Gravity - Sixty Symbols

big G is the fundamental one Newtons universal gravitational constant and that's the one that really tells you in general terms how much gravitational force there will be between two objects can you tell me what the value of big G is ah no you know you know I'm struggling especially remembering what the units are I think it's something like six point seven times ten to the minus eleven it's six point six seven times ten to the minus eleven but I always have to look up what the units are that one I'd actually have to look up why are you looking in your wallet cuz I keep it because I keep it yes I have guessed it right and I can even give you the unit's now allowance is a little kind of his wallet he's got it well this is something very useful these that I guess I guess actually from some scientific journals film mag give you this it's six point six seven times something identical something but that's not very helpful and it's got some fundamental constants I know most of these fundamental causes are three significant figures but because I don't use big G very often I usually have to look up what that is but I have just about remembered it right to two significant figures why don't I need to know them maybe because they're built into my calculator so that whenever I actually do a calculation I'll just press the big G button yes always carried around with me just in case I need it yep yeah it's very handy if you're doing if you're stuck in a in a delayed train or got a few spare hours doing something it's always good to have it there and you can doodle around think about things have the constants in front of you well a very important symbol for us is of course the strength of the gravitational force on the surface of the earth that's the thing that keeps us firmly footed on the ground well with G particularly big G it's all about gravity and I'm partial gravity because it is to my mind the most important force in the universe the quantum mechanical way of thinking about forces that the microscopic way of thinking about forces is that there are particles that kind of mediate the force a particle goes from one place to another and tells you no tells one object to be attracted but or repulsed by another object and so that's that's sort of the way we tend to picture the the forces of electromagnetism where the particle that does that of photons but also the forces the whole nuclei together and so on the nuclear forces but for gravity we don't really have that picture I mean we occasionally sort of try to sketch out a picture like that and then the particle that mediates the force that tells things how to attract each other and it's called the graviton but we don't we don't have a fool theory that actually allows us to understand gravity in that way the way we do for the other forces well if we recall those experiments that Galileo did from the Leaning Tower of Pisa so let me try and draw the Leaning Tower of Pisa well big G is what we would call the gravitational constant and it's a fundamental constant of nature comes in to Newton's laws of gravity and Einsteins laws of gravity little G is what we would talk about in terms of local gravity so the gravitational acceleration that we feel say here on earth which is 9.8 meters per second per spekt second so anyway if we imagine Gallic Galileo sitting up here with a lead ball on its sphere little G is just the force of gravity here on the surface of the earth so it's you know if you drop something on the surface of the earth it's how far something will accelerate under gravity 9.81 meters per second every second is how much faster it gets so but at some level that's a very local and kind of parochial sort of thing and let's imagine him dropping this so the sphere comes down to here under the force of gravity and the gravitational force of course is given by the mass of the red sphere times G then what G tells us is how far the object will fall from the time Galileo releases it to fall under the force of gravity and the distance s it travels s is then just equal to one-half of G times T squared where T is the time it's formed so after one second this thing will have fallen by G upon two in one second and then if we were put in there 9.8 over two that tells us it's fallen about four point nine meters if you take a smaller planet like Mars so let's imagine that we've got Mars that's smaller than the earth by about that much it's a little bit less dense as well but not much less dense they like the average density of Earth and Mars are about the same and then a bit smaller still we have we have the moon our moon and G is different on on those three different materials so on as I've said on the earth G is equal to nine point eight and we get 9.8 meters per second squared and we give it that symbol but on Mars G is about one third of G on earth and on the moon it's about one fifth and if you recall when the astronauts went to the moon they were even their heavy spacesuit was able to do very impressive jumps on the moon and that's simply because that their muscles had less gravitational force to overcome there we go so with Isaac Newton we had a very simplistic form of gravity and this is the way most of us think about gravity today which is just two massive objects pulling each other together we've moved over to this alliance tiny in view where the gravity is a distortion of space itself so for me I've used in the past this idea of gravitational lensing so let's imagine that the seat cushion here is our space-time so it's it's two dimensions of space and we're going to warp it in into a third dimension using my fist so my fist represents a very massive object say a black hole or a galaxy or even a cluster of galaxies you can see that it's made this sort of dimple in space in our analogy and what's going to happen is that any object coming near this massive object is going to feel the consequence of this space-time being warped have this this ball this is going to be another object and it's just happily making its way around through this neighborhood and it encounters this warping space-time and what happens is that its path gets deflected the exact same sort of thing can happen with light and so we have say a light source in the distance distant universe say another galaxy it's emitting light this light is following the same path that the ball did and the light path also gets deflected and so now the light changes course okay so let's imagine that we're actually sitting at a telescope we're sitting over here at a telescope in our on earth and we're looking out in this general direction we detect the light coming from this distant galaxy after it's been bent but of course we don't know it's been bent in our view light travels in a straight line so we think it's come in this direction and in fact we don't see the original source in its original location we see an image of it over here and simply by measuring the angle that this light has been deflected between where it came from and where we actually see it we can use general relativity to measure the mass of this deflecting object and it doesn't matter if this is a massive galaxy that we can see or if it's a big lump of invisible dark matter gravitational lensing is the tool that lets us measure its mass so we go back to my fist here and we've said that the light coming in this direction has been deflected and made it to our telescope over here but there's no reason that the light coming around this side of the galaxy can't be deflected as well and in that case we see not one but two objects two images of the same object but spaced out further apart on the sky so once in a while when you end up with things absolutely perfectly aligned so you end up with a object that's being lens the lens and us all in an absolutely perfect straight line then in that case the light from the object that's being lens can actually go around the lens any way any side of the lens and so instead of just producing a multiple image or distorted image you end up producing a perfect ring of light in the sky and that phenomenon is called an Einstein ring after Einstein who actually predicted that this phenomenon should be observed out there in the universe you

Channel: Sixty Symbols

Views: 460,647

Keywords: gravity, gravitational, lensing

Id: JHhoHmiiXdc

Length: 8min 46sec (526 seconds)

Published: Wed Apr 22 2009