5 ms·

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

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 9y ago
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 https://en.wikipedia.org/wiki/Quantum_threshold_theorem) is very convincing.
- rwallace 9y ago
  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 9y ago
    Is that not the problem that the threshold theorem solves?
    - rwallace 9y ago
      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?
throwawaylolx 9y ago
>But are there any cases where the expert consensus on the physical possibility of something wasn't established until it was achieved in practice? Bitcoin. It was widely believed to be impossible to have a decentralized cryptocurrency before Satoshi deployed his system.
d-jo 9y ago
Quantum Physics. "I, at any rate, am convinced that He [God] does not throw dice."
- olliej 9y ago
  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.

OscarCunningham 9y ago

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 https://en.wikipedia.org/wiki/Quantum_threshold_theorem) is very convincing.

rwallace 9y ago
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 9y ago
  Is that not the problem that the threshold theorem solves?
  - rwallace 9y ago
    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?

throwawaylolx 9y ago

>But are there any cases where the expert consensus on the physical possibility of something wasn't established until it was achieved in practice? Bitcoin. It was widely believed to be impossible to have a decentralized cryptocurrency before Satoshi deployed his system.

d-jo 9y ago

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

olliej 9y ago
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.