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4:39 AM
If the universe is expanding, does that mean that new space is being created between objects or the existing space is being stretched out?
if the existing space is being stretched out then doesn't that mean that distance between objects is gonna remain equal?
if we take "x" volume
and the space stretches to double its own size
 
@MartianCactus that doesn't really have an answer because space isn't a "thing"
 
then we will take "2x" volume? Or will we still take "x" volume but the volume itself is a larger amount now?
@JohnRennie i mean..could you just consider space as volume?
if volume itself increases
but volume itself cant increase actually..it makes no sense
 
5:38 AM
The volume increases, yes
alhough of course, volume depends on coordinates you assign
But the important thing is that, between two immobile points, the proper time it takes for a light ray to do a trip to and from increases
 
 
1 hour later…
7:06 AM
ohh alright
so space does not expand essentially, theres more of it?
if we think of space as a fabric, if you draw an 2 objects on a fabric
with paint
and then stretch the fabric, the objects will stretch too
and a photon will itself stretch?
basically its gonna take same amount of time as nothing essentially changed as everything is ON the fabric itself
guess its not like that in reality, as light takes more time to travel the distance
so the only explanation is, more fabric is being added
thus more space is being created
and nothing is stretching..
am i right?
 
So... I have finally finished reading this paper:
And my conclusion is, the hype is not exaggerated
So to start off, this is a quantum optics experiment consists of a coupled three level system $\lvert D\rangle,\lvert B\rangle,\lvert G\rangle$. Here, rabi drives were used to cause transitions between $DG$ and $BG$ respectively, with $DG$ proceed at a slower rate than $BG$
What they observed in the experiment is that, whenever the clicks die out from 0-2 $\mu s$ it is an indication that the state is trying to jump to $\lvert D\rangle$ from $\lvert G\rangle$ and hence a warning that a quantum jump had occurred
While the initiation of such warning to jump is stochastic and unpredictable, it was found that whenever the warning occurred, the jump will occur in a deterministic fashion with the ensemble passing through essentially the same trajectory in Hilbert space as it made its way from $\lvert G\rangle$ to $\lvert D\rangle$
This deterministic schrodinger equation like dynamics due to the coherence of the evolution of the jump at the short time scale (in contrast with the stochastic behaviour of the initiation of the warning at long time scales of the order 10), thus allows the experiment to use the warning to initiate some Bloch sphere operations into the system so that the jump trajectory can be reversed midway, returning the ensemble to $\lvert G\rangle$
and thus achieving the controlled reversal of a quantum jump, something that is commonly believed to be purely stochastic
Therefore, while the occurrence of the warning is stochastic, the jump evolutions that followed after those are deterministic, thus allowing them to be intercepted. This also reconcile Bohr's conception of quantum jumps with schrodinger's, that the stochastic and coherence of a quantum jump coexists at two different time scales of the dynamics
The experiment predictions are thus consistent with that of quantum trajectory theory.
:::::::
So what is quantum trajectory theory? arxiv.org/abs/1405.6694 Here is a good review of this model that is commonly used in quantum optics. Roughly speaking:
Quantum trajectory theory arises from the increasing need to model open quantum systems, systems where the quantum state of interest is coupled strongly to the environment, and that properties of the environment are sufficiently well understood that the environment played an important role in controlling the quantum state in question
What result from this is that the whole system need to be described as if the environment is a bath coupled to the quantum state, and hence an open system. In order to describe what happens to the quantum state as it evolves in time, one need to wrote the change in the density matrix in terms of all the dephasing, couplings and other interactions between the quantum system and the environment.
This forms something called the master equation, which describes how the quantum system evolves under the action of the environment
Quantum trajectory theory, which has many different versions developed by many different authors, are approaches in simplifying the computation of this master equation. All of them are based on the principle that rather than trying to compute the whole density matrix, you pick an ensemble of pure states, to be evolved in time steps through Hilbert space, and these samplings will reflect the average behaviour of the density matrix you want to compute
From these you can then deduce a lot of things from which route a state took in order to move from A to B in Hilbert space, to the observables that is expected to be measured such as when and how spontaneous emission will occur when a quantum system is coupled to a laser field
Therefore, quantum trajectory theory is not to be confused with Bohmian mechanics, though in principle, I do not see any problem that it can be used in the bohmian context
And finally, that paper is consistent with all interpretations, though that quantum jumps have a deterministic component seemed to be surprising enough for me, sharing a similar comment as the authors in the paper
It may open the door as to whether all such jumps have a warning period, or just really coherent systems like this three level rabi driven system does that, and whether they have durations that are as given by quantum trajectory theory. In the supplementary info, however, it is said that some jumps still reverses unpredictably, thus according for that 23% failure rate in the graph
Philosophically, I still think this experiment is significant. Even though it said absolutely nothing about quantum interpretations, that we can experimentally demonstrated that there is a nonzero period of determinacy that follows after almost every warning to jump can have implications on how to control what is essentially random phenomenon right when they occur
@vzn @PM2Ring @Semiclassical etc. Sorry for my extremely lengthly comment. What do you guys think of my thoughts about that experiment?
 
 
3 hours later…
11:04 AM
it's like that I dreamt of going to Beijing again or somewhere in mainland China yesterday. I often dream of there is an easy to go there, like a channel (like wormhole) which serves as a short route to go there.
 
 
1 hour later…
12:25 PM
Hm
Given a point $p$ with a sufficiently small neighbourhood $U$
If a timelike curve $\gamma$ goes through $U$, is there a deformation of $\gamma$ that is still timelike and passes through $p$?
Seems there is
I would guess it is some normal neighbourhood thing, but I'm not sure how to show it
Although... I suppose that for a sufficiently small neighbourhood, $U \subset I^+(\gamma)$, hence there is a timelike geodesic going from $p$ to $\gamma$
I guess I just need to show the $I^+$ part
And since a piecewise $C^1$ timelike curve can be deformed to a $C^1$ timelike curve it's all good
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12:44 PM
@Slereah what do you want to show?
 
@RyanUnger Geroch's proof that topology change implies either a loss of time orientability or causality
so far there is a point $p$ which is a limit point of some curve $\gamma$
which he claims can be deformed into having the curve go through $p$
the paper
(theorem 2)
 
I’ll have to look later
 
thx
 
1:00 PM
Apparently another cunning trick to have non-divergent singularities is to have directional singularities
 
1:36 PM
@Slereah what?
 
Some singularities occur because different curves to the same point end up on different values for some scalars
 
1:50 PM
@Slereah what is the name of the paper
scihub seems to be blocked at Princeton or something
 
Princeton is in the pocket of big science publishing
 
most people get papers legally :<
 
Aw man
could have tried to go but 2 weeks from now is a bit short
Visser's going
 
@Slereah so what is the issue with the paper
 
@RyanUnger What I said : At some point he says that the curve he talks about at the beginning of thm 2 can be deformed to go through $P$ while remaining timelike
Which sounds reasonable but I am trying to prove it
 
2:02 PM
what does tangent to $\xi$ mean
an integral curve?
I actually have no clue what the claim means
does he mean perturb with the endpoints fixed?
this is vague
 
I'm guessing so, yes
keep the endpoints on $\partial N$ fixed probably
 
2:21 PM
The important point is just constructing a CTC out of it
 
@Slereah but does he mean perturb it while keeping it tangent to $\xi$?
 
I'm guessing not within $N$, no
Just keeping it timelike
 
2:35 PM
@Slereah ok I buy it then
but proving it will be hard
 
Damn Geroch
Hell I could email him again about it, perhaps
 
no
it's very easy
just hard to write down
 
3:03 PM
What's the rough idea of the proof?
 
vzn
4:00 PM
@Secret it deserves a very careful look like what youve given it. think the results are breakthru and not to be downplayed or brushed aside. emerging quantum trajectory theory is essentially subquantum mechanics as sketched out by bohm. its a semiclassical theory. & despite some claims think its not really compatible with the copenhagen interpretation. its strong/ dramatic experimental evidence for non-copenhagen interpretations esp bohmian theory...
 
4:40 PM
Man, it's going to heat up to 40°C this week over here. That's not a fun summer.
 
RIP
 
At least the office has AC
 
4:55 PM
@Slereah use compactness
And continuity
Locally the vector field is almost constant
So the integral curves are almost straight lines
With slope bounded away from 45 degrees
 
Is it gonna be one of those "by continuity the norm remains < 0" sort of proof
 
Yes
 
@ACuriousMind Regarding your answer here, that doesn't imply that SUSY says that there's some physical process that can actually convert fermions to bosons & vice versa, does it? I mean, it's like CPT says that a positron is like an electron going backwards in time, but no physical process can make stuff go backwards in time, so no physical process can convert an electron into a positron. AFAIK
 
@PM2Ring Sure, just like rotation symmetry of the universe would not imply you can actually rotate the whole universe
 
@ACuriousMind Phew! You might like to add info to that effect to your answer.
 
5:07 PM
@PM2Ring Not sure whether it's necessary, but I stole your example of time reversal and edited the answer ;)
 
Oh, good. :) I do think it's necessary because in this earlier question physics.stackexchange.com/q/482074/123208 the OP says "since the fermions are turning into bosons"
 
5:39 PM
Anyone else get frustrated by a barrage of suggested edits that just add some formatting that is subjective, and like one word corrected?
 
6:06 PM
@JMac They can be annoying, especially if they bump posts that really don't deserve to be bumped. Some trivial edits are ok, but they should be done by people with enough rep to do the edit without approval. It's a waste of effort to require 4 people to edit one word. But it's hard to stop this, because it's an easy way for newbies to earn rep.
A week or so ago I had one suggested edit on a recent answer that was fairly trivial. It did fix a minor typo that I hadn't noticed, but the editor also "corrected" my British spelling of "behaviour" to the American version, which I did not appreciate.
 
@PM2Ring What bugs me is that I just went through like 4-5 edits all suggested by the same user. Most of them were all focused on another users questions. Most of the edits were replacing "-" with "," and other trivial changes. Two of them seemed maybe worth it to correct spelling mistakes, but I rejected and edited them because all the other crap they did was just changing formatting for the sake of changing it. And yes, as you just mentioned they also tried to Americanize some words
 
@PM2Ring It's fine to reject edits for making unnecessary stylistic adjustments.
 
@ACuriousMind It's fine to reject them for all the unnecessary stuff and then just put through the one good spelling correction, right?
 
Yes, if the bad/unnecessary outweighs the good it's fine to reject it and only do the good part after that. Do consider improving the edit instead of rejecting if it's not largely bad.
 
I don't like outright "rejecting" if someone did find a legitimate error... but at the same time I don't want to go through and undo the random formatting they did, so "Reject and edit" is usually much easier
 
6:13 PM
@ACuriousMind Agreed. I'm happy they spotted my typo, though. And I guess that often the stylistic changes are done mainly to get the change count above the minimal level (which I think is 6 chars).
 
That's what really bugs me, that if someone finds a typo and they feel they have to do other things to break the 6 char limit. In that case it's probably best to just leave a comment if they have the rep; but obviously not everyone would think to do that (nor would everyone read the comment)
 
hi folks, what is the remarkable evidence of cyclic model of universe?
 
You can ping editors of successful edits in the comments, although the @ autocomplete mechanism doesn't work. And of course for unsuccessful edit suggestions you can comment in the rejection reason.
 
@PM2Ring I mean if you saw a mistake but didn't have the rep to do trivial edits, you could leave a comment saying "Looks like this word was spelled incorrectly; but it's too small to edit" and presumably either the person who's post it is could see it and make the fix, or a user with the rep to make the edit might do the same
 
@JMac Yeah, I mostly prefer comments than having some newbie trampling over my answer. What's really annoying is when you're in the middle of adding further info to a fresh answer and some newbie comes along with an edit suggestion. It hasn't happened to me yet on Physics, but it has happened a few times on SO, where the pressure to post quickly is much higher.
@ACuriousMind Sure. I do try to improve such edits, I don't want to rob the poor newbie of their 2 points, if I feel that they're genuinely trying to improve the site, and aren't just doing it for the rep.
 
6:33 PM
Hm
@RyanUnger for that proof, do I just switch to Riemann formal coordinates
And then work out a small enough neighbourhood
 
no, use coordinates adapted to $\xi$
use the inverse function theorem
 
aight
 
@MoneyOrientedProgrammer "Extraordinary claims require extraordinary evidence". And there's not a lot of evidence for any cyclical universe model, AFAIK.
 
7:36 PM
@PM2Ring: If there is not a lot, could you tell me some that support CCC?
I am very interested in it.
 
7:52 PM
@MoneyOrientedProgrammer Sorry, I don't know much about CCC, although I did read an early article about it by Penrose. As Wikipedia says, empirical tests based on patterns in the CMB have not supported CCC.
 
@PM2Ring: Thank you. :-)
 
Penrose is a good reference.
 
@skullpatrol: I think we have met before in mathematics chat room. I am Jasper Loy's friend. :-)
 
probably, I haven't chatted with Jasper in a long time :-)
 
@EmilioPisanty Since you know a fair bit about metrology, I figured you might know this: why is BIPM the bureau of weights and measures? Isn't that a bit redundant? I guess the name may be due to historical and legal reasons.
 
8:05 PM
Or it is just because of the same display name.
 
@ACuriousMind that's a good reason to stay at the office.
 
8:58 PM
@PM2Ring my reading of the name is that "measures" stands for lengths
It's a good question as to the BIPM handled time standards when the name was established
 
9:25 PM
@EmilioPisanty Ah, ok. That makes sense. I think I first became aware of the BIPM when reading about time scales in this article by Steve Allen: ucolick.org/~sla/leapsecs/timescales.html FWIW, Steve is a Stack Exchange member, but he's not very active.
 
9:51 PM
The Metre Convention (French: Convention du Mètre), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations (Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States of America, and Venezuela). The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of...
it does look like at the time the BIPM it was established, the game was length and mass only
specifically the prototype meter and the prototype kilogram
(as well as the metrological chain that derives from them)
it's also important to note that the correct frame for "weights" is standardized weights as used in commerce
 
Thanks, Emilio. The question arose a few days ago in the Python room. We were discussing what will happen to the reference kilograms now that we have the new standard, and went on to discuss the whereabouts of the old metre standard bars, which are apparently on BIPM premises.
 
10:15 PM
When will we have the Natural units convention
@knzhou nice to meet you at ICTP!
 
@Hotlab When Natural units become practical for commerce & engineering, i.e., never. Also, a major impediment is that it's really difficult to measure G to more than a handful of decimal places.
 
@PM2Ring The pdg gives me 6.67408(31)e-11, how is this not impressive?
 
@Hotlab It's impressive. But as I said, it's only a handful of decimals.
 
@Hotlab that's six significant figures
Now go and look at the precision with which e and h can be measured
If you want something really impressive, look for the highest precision in frequency measurements using optical clocks and frequency combs
(hint: it's three times more significant figures)
@PM2Ring for now, the IPK is staying put. There's still metrological value in seeing how its mass drifts under the same care conditions it's been during the past decades
 
10:33 PM
@EmilioPisanty Thanks for teaching me how to count to six. Why is it your opinion that it is the number of significant figures that is interesting? Is not gravitation significuntly weaker than electromagnitism
 
@Hotlab because the precision of the primary standard is the absolute bound on the precision on anything that's metrologically downstream from it
With G, if you wanted a "natural units convention", that would be everything with a nonzero mass dimension
 
@EmilioPisanty Understood. Still, I reckon they could auction the IPK & the other PKs off, giving the proceeds to charity, with the understanding that the objects stay on BIPM premises. I bet Elon Musk would love to own the IPK. :)
 
@EmilioPisanty Then why not say c instead of e? Is that not what we define the units from?
 
@Hotlab Apologies. I understood your initial comment as indicating that you were interested in a constructive discussion, but that seems to have been my mistake. Good night!
 
@EmilioPisanty xD
 
10:47 PM
@Hotlab Hi, and welcome to Phys.SE!
 

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