A Brief History of Time

Oof. That’s an intense book.

Hawking does an impressive job of making cosmology and the Big Bang and black holes seem relatively digestable, but even for me–with a strong background in mathematics, a pretty decent one in at least undergradate level physics and a stronger study specifically in quantum weirdness, albeit more from a computational perspective–this book is hard to read at times.

Well worth the read though. I learned all sorts of crazy things. (Just read the book, but if you need even more convincing…)

Yet another proof the Earth is round:

First, he realized that eclipses of the moon were caused by the earth coming between the sun and the moon. The earth’s shadow on the moon was always round, which would be true only if the earth was spherical. If the earth had been a flat disk, the shadow would have been elongated and elliptical, unless the eclipse always occurred at a time when the sun was directly under the center of the disk.

Kepler didn’t like orbits not being perfect circles:

As far as Kepler was concerned, elliptical orbits were merely an ad hoc hypothesis, and a rather repugnant one at that, because ellipses were clearly less perfect than circles.

Relativity is measurable even on the scale of the orbit of Mercury:

For example, very accurate observations of the planet Mercury revealed a small difference between its motion and the predictions of Newton’s theory of gravity.

Mass doesn’t increase as quickly as I’d thought as you get up towards light speed:

For example, at 10 percent of the speed of light an object’s mass is only 0.5 percent more than normal, while at 90 percent of the speed of light it would be more than twice its normal mass.

Another way of looking at how large masses actually ‘curve’ three dimensional space, and ‘into what’:

The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space.

Physicists get weird when it comes to naming things:

It is believed that this force is carried by another spin-1 particle, called the gluon, which interacts only with itself and with the quarks. The strong nuclear force has a curious property called confinement: it always binds particles together into combinations that have no color. One cannot have a single quark on its own because it would have a color (red, green, or blue). Instead, a red quark has to be joined to a green and a blue quark by a “string” of gluons (red + green + blue = white). Such a triplet constitutes a proton or a neutron. Another possibility is a pair consisting of a quark and an antiquark (red + antired, or green + antigreen, or blue + antiblue = white).

Even more weirdness than expected when it comes to the speed of light being constant:

A cannonball fired upward from the earth will be slowed down by gravity and will eventually stop and fall back; a photon, however, must continue upward at a constant speed. How then can Newtonian gravity affect light?

When you get into the scale of space, numbers can get unintuitive:

The rate of energy loss in the case of the earth and the sun is very low—about enough to run a small electric heater. This means it will take about a thousand million million million million years for the earth to run into the sun, so there’s no immediate cause for worry!

Hawking has an amusing sense of humor:

Despite this, I had a bet with Kip Thorne of the California Institute of Technology that in fact Cygnus X-l does not contain a black hole! This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist.

I started to think about black holes as I was getting into bed. My disability makes this rather a slow process, so I had plenty of time.

Despite being one of the craziest events in the history of our universe, the Big Bang didn’t last very long:

Within only a few hours of the big bang, the production of helium and other elements would have stopped. And after that, for the next million years or so, the universe would have just continued expanding, without anything much happening.

Thinking about the universe or space-time itself leads to some interesting questions:

His space-time had the curious property that the whole universe was rotating. One might ask: “Rotating with respect to what?” The answer is that distant matter would be rotating with respect to directions that little tops or gyroscopes point in.

Einstein wasn’t perfect (and causality and faster than light travel have some issues):

This had the side effect that it would be possible for someone to go off in a rocket ship and return to earth before he set out. This property really upset Einstein, who had thought that general relativity wouldn’t allow time travel. However, given Einstein’s record of ill-founded opposition to gravitational collapse and the uncertainty principle, maybe this was an encouraging sign.

More humor:

There was a young lady of Wight Who travelled much faster than light. She departed one day, In a relative way, And arrived on the previous night.

Just what a Grand Unified Theory of Everything could mean:

However, if we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason—for then we would know the mind of God.

A factoid about Galileo’s relationships with the Church that I’d not heard of before:

In 1623, a longtime friend of Galileo’s became the Pope. Immediately Galileo tried to get the 1616 decree revoked.

And finally, a bit of an understatement:

In the twenty years since the last revision of this book, progress in cosmology has been rapid.

Like I said. A fascinating book. Worth the read.

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