Tuesday, August 22, 2006

How Far Is It?

Real knowledge is to know the extent of one's ignorance. ~Confucius

Sometimes disparate news items come together to highlight a point that one or more them might not have intended to make. For instance, there was an interesting little item about the discovery of a neutron star moving through space at a fantastic clip which highlights the problem of how we come to have fast-moving neutron stars. What makes the phenomenon interesting is that no one has adequately determined what sequence of event could accelerate a neutron star to such speeds. Toward the end of the article, there is a quote that caught my eye: “... the uncertainty in the distance to Puppis A means the neutron star's velocity is subject to error.”

Translation: We may be full of hot air because we can't be sure how far away the thing is.

Then alongs comes a story telling us that the universe may be a couple of billion years older and, as a result, commensurately bigger than we've been thinking it is. This possibility is raised because new observations have indicated a couple of stars in the Triangulum Galaxy are a half million light years farther away than we though they were. Because of that, something called the Hubble Constant, which describes the rate of expansion of the universe, has to be revised.

Now, the Hubble Constant is not like one of those I discussed in Inconstant Constants. Unlike things like the fine constant or the speed of light, the Hubble Constant has never been pinned down to the satisfaction of the entire astrophysical community. In fact, for some considerable period, it's assumed value created a significant complication, since the value of the constant indicated an age for the universe that was younger than the age of the oldest known stars. That meant that either the constant was wrong, or theories of star life cycles were whacked. And the star formation theories had a strong theoretical basis for being right, while the measurements that determined the Hubble Constant were under continuous assault.

See, the problem is that space is, as Douglas Adams put it, big, really, really big. It's also filled with all sorts of strange objects of which we have limited knowledge because they're so far away. Yet we try to use some of the properties of those objects to determine how far away they are.

Look at this way. You see a light in the distance. You have no way of knowing how far away it is unless I give you some information, such as telling you it's a 100-watt incandescent light bulb. If you know that and you know the intrinsic brightness of such a light, you can measure the apparent brightness you now see, plug it into a simple equation and determine instantly how far away you are. Which is fine, until I tell you that, being the inveterate joker that I am, that I lied. It's not a 100-watt light bulb but a candle. This will make you unhappy, and you will probably will to do me bodily harm.

That's sort of what keeps happening in the astronomical sciences (without the bodily harm, but not by much).

Measuring the distance to a nearby (relatively speaking) star can be done by trigonometry. You can take a position of the star when the Earth is on one side of the Sun. Then, when the Earth has moved in its orbit to be on the other side of the Sun, you take the position of the star again. You then form a triangle with the base being the distance between where you took your observations and record the apparent angle to the star. A few simple calculations, and you've determined that the star is, say, 7 light years away.

The trouble is that this method doesn't work for stars much farther than 10 light years away. Most of the stuff in the universe is a lot farther away than that. So astronomers looked for “standard candles”, objects which would have a known intrinsic brightness. Then, if you found one of these things in another place, you could tell how far away it was by measuring its actual brightness and comparing it to its intrinsic brightness.

A lot of things have been tried as standard candles, like variable Cepheids, supernovae, and even the brightness of entire galaxies. Unfortunately, each has its limitations, not to mention that things like clouds of interstellar dust than can dim an object to make it look more distant.

The debate about the Hubble Constant has often gotten acrimonious. Cosmologists work hard on their theories of the formation of the universe; they are unhappy when someone comes along with a change in the distances because that changes everything. Physicists, astrophysicists, and astronomers have held contentious conferences that ended up with colleagues becoming enemies because they differed on the Hubble Constant.

The new value isn't going to make everyone happy, and, no doubt, as I write this, someone is planning to take new observations to try to find a better means of measuring interstellar and intergalactic distances. When they think they have, someone else will start the cycle again.
This isn't a trivial exercise because so much of cosmology, physics, and astrophysics, depends on knowing the distances to things like quasars. At stake are theories of gravity, the formation of galaxies, elements of Einstein's theories of relativity, and the nature of gamma-ray-bursters (GRB), to name just a few. A lot swings on the value of Hubble's Constant.

It's difficult to imagine, given our current state of knowledge, that we can pin down the Hubble Constant to a certainty. But, that's today. Tomorrow, some breakthrough in quantum physics could drive a change in theories of star formation that could provide a key to determining a new standard candle. Perhaps we'll determine the intrinsic brightness of a quasar, or maybe the GRB's will provide a clue.

That's what makes tomorrow so exciting (cue red-haired girl with big voice).

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