
Beyond Einstein: Physicists find surprising connections in the cosmos - nyc111
https://discovery.princeton.edu/2018/12/02/beyond-einstein-physicists-find-surprising-connections-in-the-cosmos/
======
ivalm
AdS/CFT correspondence is a pretty old idea, whether it will yield something
useful is still to be seen. Our world is not an N=4 Yangs Mills CFT, and our
space is not AdS, so both sides of the correspondence are basically toy models
that have been known for a while and haven't produced verifiable predictions
(other than supersymmetric partners we haven't seen).

~~~
auntienomen
Neither of these models predicts superpartners. The AdS theory doesn't include
the standard model. The N=4 theory doesn't even have particles. They're toy
models, albeit rather sophisticated ones. Their utility is that they can give
us concrete demonstrations of various conjectures about gravity, holography,
and gauge theories.

~~~
ivalm
> The AdS theory doesn't include the standard model

I never said AdS includes standard model?

> The N=4 theory doesn't even have particles

Yang Mills is a super symmetric CFT used as a toy model for other super
symmetric models (which do have super partners; Yang Mills informs us about
our real world only in as much as it informs us about other supersymmetric
CFTs).

> Their utility is that they can give us concrete demonstrations of various
> conjectures about gravity, holography, and gauge theories.

Sure, but there is significant concern about how meaningful those
demonstrations are since the theories that are most similar to the toy
examples are themselves probably dead ends.

------
anthony_doan
The 5th dimension have already been experimentally conducted.

[https://www.youtube.com/watch?v=3HYw6vPR9qU](https://www.youtube.com/watch?v=3HYw6vPR9qU)

I won't give away the spoiler but this article is written within a few months
of the above video (oct vs dec).

The paper in reference is: "Limits on the number of spacetime dimensions from
GW170817" Pardo, Fishbach, Holz & Spergel 2018
[https://arxiv.org/abs/1801.08160](https://arxiv.org/abs/1801.08160)

~~~
mike_ivanov
Did you mean "experimentally ruled out"?

From the paper: "gravitational waves propagate in D=3+1 spacetime dimensions,
as expected in general relativity."

~~~
anthony_doan
Yep it was ruled out there is an extra dimension that's hiding gravity's
strength that was the conclusion of the paper.

------
isolli
It's difficult for a non-specialist to place this article in a broader
historical context. I thought string theory was by now more or less considered
a dead-end, so I wonder if this article is an attempt to revive it, or the
first sign that it may actually go somewhere after all. Maybe someone
knowledgeable can weigh in?

~~~
orbifold
I've worked on string theory briefly (published one paper that got a
respectable number of citations). It is not considered a dead end by people
working on it obviously.

Many of the best people in theoretical physics with Edward Witten being the
most prominent example have been working on it for ~40+ years now. None of the
public detractors have nearly the same stature that Witten has. Of course this
is basically an argument by authority and there is a chance that once titans
like Witten leave the playing field the following generations will lose more
and more influence.

I think there is a fair bit of criticism of String Theory among very good
people that is not voiced publicly because there would be no point to it, they
have nothing to gain. In many ways it is the "Only Game in Town" (literal
quote from my supervisor), just doing high-energy particle phenomenology is
not that much more exciting.

String theory has already led to a number of very high profile successes in
mathematics (Perelman's Proof of the Poincare Conjecture, Monstrous Moonshine
etc.), so it is not like the techniques that have been developed are totally
useless.

~~~
goatlover
> In many ways it is the "Only Game in Town" (literal quote from my
> supervisor),

Aren't there a few alternatives such as Quantum Loop Gravity?

~~~
auntienomen
Yeah, loop quantum gravity is pretty badly broken as a theory of gravity. It
doesn't have the right classical limit (i.e., general relativity). Doesn't
even have thermodynamics compatible with the right classical limit.

I'd be extremely hard pressed to name a significant result loop quantum
gravity has exported to the rest of physics and mathematics.

There's also the asymptotic safety program, which isn't completely
implausible, but is kind of stuck in the 'steal underpants' phase.

------
mike_ivanov
"black holes have a temperature that arises because each particle that falls
into a black hole has an entangled particle that can escape as heat"

o_O

~~~
rhn_mk1
I believe this is referring to Hawking radiation.

I'm not an expert, but normally particles don't have an entangled counterpart.
Only virtual ones do, and those which fall in can have a counterpart which
ends up being "radiated".

Perhaps the confusion is caused by the current understanding that any
information (and mass?) that comes in will eventually get emitted back away.

~~~
mike_ivanov
"virtual ones .. a counterpart which ends up being "radiated"." \-- but this
is not how Hawking radiation works.

~~~
rhn_mk1
Care to point out the error?

~~~
mike_ivanov
It's a form of redshift effect in curved spacetime. Any massive object
stretches the spacetime in a way that the spectrum of the light emitted by
that object shifts down from the point of view of a remote observer (remote -
this is important). The larger the mass, the stronger the effect.

A black hole would create an infinite redshift (due to singularity inside of
it) -- but only if infinitesimal EM frequencies were possible, which in turn
would imply continuous spectrum. The energy of photons is quantized, therefore
no truly infinite redshift is possible. The same applies to other particles.

Basically it's a raster effect that becomes apparent when you zoom up on a
magazine photograph, or equivalently -- when you stretch a photograph printed
on a rubber sheet, the only difference is that you stretch spacetime, not just
space.

~~~
Pharmakon
That’s part of it, but then you have to account for the role the event horizon
plays, and inherently complicated things like wave packets scattering off of
it. I can’t pretend to understand the process well enough to talk about it
without resorting to heuristics.

------
TomMckenny
Given this explanation of gravity, can anyone hint (to a layman) how the-
force-due-to-acceleration works in this model?

------
blaze33
Does assuming some ultimate law ruling the universe what mainly drives
research?

Or, what if our so-called laws of nature, just like life, only emerged from
the initial big bang? Was there a natural selection of the fittest particles
after the big bang?

~~~
goldfeld
M-Theory posits that yes, the laws of nature and physics have evolved as the
miltudimensional space evolved, they may not always have been the same as now.

~~~
blaze33
Thanks, didn't knew about M-theory, I'll look it up!

Science taught me about how everything is made of particles while their
interactions follow the laws of nature. Much later, reading Wittgenstein, I
understood that the world is not so much about it's particles but more about
how things interact together. Because if something never interact at all with
anything, it may as well not exist at all. So the very existence of something
always comes with its interactions to the world and whatever rules they happen
to follow...

