Comment by cevi | Hacker News Reader

Comment by cevi | Hacker News Reader

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Unlike perpetual motion or free energy, the math and physics behind quantum computing is rock solid - the possibility is baked into the framework of quantum mechanics at an absolutely fundamental level. Even if it turned out that all of quantum field theory was completely bogus, I would still expect the basic framework of quantum mechanics (i.e. unitary evolution, the Born rule for converting amplitudes to probabilities, density matrices describing mixed states) to be correct.

If quantum computing was not possible, that would mean that everything we thought we knew about physics was hopelessly wrong. Physics would have to be much stranger - we'd have to give up on either locality (i.e. causation being limited by the speed of light), reversibility (closely related to conservation of energy), or nondeterminism in order to make the math work out. There are several papers in the literature that derive quantum mechanics from these or related assumptions (often together with some extra technical detail, that differs between approaches):

https://arxiv.org/abs/quant-ph/0101012 https://arxiv.org/abs/1011.6451 https://arxiv.org/abs/0911.0695

I think that quantum mechanics is a useful model and there is a lot of truth in it, but it is not the truth.

Using rock-solid math and physics I think you can do time travel and speculate about parallel universes, but I don't think we should blindly believe the models.

We should never confuse the map and the territory.

Then how do you explain the extraordinary predictive power of QM, and why do you believe that the same maths that gives us measurements accurate to 23 decimals is completely inaccurate in its predicition of QC?

30 years is nothing given how complex the engineering problem is of actually building a working QC. Fusion is much simpler and very well understood and yet it has been more than 50 years since research into fusion power plants has started, and the most optimistic estimates say we'll have the first ever research power plant 30 years from now, if everything goes well (EUfusion's DEMO plant).

There is no solid math or physics which supports time travel. There are a lot of cases where people claim to have found some loophole or another in the impossibility proofs for time travel, sketching out a model that looks plausible at first glance, but they all end up postulating some blatantly non-physical entity somewhere along the way (i.e. cosmic strings, negative energy density, ... - I've gotten my hopes up quite a few times).

Speculating about parallel universes is a different story - even if they exist, we can't travel to them, so there is no sense in claiming that we can somehow harness their power. It's a philosophical question, not a physical one. In contrast, the rules of quantum mechanics (when phrased in terms of density matrices, at least) only describe things we can actually measure and observe, and people actually do test every single aspect of the predictions quantum mechanics makes.

> There is no solid math or physics which supports time travel.

This is false. Time travel to the future is possible and proven[0]. Perhaps you are referring to traveling to the past.

[0] https://en.m.wikipedia.org/wiki/Experimental_testing_of_time...

As a nitpick, negative energy densities are indeed uncommon but they aren’t unphysical. In fact, every quantum field theory admits at least one state with negative energy density—its a theorem. But even in quantum field theory there isn’t enough negative energy density to actually form acausal (faster than light) shortcuts through the spacetime.

Oh, I didn't know about this! "Get past chapter four of any textbook on quantum field theory" has been on my to-do list for quite a while.

Sure, experiments will always be the ultimate arbiter of truth. But if you don’t “visit the territory”, you’ll never know that your map is wrong. The map has been challenged in many many ways that seemed more impossible than large scale QC, and the map turned out to be right.

Backwards time travel isn’t possible in general relativity, though, making very mild and physically plausible assumptions about the spacetime. Same in semiclassical quantum gravity.

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