Tuesday, May 26, 2009

The Copernican Principle

This weekend I relaxed and caught up on some recreational reading, finally getting through my stack of magazines from April. Now I know why the magazines in so many doctors' waiting rooms are so old; it takes the doctors months to get around to reading them in the first place.

In the April issue of Scientific American, there is an article by Timothy Clifton and Pedro Ferreira pondering if dark energy really exists, or if we perhaps live in the middle of a billions of light-years wide void. A void is a region of space where there is less matter than average, therefore the force of gravity would be less than on average, so the expansion of the Universe would appear to be faster than we would expect, which is just what we see with dark energy. (For what I think is a simple and honest explanation of dark energy, listen to or read the May 12 episode of the 365 Days of Astronomy podcast.)

The authors of the article do what I think is a very good thing: they present a pretty balanced review of both sides of the argument for and against giant voids. One of the arguments against a giant void is that it would appear to violate the Copernican principle.

The Copernican principle is one of the primary pillars of the science of astronomy. It says simply that we do not occupy a privileged location in the Universe. As you might guess, the principle comes out of the Copernican view of the Solar System, that the Earth is not the center of the Solar System, the sun is. In other words, we are not the center of the Universe, but rather we are just a small part in the larger whole.

The Copernican principle has been invoked many times. When astronomers first started to make maps of the Milky Way galaxy, the Sun seemed to be very close to the center (see, for example, this map by famed astronomer William Herschel). This raised alarm bells among some astronomers, because the Copernican principle said we shouldn't be someplace special, and the center of the entire galaxy would certainly seem like a special place, especially since we didn't know that other galaxies existed. This issue was resolved when Harlow Shapley, a great early 20th-century astronomer, used globular clusters to show that we were not at the center of the Milky Way, but way off to one side.

Barely had that crisis been resolved when a new one arose. Astronomer Edwin Hubble discovered that almost every galaxy in the Universe was moving away from the Milky Way, and the further away it was, the faster it was moving. This would seem to put the Milky Way at the center of the Universe, which is again against the Copernican principle! This conundrum was solved when it was recognized that the entire Universe was expanding. In an expanding Universe, no matter where you are, it looks like every other thing is moving away from you. (This is one of those concepts that is hard to grasp; the time-tested "raisin bread" analogy seems to work best, though since many people these days have never seen a loaf of bread rise, a balloon analogy also helps.)

So, back to the voluminous void from the article. For the void explanation of dark energy to work, the Milky Way would need to be located at almost the exact center of a gigantic void. How big? The void would need to be a large fraction of the observable universe! So, we would need to be near the center of a structure that is a good fraction the size of the observable Universe. That doesn't sit well with the Copernican principle. This argument alone doesn't rule out a giant void (the authors admit to many other problems with the void idea), but it makes it hard to swallow.

But I do think we need to be careful in applying the Copernican principle. The principle can be taken too far. For example, we see lots of stars near the center of our galaxy. If any of those stars harbor life-bearing planets, then any intelligent creatures on those planets will find themselves at the center of a galaxy. It's only when they discover other galaxies that they would realize they are not in a special place.

Likewise, if there is a giant void in our Universe, some galaxy will likely be near the middle of it. And if there is life on a planet around a star in that galaxy, they will find themselves in a fairly special place in the Universe.

In summary, the Copernican principle is one of the primary philosophical underpinnings of the science of astronomy. It is not physical law in and of itself, but it does require us to use an abundance of caution around any hypothesis that requires us to live in a special place of the Universe.

The more I think about it, the more I've been imagining the Copernican principle as the Robot from Lost In Space, running around and saying, "Danger! Danger, Will Robinson!" When Copernicus speaks, we should all listen.


  1. Your blog records an argument I've seen over and over again that attempts to use the Copernican principal to prove metric expansion of space as the logical conclusion from Hubble's experiments, but I don't see the reasoning.

    Hubble's observations showed all galaxies are moving away with a speed proportional to their distance from Earth. From what I can tell, your reasoning (and the reasoning I have seen implicit in other people's arguments) is the following:

    1. Either the redshift seen by Hubble was

    i)due to the Earth honestly being at the center of the universe and everything was receding from it, in which case the redshift is from the Doppler Effect.


    ii) The redshift is not from the Doppler effect but is due to metric expansion of the universe itself (the stretching of space...in which case the balloon analogy is used to explain how this is different from mundane velocity that would show up as the Doppler effect)

    2. The Copernican Principle rules out option "i)" above because we are not at the center of the universe.

    3. Hence, "ii)" is correct and the red shift is due to metric expansion.


    But this just looks like shoddy logic. From what I can tell, there is absolutely nothing in option "i" that requires Earth to be at the center of the universe.

    Indeed, simple vector algebra should show that if everything is doppler-shifted away from any point in space in a way consistent with Hubble's law, then the same applies to all points without resorting to metric expansion. If Alex if flying away from me at 45 miles per hour, than (in Alex's rest frame) I am flying away from Alex at the same speed.

    If Bob is halfway between me and Alex, and I see Bob running away from me toward Alex, but only at half the speed, then Alex (in his rest frame) see's Bob running away from him toward me, but only at half the speed.

  2. The argument I put forth here is common because it is the most understandable argument for a general audience. Certainly it is not proof. Better arguments require a deeper knowledge exploration of mathematics, physics, and even philosophy than most people have, and I don't have the space nor the time here to lay those arguments out.

    I would rephrase your summary of my reasoning:

    * If the redshift seen by Hubble (and all astronomers looking at distant galaxies) is due to the Earth honestly being at the center of the universe and everything was receding from it, the redshift is from the Doppler Effect, and this would violate the Copernican Principle.

    * If the redshift is not from the Doppler effect but is due to metric expansion of the universe itself, this results in a Universe that has no special location. This instance would not violate the Copernican Principle.

    * If the Copernican Principle is true, then the first case cannot be true, while the second case could be.

    * No "QED", because I have not proven the second case is true, nor that the first case is false. This is the starting point of a much larger line of reasoning that doesn't fit here.

    In regard to your proposed scenario, I have two points. First, the Copernican Principle, as I carefully state in paragraph 4, does not require that we not be at the center of the Universe, but that we not occupy some unique position within the Universe.

    Second, if I generalize your scenario to the Universe at large, there is the unsatisfying characteristic of it being ad hoc. Why are all the galaxies set up in just such a way that they all appear to be flying apart from one another at a speed proportional to the distance between them?

    This is the question that faced Hubble and other astronomers first looking at his data. An expanding metric, which is a solution to Einstein's General Relativity, naturally gives this sort of expansion. In fact, prior to Hubble's observations, Einstein had realized that the Universe should be either expanding or contracting in just such a fashion and inserted his cosmological constant to keep this expansion/contraction from happening. In addition, this homologous expansion also results somewhat naturally from certain types of explosions, which was one line of reasoning that led to the idea of a Big Bang.

    So, my argument would be that the application of the Copernican Principle to the observed redshifts of galaxies led to the development of a Big Bang theory based on General Relativity. That theory, in turn, made several specific predictions (for example, the presence of a microwave background and the relative abundance of hydrogen and helium in the Universe) that were later shown to be correct.

    But even then, it took decades and thousands of research projects to convince the vast majority of astronomers that this expanding Universe is, in fact, reality. And as dark energy as shown us, we still don't fully understand everything about the expanding universe. So I still would withhold that "QED".

  3. Anonymous1:51 PM

    I have a blog against the expansion of the universe, with arguments that show this is impossible.

    How the people consider there are evidences for the Big Bang I study these evidences.

    I have arguments and Hypotheses in: http://bigbangno.wordpress.com


  4. Anonymous5:21 PM

    The whole principle should just be scraped for the utter nonsense it is. It has nothing to do with anything and doesn't even reflect, in any way, how even Humans think let alone God.

    And Gods the key word here. This is the only reason this lame principle continues. It is to say God would put us smack dap in the middle of the universe. Goodness gracious...human dont even do things this way. Is Rome in the center of Italy? Do we live in the center of the earth? Is your TV in the middle of your room? Nothing in design values the center and if does go plant a tree in the middle of your lawn.
    I understand the principle has moved into deeper territory but this is its crux and its, frankly, the work of a simpleton. It shouldnt be applied in way shape or from to anything.
    Its just another way naturalists try and hammer home the thought that we are insignificant when a 3rd grader can see we are in all of 3 seconds. In fact we are so significant that these same people are now finally accepting the odds that everyone already saw in split second, so they are postulating multiverses and infinities to explain what they were claiming is so insignificant.