a circle (like the Earth orbiting the sun) but really be traveling in a straight line.
Gravity, according to Einstein, doesn’t really pull on objects in the same way that Isaac Newton believed. Instead, the gravitational field condenses space itself. The stronger the gravity, the denser the space. Newton discovered this equation to calculate the gravitation force between two objects:
F = (G)(m1)(m2)/r^2
Where F is the gravitational force, m is the mass of the two objects in question, G is Newton’s constant, and r is the distance between the two objects. This explains why gravity has a much lesser effect over larger distances. Due to this, the closer you get to an object, the more its gravity condenses space.
Before I tie all of these concepts together I have one more thing to bring up: circles. Everybody loves them. One very important thing about circles is that the circumference of the innermost edge of the circle will always be less than the outermost edge. Now lets imagine we had a sharpie with a tip as big around as the diameter of the Earth. Now that’s a sharpie. If you use that sharpie to draw a circle, the innermost edge of that circle will be WAY smaller than the outermost edge. Therefore when the planet Earth is orbiting the sun, the innermost part of it is traveling less distance. Except for the fact that the innermost part is considerably closer to the sun, and therefore that space is denser. So it looks like it is traveling less distance, but because of how the sun is deforming and condensing space, the innermost part of the Earth and the outermost part of the Earth are traveling the same equivalent distance. Since both sides are traveling the same distance, the Earth isn’t moving in a circle at all, but instead a straight line. The illusion of circular movement is caused by our own inability to properly perceive space, but the math is pretty clear, and Relativity is fairly bulletproof.
References:
Leonard Susskind
Albert Einstein
Isaac Newton
Picture courtesy of criticalmass.uk.com
In a Newtonian universe the answer is pretty simple. To
start with, anything that wasn’t orbiting around the super-massive black
hole located in the center of the galaxy would simply fall in a
straight line to the black hole. The reason why is the exact same reason
why if you drop a pencil it will fall in a straight line to the Earth,
which is likely the most dominant gravitational force nearby. By contrast anything that is orbiting the black hole has what is called centrifugal force. Centrifugal force is why if you hold a weight and swing your arm in a circle the weight feels like it is pulling on your hand even when it is upside down. There is of course a very long and perfectly well reasoned explanation for why centrifugal force appears to disregard gravity, but that’s a post for another time.
To go back to galaxy, all of the objects that are orbiting the galaxy have centrifugal force. That force is constantly pushing on them in the opposite direction as the gravity of the black hole, thus allowing a stable or semi-stable orbit.
Now Isaac Newton was a brilliant man and all, but he still spent most of his life in the seventeenth century. Since we are all in the twenty-first century, things have gotten a good bit more complex. This will get kind of technical, so read at your own risk.
Albert Einstein was the first physicist to suggest that space-time could be non-uniform in density. He basically took Minkowski’s idea of space-time and added the (very important) concept of non-uniformity. What that means is that if you have on cubic meter of empty space (a complete vacuum), it is possible for half of it to be denser than the other half. This probably doesn’t make much sense. It isn’t that the other half has more “space” in it, because it is still half a cubic meter. It doesn’t have more “nothing” either, because a vacuum is a vacuum, that’s about as empty as it gets (though how empty a vacuum is highly debatable. But that is a post for another time). The best way to explain it is with the analogy of a graph. It would be like if you taped to half pages of graph paper together, where one half has four squares to an inch and the other half has five squares to an inch. You still have one sheet of graph paper, but half of it is denser.
If that didn’t make much sense to you, it only gets worse. Because using Einstein’s understanding of the universe, the objects in a galaxy don’t orbit. For that matter, the Earth also does not orbit. The reason why is simple: An orbit is a circle. According to Einstein, gravity only causes objects to travel in straight lines. Never circles or arches. This is probably contrary to everything you know about how anything from a solar system to a galaxy works. But fear not, for an explanation is ahead.
The concept of non-uniform density in space is the reason why something can look like it is traveling in a circle (like the Earth orbiting the sun) but really be traveling in a straight line.
Gravity, according to Einstein, doesn’t really pull on objects in the same way that Isaac Newton believed. Instead, the gravitational field condenses space itself. The stronger the gravity, the denser the space. Newton discovered this equation to calculate the gravitation force between two objects:
F = (G)(m1)(m2)/r^2
Where F is the gravitational force, m is the mass of the two objects in question, G is Newton’s constant, and r is the distance between the two objects. This explains why gravity has a much lesser effect over larger distances. Due to this, the closer you get to an object, the more its gravity condenses space.
Before I tie all of these concepts together I have one more thing to bring up: circles. Everybody loves them. One very important thing about circles is that the circumference of the innermost edge of the circle will always be less than the outermost edge. Now lets imagine we had a sharpie with a tip as big around as the diameter of the Earth. Now that’s a sharpie. If you use that sharpie to draw a circle, the innermost edge of that circle will be WAY smaller than the outermost edge. Therefore when the planet Earth is orbiting the sun, the innermost part of it is traveling less distance. Except for the fact that the innermost part is considerably closer to the sun, and therefore that space is denser. So it looks like it is traveling less distance, but because of how the sun is deforming and condensing space, the innermost part of the Earth and the outermost part of the Earth are traveling the same equivalent distance. Since both sides are traveling the same distance, the Earth isn’t moving in a circle at all, but instead a straight line. The illusion of circular movement is caused by our own inability to properly perceive space, but the math is pretty clear, and Relativity is fairly bulletproof.
References:
Leonard Susskind
Albert Einstein
Isaac Newton
Picture courtesy of criticalmass.uk.com
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