The h Bar

kyle campbell

00:34

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.

bolbteppa

@kylecampbell feynmanlectures.caltech.edu/I_15.html and feynmanlectures.caltech.edu/I_16.html

kyle campbell

Thanks :)

Physics Meta

03:06

0

Q: How to handle references to literature in predatory journals

Are 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?

I 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...

discussion

Sir Cumference

04:42

Anyone know what the red review queue circles mean?

vzn

04:56

@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...

Secret

05:32

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

John Rennie

@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.

Secret

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

Slereah

07:56

Morning

Akash. B

08:19

@Slereah morning

Slereah

08:47

Trying to do a more or less self-contained proof in differential geometry ain't easy

Slereah

10:00

0

A: What are the allowed topologies for a FRW metric?

As 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

Secret

10:11

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...?

Slereah

12:24

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

Secret

12:46

hmm... a spacetime with a different time topology and yet still foliatable...

Qmechanic

I would appreciate it if someone could explain-to-OP/mediate/step-in/vote-to-reopen/vote-to-close here.

Secret

iopscience.iop.org/article/10.1088/1367-2630/aaaa17/pdf

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

Secret

13:11

arxiv.org/ftp/gr-qc/papers/0211/0211066.pdf

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

sciencedirect.com/science/article/pii/S0926224507001039

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)

Slereah

@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

Slereah

13:33

pitt.edu/~jearman/Earman_1995BangsCrunches.pdf

chapter 7

Secret

13:45

p.215: Very interesting, there are temporally neither open nor closed spacetimes, and also temporally clopen spacetimes

bolbteppa

@Slereah more additions to edpif.org/en/recrutement/prop.php#view169

This one looks good

Slereah

thx

Secret

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

bolbteppa

'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

Slereah

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

bolbteppa

14:01

Yeah don't go for it just for the sake of it I guess, need to want to do it etc

Slereah

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

Sir Cumference

14:34

"Matter" does not have any real physical meaning right?

JMac

@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.

Slereah

Matter is one of those term that depends on the context

In GR it usually means any non-gravitational term to the action

Sir Cumference

@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

JMac

That just seems like a crappy tag IMO. What wouldn't get a matter tag in astronomy, pure math?

Sir Cumference

14:55

@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

Slereah

well it would exclude gravitational and EM fields also

Kyle Kanos

16:11

10

Q: What is the temperature of the clear night sky from the surface of Earth?

Before 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?

temperature radiation atmospheric-science geophysics

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?

JMac

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]"

Semiclassical

Tbf, that title lends itself to such

Kyle Kanos

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

Semiclassical

A title like “what are typical values for X” might convey that better

Tobi

Anyone know how to normalise numbers logarithmically

Semiclassical

16:18

That gets wordy tho

@Tobi example?

Tobi

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

Semiclassical

I can’t tell what mapping you’re doing there

Tobi

from: 0-infinity

to: 0-1

Semiclassical

Ok, but log(1)=0

Tobi

?

Semiclassical

16:21

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]

Tobi

0

Q: Map [0, infinity] to [0, 1]

m 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, ...

logarithms exponential-function normal-distribution exponentiation

i can inverse and map 0,1 -> -infinity, 0

Semiclassical

Ok, so you’re looking for a mapping that does something like what you propose in the post

Tobi

how would that work

Semiclassical

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)

Tobi

1/x works doesn't it

Semiclassical

16:30

That’s undefined at 0

Tobi

I'd take it as infinity

wait that's not between 0 and 1

Semiclassical

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

Tobi

1/(1+x)

seems best, no?

Semiclassical

If all you’re after is a convenient mapping, tho, that kind of arbitrary choice isn’t an issue

Define “best.”

Tobi

oh you same, 1-1/(1+x) which is basically the same

Semiclassical

16:35

It’s certainly a satisfactory choice

Yeah

Tobi

its performant for a computer to calculate

Semiclassical

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

Tobi

I've got some crazy looking fractals

using this technique

i don't know if theyre complete nonsense or not

Semiclassical

neat.

Tobi

imgur.com/NGvw0Tx

Kyle Kanos

19:26

What a waste if a review: physics.stackexchange.com/review/low-quality-posts/259137

Both guys did this for both non-answers, disputing my flags.

JMac

19:56

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

Kyle Kanos

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

JMac

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.

Emilio Pisanty

22:19

@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

JMac

23:29

@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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