Batman
Not all those who wander are lost...
I made this post in response to a debate I saw in the shoutbox earlier today. The debate centered on the nature of gravity and ‘how we know what gravity is’. I thought I’d share a very brief and straightforward explanation of what we currently know about gravity. To start off, I’ll give you my credentials: I have a M.S. in Applied Particle Physics from Cal Tech and am currently pursuing my PhD in Quantum Cosmology. Again, I’ll make this accessible to everyone; no technical stuff here.
First of all, what is gravity? Isaac Newton is famous for formulating his three laws of gravitation but is often mistakenly credited with unlocking the secrets of gravity. What Newton did was unlock the secrets of the “effects” of gravity. Gravity, no matter what it is, has a measurable effect on the universe. Newton figured out how gravity behaves in the universe (by uncovering its mathematical secrets); he did not figure out what gravity is. An understanding of what gravity is; an understanding of what is causing these attractive force effects that we see in the universe, wouldn’t be understood until much later; all thanks to the work Albert Einstein. Albert Einstein’s theory (now a law) of General Relativity is perhaps the greatest scientific breakthrough in human history. Einstein created a framework that finally explained what gravity is. “General relativity” is a geometric framework that explains gravity to a degree that Newton could have only dreamed, and has been with us since 1915. Einstein theorized that gravity was in fact not some mystical force that “pulled” things in attraction (in a proportion to the relative masses of the bodies involved). Gravity was in fact a force, but one that was generated by a unique phenomenon. Einstein discovered that gravity is just the byproduct (the cause of an effect) generated by the warping of space-time. We all know what space is: it’s the 3-dimensional plane in which all things exist. And we all know what time is: it’s the dimension in which the change of the states of objects occurs. Einstein figured out that in fact, the 3 dimensions of space and the one of time are actually part of the same fabric called “space-time”. Everything in the universe can be determined by 4 coordinates; length, width, depth, and time.
So, this fabric of space-time becomes warped (stretched and molded) in the presence of mass (see the pic below). Anything with mass warps the 3 dimensions of space it occupies. This warping of the space is what we call gravity. Objects with mass get “pulled together” because the space between them has been bent in a way that brings them closer together. So here on Earth, it’s incorrect to think that the Earth’s gravity is pulling us to the ground. It’s more correct to say that the Earth’s mass has warped the space around it so much, that we puny humans are being “pushed toward it”. Same goes for everything else in the universe. The Earth is not being pulled toward the sun; the sun’s gravity has warped the space around Earth so much that our planet is being pushed toward it. The fact that we don’t just plunge into the sun is because our planet is moving through space really really fast. The angular momentum of our earth is just enough to resist being pulled completely into the sun. Think of a spinning a ball on a string that you are slinging over your head. Earth’s gravity can’t pull the ball down to the ground because the speed at which you are slinging the ball creates an angular momentum that is greater than the gravitational influence of the Earth.
Not only does gravity warp space though. Remember that space and time are in fact one physical fabric called space-time. So in fact, when mass is present, space AND time gets warped! That means that the larger the gravitational impact, the more of an effect it has on the duration of time. Thus where there is less gravity, time beats faster. Where there is a lot of gravity, time beats slower. In effect, when you are standing next to a mountain, the mountain’s gravity has warped space-time around you and means that time is beating slower for you relative to someone on the ocean. Granted, the difference is extremely extremely small on the scale of things here on Earth. But in more extreme situations in the universe (near a star or a black hole for instance) it can have an amazing effect.
You may ask the question, “what about mass causes space-time to warp?” and that is a really good question. This is where science’s knowledge about gravity gets theoretical. We think that space-time is warped in the presence of mass because of the “graviton”. The graviton is a theorized subatomic force-carrying particle (like the photon for light). It is produced in the presence of mass. Mass generates the graviton which warps the space around the mass that produced it.
So, what does this all mean? It means that the effects of gravity (i.e. the warping of space-time in the presence of mass) is a law of nature. It’s in every since of the word a “fact”. We know that thanks to the theories of Isaac Newton and Albert Einstein (and just simple observation), and the literally thousands of experiments that have been conducted over the last few hundred years to prove their validity.
But it also means that the exact “nature” of gravity (what it is at the fundamental level) is still not fully understood. Thanks to over a hundred years of particle experiments peering into the laws of quantum mechanics, we understand the basic building blocks to all things baryonic in the universe. This model, called the “standard model” has outlasted every test ever thrown at it, and if it is correct, then the “graviton” must exist; even though we haven’t found it yet. But still…physicists aren’t comfortable until we get convincing direct or indirect observational evidence of its existence. So in this matter, the fundamental nature of gravity is still theoretical. But the effects and geometry of gravitational phenomena is well understood and has been “proven”. Proven in the sense that all experiments conducted have been in accordance with the theoretical frameworks to such an enormous degree, that we can take it to the bank. And proven in the sense that we can directly and indirectly observe the effects of an attractive force…the one we call gravity.
I'll be happy to answer any questions you may have.
First of all, what is gravity? Isaac Newton is famous for formulating his three laws of gravitation but is often mistakenly credited with unlocking the secrets of gravity. What Newton did was unlock the secrets of the “effects” of gravity. Gravity, no matter what it is, has a measurable effect on the universe. Newton figured out how gravity behaves in the universe (by uncovering its mathematical secrets); he did not figure out what gravity is. An understanding of what gravity is; an understanding of what is causing these attractive force effects that we see in the universe, wouldn’t be understood until much later; all thanks to the work Albert Einstein. Albert Einstein’s theory (now a law) of General Relativity is perhaps the greatest scientific breakthrough in human history. Einstein created a framework that finally explained what gravity is. “General relativity” is a geometric framework that explains gravity to a degree that Newton could have only dreamed, and has been with us since 1915. Einstein theorized that gravity was in fact not some mystical force that “pulled” things in attraction (in a proportion to the relative masses of the bodies involved). Gravity was in fact a force, but one that was generated by a unique phenomenon. Einstein discovered that gravity is just the byproduct (the cause of an effect) generated by the warping of space-time. We all know what space is: it’s the 3-dimensional plane in which all things exist. And we all know what time is: it’s the dimension in which the change of the states of objects occurs. Einstein figured out that in fact, the 3 dimensions of space and the one of time are actually part of the same fabric called “space-time”. Everything in the universe can be determined by 4 coordinates; length, width, depth, and time.
So, this fabric of space-time becomes warped (stretched and molded) in the presence of mass (see the pic below). Anything with mass warps the 3 dimensions of space it occupies. This warping of the space is what we call gravity. Objects with mass get “pulled together” because the space between them has been bent in a way that brings them closer together. So here on Earth, it’s incorrect to think that the Earth’s gravity is pulling us to the ground. It’s more correct to say that the Earth’s mass has warped the space around it so much, that we puny humans are being “pushed toward it”. Same goes for everything else in the universe. The Earth is not being pulled toward the sun; the sun’s gravity has warped the space around Earth so much that our planet is being pushed toward it. The fact that we don’t just plunge into the sun is because our planet is moving through space really really fast. The angular momentum of our earth is just enough to resist being pulled completely into the sun. Think of a spinning a ball on a string that you are slinging over your head. Earth’s gravity can’t pull the ball down to the ground because the speed at which you are slinging the ball creates an angular momentum that is greater than the gravitational influence of the Earth.
Not only does gravity warp space though. Remember that space and time are in fact one physical fabric called space-time. So in fact, when mass is present, space AND time gets warped! That means that the larger the gravitational impact, the more of an effect it has on the duration of time. Thus where there is less gravity, time beats faster. Where there is a lot of gravity, time beats slower. In effect, when you are standing next to a mountain, the mountain’s gravity has warped space-time around you and means that time is beating slower for you relative to someone on the ocean. Granted, the difference is extremely extremely small on the scale of things here on Earth. But in more extreme situations in the universe (near a star or a black hole for instance) it can have an amazing effect.
You may ask the question, “what about mass causes space-time to warp?” and that is a really good question. This is where science’s knowledge about gravity gets theoretical. We think that space-time is warped in the presence of mass because of the “graviton”. The graviton is a theorized subatomic force-carrying particle (like the photon for light). It is produced in the presence of mass. Mass generates the graviton which warps the space around the mass that produced it.
So, what does this all mean? It means that the effects of gravity (i.e. the warping of space-time in the presence of mass) is a law of nature. It’s in every since of the word a “fact”. We know that thanks to the theories of Isaac Newton and Albert Einstein (and just simple observation), and the literally thousands of experiments that have been conducted over the last few hundred years to prove their validity.
But it also means that the exact “nature” of gravity (what it is at the fundamental level) is still not fully understood. Thanks to over a hundred years of particle experiments peering into the laws of quantum mechanics, we understand the basic building blocks to all things baryonic in the universe. This model, called the “standard model” has outlasted every test ever thrown at it, and if it is correct, then the “graviton” must exist; even though we haven’t found it yet. But still…physicists aren’t comfortable until we get convincing direct or indirect observational evidence of its existence. So in this matter, the fundamental nature of gravity is still theoretical. But the effects and geometry of gravitational phenomena is well understood and has been “proven”. Proven in the sense that all experiments conducted have been in accordance with the theoretical frameworks to such an enormous degree, that we can take it to the bank. And proven in the sense that we can directly and indirectly observe the effects of an attractive force…the one we call gravity.
I'll be happy to answer any questions you may have.
His laws are still holding up strong. However, for a little while, physicists across the world were holding their breath. We were nervous and excited at the same time.
