[andybak]

Yes. From my layman's view the fact that there are serious scientists in the field who still doubt that practical quantum computation is even realistically possible surely indicates we're not on the verge of being able to order one for delivery.

[OscarCunningham]

On the other hand I'm sure you could find lots of other achievements where some scientists were denying it was possible right up to the point it was achieved.

[rwallace]

Could you?

Sure, we can all think of cases where someone said something was impossible and then eventually it was achieved.

But are there any cases where the expert consensus on the physical possibility of something wasn't established until it was achieved in practice? I can't think of one off the top of my head.

Nuclear energy? No, the physical possibility of this was understood in the thirties; the objection was that, as one scientist put it, you would have to turn a whole country into a uranium refinery. Which wasn't exactly wrong; the Manhattan project took engineering resources in the ballpark of a small country.

Supersonic flight? No, artificial objects had been going supersonic for centuries.

Spaceflight? No, the physics of this was well understood long before it was achieved.

Any that I'm missing?

[OscarCunningham]

Right, people had consensus on the physical possibility, but not on whether the engineering problems were surmountable.

But I think there is a consensus on the physical possibility of quantum computing in principle. The [threshold theorem](https://en.wikipedia.org/wiki/Quantum_threshold_theorem) is very convincing.

[rwallace]

Be careful: there's an important distinction. There is a consensus that quantum computers can, for certain problems, perform computation exponential in the number of qubits. But the often unspoken assumption is that the difficulty of getting the computation to stay coherent, is polynomial in the number of qubits. That's currently the big unknown. If it's not, then that's what would be meant by practical quantum computers being physically impossible.

[OscarCunningham]

Is that not the problem that the threshold theorem solves?

[rwallace]

As I understand it (disclaimer: not a physicist), that's the open question: does there exist a stable configuration of atoms that will create conditions such that the threshold theory applies in full, errors are adequately corrected, and an N-qubit quantum computer can be built and operated for cost polynomial rather than exponential in N? Or does that line of thinking rely on assumptions that can't actually be met?

[d-jo]

Quantum Physics.

"I, at any rate, am convinced that He [God] does not throw dice."

[olliej]

Errr, Eistein fully accepted (as did most others fairly quickly) that quantum mechanics was real -- in fact the duality of wave + particle for photons was something he developed (as part of his developing the theory that explained the photoelectric effect).

The "does not play dice" was mostly with regard to Heisenberg, in which he was determined to find a way to avoid the predicted lack of predictability of the future.

[andybak]

"If an elderly but distinguished scientist says that something is possible, he is almost certainly right; but if he says that it is impossible, he is very probably wrong."

https://www.brainyquote.com/quotes/arthur_c_clarke_100793

> On the other hand I'm sure you could find lots of other achievements where some scientists were denying it was possible right up to the point it was achieved.

True and I'm very aware that I'm creeping towards the age where I'm likely to be on the wrong side of these debates. (Not that I'm distinguished or even a scientist)

However - the distinguished, elderly scientists are sometimes right.

[Retric]

That only seems true with hindsight bias, as distinguished elderly scientists are most memorable when they are wrong. Being right 99.9% of the time would not be enough if there are millions of you and people only care about a few quotes.

Granted, you can't be proved correct if you say some action is impossible as someone can always wait to see if you're wrong.

[coldtea]

Or you know, that is a BS quote, extrapolating from too few historical examples (and even them, not understood well), and not some scientific law.

[akvadrako]

That's missing the point. Until it's actually achieved it isn't "finally here".

[posterboy]

It's reverse psychology. Surely, if they are in the field, they want it to be true. I frequently find myself invariably assuming a contrarian position in debate just to provoke more arguments, effectively contradicting myself over the course of different debates. In that sense, I just wanna know if that has a name in psychology as a normal or pathological condition. I mean, it might be purely rhetorical.

Also, if it's not "crypto season", what is it actually, nano, cryo, opto, ..?

[neltnerb]

Any good scientist should be the biggest skeptic of their own work. Though I usually poke holes in my own ideas in private ;-)