Thursday, July 1, 2010

The Impossibility of Why

So I think we've all been rather busy here.  Hence the lack of posts.  I'm going to try to keep this one short but sweet.  A lot of people think that physics tells us why things happen.  Why is the sky blue?  Why does the earth orbit the sun?  Why does copper transmit electricity so well?  These all seem like perfectly reasonable questions to ask.  Questions that we, as physicists can answer.  Yet, I entitled this post the impossibility of why.

In general, questions about why are not good questions for physicists   More accurately we answer questions about how.  Or what.  What phenomena causes use to see the sky as blue?  What forces cause the earth to orbit the sun?  How does copper transmit electricity so well?  In general, we can't answer a question of why.

A friend once asked me (at the end of a talk I gave, nonetheless) 'why can't two electrons be in the same quantum state, while two bosons can?'.  That's an example of a question that we as physicists can't answer. We have no idea about why that is.  The best answer we can give is 'I don't know.  However, experiment tells us that's what happens.'

In general, physics cannot be built in a vacuum.  We cannot sit down and write down the laws of nature.  Not without looking at nature.  That is the difference between physics and mathematics.  Mathematicians can construct arbitrary logical systems.  Anything with a given set of logical axioms that is consistent can be a valid system of mathematics.  Of course, only a few are useful systems.  This allows mathematicians to generate exceedingly interesting playgrounds for the mind.

Physics is meaningless without experiment.  We have to test our theories against the world as we know it.  We can no more explain why F = ma or the Pauli exclusion principle is the way of the world than we can answer why the universe exists.  We can (at least we hope) tell you how the world works, what to expect, cause and effect.  But why we found this particular set of rules and not another, different but consistent system, is something we can't, and usually don't try to answer.

In many ways, I think this reduces much of the perceived conflict between religion and science.  As long as one doesn't read God's (or gods, depending on your religion) word literally, religion is an attempt to explain why.  Physics is an attempt to explain how.  There's no inherent conflict in that.  At least, that's my thinking.


  1. Well, I don't think anything can ultimately answer a long chain of why questions. That's why kids can have so much fun by repeatedly asking their parents "why?" after everything they say. They are bound to get stuck eventually.

    In science, I think we can answer why questions to a certain extent, but only by replacing them by another why question. For example, in your bosons vs. fermions question you could have answered that it follows from the spin-statistics connection. However, then it would be replaced by the question of why does the spin-statistics connection hold, and so on ad infinitum.

    Religion is similar to the extent that it also leads to a long chain of why questions that end with something unanswerable, e.g. Q: Why does something exist rather than nothing? A: Because God willed it so. Q: Why did God will it so? A: We can't answer questions about God's will. Q: Why can't we answer questions about God's will? A: We can't answer questions about God's will. etc.

    The main difference here is that in the case of religion the chain ends in some sort of mystical refusal to answer further questions, whereas in the case of science we just admit that we don't know, but we don't deny the possibility that we might be able to take the chain further sometime in the future.

    In conclusion, religion doesn't answer why questions either. Instead, it just blocks the chain of questions dead by appeal to some sort of mystical principle.

  2. I think that you conclusion is expressing a bias. Religion successfully answers the question of why, assuming that you accept the answer. It is an answer that I, and it sounds like you, are not ready to accept, but since it is an answer that we cannot prove or disprove, it is as valid an answer as anyone can give.

    Religion has answer the question, and answered it satisfactorily within their framework and thinking. To those of us outside the framework, it may not be satisfactory. However, the very fact that they can provide an answer to the question differentiates them from physics (or any science), where that kind of question is inherently unanswerable. We can't give reasons for why the universe exists, we can only explain how it works. Religion (and now I show my bias) seems able to do the opposite, they can't explain how the universe works, but they can give reasons for why it exists. Whether you accept their reasons is another, more personal, question.

  3. This reminded me of the classic Feynman clip where he gets a "why" question.

  4. More broadly, questions of "why" are predicated on the assumption of purpose or meaning (of a phenomenon). Questions of "how" are built on the assumption of causation behind phenomena.

    Historically, we started trying to understand the world looking for explanations of the former type. It typically gave us explanations with not much predictive power. And it crumbled once we realized the anthropomorphic nature of purpose and meaning. So now we stick to the latter type of question and answers. It leaves some people cold, but its probably a wiser way to understand the world.