
A Jewel at the Heart of Quantum Physics - milkshakes
https://www.simonsfoundation.org/quanta/20130917-a-jewel-at-the-heart-of-quantum-physics
======
gruseom
Is anyone here familiar with this work? I would like to hear more about it. At
a minimum, this article is very much better than the usual science blog
filler—it contains signs of a genuine conceptual breakthrough. For example:

 _“You can easily do, on paper, computations that were infeasible even with a
computer before.”_

That doesn't happen very often! Or this:

 _[T]he new geometric approach to particle interactions removes locality and
unitarity from its starting assumptions. The amplituhedron is not built out of
space-time and probabilities; these properties merely arise as consequences of
the jewel’s geometry. The usual picture of space and time, and particles
moving around in them, is a construct._

That is exactly the kind of thing that happens when one model is replaced with
a deeper one.

~~~
redhat-reallyp
"it contains signs of a genuine conceptual breakthrough."

It contains claims of such signs, which seems to be the point of this fluff
piece.

Alarm bells should go off when you read statements like this:

"...giving up space and time as fundamental constituents of nature and
figuring out how the Big Bang and cosmological evolution of the universe arose
out of pure geometry."

Ah yes, well that solves the mystery, doesn't it. The universe arose out of
pure geometry.

A more concrete sign of the limitations of this work can be seen in the
paragraph prior to the above quote:

"Physicists must also prove that the new geometric formulation applies to the
exact particles that are known to exist in the universe, rather than to the
idealized quantum field theory they used to develop it, called maximally
supersymmetric Yang-Mills theory."

The problem is that maximally supersymmetric Yang-Mills theory conflicts with
experiments (see e.g. [http://news.discovery.com/space/lhc-discovery-maims-
supersym...](http://news.discovery.com/space/lhc-discovery-maims-
supersymmetry-again-130724.htm)). This is why they hurry to qualify this point
by saying:

"This model, which includes a 'superpartner' particle for every known particle
and treats space-time as flat, 'just happens to be the simplest test case for
these new tools,' Bourjaily said. 'The way to generalize these new tools to
[other] theories is understood.'"

Well that's good, because they're going to need that generalization. Now all
they need is to actually produce it.

This is the sort of thing that leads these gee-whiz models to end up on the
ash heap of history.

Don't get me wrong, it would be fantastic if the amplituhedron revolutionizes
quantum theory. But based on the red flags in this article, I wouldn't hold my
breath.

~~~
gaze
Things that work on simple models motivate work on more complex models. He
STARTS OUT by saying N=4 SYM is a toy model. The klein-gordon equation is
unphysical but motivated a lot of good physics.

~~~
selimthegrim
In condensed-matter physics, toy models play with you!

[http://arxiv.org/abs/cond-mat/0007254](http://arxiv.org/abs/cond-mat/0007254)

------
scotty79
From what I get they managed to vastly simplify calculations of super-
symmetric Yang-Mills theory by representing results as volumes of geometric
object.

Super-symmetric Yang-Mills theory is not shown to represent reality (yet?) but
they hope they can use similar method to simplify calculations of the quantum
field theories that are currently believed to represent reality.

~~~
evanb
It's worse than that--- N=4SYM is known not to represent reality. However, it
has some features that make calculations particularly easy to do. The
analogous calculations are doable for other theories, but are trickier. So
they've done the simplest example with this new framework.

------
011011100
Discussion by people who are more likely to know what's going on:

[http://physics.stackexchange.com/questions/77730/what-is-
the...](http://physics.stackexchange.com/questions/77730/what-is-the-actual-
significance-of-the-amplituhedron)

------
noonespecial
Every once in a while I get the feeling that the greatest discoveries are
still ahead and these next few decades might be the real "golden age of
science". The "individual genius" phase is just behind us and the
"collaborative brilliance" phase is just beginning.

~~~
has2k1
It is possible that Gravity as we know it is an illusion. For example, the
Newtonian take on Gravity was that it was a force. That according to Einstein
was illusory, Gravity was really a geometric manifestation due to the
curvature of space-time. With that take Gravity as we know it is different
from the Gravity as was once known. However the Newtonian view that Gravity is
a force can and still does facilitate useful everyday computations.

The go to illusion when we think of illusions is the mirage, you can see as
water complete with the wavy nature of water and if you had no way of getting
closer to inspect it, it might as well be the real thing. Depending on your
philosophical take either it is always an illusion, or it becomes an illusion
when you find out that it is not water.

A note of agreement with what seems like your main query, we might as well
drop the word illusion when it comes to the physical description of the
universe. Any framework of description is full of illusions that may never be
proven otherwise.

I took QFT some years back and it also went over my head. There is still that
mental gap of where the only way you can think about time is via
time-(in)dependent wave function.

~~~
rational_me
>I took QFT some years back and it also went over my head. There is still that
mental gap of where the only way you can think about time is via
time-(in)dependent wave function.

Total bullshit. The wave function is time dependent.

~~~
has2k1
Given psi(x, t), if the relation between x and t is not non- linear. Then you
can have a time-independent Schrodinger equation with a time-independent wave-
function i.e psi(x).

However, you do not need start with a time dependent wave function to justify
the time-independent wave function.

------
mrcactu5
Nima Arkani-Hamed is a nortorious loudmouth.

I went to his lecture from 1-3 and we got an encore from 3-5 and another from
5-6. It is no surprise his paper is verbose clocking at 154 pages.

Scattering Amplitudes and the Positive Grassmannian
[http://arxiv.org/abs/1212.5605](http://arxiv.org/abs/1212.5605)

Alejandro Morales has some lecture notes on a course on the Positive
Grassmanian The professor was Alexander Postnikov at MIT.

[http://www.thales.math.uqam.ca/~ahmorales/18.318lecs/lecture...](http://www.thales.math.uqam.ca/~ahmorales/18.318lecs/lectures.pdf)

Feynman famously said "I think I can safely say that nobody understands
quantum mechanics." That was in 1965 but it's still true.

The scattering amplitudes in a certain case of N=4 Super Yang-Mills theory are
more symmetric than people first expected. This has applications to "total
positivity" and "integrable systems".

This will not help you build an app, but it's beautiful and represents an
important shift in mathematics.

------
freyrs3
The paper from the arxiv mentioned in the article:
[http://arxiv.org/pdf/1212.5605v1.pdf](http://arxiv.org/pdf/1212.5605v1.pdf)

~~~
selimthegrim
I was talking to a friend in the physics department at Stanford a few months
ago about how one of the people whose work is cited in this paper (Greg Moore
of Rutgers) had found analytic expressions purely rooted in geometry for Bethe
Ansatz[1] results in (1+1) dimensional systems. Based on that, I told my
friend I believed these "physical mathematics types" (as they call themselves)
were on to something. He concurred that their methods would be very important
in the future.

(This was in May, lol)

[1] BA is a nifty numerical method involving a lot of number crunching to find
scattering matrices for these theories, and is very useful in low-dimensional
electronic systems.

~~~
ssivark
I think there's a lot of exciting research going on in the context of
integrable systems (~Bethe Ansatz) and N=2 quantum field theories. However,
some people are also working on integrability in N=4 theories, which might be
relevant to the ideas in this article. It's a thought some people (including
Arkani-Hamed) have expressed, but it's too early to tell.

~~~
selimthegrim
You don't know any of Neitzke's grad students by any chance, do you? A certain
fellow with a fondness for army boots and camo fatigues used to live down the
hall from me at Caltech....

~~~
ssivark
A certain fellow working down the hall from me is quirky enough to match that
description :-)

------
Xcelerate
This is really cool. I didn't understand most of it, even though I'm taking
QFT right now. Shows you how deep the subject goes.

On another note, if I see the word "illusion" or "illusory" one more time in
these science blogs, I'm going to have a fit. This must be the meaningless new
buzzword of the day. 1) Spacetime is not an "illusion" 2) Gravity is not an
"illusion" 3) <insert sciency word> is not an "illusion". That doesn't even
make sense.

~~~
crntaylor
I think what they mean is that the force we call gravity is an illusion that
we experience because we perceive spacetime as flat. In a curved spacetime,
you don't need to posit a force to explain motion due to a nearby mass - the
mass curves spacetime, and objects move along geodesics in the curved
spacetime. So while it would be incorrect to say "gravity is an illusion" I
think it makes sense to say "the force of gravity is an illusion".

~~~
ars
> and objects move along geodesics

You say that like it's so obvious. But why do they move in the first place?

~~~
textminer
Good discussion of your question:
[http://physics.stackexchange.com/questions/24359/why-do-
obje...](http://physics.stackexchange.com/questions/24359/why-do-objects-
follow-geodesics-in-spacetime)

~~~
textminer
I thought those seemed part and parcel. :) "I'm in the gravity well of a star"
leads to "this alters the geodesic of spacetime that the I tumble down", which
made me wonder "wait, why does it tumble down geodesics to begin with".

The answers seems to be "physics doesn't have to abide by your intuition or
need for narratives, human"

------
wbhart
The article states about unitarity and locality that "both are suspect". I'm
happy for locality to be suspect, but in what sense is unitarity suspect?
Surely only in the sense that it is apparently a consequence of more
fundamental things. And when you think about it, that's philosophically
amazing, that probabilities summing to one may have an underlying (timeless?)
explanation. Or maybe I should infer that there has been some exaggeration!?

~~~
stan_rogers
It's that "only" in "... only in the sense that ..." upon which the whole
thing hangs. Presupposing unitarity (and locality) and constraining models to
conform to that presupposition may be that complication, that blinder, that
prevents discovery of the simpler underlying rule from which either or both
emerge. And either or both may be _necessary_ consequences of the much
simpler, underlying rule (if there is one to be found), at least on the
observable side of any predicted phenomena, but that can't be determined
unless one is willing to first throw away the constraints that keep you from
finding it. (In effect, it's analogous to simply adding two natural numbers
directly rather than using Peano arithmetic and checking at each iteration
that an increment occurs. Either way you will arrive at the same answer, but
the unconstrained rules for addition allows for negative numbers and
fractions.)

------
Lerc
As a Layman, the concept seems reminiscent of Garrett Lisi's E8 idea. Are they
comparable approaches?

~~~
yk
I did not read the paper, but from the article; probably not. So the E8 idea
is stab at a quantum field theory of gravity. On the other hand these
amplituhedron idea seems to be 'just' a different idea to calculate usual qft
calculations easier. The only connection which may be there is, that
apparently the amplituhedron idea can be turned on its head and then serve as
a new way of looking onto qft. And this may or may not lead into a interesting
direction for quantum gravity theories.

------
tlb
Rejecting unitarity is a bold thing to do. What can it mean for the
probabilities of alternatives to sum to more than 1?

~~~
ssivark
No, they _do not_ reject unitarity. The theory is still unitary. However,
unitarity is not built-in to the theory... the theory talks about some other
ideas and structures, and it turns out that all the answers you compute
respect unitarity -- like it should!

~~~
TeMPOraL
I understand it's kind of like "let's start without unitarity and locality
baked in explicitly in the model and see where it leads and if they come out
implicitly in the results".

------
ivan_ah
A video lecture by Nima Arkani-Hamed here on the topic:

[http://susy2013.ictp.it/video/05_Friday/2013_08_30_Arkani-
Ha...](http://susy2013.ictp.it/video/05_Friday/2013_08_30_Arkani-
Hamed_4-3.html)

~~~
Create
"How should we make it attractive for them [young people] to spend 5,6,7 years
in our field, be satisfied, learn about excitement, but finally be qualified
to find other possibilities?" \-- H. Schopper

(btw: entire careers have been based on susy, without a spec of experimental
evidence: on might say, it is not even wrong)

------
snarfy
It reminds me of heim theory. Supposedly, in heim theory you could derive
particle mass by from quantum numbers by a relationship in a six dimensional
geometric structure.

[http://en.wikipedia.org/wiki/Heim_theory](http://en.wikipedia.org/wiki/Heim_theory)

Wiki doesn't have a lot of info, but it's mostly been debunked from what I've
read.

------
devx
So this guy was right? The geometry-based particle interaction theory sounds
similar to what this guy was saying a few years back on TED:

[http://www.ted.com/talks/garrett_lisi_on_his_theory_of_every...](http://www.ted.com/talks/garrett_lisi_on_his_theory_of_everything.html)

~~~
gaze
Completely different thing. Garrett Lisi threw us some weird looking thing
that when you turn it to the left and spin it 3 times and do a backflip looks
like an electron... and if you turn it up and around and up town it looks like
a quark. However, if you're gonna propose a structure like this, it better
behave in a way that any way you look at it, it better look like something you
might see in the universe. This thing predicted WAY too much shit to have
physical relevance.

The thing in this post is a new way to calculate scattering amplitudes that
has nothing to do with space, time, and isn't motivated by unitarity or
locality.

------
robbiep
My favourite line, and something I wasn't aware of (as a layman)

 _In 1986, it became apparent that Feynman’s apparatus was a Rube Goldberg
machine._

I think Feynman would have loved to have heard his doodles to be described in
such a way (maybe he did, he had a few years left in him at that point!)

------
axilmar
What are the consequences of it regarding P = NP? can this be used as a proof
that P = NP since a 9 page formula is reduced to a simple function? can we use
geometry to solve programs in the NP space?

