Is a Proof a Proof If No One Can Check It?

http://www.nytimes.com/1988/12/20/science/is-a-math-proof-a-proof-if-no-one-can-check-it.html

by kqr2 • 17 years ago

32 29 17 years ago

pj2• 17 years ago

As the singularity trots backward in time toward us, these types of computer found solutions that humans can't comprehend are going to increase.

xenophanes2• 17 years ago

Huh? Singularities do not trot or time travel.

jderick• 17 years ago

Try using your imagination

enum• 17 years ago

If you're willing to understand the program behind the proof, I think it's more acceptable than a handwritten proof.

bitdiddle• 17 years ago

reminds me of when the four color theorem was proved using a computer program. They had to convince everyone the proof was valid. There were lots of questions about the validity of the program.

shib71• 17 years ago

There are proofs (by humans) that are too large for all but a rare few to check. Wiles' proof of "Fermat's Last Theorum" was 100 pages. Courtesy of Wikipedia:

"Because Wiles had incorporated the work of so many other specialists, it had been suggested in 1994 that only a small number of people were capable of fully understanding at that time all the details of what Wiles has done." [http://en.wikipedia.org/wiki/Wiles%27_proof_of_Fermat%27s_La...]

No one can argue that an unconfirmed computer proof is more trustworthy than an unconfirmed proof from a human.

telegraph• 17 years ago

I think you're overlooking the real thrust of the situation. Wiles' proof was relatively lengthy and involved, but it can be examined and studied by humans in an extremely reasonable amount of time. It only took three days for Wiles to present his original proof. There aren't many people in the world who can understand it, and there are fewer who are knowledgeable enough to confirm its validity, but they exist.

Conversely, the proof described in the NYTimes article is of such length that no single mathematician could confirm its validity -- rather than deducing the non-existence of the object in question by a logical argument, it examined a huge number of possible cases. In that respect, it is far more similar to the proof of the four color map theorem. The issue is not so much whether or not we trust the computer's result, but moreso what it means for mathematics to proceed with results that we do not, in a traditional sense, understand.

skermes• 17 years ago

This question came up in some of my more abstract classes in college. A few professors in my department were working on slightly different problems in the same general domain of automated mathematical problem solving and proof construction. The general consensus as I remember it was that the simplest way to get around the problem of no human being able to survey these proofs was to do something like this:

1. Define a machine read- and writable logic that can express your theorem and the steps you think it'll take to get there. 2. Write an automated proof-checker that can verify that proofs in this language are correct. 3. Prove the correctness of the checker. 4. Write a program that starts with your axioms and writes out a proof that ends in your theorem. 5. Verify it with the checker.

Now the only proof that needs to be human-surveyable for us to be certain that everything is correct is the one in step 3. The proof created by step 4 can fill up a skyscraper full of hard disks, and as long as the proof checker verifies it we know that it must be correct. Given a simple enough proof language (FOPL, for example) and a suitable programming language (the choice at the time was lisp, I believe) step 3's proof is easily short enough for a human to verify.

The only hole left is the possibility of a subtle machine malfunction that causes the checker to falsely categorize a proof as correct. On modern hardware this possibility is remote enough that once a proof has been verified a number of times by independent researchers on their own hardware we can safely ignore it.

timwiseman• 17 years ago

You have some magnificent points.

At least to me, this is a nonissue. If the program can be proven to be correct (in the sense that the logic can be confirmed so that it will result in a correct answer), and the hardware can be shown to be able to properly execute the program, and the programs answer is proven.

Even the already tiny chance of a hardware error can be eliminated in all practicality by the combination of error checking within the program itself and repetition on independent hardware.

russell• 17 years ago

Cray 1S? Isn't that about the power of today's desk top? The Cray could do 160 to 250 MFLOPS depending on the problem (Wikipedia). That seems pretty slow. I answered my own question http://forums.techpowerup.com/showthread.php?t=94721. There are processors doing up to 60 GFLOPS. Must be an error in the story.

EDIT: I have to start paying attention to bylines. There are archeologists among us.

goodside• 17 years ago

No error. The story is from 1988.

J_McQuade• 17 years ago

Well, the proof is perfectly valid, as on some abstract level a computer == Turing machine == something you can reason about. If you can prove that your program works in the required way, then your proof holds.

However, brute force solutions are ugly and shed little (if any) new light on the problem, and in the words of G.H.Hardy - "There is no permanent place in the world for ugly mathematics". With things like this I just can't help but think that an elegant and enlightening proof will unexpectedly pop up some time in the distant future when we're all off researching some sort of new-age hypermaths or something...

... well, I suppose a man's got to dream, right?

andrew1• 17 years ago

I'd agree with your dislike of ugly solutions. I guess the benefit of this being accepted as a proof is that all the conjectures that begin 'Assuming that x is true...' can now be converted to theorems saying 'x is true, therefore...', which might tidy up a lot of loose ends.

hexis• 17 years ago

If it's called a "proof" shouldn't it prove something to someone? If it can't be checked, it seems like it should be called something else.

KaiP• 17 years ago

Explanation of the proof by the person who implemented it:

http://www.cecm.sfu.ca/organics/papers/lam/paper/html/paper....

justin_vanw4• 17 years ago

For brute force proofs, the proof basically nothing to do with the output of the program. I'll explain:

A mathematical proof is just some utterance or depiction that will convince your audience that you have proven something. The actual utterance that might 'count' is totally dependent upon who is in the audience. If you are a topologist and you are talking to another topologist working on very similar stuff, your proofs would be incomprehensible to even other mathematics researchers, who could not determine if you were giving a valid proof or not, while the other topologist could easily tell.

If your audience is a really smart physics professor, that same topology proof would have to suck in many other facts, and explicitly quote accepted theorems, or you would risk the physicist jumping up and saying "not so fast" every time he see's a break in the reasoning. At least the physicist will be comfortable with the existence of theorems.

Now, a proof of the same thing, given to an alien with an IQ of 5000 but who's race never thought up the concept of a theorem would be exquisitely painful. You might have to actually give the proof of each theorem you use, and each theorem each of those theorems use. Otherwise, he would say "your logic is flawless, but you assume facts you have not proven, so this proof is incomplete."

If people were way smarter, proofs would be way shorter, since nobody writes down things that are really obvious. Nobody says something like "since the real numbers are commutative under multiplication..." because there is no need to say it. If people were 50 times smarter or something, way more complex things would just 'go without saying'.

So, what does some kind of brute force proof really consist of? It's a program that runs on a computer generally. So the proof is really the source code. If people are convinced the program does what you claim, and then you run it, that is certainly a proof.

If you can't count that, you also can't count any other non-constructive proof. I would say that they are basically pointless, since you can use the result of the proof in other proofs, but you can't build on it very much.

eru• 17 years ago

I agree with your notion of "proof as conversation". That's the way presenting proofs verbally works in practice. (And I am a mathematician.)

justin_vanw4• 17 years ago

I got most of this from conversations with Rich Schwartz. http://www.math.brown.edu/~res/

hc2• 17 years ago

no, this is really begging the question. youve reduced "the proposition is true" to "the algorithm is correct, and the algorithm returns 'true'". once you prove the former you still have to do the hard part of actually _proving_ the latter. i suppose this amounts to mathematically proving, among other things, that your pentium 4 or whatever is incapable of executing a program incorrectly.

justin_vanw4• 17 years ago

You miss my point completely. You are thinking that mathematical proofs are like airtight and completely unassailable and locked in for all time. In reality (practice) they are pretty sloppy, and are really just a form of persuasion.

Now, persuading a group of mathematicians to believe a claim is a high bar, and they are very good at finding logical holes, gaps, and mistakes. As Nietzsche points out in Beyond Good and Evil, it is quite possible that humans as a species have all sorts of blind spots in our reasoning abilities, and that such blind spots are essential for life itself.

In fact, we know of many of these blind spots, or failures of human reason. One great example is the Monty Hall problem. Others are the lottery, gambling, stock market events, advertising (the effects of), and all sorts of experiments that economists do these days.

So before you get concerned about the Pentium IV's accuracy, and 'mathematically proving' it is correct, consider that your brain is a very imperfect reason engine, and there is no reason to suspect that our intuition is accurate except in (and still imperfectly in) those situations where those intuitions improved the reproductive chances of our ancestors.

ErrantX• 17 years ago

I cant help thinking Douglas Adams brought this exact problem up ages ago when Deep Thought found the Answer to Life, The Universe and Everything.

As Adams pointed out the answer is sometimes useless because the process (or the question) was incomprehensible to the "beings" who sought the answer.

Then later when Earth is destroyed before the Answer is calculated he makes an even stronger point: It doesnt matter

The answer gave occupation to the philosophers for years before it was discovered, it provided millions of years of purpose for the "mice" in finding the answer. And Ultimately the lack of the answer didnt matter anyway.

Perhaps we can just accept the answer (knowing that it could be innacurate and keeping an eye out for indicators) and make use of it? Surely that is more worthwhile - it's something of a leap to completely trust the computer, but it seems more dangerous to refuse to trust it at all :)

sireat• 17 years ago

In some ways this kind of proof is similar to computer generating massive end-game databases for chess.

The resulting perfect play for some endgames is incomprehensible to a human GM and can last for more than 100 moves.

http://www.xs4all.nl/~timkr/chess/perfect.htm

Is a Proof a Proof If No One Can Check It?

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