
Einstein Was Right: Space-Time Is Smooth, Not Foamy  - ekm2
http://www.space.com/19202-einstein-space-time-smooth.html
======
jessriedel
ArXiv: <http://arxiv.org/abs/1109.5191>

I don't understand how photons with wavelength ~ 10^-17 meters are supposed to
probe the planck scale, p_l ~ 10^-35 meter. I've skimmed the paper, but it
seems most of that explanation is buried in references 11, 14, and 15.
(Especially 15.) From their language, my hunch is that this is rather model
dependent, i.e. the observations only bounds specific models of spacetime foam
which happen to have the nice property of amplifying the dispersive effects.

(I'm a physicist, but this is _way_ out of my area of expertise.)

~~~
yk
(I did not read the paper yet.) It is not much model dependent, the general
idea is that a energy dependent speed of photons should be something like

c_gamma=c_0 + a ( E_gamma / m_p) + b ( E_gamma / m_p)^2 + ...,

with c_gamma the speed of a photon of energy E_gamma, c_0 the speed of light (
for low energy photons), m_p the Planck mass and a,b are some parameters
determined by your model. This style of papers then sets limits on a,b. And
the general expectation of some space time foam model is, that a,b should be
roughly 1.

From this the way they can probe the Planck scale is then simply a huge
baseline, in this case 7e9 light years, compared to a rather short duration of
the event ( ~ days).

~~~
jessriedel
Thanks.

> And the general expectation of some space time foam model is, that a,b
> should be roughly 1.

OK. Any idea how generic this expectation really is?

> compared to a rather short duration of the event ( ~ days).

What's this timescale have to do with it? They must be assuming they can
identify photons emitted at the same time since they're measuring the arrival
time difference in milliseconds.

~~~
Steuard
>> And the general expectation of some space time foam model is, that a,b
should be roughly 1.

> OK. Any idea how generic this expectation really is?

I can't comment on this case specifically, but the entire notion of the
"Planck energy" is based on that idea: that once you've found the combination
of fundamental constants with the right units, the numerical coefficient that
multiplies it will wind up being within one or two orders of magnitude of 1.
By some miracle, the vast majority of systems in physics seem to match such
expectations.

~~~
jessriedel
No no, I get the basic idea of making estimates based on dimensional analysis.
My question is: how generically do we expect the dispersion relation to be of
this form? I could imagine, for example, that differences in speed are
exponentially suppressed by the ratio E_gamma/M_p.

~~~
yk
It is just a Taylor expansion around c_0. And since we do not observe any
strange effects we know that it should kind of work. (That the speed of light
should be well behaved.) In addition the planck mass is the only constant,
that we know of, which fits there.

------
lobster_johnson
This is interesting, since the MAGIC telescope [1], a gamma ray detector, had
some potential evidence _for_ quantum foam:

> A much more controversial observation is an energy dependence in the speed
> of light of cosmic rays coming from a short burst of the blazar Markarian
> 501 on July 9, 2005. Photons with energies between 1.2 and 10 TeV arrived 4
> minutes after those in a band between .25 and .6 TeV. The average delay was
> .030±.012 seconds per GeV of energy of the photon. If the relation between
> the space velocity of a photon and its energy is linear, then this
> translates into the fractional difference in the speed of light being equal
> to minus the photon's energy divided by 2×1017 GeV. The researchers have
> suggested that the delay could be explained by the presence of quantum foam,
> the irregular structure of which might slow down photons by minuscule
> amounts only detectable at cosmic distances such as in the case of the
> blazar. [2]

[1] <http://en.wikipedia.org/wiki/MAGIC_(telescope)>

[2] <http://www.news.ucdavis.edu/search/news_detail.lasso?id=8364>

~~~
Benferhat
> potential evidence

That's the most forceful language you can really use in this situation.

------
mtgx
Did other scientists have similar ideas at the time, that would lead us to the
same conclusions today, but Einstein always got the spotlight because he was
famous, or would 2013's technology and science look different today if it
weren't for Einstein?

I mean is it more like with Edison "inventing the lightbulb" (many others did
at the same time, or would've done it soon anyway), or would things be
fundamentally different today?

~~~
ghc
Yes, other scientists had similar idea at the time, but the papers Einstein
wrote were groundbreaking all the same. Those papers made Einstein famous, so
it's not as if others' contributions are overlooked because of his prior fame.
The only rumored stain on his character is that he "stole" ideas from
Poincare's and Pauli's prior papers, but people who make those claims don't
really understand how the scientific publishing process works. Even Edison
[edit: I meant Einstein, not Edison] stood on the shoulders of giants.

~~~
pretoriusB
> _Even Edison stood on the shoulders of giants._

Sorry? "Even" Edison? Especially Edison. The man was a charlatan and a crook.

~~~
derleth
From vor_, who is dead

> This article might interest you:
> [http://www.forbes.com/sites/alexknapp/2012/05/18/nikola-
> tesl...](http://www.forbes.com/sites/alexknapp/2012/05/18/nikola-tesla-
> wasnt-god-and-thomas-edison-wasnt-the-devil/)

~~~
Falling3
And this point-by-point rebuttal would definitely interest you:
<http://theoatmeal.com/blog/tesla_response>

------
skwosh
Can anyone comment on the impact of this result on the theory of Loop Quantum
Gravity (LQG)?

My understanding is that one of the predictions of LQG is that photons of
higher energy are likely to lag behind those of lower energy due to
propagation through spin-foam (meaning velocity isn't c but dependent on
energy or frequency).

It seems like this is what they're getting at in [1]...

Would something like this categorically invalidate something like LQG or
merely some of its assumptions?

[1] [http://phys.org/news/2013-01-spacetime-smoother-brew-
knew.ht...](http://phys.org/news/2013-01-spacetime-smoother-brew-knew.html)

------
huggah
It's really not acceptable that the author of this article can't even
consistently use 'photon' instead of 'proton'. Can we stop advertising pop sci
summaries that fail to handle summarizing papers?

------
brutuscat
Interesting! I've read in one Scientific American edition about this and about
what the Prof. Craig Hogan is studying:

<http://arxiv.org/find/astro-ph/1/au:+Hogan_C/0/1/0/all/0/1>

In his current work he is developing the theory of a proposed new phenomenon,
which he calls “holographic noise”, a fundamental, universal uncertainty in
the fabric of spacetime, akin to pixelation in an imperfectly sampled digital
audio file or video display. The theory may lead to the development of
experiments that could allow a direct measurement of the minimum interval of
time.

<http://astro.fnal.gov/people/Hogan/>

------
chmike
Space Time may look smooth at the Plank resolution, but space time
quantification at a much lower resolution is still possible. Beside if the
quantification is organized as a regular 3D lattice and not "foam bubbles"
then the space time quantification theory [1][2] would not be invalidated by
the experiment.

[1] : <http://www.meessen.net/AMeessen/STQ/STQ.pdf> [2] :
<http://www.meessen.net/AMeessen/STQ/STQ2.pdf>

------
CamperBob2
_If this foam indeed exists, the three protons (sic) should have been knocked
around a bit during their epic voyage. In such a scenario, the chances of all
three reaching the Fermi telescope at virtually the same time are very low,
researchers said._

How in the world do you get from point A to point B in this line of reasoning?
They received three photons of similar wavelength at a similar time.
Everything else seems to be some stuff they made up.

~~~
dalke
This is a summary of a presentation, not a paper, so it doesn't have all the
details.

I thought the model was pretty simple to infer. Gamma rays are rare. GRBs dump
a lot of energy out at the same time; enough that we can get a spectrum. Given
the observation date, it's easy to find that this was GRB 090510, which was a
short burst of 0.33 seconds. It was detected by Fermi-LAT, so that gives
somewhere under a degree of angular resolution.

Few gamma rays normally, huge numbers at once, from the same direction, means
that few of huge numbers are going to be background, and likely means that
those gamma rays came from the same source.

Still, it might be a coincidence, which is why the summary we both read
includes the phrase "There is a possibility of a statistical fluke, or that
space-time foam interacts with light differently than we imagined," and
mentions "If future gamma-ray bursts confirm this."

How did you not draw this same inference?

~~~
CamperBob2
_How did you not draw this same inference?_

The problem I have isn't with the inference that the photons came from GRB
090510, but with the idea that anything can be inferred about the medium they
passed through.

If quantum-foam fluctuations can be assumed to be uniformly distributed along
the paths of all three photons, then the number of disturbances encountered by
a photon is proportional only to the photon's path length. Here, the path
lengths of all three photons are enormous and essentially identical, so it
seems reasonable to assume that each one encounters a similar number of
quantum "potholes," and that no difference in their arrival
energies/times/whatever would be noticeable on a millisecond scale.

~~~
Florin_Andrei
They would drift apart _from each other_ , in a manner somewhat similar to the
brownian motion. Essentially, small-scale foam would be similar to a slight
inherent "blur" of the vacuum for gamma rays.

~~~
dalke
As well, some gamma rays which weren't exactly directed towards the earth
would be nudged towards us, given it a slightly longer path.

------
tocomment
Does that mean Hawking radiation [1] doesn't exist? Matter, anti-matter pairs
being created on opposite sides of the event horizon of a black hole and thus
not being able to pop out of existence.

[1] <http://en.wikipedia.org/wiki/Hawking_radiation>

~~~
digeridoo
The article is a bit dodgy in this regard. It just means that at a quantum
level, the energy fluctuations (as observed in this experiment) are not big
enough to cause significant space-time alterations.

~~~
ZoFreX
Surely over such a long distance, the staggeringly huge number of extremely
small genuinely random changes would cancel out?

~~~
Cogito
I'm interested why you would think that.

If the situation was chaotic (which I would presume a system with many random
perturbations to be) the expectation is for the repeated small changes to have
a significant impact.

~~~
sageikosa
My guess would be this would be true only if they were coherent, and the
resonant frequency were some multiple of the Planck length. The many small
random perturbations would largely cancel each other out and the remainder
would be insignificant (not energetic enough) to meet the lowest energy
requirements to nudge a photon by even a miniscule amount.

------
wheaties
This is what I sometimes lament about astrophysics, N=1 is sufficient for
proof. It's one observation of one event. You can't make any claims about that
unless it's a direct violation of some which will occur 100% by theory. In
this case there is no such guarantee.

~~~
digeridoo
Proof is a word that shouldn't be used lightly. I don't think anyone would see
this as proof that quantum foam doesn't exist. The laws of physics allow for
quantum foam to happen, which may lead us to start looking very differently at
the structure of the universe. However, we now have some strong evidence from
measurements over a reasonably large distance that quantum foam is not an
immediately noticeable factor in the universe.

I kind of liked quantum foam as a possible way of explaining gravity
observation that are not explained by the amount of visible matter and thus
attributed to 'dark matter', because 'matter' is the only thing we know that
would cause gravity. This tells me I shouldn't get too excited about that
possibility.

------
methodin
It seems to me the art of reasoning is one that is slowly dissipating as the
quick access to information continues to infiltrate all aspects of our lives
and replace the need to deduce. It is quite amazing that someone who lived
quite a while ago, before the boom of science/technology, could be so accurate
simply through the power of observation, logic and reasoning.

