Wonders of the Universe - Brian Cox [78]
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So what, then, is gravity? The explanation in Einstein’s theory is beautifully simple: gravity is the curvature of spacetime.
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In technical language, Einstein would have defined the Vomit Comet, during its time in freefall, as an inertial frame of reference – which is to say that it can be legitimately considered to be at rest, with no forces acting on it.
The assertion that sitting in a falling aircraft should be considered as being absolutely equivalent to floating around in space, far beyond the gravitational pull of any planet or moon, can be used to explain why all objects fall at the same rate.
Why? Simply because there are two equally valid ways of looking at what is happening. From the point of view inside the plane, nothing at all is happening; everything is simply floating, untouched by any forces of any kind. If no forces are acting, then everything naturally stays where it is put. Shift outside the plane, however, and things appear different; everything is falling towards Earth, accelerating under the action of the force of gravity. But, very importantly, the reality of the situation cannot change depending on which point of view we adopt – everything has to behave in the same way in reality, irrespective of how you look at it. If plastic Albert and the globules of water float in front of my face when viewed from my vantage point inside the plane, then plastic Albert and the globules of water had better float in front of my face when viewed from a vantage point outside the plane. In other words, we had all better be accelerating towards the ground at exactly the same rate! Notice that we’ve made no assumptions about the equivalence between gravitational and inertial masses here; we’ve just said that a freely falling box in Earth’s gravitational field is indistinguishable from a freely falling box in space, or indeed any freely falling box anywhere in the Universe, around any planet, any star, or any moon.
So what, then, is gravity? The explanation in Einstein’s theory is beautifully simple: gravity is the curvature of spacetime. What is spacetime? Spacetime is the fabric of the Universe itself.
A good way to picture spacetime, and what it means to curve it, is to think about a simpler surface; the surface of Earth. Our planet has a two-dimensional surface, which is to say that you only need two numbers to identify any point on it: latitude and longitude. Earth’s surface is curved into a sphere, but you don’t need to know that to move around on it and navigate from place to place. The reason we can picture the curvature is that we are happy to think in three dimensions, so we can actually see that Earth’s surface is curved. But imagine that we were two-dimensional beings, confined to move on the surface of Earth with absolutely no concept of a third dimension. We would know nothing about up and down, only about latitude and longitude. It would be very difficult indeed for us to picture in our mind’s eye the curvature of our planet’s surface.
Now let’s extend our analogy to see how the curvature of something can give rise to a force. Imagine that a pair of two-dimensional friends are standing on the Equator and decide to take a journey due north. They decide to walk parallel to each other, with the intention of never bumping into each other. If they both keep walking, they will walk up parallel lines of longitude, and they will find that as they get closer and closer to the North Pole they will get closer and closer together. Eventually, when they reach the North Pole, they will bump into each other! As three-dimensional beings, we can see what happened; Earth’s surface is curved, so all the lines of longitude meet at the poles. However, from the perspective of our two-dimensional friends, even though they kept assiduously to their parallel lines they still were mysteriously drawn together. They may well conclude