
Holometer rules out first theory of space-time correlations - jonbaer
http://www.symmetrymagazine.org/article/holometer-rules-out-first-theory-of-space-time-correlations
======
danbruc
Probably biased by the physicist I mostly listen to because they have a lot of
lectures and talks online, Leonard Susskind and Nima Arkani-Hamed.

The idea that space is quantized is pretty unlikely to be true because of
relativity, i.e. because of length contraction the size of those space quanta
depends on the motion of the observer. So if you find those space quanta have
a specific size in your reference frame I can just go to a reference frame
moving relative to yours and I will see length contracted space quanta and we
will therefore disagree on their size.

It is actually believed that neither space nor time are fundamental but that
they emerge from something more fundamental because spacetime, relativity and
quantum physics taken together are not really compatible, see for example
"Space-time is doomed. What replaces it?" [1]. Susskind explores the idea that
entanglement is what holds space together under the name "ER = EPR" [2].

[1] [http://www.cornell.edu/video/nima-arkani-hamed-spacetime-
is-...](http://www.cornell.edu/video/nima-arkani-hamed-spacetime-is-doomed)

[2]
[https://youtube.com/watch?v=OBPpRqxY8Uw](https://youtube.com/watch?v=OBPpRqxY8Uw)

~~~
qrendel
I'd be curious to know why the Planck length wouldn't be reference frame
independent, though... (which may be explained in your links, but it will take
some time to go through them).

~~~
yk
Roughly because of Lorentz boost, aka length contraction. So if I take some
distance, say 1m, and I look at it from relativistic speed it gets contracted.
And if I do that from high enough speeds, it gets contracted below the Planck
length, so a fundamental length constant would violate Lorentz symmetry.

Some people tried to construct such a theory, but as far as I understand, they
are going nowhere. See

[https://en.wikipedia.org/wiki/Doubly_special_relativity](https://en.wikipedia.org/wiki/Doubly_special_relativity)

~~~
mlonkibjuyhv
How about if length contraction means that 1 meter is now fewer length quanta
than before?

I mean if spacetime is emergent why limit ourselves to an intuitive sense of
what distance is.

~~~
yk
Well, if you have less length quanta then we could call them a unit of length,
they would no longer be any kind of fundamental property. ( But actually the
notion of a particle is a rather big problem of quantum field theory in curved
backgrounds.)

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qrendel
> _If the universe were similarly segmented, then there would be a limit to
> the amount of information space-time could contain._

I thought there is a limit to the amount of information spacetime can contain:
the Bekenstein bound[1], and its variations[2]. Though it's related to the
surface area bounding the region, and not the volume enclosed itself. For a
spherical cubic cm, about 10^66 bits, iirc.

[1]
[https://en.wikipedia.org/wiki/Bekenstein_bound](https://en.wikipedia.org/wiki/Bekenstein_bound)

[2]
[http://www.scholarpedia.org/article/Bekenstein_bound](http://www.scholarpedia.org/article/Bekenstein_bound)

~~~
lpage
I'm wondering about that as well. The closely related covariant entropy bound
is predicated on both the holographic principle and our current understanding
of quantum gravity, so I'm not sure if this finding calls that into question
or not.

There was a good HN discussion on the theoretical limits of information
density a few years ago:
[https://news.ycombinator.com/item?id=6466430](https://news.ycombinator.com/item?id=6466430)

------
powertower
The results seem to disproves both:

1\. A quantum foam space-time.

2\. A networked/graphed space-time in which the nodes can have plank-scale (or
larger) movements and re-arrangements.

But I'm not sure if this experiment disproves a space-time of _fixed-position_
nodes which have additional properties (such as a field of scalars / vectors /
spinors / etc).

To test the above I think measuring the smallest possible change-of-angle that
a laser can make off a reflective surface (compounded X times) would do well.
If the arrangement is as above, rotating the reflective surface by the
smallest amount will affect the produced angle in a non-linear way (compared
to larger amounts).

~~~
c-slice
Also seems to provide evidence against simulated reality.

~~~
breakyerself
You can zoom into a fractal image forever.

~~~
powertower
In a simulated environment the cost of computation grows exponentially. There
would be a cutoff point.

~~~
coldtea
Depends on the implementation.

You don't have to describe infinite (or very many) ever smaller layers at all
times -- as with the fractal example, you can just render them when someone
looks at them (which is almost always never below some point) and only at
those areas and levels in which they look (make measurements) at a particular
moment.

Besides, we assume a simulation that's uniformly behaving at all points. They
designer of the simulation could very well hard-code the result of that
experiment to be what it is.

~~~
dangirsh
What counts as a "someone"?

~~~
chriswarbo
As always, observer-dependent effects are actually information-dependent
effects.

In this case, the Universe is simulated as a single entity. Since the
behaviour of the Universe depends on its contents (i.e. we can gain
information about the contents by observing the overall entity), this coarse-
grained simulation is forced to sub-divide into smaller regions. The
simulation keeps subdividing (some regions more than others) until it reaches
a level where some hard-coded criterion is reached that further subdivision
will have no effect on the outcome. For example, if an entire planet is
getting swallowed by a star, it is not useful to simulate each atom of the
planet; just treat the whole thing as one entity.

Unfortunately, the simulation has to spend a lot of resources simulating part
of the Milky Way galaxy, because it contains a chaotic mechanism of "life",
which may cause an observable effect on the Universe as a whole, e.g. by
seeding galaxies with star-engulfing, self-replicating megastructures. In the
region around Earth, the simulation was forced to sub-divide to a level we
might call "classical mechanics", in order to calculate outcomes with enough
precision.

Unfortunately, during the 20th Century, more subdivision has become necessary,
as "life" has begun performing "experiments" with a precision beyond that of
the "classical mechanics" level. These experiments act as amplifiers, turning
very small effects (such as the discrete nature of space) into large ones
(such as the publication of scientific papers, development of technology,
colonisation of intergalactic space, etc.).

It turns out that the Universe is written in Haskell, and a "print" statement
caused a thunk to be forced ;)

------
Estragon
Good to see someone getting some press for a negative result.

~~~
jonbaer
What I am wondering about is what the implications (and reactions) would have
been if it were a positive result ...

~~~
Estragon
From a purely scientific perspective the rational response would be to assume
it's a yet-to-be-accounted-for experimental artifact, but no one in the
reporting chain has an economic incentive to approach it that way.

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shmerl
_> If the universe were similarly segmented, then there would be a limit to
the amount of information space-time could contain._

Not really, if those "pixels" are a countable set. Is still wouldn't be
continuous, but it could hold any information. Symbolic elements ("pixels",
letters etc.) actually are basis of information, so it kind of implies a
discrete space when information is involved.

~~~
graycat
> countable

Maybe you have in mind _countable dense_?

~~~
shmerl
Not sure if it has to be dense. It can still hold any information if it's not
finite.

------
jonbaer
The first time I learned about this project was from the BBC Documentary on
Reality (excellent BTW) ... here is the holometer segment ...
[https://youtu.be/DbqB0--Td28?t=2346](https://youtu.be/DbqB0--Td28?t=2346)

------
Kinnard
This pairs very well with Stephen Wolfram's recent piece on the true nature of
space-time: a network

[https://news.ycombinator.com/item?id=10664272](https://news.ycombinator.com/item?id=10664272)

Published acoincidentally on the 100th anniversary of Einstein's original
General Relativity Paper.

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yCloser
TL;DR: background-color: #e45950

~~~
TranquilMarmot
My eyes!

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barrystaes
Reading how sensitive this equipment can measure short duration displacements,
and the distance the laser beams travel.. I wonder if - and how - they would
adjust this for the drift due to the earth rotating. (and its speed is not
even constant)

~~~
danbruc
It's probably even way worse. I think it was one of the gravity wave
experiments and they talked about how they picked up signals from people
walking in neighboring building, cars driving on the highway and even the
ocean waves hitting the beach while the beach was fifty or a hundred or so
kilometers away. Maybe it's as simple as a high pass filter but I'd also like
to know more about the details.

EDIT: It was probably not this exact article [1] but it matches my memories
pretty well. They also mention some of the countermeasures.

[1] [http://www.nature.com/news/physics-wave-of-the-
future-1.1556...](http://www.nature.com/news/physics-wave-of-the-
future-1.15561)

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iopq
I have not reached for the Inspect Element faster than seeing this
abomination. Who thought that an all red webpage might be difficult on the
eyes?

~~~
ansgri
This is more like 'salmon' than 'red'. Seems rather readable on a good display
with low brightness. Unsafe choice for not art-specific website though.

