Why Does E=mc2_ - Brian Cox [23]
The discovery that the passage of time can be slowed down and distances can be shrunk is strange enough when applied to the realm of subatomic particles, but Einstein’s reasoning applies equally well to things the size of humans. One day we may even come to rely on this strange behavior for our survival. Imagine living on the earth in the far future. In a few billion years’ time, the sun will no longer be a stable provider of life-sustaining illumination to our world, but a seething, unstable monster of a star that may well engulf our planet as it swells in its final reddening death throes. If we have not become extinct for some other reason by then, it will be necessary for humans to escape our ancestral home and journey to the stars. The Milky Way, our local spiral island of a hundred billion suns, is 100,000 light-years across. This means that light takes 100,000 years to journey across it, as determined by someone on Earth. Hopefully, the need for the last caveat is clear given all that we have been saying. It might seem that humanity’s possible destinations within the Milky Way will be forever restricted to a tiny portion of the stars very close to our home (on astronomical scales) because we could hardly be expected to undertake a journey to distant corners of the galaxy that would take light itself 100,000 years to reach. But here is where Einstein comes to the rescue. If we could build a spaceship that could whisk us into space at speeds very close to light speed, then the distances to the stars would shrink, and the amount of shrinking would increase the closer to light speed we could travel. If we managed to travel at 99.99999999 percent of light speed, then we could travel out of the Milky Way and all the way to the neighboring Andromeda galaxy, almost 3 million light-years away, in a mere fifty years. Admittedly, that looks like a tall order and indeed it is. The big obstacle is figuring out how to power a spaceship so that it could get up to such high speeds, but the point remains: With the warping of space and time, travel to distant parts of the universe becomes imaginable in a way it never was before. If you were part of humanity’s first Andromeda expedition, arriving in a new galaxy after a fifty-year journey, your children born in space might wish to return to their home world and gaze upon the earth with their own eyes for the first time. For them, the Blue Planet would be nothing more than a bedtime space story. Turning the spaceship around, and traveling back to Earth for fifty years, the entire journey to Andromeda and back would have taken one hundred years. By the time they arrived back in Earth orbit, however, a shocking 6 million years would have passed by for the inhabitants of the earth. Would their progenitor civilization have even survived? Einstein has opened our eyes to a weird and wonderful world.
4
Spacetime
In the previous chapters we followed the historical road to relativity, and in fact our reasoning was not too far from what Einstein originally presented. We have been forced to accept that space is not the great stage upon which the events of our lives are played out. Likewise, time is not something universal and absolute. Instead we moved toward a picture of space and time that is much more malleable and subjective. The great clock in the sky, and in some sense the sky itself, has been banished. It might feel to us like the world is a box within which we go about our business, because that picture allows us to make sense of it quickly and efficiently. The ability to map the movement of things against an imaginary grid is what we might call spatial awareness, and it is clearly important if you are to avoid predators, catch food, and survive in a dangerous and challenging environment. But there is no reason why this model, buried deep within our brains and reinforced over millions of years by natural selection, should be anything other