Scientists Discover a Jewel at the Heart of Quantum Physics | Hacker News Reader

Scientists Discover a Jewel at the Heart of Quantum Physics

wired.com

83 pointsepenn13 comments

> Unitarity says the quantum mechanical probabilities of all possible outcomes of a particle interaction must sum to one. To prove it, one would have to observe the same interaction over and over and count the frequencies of the different outcomes. Doing this to perfect accuracy would require an infinite number of observations using an infinitely large measuring apparatus, but the latter would again cause gravitational collapse into a black hole. In finite regions of the universe, unitarity can therefore only be approximately known.

I'm sure the physicists have something coherent in mind, but the discussion of unitarity in the article makes no sense at all. The idea that the sum of all probabilities of all possible outcomes of some situation is 1 isn't even part of physics; it's the definition of probability. It's not necessary to make an infinite number of observations, or even a single observation, to know that the sum over all the possibilities is 1 (also phrasable as "something will happen"). If we did make an infinite number of observations, the only possible sum over all possibilities would be 1, because it's 1 by definition. No matter what we observed, it would still be 1, and we would assign probabilities based on our observations so that the sum of them all would be 1.

The previous HN discussion linked by Hopka seems to state that in fact unitarity is not in doubt, that it's just that under the system described in the article no one has yet proved that a non-unitary solution cannot be generated. But how could unitarity possibly be "suspect"? It's a definition.

There are two things, one is the definition of a probability and the other is unitarity. Unitarity is observable, that is closely connected to the information content of the system, and if memory serves to the evolution of the normalization factor of the wave function. So if unitarity is 1, then the interpretation of quantum field theories as probability distributions is straight forward. If it is not 1, then the theories look strange.

Yeah, this is an issue of "probability" in the physical sense not being the same thing as probability as defined by the Komolgorov axioms -- essentially, a sample space that does not have unit measure. This can end up being "resolved" by redefining the sample space, and the notion of what an "elementary events" is for physical interactions, for something we can define (and normalize) a total measure over, or by accepting the fact that probability isn't an accurate/complete description of what's going on (in the same way that quantum mechanics resulted in accepting that measurement wasn't a complete description of what's going on).

> Kol-mo-go-rov

(I get it wrong too every other time)

The excitement would be if spacetime and quantum mechanics can be shown as emergent from something more fundamental. Locality and unitarity emergent from an underlying principle. Right now, with Feynman diagrams, locality and unitarity are built in. So this new scattering structure formulation doesn't need Hilbert space, spacetime and quantum mechanics.

The same article was discussed here ~3 months ago: https://news.ycombinator.com/item?id=6403285

Yes, this one has a date of yesterday, still, it's very familiar

Not sure if this one has any new information

Where did you even see the date ... It's not apparent on my phone but I remember reading about this here on HN as well.

From Wired:

  > Scientists Discover a Jewel at the Heart of Quantum Physics
  >
  > By Natalie Wolchover, Quanta Magazine 12.11.13 12:46 PM

From Simons Foundation:

  > A Jewel at the Heart of Quantum Physics
  > 
  > By: Natalie Wolchover	
  > 
  > September 17, 2013

I spoke with Nima about this a few months ago.

He isn't saying that sometimes all probabilities don't add up to one, he is constructing a physical theory without assuming that as a precondition.

It is a consequence of the theory and develops, but he said that if he can explain unitarity in terms of other phoenomena, then there is probably something more fundamental happening, possibly new physics.

For the Scott Aaronson parody post, see http://www.scottaaronson.com/blog/?p=1537

The same author published a similar article back in September on the same subject. Very intriguing!

https://www.simonsfoundation.org/quanta/20130917-a-jewel-at-...

Another one?!

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