
Scientists Discover a Jewel at the Heart of Quantum Physics - epenn
http://www.wired.com/wiredscience/2013/12/amplituhedron-jewel-quantum-physics/all/
======
thaumasiotes
> 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.

~~~
wickedshimmy
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).

~~~
3rd3
> Kol-mo-go-rov

(I get it wrong too every other time)

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

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

Not sure if this one has any new information

~~~
smoyer
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.

~~~
pyre
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

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neaanopri
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.

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

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kevrone
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-...](https://www.simonsfoundation.org/quanta/20130917-a-jewel-at-the-heart-
of-quantum-physics/)

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rbanffy
Another one?!

