Vrftsjtryk

Does anyone know how efficient was the first Turing machine that Alan Turing made? I mean how many moves did it do per second or so... I'm just curious. Also couldn't find any info about it on the web.

Turing never built a physical Turing machine. The point of Turing machines was not to be a practical physical computer but to formalize what it's possible to compute and, indeed, to formalize what "computation" even means.

"Turing machines" (or "a-machines") are a mathematical concept, not actual, physical devices. Turing came up with them in order to write mathematical proofs about computers, with the following logic:

• Writing proofs about physical wires and switches is extremely difficult.


• Writing proofs about Turing machines is (relatively) easy.


• Anything physical wires and switches can do, you can build a Turing machine to do (*).

But Turing never built an actual machine that wrote symbols on a paper tape. Other people have, but only as a demonstration: here's one you can make out of a business card, for example.

Why did he never build a physical Turing machine? To put it simply, it just wouldn't be that useful. The thing is, nobody's ever come up with a model of computation that's stronger than a Turing machine (in that it can compute things a Turing machine can't). And it's been proven that several other models of computation, such as the lambda calculus or the Python programming language, are "Turing-complete": they can do everything a Turing machine can.

So for anything except a mathematical proof, it's generally much more useful to use one of these other models. Then you can use the Turing machines in your proofs without any loss of generality.

(*) Specifically, any calculation: a Turing machine can't turn on a lightbulb, for example, but lightbulbs aren't very interesting from a theory-of-computation standpoint.

The TM only exists on paper. It is a theoretical model of computation. It actually can't be built (because the tape is infinitely long).

So, the answer is: no, Turing never built a TM in real life, because he can't.

This is not exactly an answer to the question. However, I cannot help making some serious fun.

Claim One: Lots of Turing machines have been built, by Alan Turing or by many others.

Proof. I am sure Alan Turing might have pointed to a piece of junk and said, "look, this is a Turing machine that just halt at its very first step given any input". Me too.

Claim Two: It is undecidable whether someone has built a Turing machine with unbounded tape.

Proof. Here I claim that I have built one. However, to show each additional bit of its tape, it will take another day. Even I am not certain whether I have built a Turing machine or not.