Friday, September 07, 2012

Astronomers are the real time travelers

kw: book reviews, nonfiction, astronomy, cosmology

This will be the seventh book "about" cosmology that I've reviewed here: How it Began: A Time-Traveler's Guide to the Universe by Chris Impey. I never tire of astronomy or cosmology. However, it is getting harder to find a book that covers new ground. Steven Hawking's A Brief History of Time is a hard act to follow! Dr. Impey manages to follow very, very well.

The plan of the book ensures nothing will be missed.
  • Part 1, "Proximate", takes us from Earth and the Solar System out into the Galaxy. 
  • Part 2, "Remote", covers the rest of the Universe, with frequent reminders that the farther we look, the further back in time we see. 
  • Part 3, "Alien", doesn't deal with E.T.'s hardly at all, but with the hierarchy of increasingly unfamiliar regimes in which we find ourselves as we approach the first year, the first second, the first picosecond, and back to the Planck time interval after the Big Bang began…and perhaps a bit beyond.
Each Part has five chapters. One device the author uses to bring us into the realm of each chapter is to open and close with a "you are there" vignette, told in first person. When the "I" becomes an atom or proton doing its best to avoid annihilation by its antiparticle, the language gets a bit strained. Nonetheless, it does get you thinking. I am not sure what the "I" is in the earliest moment, probably a gamma ray photon.

A significant feature of the book is great analogies. For example, while explaining the fusion furnace in the Sun's core, Impey notes that the power generated is about 280 watts per cubic meter, "the same as a compost heap." The fact that a cubic meter of solar core weighs about 150 tons per cubic meter is elided over for the moment. Thinking about that, I realized that pound for pound, a compost heap is hundreds of times as energetic as the core of the Sun! Of course, a compost heap's heating cycle is over in about a year. That cubic meter of star stuff can cook along for millions of years before it needs to import a little hydrogen from further up to keep going. And just by the way, this helps us understand why "doing" fusion in a Tokamak is so hard; we're trying to devise a fusion system that is thousands of times as rapid as Solar fusion (I wonder just how many people realize that the Sun's volume is 1.3 million times the volume of Earth, and it weighs 333,000 times as much—most of it is less dense than water).

Dealing with light and special relativity, early in the 9th chapter there is a scene discussing "slow light", in which the speed of light is about a meter per second (around 2.2 mph). You can walk faster than that, though it is dangerous to do so; turn on a switch and you must wait a few seconds to see what is in the room; waving at a friend across the street may be futile because the friend might be half a block away before seeing your wave; driving and riding a bicycle are probably impossible, and even jogging would apparently have you seeing events in reverse order. In a note among the endnotes, Impey jokes that he had to "leave Einstein bound and gagged in a corner and play fast and loose with his theory." Actually, the scene had to be in a classical Newtonian universe, so you could outrun (or outwalk) the speed of light. In such a universe, approaching anything at any speed would just make you move closer and closer in time to its "present". In other words, you would seem to move faster forward in time, not backwards in time. You could not see events in reverse, because the light that is "behind" you could not catch up to you.

So let us at least ungag Einstein in his corner, and consider the scene if "slow light" of 1 m/s were the case. It isn't too hard to stroll along at speeds up to 0.9 (2 mph), but trying to get all the way up to light speed, you'd find your feet dragging, as though you were pushing something heavy—and you are: you. Even at 0.9 your inertial mass would be more than doubled. There'd be no brisk walking, no jogging, and certainly no bike or auto (if they existed) could go any faster than a medium walk. You'd also notice that the world seemed compressed ahead of you, and brighter and bluer, and things to the side would seem to whip by and dim behind, fading redly. In fact, pushing as hard as you could toward a bright light would be inadvisable. You'd likely get a bad sunburn from the excess UV.

By the way, it is stated on page 193 that light illuminates a room in a ten-billionth of a second. Not so fast, Dr. Impey! Light travels about a foot per nanosecond, or a meter in 3 ns. A medium-sized bedroom is thus filled with light from a central lamp in about 8 ns, and appears fully illuminated to you, standing it the doorway by the light switch, by 20-26 ns (depending on whether the door is in a corner or mid-wall). Fast enough for practical purposes, but 200 times slower than the author suggests.

Later in the same chapter is a great explanation of the extra dimness of distant galaxies because of Hubble expansion. Ordinarily, if a bright object is twice as far away, you receive ¼ the amount of light, simply because it covers ¼ the visual solid angle while its surface brightness, in lumens per square degree, is the same. However, if the first distance is a few billion light years, the light would be somewhat redshifted, and doubling the distance more than doubles the redshift (it is not a linear relationship, and a chart on p.203 shows the function). For example, looking back 5 billion years (a visual distance of 5 billion light years from our perspective), the redshift is about 0.5, but at 10 gy, the redshift is close to 1.8, rather than 1.0.

By definition, at a lookback time of 13.72 gy, the actual instant of the Big Bang, the redshift is near-infinite (we are not sure whether it is actually infinite). The redshift is related to the factor by which light is stretched by the stretching of space. To conclude, two exactly similar galaxies, one at 5 gy and one at 10 gy, would not differ in brightness by a factor of 4, but by a factor of close to 7.5. In particular, not just the total light from the further one would be less, its surface brightness would be about 45% less. This is why it takes such huge telescopes to see the most distant galaxies; not just because they are visually small, but because they are extra-dim (and we haven't even touched upon the extra dimming by intergalactic gas).

In the explanation of Dark Energy, which is the current explanation for an apparent acceleration of Hubble expansion since 8 billion years ago, I was struck by an odd thought. This mysterious Energy is apparently constant on a volume basis, so its total quantity increases as the Universe expands. Now that sounds a lot like a kind of continuous creation! Fred Hoyle would be pleased.

Dr. Impey doesn't ignore philosophy. The chapter titled "Something Rather Than Nothing" is pretty much all philosophy. After all, we have no empirical evidence (yet) why or how a Universe could contain matter rather than just a sea of photons. And at the book's writing, the Higgs Boson hadn't been detected yet (Now it has: for more, go here and search for ATLAS). He spends a quarter of the chapter on the fine-tuning problem, AKA the Anthropic Principle, which states that if certain physical quantities were a little different, we could not exist. Actually, most of the "fine tuning" turns out to be in a rather wide range, so the Penrose Number of 10 to the 10 to the 123d power "against", is rather too pessimistic (to put it extremely mildly!). In perspective (sorry, I can't stop myself), the fanciful number Googol (NOT Google) is 10 to the 100th power and Googolplex is 10 to the Googol-th power. Let's coin the Gargle as 10 to the 123d power. The Penrose Number is a Gargleplex.

Finally, here is a great graph (p 342) that summarizes just why the Planck Time (~10-43s) and Planck Length (~10-35m) represent an absolute limit to how far we can probe the beginning of time. Unless… unless the Universe is part of a Multiverse, and new quantum events trigger the creation of new Universes, so that ours is just a bubble blown off from a corner of an earlier one. I know Chris Impey had to take the obligatory excursion into Strings and superstrings and multiverse theory and all that. Considering that the number of possibly viable "string theories" is a Googolplex to the fifth power, it bears more resemblance to Medieval monks deciding how many angels could dance on the head of a pin. Fortunately, Dr. Impey seems to keep his tongue firmly in his cheek, and manages to distance himself from astro-theological speculations.

There are things we know, and things we don't know, and very likely there are things we cannot know, even if we survive for millions or billions of years. The little triangle at bottom center of the chart above represents a region of "cannot know, whether there is something to know or not". A smidgen of humility is a salutary attribute for a scientist.

The book's main text is 362 pages, but do be sure to read the endnotes. They are equally entertaining, and take you to page 416 with the greatest of ease.

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