
The quantum source of space-time - joshus
http://www.nature.com/news/the-quantum-source-of-space-time-1.18797
======
taliesinb
Here's a recent lecture by Erik Verlinde in which he tries to explain both
dark energy and dark matter using entropic gravity:

[https://www.youtube.com/watch?v=PSYXt3Xu3xI](https://www.youtube.com/watch?v=PSYXt3Xu3xI)

I'm not a physicist, but he seems to be invoking similar ideas to Raamsdonk:
that spacetime and gravity are emergent properties that boil down to
entanglement somehow. But the loop quantum gravity people have been saying
something like that for ages. And Stephen Wolfram and Konrad Zuse etc. And we
have people like Lubos Motl on the other hand to ridicule all of the above.

Anyone able to give a big picture overview of what's been happening the last
few years in this somewhat rarefied world? Who are we, the intelligent lay
public, to trust?

~~~
madaxe_again
I am a physicist...

The most interesting (to me, at any rate) bit of this article are susskind's
observations on computational complexity. As a precocious undergrad I put
forward the idea that gravity and time dilation were the same thing, and both
stemmed from the universe needing a constant amount of "time" to compute the
interrelations between the particles in a volume of space - and if there were
more particles more time would be needed as the graph of particle interactions
would grow non-linearly - and therefore time "slows down" in a matter
(information) dense area of spacetime.

What really got me gunned down was then going on to argue that this suggested
a simulated universe, running in a substrate.

Either way, interesting to see someone else who actually has some clout having
similar ideas.

~~~
hellofunk
Beautiful stuff, man. One of the more interesting questions I've seen asked in
my casual hobby readings lately is: "Is the Universe quantized?" Meaning of
course, is there a final limit to how small things get, and, with the passing
of time, does it flow freely or pass in very tiny quantized periods?

~~~
progman
> is there a final limit to how small things get

AFAIK yes, the limits are known as Planck Constants. It means that we are
living in a digital universe.

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

There are also conjectures about a _holographic_ universe.

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

[https://www.youtube.com/results?search_query=holographic+uni...](https://www.youtube.com/results?search_query=holographic+universe)

~~~
flatline
The existence of the Planck length does not imply quantized spacetime, which
is still very much an open question in the field of physics. To the best of my
knowledge most models still posit a continuous spacetime. cf.
[https://www.quora.com/Is-time-discrete-or-
continuous](https://www.quora.com/Is-time-discrete-or-continuous)

------
cperciva
_If any two particles are connected by entanglement, the physicists suggested,
then they are effectively joined by a wormhole. And vice versa: the connection
that physicists call a wormhole is equivalent to entanglement. They are
different ways of describing the same underlying reality._

In other words, there's no such thing as "spooky action at a distance",
because with particles joined by a wormhole there is no distance between them?

~~~
taliesinb
Information transfer was never a problem with spooky action. But if all
entangled particles were linked by some kind of funky wormhole it seems a
harder task to explain why FTL information transfer _isn 't_ routinely
possible.

~~~
jerf
All of the arguments about why it's not possible remain true, even if
connected by some sort of wormhole that means they "really are" right next to
each other in some sense. You still have the fundamental problem that you
can't select _what_ you collapse, so you still have to send a conventional
signal to "decode" the putative instantaneous signal.

Remember the correspondence principle will still be in play; future quantum
theories will still have to limit out to what we know today, because the QM of
today is arguably the most rigorously tested theory in humanity's history, by
number of significant digits. FTL communication still will have all of the
problems conventional relativity says it will, for all the same reasons.

Entanglement has suggested for a long time that there may (in clumsy English
words) be some sort of "real reality" that isn't necessarily constrained by
what we think of as space and time. In fact even relativity looked at in a
certain manner has suggested this; you can travel from any point in the
universe to any other, barring black holes, along null spacetime intervals.
Null spacetime intervals have no distinction between the points in them,
because they all come out the same 0 in the metric measurement. I wouldn't
expect that any of this new math is going to change anything; it may explain
where the constraints of space and time come from, but explaining the
constraints doesn't mean that the explanation will come with a way to get
around them!

In fact my personal observation is that the fundamental limits of space and
time have been getting stronger as we learn more about physics, not weaker; a
mathematically rigorous derivation of the fundamental speed-of-light limit
from a "more fundamental" level of reality locks the door even tighter, it
doesn't open it.

------
ajmurmann
Do I understand this right that if the conjecture can be proved correct and
the remaining issue about time gets solved we effectively have a grand unified
theory?

~~~
abdullahkhalids
Probably not. But it will be a big step in the direction.

There are two unifications that need to happen. One is the unification of the
strong force with electro-weak force(the unified version of electromagnatic
force and weak force). The other is gravity with the unification of the three
forces.

As far as I understand, this work is concentrating on the second unification,
and not the same. Though it might open the door for the first one too. Someone
closer to this area should comment.

------
ars
"The theory holds that gravity is geometry: particles are deflected ... not
because they feel a force ... but because space and time around the object are
curved."

Can anyone explain to me why it _can 't_ simply be a force? If you accept that
gravity is caused by energy and not mass (which is obviously the case), then I
see no reason a photon can not experience a force.

So what is the reason to insist it's geometry?

And related to that, I still have never seen an explanation of how geometry is
supposed to cause something to start moving without a force. Explaining how it
_deflects_ something, I get. How does it start moving in the first place?

The only explanation I've gotten is that things are always moving - through
time, and the geometry just transfers some of that motion into physical
motion. But that explanation is very very very lacking since all things do not
move through time at the same rate (because of the various types of time
dilation that are possible), yet the gravitational force is identical.

~~~
cjg
That explanation that "things are always moving - through time" is exactly
correct. You need to think of the full four dimensional spacetime. It is that
spacetime which is curved rather than just space.

The crucial concept is Newton's first law (objects continue on their
trajectory if a force is not applied). The straight lines in the 4D spacetime
(geodesics) - the lines that an object would follow if no force is applied -
correspond to the paths that look as if a gravitational force is applied.

~~~
mrob
>It is that spacetime which is curved rather than just space.

This is the crucial point that many discussions overlook. The concept of
"curvature of space" is obvious nonsense because curvature is measured with
respect to space. For space itself to curve implies the existence of meta-
space (and as many nested metaspaces as you like). But "spacetime" does not
work like "space", and there actually is no metaspace. Talking about "space"
curving or expanding is unnecessarily confusing.

~~~
danbruc
_The concept of "curvature of space" is obvious nonsense because curvature is
measured with respect to space._

This is not really true. If you take a flat two dimensional space, a sheet of
paper, this space has no intrinsic curvature. You surely now imagine this
sheet of paper laying flat on a table in front of you embedded in our usual
three dimensional space. Now pick it up and role it to form a cylinder. This
gives extrinsic curvature to the space, curvature in the space the sheet is
embedded in. But the sheet has still no intrinsic curvature, if you were a two
dimensional creature living on the sheet you can not tell whether it is laying
flat on the table or is rolled up to a cylinder, at least ignoring the fact
that the cylinder connects two opposite edges of the space.

The surface of the earth on the other hand has intrinsic curvature, angles of
triangles don't add up to 180 degrees for example. And this intrinsic
curvature is a feature of the space not of the embedding into a higher
dimensional space. It is not easy to visualize if possible at all, but spaces
can have intrinsic curvature independent or even without an embedding, our
three dimensional space can be curved without being embedded in a higher
dimensional space and the same of course holds for space-time.

~~~
mrob
"Extrinsic curvature" is normally called just "curvature". "Intrinsic
curvature" isn't normally called anything because it has minimal relevance to
everyday life. If you talk about "curvature of space" the natural assumption
is that you mean extrinsic curvature.

~~~
danbruc
But in general relativity extrinsic curvature is the irrelevant thing, it's
all about intrinsic curvature. We don't think of space-time being embedded in
a higher dimensional space but we still talk about curvature of space-time,
its intrinsic curvature.

These demonstrations with balls rolling on a stretchable rubber sheet to
visualize gravity are really misleading in this regard because they use a good
deal of extrinsic curvature to make things work but in reality mass doesn't
deform space-time into a fifth dimension or at least it doesn't necessarily do
so.

------
edem
After reading this article I somehow feel a little correlation between this
and the short story "That alien message" on LessWrong which suggested that we
are living in a simulated universe.

------
lisper
"Entanglement lets the measurement of one particle instantaneously determine
the state of a partner particle, no matter how far away it may be"

No. No! No!!! This meme has really got to die. A measurement of one entangled
particle does NOTHING to the other particle. Its state is exactly the same as
it was before. In fact, the whole concept of "before" and "after" a remote
measurement doesn't even make sense because it depends on your frame of
reference!

~~~
zby
'determine' does not mean 'change' \- does it?

~~~
lisper
The issue is not so much with the word "determine" as it is with the word
"instantaneously". You could say "Entanglement lets the measurement of one
particle instantaneously frob the state of a partner particle" without knowing
or specifying what the word "frob" means and you'd have the same problem:
you're claiming that measuring one particle _does something_ to the other
particle, and does it _instantaneously_ and, moreover, that this is mysterious
because the two particles are far apart (hence the slogan "spooky action at a
distance"). All this is wrong. There is no "spooky action at a distance"
because there is no _action_. Whether that action is "determining" or
"frobbing" the state of the other particle is irrelevant.

The correct story is that measurement and entanglement are the same physical
phenomenon. The creation of an EPR pair is the first step in _any_ measurement
process. The correlations in EPR measurements derive from exactly the same
physical process as the correlations in "ordinary" measurements (I put
"ordinary" in scare quotes because, as I said, even "ordinary" measurements
start with the creation of an EPR pair). When you "measure" the two halves of
an EPR pair what you are really doing is performing two measurements on
whatever system produced the EPR pair to begin with. When you look at it that
way it is not at all surprising that the measurements should be correlated.

For more details see:

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

[http://www.flownet.com/ron/QM.pdf](http://www.flownet.com/ron/QM.pdf)

~~~
Xcelerate
No one denies that when a measurement is performed, the measuring system and
the measured system become entangled (and the aggregate system containing both
continues evolving unitarily).

> When you "measure" the two halves of an EPR pair what you are really doing
> is performing two measurements on whatever system produced the EPR pair to
> begin with.

Here's the problem with that. Consider the measurements of a pair of space-
like separated entangled photons. A _choice_ can be made about the details of
these measurements by the experimenters at the last second possible. For
example, the rotation angle of a polarizer can be randomly chosen the instant
before a photon strikes it, yet correlations with this choice show up in the
_other_ measurement that was taken far away. (Note that these correlations
still don't allow you to send information faster than light.) But the "which
angle" information was never contained in whatever system produced the two
photons.

~~~
lisper
>No one denies that when a measurement is performed, the measuring system and
the measured system become entangled (and the aggregate system containing both
continues evolving unitarily).

It's not true that no one denies this. Adherents of the Copenhagen
interpretation deny it. They claim that measurement involves a non-unitary
phenomenon called "wave function collapse."

> I can't quite tell exactly what you all are arguing about.

Exactly this. You may not realize it, but not everyone understands that
measurement and entanglement are intimately related (in fact, the exact same
physical phenomenon). In fact, some people vehemently deny it. There are even
some card-carrying physicists who vehemently deny it.

> entanglement is not a phenomenon that can be explained classically.

That is certainly true (though I've met people who deny this as well).

~~~
Xcelerate
My apologies, I was editing my post while you were replying to it.

> the _aggregate system_ containing both continues evolving unitarily.

The Copenhagen interpretation is that the _measured subsystem_ collapses in a
non-unitary way. That doesn't imply the overall system did. I have no opinion*
on whether a measured subsystem collapses unitarily or not — the answer to
that question does not currently seem to be experimentally testable. But it is
testable that the overall system continues evolving unitarily during a
subsystem collapse (well, at least up until the overall system is measured),
and this has been tested and verified many times.

(* Ok fine, I do have an opinion. I don't think unitarity breaks — ever. I
think it _appears_ to break through the process of decoherence. But I'm
certainly not going to claim that as fact unless experiment can prove it
somehow.)

------
sevenless
Does this imply we could possibly manipulate space-time or gravity by creating
artificial entanglements (like in a quantum computer)?

------
tambourine_man
What a fascinating article. Thank you.

------
crontumCombudor

      Entanglement lets the measurement of one particle 
      instantaneously determine the state of a partner 
      particle, no matter how far away it may be — even 
      on the other side of the Milky Way.
    

Big deal.

If I have a bag of coins, and I take out all of the copper coins, it's not
very mysterious that the bag now contains only silver coins, no matter whether
I hide it under my bed, or in the freezer, or hang it out the window.

Yes, all the copper coins are now absent from the bag, forcing it into an all
silver state, and I can know this, without even looking at the bag. I take the
bag, and I drive it across town. I hide it under a rock. I go home. I think
about the bag under the rock. I can instantaneously know that the bag contains
only silver coins, even though it's across town, hidden under a rock.
Instantaneous. I determine the state of the bag. With my mind. Just by
thinking. Even across town. So amazing.

Why do journalists and scientists so deeply covet the seeming appearance of
the arcane?

~~~
fizx
Say we're measuring spins of entangled particles with random entangled spins.

So the real problem isn't that "when one is up, then the other will magically
be up too." That could be accomplished with local hidden variables (e.g.
shared seeds on a PRNG, or your examples).

The real problem is that when you measure A in the "up" direction, and then B
in the "10 degrees east of up" direction, then B seems to know that you
measured A in the "up" direction.

That is to say: B's probability distribution as a function of the direction
its being measured is correlated to the direction that A is measured. There's
no way to construct an "A-independent" probability distribution of B's results
for arbitrary directions. The probabilities won't sum to 1 and still match
experimental results.

It's unfortunate that "A up" therefore "B up" is a degenerate case of this
reality where classicality actually works, because it leads to confusion.

(Also the reason you can't use this magic to communicate FTL is that you can
only ask one yes/no question of each particle, and because B's probability
distribution is distorted in a symmetric way based upon A's measurement,
you're still going to get a 50/50 response for yes/no questions asked of
random entangled particles)

Feel free to comment, as I paste this on every misunderstanding of Bell's
Theorem here, and I edit to make more clear each time.

~~~
pacala
What if I have a million A/B pairs, measured in parallel. Each bit is 100 A
measurements, 1 for up and 0 for down. Why is this not FTL communication of
10k bits?

~~~
philipov
Because you can't use it to pass information to someone listening to only a
single end of the pair. You have to measure both particles to detect the
correlation, or else all you're getting is random noise.

Imagine I'm flashing a light at you on and off, randomly. Some of the flashes
aren't random and contain a message. To read the message, or even detect its
existence, you have to know which flashes were random, and this is what
observing both particles in an entangled pair allows us to do. There is no
signal with only a single particle.

------
dschiptsov
What is the proof of existence of 'universal time' as physical phenomena
instead of being a 'derived property' of some or other physical process?

Rotation of Earth around Sun is just a process, there is no time in it apart
from the mind of an external observer. Similarly with any so called "atomic
clocks" or any other physical process, including so called expansion of the
Universe.

Time, of course, could be derived as a concept from an observable change (a
process) by mind, but this does not imply its physical existence, like it is
with what we call physical forces.

Space is more subtle, but it is also require an observer and at least two
particles related via this or that set of forces.

Two random Photons share nothing, and without an observer there is neither
space nor time among them.

Mathematics does not imply existence of described abstraction.

~~~
dools
Mathematics doesn't imply anything. You can make a model using mathematics and
then use it to make predictions, then test those predictions experimentally.
By doing this a lot you can discover things and give them names like time, and
proper time, and space and whatnot.

~~~
dschiptsov
Is this a proof?

Western philosophy, so far, succeed in separating these concepts as being _a
priory_ which is perfectly reasonable, considering that any sensory input
which is subsequently used to train and condition our mind is coming
_serialized_ by out sense organ, so, for a mind, which is a result of
conditioning by the senses, the notion of succession is _a priory_.
Conditioning by shared physical environment gives us cycles, so the notion of
a cycle is also _a priory_. Succession days into nights in the environment and
ageing of other people's bodies gives us notion of a continuous change. But
change is not time.

Eastern philosophy (and modern cognitive neuroscience) would suggest that this
_a priory_ is related to our minds, not to photons or forces. So?

~~~
Retra
What would you ask of a proof? If you're reading some text, how do you
determine if the text constitutes a proof?

~~~
dschiptsov
To "see" Universe as it is we have to remove an observer - a mind which
creates appearances and conditions itself with them.

It is a hack, so to speak.)

