Science In Real Life: Gravity
I have a weighty matter to discuss with you all today: gravity. This is another one of those so-persistent-we-don't-notice-it kind of phenomena most of the time. Ask a fish sometime if they realize they're swimming in a giant mass of water, and the confused blank stare you receive should remind you of how aware we all are of the gravitational field holding us to terra firma.
Let's get the riffraff out of the way, and I don't mean the kids who sneak into the movie theater. We have to draw a distinction between the weight of an object and the mass of an object. The mass of an object is proportional to how many pies it has eaten much raw material is there, i.e. how many atoms are present. The weight of an object is equal to its mass times how much gravity is pulling on it. So an object's weight can change if it goes to the moon, but its mass does not.
Gravity is a universal force. Every piece of matter attracts every other piece of matter. The only variables are how much matter there is and how it is arranged. How does this happen? Well ... we don't really have it nailed down. But we do have a pretty good guess, and it comes to us from Einstein's Theory of General Relativity. But before we get to that I need to introduce a new concept: the spacetime continuum. As I mentioned in the Time essay, space and time share certain similarities despite significant differences, like identical twins who can never agree on what movie to watch. Eventually physicists found it convenient to simply treat space and time not as separate entities but as different aspects of a coherent whole. Then, when Einstein made predictions based on that notion (among other things), they turned out to be correct.
The other key innovation Einstein made is that spacetime can bend. That's right, the very geometry of space and time is a flexible, dynamic thing. How does one bend spacetime? You don't send it to a yoga class, you just have to put some matter in it. This was Einstein's biggest insight: that the force of gravity and the bending of spacetime are one and the same phenomenon. In a nutshell - matter (and energy, for that matter) tell spacetime how to bend. The bent spacetime then tells matter how to move. Moving matter creates a new gravitational arrangement, and the process repeats all over again with more inexorability than your in-laws coming to visit for the holidays.
It's not immediately apparent how the bent spacetime tells matter how to move. Remember that when you get right down to brass tacks, space is geometry. The definition of a straight line, or what appears to be a straight line, is contingent on the shape of spacetime. Take light, for example. Light rays always always always travel in a straight line, or at least a straight line as far as they can determine. But if I bend all the lines, what looks like a straight line for the light ray looks bent to us. It's relative.
How does this impact you, the consumer? Not much. New theories in physics don't so much prove old theories wrong as subsume and augment them. Newton's theory of gravity, developed umpity-ump years ago, still works just fine as long as you don't go too fast or get near too large a planet. In fact, Newton's theory is all you need to send men to the moon. But physicists like Einstein's theory better because it explains a wider range of situations. One such situation is the orbits of global positioning satellites. The Gee Pee Esses move fast enough and close enough to the Earth that you need to make relativistic corrections in order to accurately measure their positions.
One other thing of note is how weak gravity is. For being such a universal force it's remarkably wimpy. Grab an ordinary refrigerator magnet and use it to pick up some paper clips. It lifts them right off the table, doesn't it? That puny magnet just won a tug of war with the ENTIRE PLANET EARTH. The electromagnetic force is so much stronger than the gravitational force that it's really no contest. It's so much stronger, in fact, that if you were to decide today to end it all by jumping off the Empire State Building, you would never actually touch the ground. Remember how everything is made of atoms? And that atoms have an outer layer of negative electrons? Your electrons and the sidewalk's electrons repel each other so strongly that they never actually make contact even though you're traveling at a fatal velocity.
Of course, you'll still be street pizza, but that's because of inertia, which is a topic for another day. Mmm, pizza.
Don't forget, the floor is open to questions!
Let's get the riffraff out of the way, and I don't mean the kids who sneak into the movie theater. We have to draw a distinction between the weight of an object and the mass of an object. The mass of an object is proportional to how many pies it has eaten much raw material is there, i.e. how many atoms are present. The weight of an object is equal to its mass times how much gravity is pulling on it. So an object's weight can change if it goes to the moon, but its mass does not.
Gravity is a universal force. Every piece of matter attracts every other piece of matter. The only variables are how much matter there is and how it is arranged. How does this happen? Well ... we don't really have it nailed down. But we do have a pretty good guess, and it comes to us from Einstein's Theory of General Relativity. But before we get to that I need to introduce a new concept: the spacetime continuum. As I mentioned in the Time essay, space and time share certain similarities despite significant differences, like identical twins who can never agree on what movie to watch. Eventually physicists found it convenient to simply treat space and time not as separate entities but as different aspects of a coherent whole. Then, when Einstein made predictions based on that notion (among other things), they turned out to be correct.
The other key innovation Einstein made is that spacetime can bend. That's right, the very geometry of space and time is a flexible, dynamic thing. How does one bend spacetime? You don't send it to a yoga class, you just have to put some matter in it. This was Einstein's biggest insight: that the force of gravity and the bending of spacetime are one and the same phenomenon. In a nutshell - matter (and energy, for that matter) tell spacetime how to bend. The bent spacetime then tells matter how to move. Moving matter creates a new gravitational arrangement, and the process repeats all over again with more inexorability than your in-laws coming to visit for the holidays.
It's not immediately apparent how the bent spacetime tells matter how to move. Remember that when you get right down to brass tacks, space is geometry. The definition of a straight line, or what appears to be a straight line, is contingent on the shape of spacetime. Take light, for example. Light rays always always always travel in a straight line, or at least a straight line as far as they can determine. But if I bend all the lines, what looks like a straight line for the light ray looks bent to us. It's relative.
How does this impact you, the consumer? Not much. New theories in physics don't so much prove old theories wrong as subsume and augment them. Newton's theory of gravity, developed umpity-ump years ago, still works just fine as long as you don't go too fast or get near too large a planet. In fact, Newton's theory is all you need to send men to the moon. But physicists like Einstein's theory better because it explains a wider range of situations. One such situation is the orbits of global positioning satellites. The Gee Pee Esses move fast enough and close enough to the Earth that you need to make relativistic corrections in order to accurately measure their positions.
One other thing of note is how weak gravity is. For being such a universal force it's remarkably wimpy. Grab an ordinary refrigerator magnet and use it to pick up some paper clips. It lifts them right off the table, doesn't it? That puny magnet just won a tug of war with the ENTIRE PLANET EARTH. The electromagnetic force is so much stronger than the gravitational force that it's really no contest. It's so much stronger, in fact, that if you were to decide today to end it all by jumping off the Empire State Building, you would never actually touch the ground. Remember how everything is made of atoms? And that atoms have an outer layer of negative electrons? Your electrons and the sidewalk's electrons repel each other so strongly that they never actually make contact even though you're traveling at a fatal velocity.
Of course, you'll still be street pizza, but that's because of inertia, which is a topic for another day. Mmm, pizza.
Don't forget, the floor is open to questions!