[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.