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12:34 AM
In chapter 1 (equation 1.2) of Feynman feynmanlectures.caltech.edu/II_01.html he mentions that you can find the motion of a particle given its force. Then he says that the derivative of the momentum is that force (fair enough, just a definition). But where does is he getting the function for p?
Is he just treating it as a given from special relativity? I don't really get it.
Perhaps because he already introduced special relativity in chapter 1.
 
Thanks :)
 
 
2 hours later…
3:06 AM
0
Q: How to handle references to literature in predatory journals

ZeroTheHeroAre questions or answers that cite, refer to or are based on work published in predatory journals to be tolerated? (If no, how do we check if a journal is predatory?)

0
Q: Has Stack Exchange changed how it counts daily visits?

BuzzI realize that this is almost certainly a network-wide question, but I am asking it here, because this is the only site on which I have seen it manifest: Has there been a change in how daily visits are calculated? (I noticed this in the context of the Fanatic badge on the physics meta site.) I...

 
 
2 hours later…
4:42 AM
Anyone know what the red review queue circles mean?
 
vzn
4:56 AM
@JMac interesting, did a little websurfing & was not aware MR headsets had advanced so far. wheres the killer app games for them anyway? and bet they will have some substantial Physics related apps someday. toms hardware was not too impressed with the HP version tomshardware.com/reviews/… www8.hp.com/us/en/campaigns/mixedrealityheadset/overview.html there used to be some epic video game chat sessions in the past around here...
 
5:32 AM
Reread the quantum theory cannot consistently applied to itself paper. The detailed argument is still analysed. But a couple of preliminary comments:
1. I think Consistency(C), Quantum(Q) or both consistency and quantum has to be violated. For C, it is relatively easy to understand as context is often needed to reason about an outcome, thus it will not surprise me that outcomes are subjective to the measurement devices
For Q, i think nonlocality similar to Bohm is most likely. It seems somehow, studying the politics of libertarian theory and slavery arguments actually helped me to understand nonlocality better: For any agent who only have access to local information, the nonlocality manifests as a unaccounted for constraint in the system that preconditioned the system, hence the choices that the agent can made is really a constrained set compared to one where there is no nonlocal influence
 
@SirCumference the red dot indicates the category has increased rapidly recently i.e. that the SE thinks this needs urgent attention. However my experience is that the red dot is largely useless and can be ignored.
 
It is thus not surprising that a contradiction arises since there may be parts of the environment that has not been accounted for and cannot be accounted for in the experiments
2. As for S, I think it is even less likely now, as that recent updated analysis of the group showed not even the universal wavefunction of many worlds can resolve the contradiction
3. It will be interesting to see whether my own way of reason about quantum mechanics (which Acuriousmind said mine is not an interpretation) can recover the conclusion of that paper. Because if it is so, then it means there is definitely something inconsistent about quantum mechanics
at least in its current form
 
 
2 hours later…
7:56 AM
Morning
 
8:19 AM
@Slereah morning
 
8:47 AM
Trying to do a more or less self-contained proof in differential geometry ain't easy
 
 
1 hour later…
10:00 AM
0
A: What are the allowed topologies for a FRW metric?

Colin MacLaurinAs you know, 3-dimensional space need not be infinite even if the spatial curvature has sign $k = 0$ or $k = -1$. As George Ellis writes in "Issues in the philosophy of cosmology", in Butterfield & Earman eds. (2006): Misconception 5: The space sections are necessarily infinite if $k = 0$ o...

That guy replied to exactly the bit I already knew about :V
 
10:11 AM
I wonder what an FRW metric means for a spacetime that cannot be foilated (e.g. the time topology contains CTCs and other nontrivial curves)
an expanding universe that loops upon itself...?
 
 
2 hours later…
12:24 PM
well as said
I don't think you could do it without foliation
Otherwise I don't think "spatially homogeneous" would make sense without spacelike hypersurfaces
Krasnikov's book ain't bad but you feel he runs out of Known Things by the end
the ending gets fairly vague
 
12:46 PM
hmm... a spacetime with a different time topology and yet still foliatable...
 
I would appreciate it if someone could explain-to-OP/mediate/step-in/vote-to-reopen/vote-to-close here.
 
pretty sure you don't just want a different one in terms of going from continuous time translation to discrete time translation, but something that is more global
 
1:11 PM
The Time orientability section of this paper seemed to be part of the puzzle: We cannot have orientable closed spacetimes without CTCs (those will rule out foliations). Otherwise we can have spacetimes which the hypersurfaces orthogonal to the direction the proper time is pointing can have a lot more complexity than merely translating from one leaf to another
This paper talks about the conditions when generalised FRW spacetimes where the mean curvature of the spacelike hypersurface remains unchanged. However I don't see any alternate topology of time in there, other than possibly buried in this sentence:
> Then the vector field (...) is conformal, timelike and closed (in the sense that its dual 1-form is closed)
 
@Secret Minkowski cylinder
is the simplest one
There are specific terms for spacetimes with a different "time topology"
"closed" ones don't necessarily have CTCs
Earman discusses them a bit
 
1:45 PM
p.215: Very interesting, there are temporally neither open nor closed spacetimes, and also temporally clopen spacetimes
 
@Slereah more additions to edpif.org/en/recrutement/prop.php#view169
This one looks good
 
thx
 
Ok after reading chapter 7, I am starting to wonder, since spacetime has 3+1 dimensions, perhaps more complicated temporally closed spacetimes may be possible, such as those where the time direction is tied into a trefoil knot. A trefoil knotted time should be pretty different from a circular one since the trefoil cannot be continuously deformed into a circle (unknot). However googling so far gave me no spacetimes of that context
 
'The purpose of this thesis is to examine whether a very large number of these vacua are in fact unstable or inconsistent with the experimental data coming out of the Large Hadron Collider. This will be done by analyzing one of the key ingredients of the Multiverse construction - the uplifting of the cosmological constant, and taking into account the embeddings of Standard Model physics in String Theory.'
'Applicants are expected to have a solid background in general relativity and quantum field theory' etc
 
I'll give it a shot, thx
Although I won't lie
I haven't done shit wrt finding a thesis in the past 4 months
Kind of in a slump
 
2:01 PM
Yeah don't go for it just for the sake of it I guess, need to want to do it etc
 
I need to get my groove back
as it were
 
user351417
@Qmechanic I don't see any pending items in my reopen queue. And it doesn't appear to be in my history.
 
rob
It looks like edits only bump "on-hold" questions to the reopen-review queue. Five days after the question is put on hold, its status changes to "closed." This question wasn't edited until seven days after being put on hold, so it may not have automatically entered the reopen queue. — rob ♦ 19 secs ago
 
2:34 PM
"Matter" does not have any real physical meaning right?
 
@SirCumference Classically it does. That depends on what you mean by "physical meaning" and how philosophical you want to get with it I would imagine.
 
Matter is one of those term that depends on the context
In GR it usually means any non-gravitational term to the action
 
@JMac Well I'm deciding whether to accept this, since it seems a little specific on the definition
Sigh uploading a screenshot is so much harder on mobile
 
That just seems like a crappy tag IMO. What wouldn't get a matter tag in astronomy, pure math?
 
2:55 PM
@JMac I guess it's for topics more about astrophysics than e.g. telescope use, astronomical conventions, spacecrafts, etc.
Though yes, it is a very vague tag as I expected
Tag definition seems overspecific imo
 
well it would exclude gravitational and EM fields also
 
 
1 hour later…
4:11 PM
10
Q: What is the temperature of the clear night sky from the surface of Earth?

user56903Before you all jump in with 2.73 K or thereabouts, this is more of an experimental question. It will obviously depend on humidity and radiation being scattered back towards the surface of the Earth. Any ideas? Anyone ever pointed a pyrometer or similar at the night sky?

There are two answers there if the form "I measured it locally to be X" from new users (1 two years ago, the other today)
Those really doesn't answer the question, right?
 
I noticed that a few minutes ago too and was thinking those probably shouldn't be answers
Or else we could very quickly get a lot of "answers" that are "I just checked and got [temperature]"
 
Tbf, that title lends itself to such
 
Okay, I probably will start a delete/LQ vote on those with custom comment about them not being answers
Also, my edit comment on the question is kinda funny
 
A title like “what are typical values for X” might convey that better
 
Anyone know how to normalise numbers logarithmically
 
4:18 PM
That gets wordy tho
@Tobi example?
 
i.e. 1 4 9 to 0.1, 0.2, 0.23
between 0 and 1
obviously 1 can't be reached
unless using infinity
I have no idea how
 
I can’t tell what mapping you’re doing there
 
from: 0-infinity
to: 0-1
 
Ok, but log(1)=0
 
?
 
4:21 PM
I mean, I don’t see what logs have to do with this. Regardless of base, one has log mapping (0,infinity) to (-infinity, infinity)
With exponentiation doing the opposite
At best, you’ll have log mapping (0,1] to (-infinity,0]
 
0
Q: Map [0, infinity] to [0, 1]

Tobim creating a fractal visualisation. I want the colour per point to be based off the iteration final value, f(zn), instead of the traditional: number of iterations before reaching a cut-off (usually when any component, real or imaginary, of z is >2). How could I from from something like: 0, 1, ...

i can inverse and map 0,1 -> -infinity, 0
 
Ok, so you’re looking for a mapping that does something like what you propose in the post
 
how would that work
 
I mean, right now, there’s not going to be a unique answer to your question
There’s an infinitude of smooth maps from [0,infinity) to [0,1)
 
1/x works doesn't it
 
4:30 PM
That’s undefined at 0
 
I'd take it as infinity
wait that's not between 0 and 1
 
Yeah
One that’d work is 1-e^(-x)
Or any 1-b^(-x), really
(With b>1 at least.)
Another should be f(x)=1-1/(1+x)
Which you can also modify in a number of ways
the basic point is that there’s no one way of doing it. There’s an infinitude of ways to choose that map
 
1/(1+x)
seems best, no?
 
If all you’re after is a convenient mapping, tho, that kind of arbitrary choice isn’t an issue
Define “best.”
 
oh you same, 1-1/(1+x) which is basically the same
 
4:35 PM
It’s certainly a satisfactory choice
Yeah
 
its performant for a computer to calculate
 
Dunno, then. Depends on whether division is faster than exponentiation
Note, for instance: f(x)=1-10^-x
Sends 0,1,2,3,... to 0,0.9,0.99,0.999, etc
Which does fulfill your criterion but
Probably not useful as a representation, since those values are already so close to 1
 
I've got some crazy looking fractals
using this technique
i don't know if theyre complete nonsense or not
 
neat.
 
 
3 hours later…
7:26 PM
Both guys did this for both non-answers, disputing my flags.
 
7:56 PM
I flagged both for attention with "Single point experimental data with minimal details of setup or reasoning for results doesn't meet quality standards for answers on this site." because those answers don't really say anything from a physics standpoint
 
I wonder if I had an argument like that posted as the comment it'd have been marked for deletion. Instead it "Looks ok". Insert snarky eyeroll
 
Honestly, probably. I tried to frame it in a way that implied as heavily as possible that there was no real value in the numbers.
 
 
2 hours later…
10:19 PM
@KyleKanos that one looks like an answer to me
A bad one, and with a well deserved score, but still an answer, I should think
@JMac comment, downvote, move on
 
 
1 hour later…
11:29 PM
@EmilioPisanty It doesn't really make sense to me to even keep answers like that around. It's not like it's downvoted because it's wrong, but keeping around answers like "I just took a measurement of it and got this value" doesn't seem right to me
Many questions could be answered that way, and it would be a huge clutter
 

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