Picture this: you're walking down a road on a dark, moonless night. In the distance, you see a light. How far away is the light?
The problem is obvious. You can only make a good guess about the distance between you and the light source if you know how bright the light actually is. A close-by dim light will have the same apparent brightness as a faraway bright light. (The opposite would be true, too, of course. You could only estimate the light source's intrinsic brightness if you knew how far away it was.)
That, in a nutshell, is the difficulty with making distance measurements of astronomical objects. There are three tools, though, that can help to get around this problem.
The first only works for relatively nearby objects. It's called parallax, and it has to do with the apparent motion of objects when you are actually what's moving. You've all seen this; when you're driving down the freeway, nearby objects (such as the fence running along the side of the road) seem to zoom past a lot faster than distant ones (such as the mountain in the distance). To figure out something's distance using parallax, you need two measurements of its apparent position relative to the unmoving background. Then, using the distance you know that you have traveled, it's a matter of simple trigonometry to figure out how far away the object is.
Even nearby stars, though, exhibit such a tiny parallax that it requires a very long baseline -- such as the position of the Earth between June 21 and December 21. By that time, it's halfway around its orbit, and the baseline is the orbit's circumference -- about three hundred million kilometers. However, objects farther away than about ten light years have such a minuscule parallax that it's effectively undetectable.
The second, discovered by astronomer Henrietta Swan Leavitt in the early twentieth century, is a peculiarity of a type of variable star called a Cepheid variable. Cepheid variables have a regular rise and fall in brightness, and Leavitt discovered (using fairly nearby ones) that their pulsation rate is directly proportional to how bright they actually are. And, as I pointed out above, once you know how bright a light source is, you can estimate how far away it is. (Making Cepheids one of the most commonly used "standard candles" in astronomy.)
The third sprang right from Leavitt's discovery. When the light from distant galaxies was analyzed, astronomer Edwin Hubble observed something strange; it was red shifted. Red shift is the electromagnetic version of the Doppler effect -- the wavelengths of light get stretched out (move toward the red end of the spectrum) if an object is moving away from you. The more the shift, the greater the velocity. But the kicker occurred when Hubble used Leavitt's discovery of the relationship between a Cepheid variable's pulsation rate and intrinsic brightness to figure out how far away these galaxies were, and found another interesting correlation; the farther away the galaxy was, the greater the red shift -- and therefore, the faster it was moving away from us. This led directly to the Big Bang/expanding universe model, and marks the origin of modern cosmology.
There's a fourth method, though, only recently discovered, but which was the technique used in a study that appeared last week in the Astrophysical Journal to determine the distance to five hundred distant galaxies. It's called echo mapping, and it works like this.
Many, if not all, galaxies have a massive black hole at the center. Black holes are not amenable to any of the standard methods of distance calculation. They don't emit light, so even the red shift method won't work. But one feature of most massive black holes is that they are surrounded by a torus-shaped dust cloud of debris. The intense gravitational pull of the black hole draws matter into it, heats it up, and causes it to emit radiation in sudden bursts. That radiation flashes outward and is absorbed by the inner surface of the dust cloud, warming it and creating an infrared signal that is detectable by telescopes on Earth.
Well, we know that light travels at three hundred thousand kilometers per second, and also that light's intensity drops off as a function of the inverse square of the distance from the source (twice as far means four times dimmer, three times as far means nine times dimmer, and so on). Dust only forms if the temperature is below twelve hundred degrees Celsius -- any hotter and the molecules are torn apart by the thermal energy. So a large black hole, with a large radiation output, would generate a dust cloud with a larger inner radius -- just as campers sitting around a campfire need to be closer to a smaller fire to be as warm as someone farther from a bigger fire.
So that's all the pieces. If you know the time between the initial flash of radiation from the black hole and the subsequent infrared signal emitted by the dust cloud, you can figure out the circumference of the dust cloud. Knowing the circumference tells you how intense the radiation source is (bigger circumference = more intense radiation source). This gives you the actual luminosity of the accretion disc around the black hole -- and therefore how far away it is.
What never fails to impress me about scientists, and science in general, is the cleverness with which problems are approached. Some of the best solutions to scientific questions have come from completely out-of-the-box ideas, or (as in the case of Henrietta Swan Leavitt's discovery about Cepheid variables) using something that at first appears to be a trivial factoid to illuminate something truly enormous.
I don't know about you, but whenever I see stuff like this, I always think, "I would never have thought of doing that." I know that part of it is that, being a non-scientist, I haven't been steeped in one subject for years. But I think the really successful scientists, the ones who make the major breakthroughs, are the ones whose brains are able to bring together what initially appear to be entirely disparate bits of information, and generate a synthesis that is way bigger than the sum of the parts.
In other words, science is primarily a creative act.
A fitting way to end this post is a quote from the brilliant Austrian physicist Lise Meitner:
One of my favorite TED talks is by the neurophysiologist David Eagleman, who combines two things that don't always show up together; intelligence and scientific insight, and the ability to explain complex ideas in a way that a layperson can understand and appreciate.
His first book, Incognito, was a wonderful introduction to the workings of the human brain, and in my opinion is one of the best books out there on the subject. So I was thrilled to see he had a new book out -- and this one is the Skeptophilia book recommendation of the week.
In Livewired: The Inside Story of the Ever-Changing Brain, Eagleman looks at the brain in a new way; not as a static bunch of parts that work together to power your mind and your body, but as a dynamic network that is constantly shifting to maximize its efficiency. What you probably learned in high school biology -- that your brain never regenerates lost neurons -- is misleading. It may be true that you don't grow any new neural cells, but you're always adding new connections and new pathways.
Understanding how this happens is the key to figuring out how we learn.
In his usual fascinating fashion, Eagleman lays out the frontiers of neuroscience, giving you a glimpse of what's going on inside your skull as you read his book -- which is not only amusingly self-referential, but is kind of mind-blowing. I can't recommend his book highly enough.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]