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Dark energy

Dark energy (edit | talk | history | protect | delete | links | watch | logs | views)

IP users at this page keep asserting, without a source, that dark energy, particularly the cosmological constant version, violates conservation of energy because the total amount of it increases as space expands. No article in Category:Dark energy seems to discuss this apparent paradox. How do academic sources resolve it? –LaundryPizza03 (dc̄) 06:39, 14 December 2021 (UTC)

There may be problems with defining total energy in the general theory of relativity. Wiser counsel needed. Xxanthippe (talk) 08:34, 14 December 2021 (UTC).

This is a perennial controversy in general relativity. If you define energy from the usual stress–energy tensor, then it is not conserved. A pair of orbiting black holes will lose energy by emitting gravitational waves and eventually collide. This is well-established experimentally. Now, personally I find it silly to call this a violation of conservation of energy, because it clearly went to the gravitational waves, and in fact one extracts energy from them when you detect them with LIGO. The problem is that it's really not easy to define the energy of the gravitational radiation. The usual way is with a stress–energy–momentum pseudotensor, which is not a tensor, but a pseudotensor, which is not covariant, and leads to all sorts of trouble. If you can stomach that, then energy is conserved. The venerable Physics FAQ explains it well. Tercer (talk) 09:36, 14 December 2021 (UTC)

See the section Friedmann–Lemaître–Robertson–Walker metric#Cosmological constant and the immediately following section Friedmann–Lemaître–Robertson–Walker metric#Newtonian interpretation. The dark energy increases due to the work done by the expansion of the universe against the tension which pervades space. Since the universe is infinite, it makes no sense to talk about the total energy of the universe. The conservation of energy makes sense only when applied to a finite region (whether constant in size or expanding does not matter). JRSpriggs (talk) 21:05, 14 December 2021 (UTC)

Another way to look at it is: the increase in positive energy (dark energy) is balanced by the increase in negative energy (gravitational potential energy). JRSpriggs (talk) 21:26, 14 December 2021 (UTC)

Okay, until a source is found that specifically discusses this notion of energy conservation in GR with regards to dark energy, I've created an FAQ on the talk page. Does it make sense, and should it have ref tags? It would be best to ensure that WP:OR is not violated here. Are there any other FAQs that you think should be clarified w.r.t. this topic? –LaundryPizza03 (dc̄) 08:02, 16 December 2021 (UTC)

Number states

Would someone from the project please look at Draft:Number states and make a recommendation? DGG ( talk ) 06:36, 15 December 2021 (UTC)

I don't see a problem in principle with having an article specifically about photonic number states. However, the draft is pretty bad, and the material it contains duplicates what we already have at Fock state and quantum harmonic oscillator. I would delete the draft and redirect it to Fock state. Tercer (talk) 10:36, 15 December 2021 (UTC)

Agree. WP:Nottextbook. Xxanthippe (talk) 21:46, 15 December 2021 (UTC).

done. DGG ( talk ) 07:31, 18 December 2021 (UTC)

Wikipedia:Articles for deletion/Enzo Tonti

This deletion discussion may be of interest to the community here. XOR'easter (talk) 03:20, 16 December 2021 (UTC)

"Algebra of physical space"

These appear to be rather grandiose portrayals of some rather marginal ideas in the geometric algebra tradition:

• Algebra of physical space (edit | talk | history | links | watch | logs)
• Dirac equation in the algebra of physical space (edit | talk | history | links | watch | logs)

Thoughts? XOR'easter (talk) 17:52, 17 December 2021 (UTC)

Also this:

• Spacetime algebra (edit | talk | history | links | watch | logs)

It seems like we have more pages than strictly necessary devoted to something that's never really caught on. XOR'easter (talk) 17:52, 18 December 2021 (UTC)

A quick investigation shows that these pages were mostly written by the SPAs Rcabrera, Cabrer7, 137.207.80.65 (talk), and 24.57.20.112 (talk), which are all operated by the same person. I'm not in the mood of cleaning up this mess, though. An easy thing to do would be to redirect Dirac equation in the algebra of physical space to Algebra of physical space. Tercer (talk) 13:30, 19 December 2021 (UTC)

That seems like a good first step, at least. Duly redirected. XOR'easter (talk) 03:48, 20 December 2021 (UTC)

Multiphysics

The page Multiphysics could stand some attention. It seems to be what happens when you leave different usages of a buzzword together in the fridge for too long. XOR'easter (talk) 22:33, 24 December 2021 (UTC)

Dear lord. That's.... quite something. It's a shame it's probably a notable concept, because AfD would be so much easier than riding a lawnmower into THAT thicket. I'll give it some thought next week after the holiday. PianoDan (talk) 22:42, 24 December 2021 (UTC)

Should it perhaps be renamed multiphysics analysis or multiphysics simulation to narrow the topic and clarify the scope?--Srleffler (talk) 03:41, 25 December 2021 (UTC)

I could get on on board with that, depending on what the references support. PianoDan (talk) 16:04, 25 December 2021 (UTC)

Yes, that sounds reasonable. XOR'easter (talk) 16:33, 25 December 2021 (UTC)

Here's a draft: User:PianoDan/sandbox Got rid of all the meaningless doublespeak, and there wasn't THAT much left. It could certainly be expanded, but for now, this strikes me as more encyclopedic. Any major concerns before I put it back in article space? @XOR'easter:, @Srleffler:? PianoDan (talk) 22:53, 28 December 2021 (UTC)

Looks good to me. The references need work (it looks like the arXiv item might have been published formally later [1]), but fixes like that can be done in mainspace. XOR'easter (talk) 22:58, 28 December 2021 (UTC)

Looks good.--Srleffler (talk) 03:55, 29 December 2021 (UTC)

OK, I've brought the short version back over to mainspace, moved the page to multiphysics simulation, and done some additional cleanup. XOR'easter (talk) 04:44, 29 December 2021 (UTC)

Thanks! PianoDan (talk) 17:20, 29 December 2021 (UTC)

Digital philosophy

The page Digital philosophy (despite the name, mostly about fringe universe-as-computer speculations) had been in a bad state with no efforts to improve it months after being tagged, so I went ahead with the Gordian-knot approach. Further suggestions on what to do with it are welcome. XOR'easter (talk) 18:13, 28 December 2021 (UTC)

Srleffler, your changes look good. Thanks. XOR'easter (talk) 19:32, 28 December 2021 (UTC)

It seems that the more popular name for use of computers in philosophy is computational philosophy; for instance there is an SEP article on Computational Philosophy that looks like it includes the use of computers to investigate the network models of philosophy mentioned in Digital philosophy. It may be best to merge this article into the computational philosophy article. --{{u|Mark viking}} {Talk} 19:37, 28 December 2021 (UTC)

That's not a bad idea; thanks for the suggestion. XOR'easter (talk) 21:00, 28 December 2021 (UTC)

I proposed the merger on the page. While I was at it, I also proposed merging Digital physics into Simulation hypothesis.--Srleffler (talk) 21:23, 28 December 2021 (UTC)

I ended up pruning the physics section of Simulation hypothesis quite a bit. It appears to have been original research by a single-purpose account.--Srleffler (talk) 22:08, 28 December 2021 (UTC)

An entertaining old quote I found: In many respects this point of view may be nothing more than a result of the fact that the notion of computation is the disease of our age—everywhere we look today we see examples of computers, computation, and information theory and thus we extrapolate this to our laws of physics. Indeed, thinking about computing as arising from faulty components, it seems as if the abstraction that uses perfectly operating computers is unlikely to exist as anything but a platonic ideal. Another critique of such a point of view is that there is no evidence for the kind of digitization that characterizes computers nor are there any predictions made by those who advocate such a view that have been experimentally confirmed. [2] XOR'easter (talk) 22:50, 28 December 2021 (UTC)

It reminds me of how in the steam age they conceptualized lots of things as being like steam engines. We are all prisoners of our time.--Srleffler (talk) 18:31, 29 December 2021 (UTC)

The word "digital" implies the Turing machine, which has a finite set of states and a finite set of state transitions (and an infinite tape). This is in contrast to quantum computers, which have an uncountable number of state transitions (an uncountable number of operator (mathematics)) acting on an uncountable space (complex projective space, in the case of QM), organized into finite-length strings of operations applied in sequence. There is no "tape"; the no-cloning theorem implies that there cannot be a tape. In summary: we can have "computers" that are not "digital", and conflating the two concepts leads to popular misconceptions and confusions. We may live in a simulation, but the simulation hypothesis does not suggest that it is digital. 67.198.37.16 (talk) 17:57, 30 December 2021 (UTC)

I imagine most editors here understand this. Digital physics is a really stupid idea, but it appears to be a notable stupid idea.--Srleffler (talk) 04:35, 31 December 2021 (UTC)

Actually, quantum mechanics is no counterexample. One just needs to use a quantum Turing machine instead of a classical one. It seems a really deep fact of Nature that all of physics can be efficiently simulated with a universal digital quantum computer. In principle one might need the continuum, or a non-programmable computer. Tercer (talk) 18:52, 31 December 2021 (UTC)

It seems a really deep fact of Nature that all of physics can be efficiently simulated with a totally imaginary device, that we imagine to be capable of simulating all of physics.--Srleffler (talk) 23:33, 31 December 2021 (UTC)

Nothing imaginary about quantum computers, the theory is clear that they are possible and good prototypes already exist. Tercer (talk) 23:35, 31 December 2021 (UTC)

Electromotive force: uppercase or lowercase?

When abbreviating electromotive force as emf/EMF, do we use uppercase or lowercase?

I see that this has been asked on the article talk page in several sections without an entirely clear answer. The only somewhat clarifying comment I see (from Dicklyon in 2018) states that emf is more commonly seen in textbooks and style guides, which matches what I've seen (anecdotally), though capitalization recommendations are not consistent among other sources. The article on electromotive force uses primarily emf, though many linked articles use EMF, and I saw a handful of relatively recent edits (this, for example) changing capitalization, though it is unclear which is more correct.

Also, the uppercase EMF can also be used to abbreviate electromagnetic field, possibly a source of confusion if both terms appear in the same article. ComplexRational (talk) 22:25, 4 January 2022 (UTC)

Just to muddy the waters, I'll mention that the most common abbreviation I've seen for this in textbooks is a italicized SCRIPT E: ℰ PianoDan (talk) 22:45, 4 January 2022 (UTC)

Here's the n-gram stats. Looking at books, the recent rise of capped EMF is almost all about electromagnetic fields, it appears to me, while electromotive force is still usually emf. Dicklyon (talk) 01:56, 5 January 2022 (UTC)

@Constant314: You changed emf back to EMF. You have strong feelings about this question, or just what your edit summary said? Dicklyon (talk) 01:38, 5 January 2022 (UTC)

I try to stick with the manual of style. No strong feelings. Constant314 (talk) 03:31, 5 January 2022 (UTC)

Well, this is not an ordinary acronym, but something that appears almost always in all lowercase. OK? Dicklyon (talk) 06:10, 5 January 2022 (UTC)

Is there any reason not to just refer to it as "voltage"? Or is there a difference? JRSpriggs (talk) 17:32, 5 January 2022 (UTC)

Often "voltage" is reserved for a difference in static potential, whereas electromotive force may come from a changing magnetic field. --Trovatore (talk) 18:51, 5 January 2022 (UTC)

Comments welcome on Gravitational lens

See Talk:Gravitational lens#Half the value given by general relativity. Comments would be welcome. TIA. - DVdm (talk) 22:58, 9 January 2022 (UTC)

Articles needing attention

I've been looking at the "articles needing immediate attention" list, as opposed to the "articles needing expert attention." Membership in that list is generated when someone sets a flag on the project template on a talk page. But given that it's just a flag, what's the appropriate response when you get to a flagged page and there's no comment, and it's not obvious what the issue is? I mean, in theory, EVERY page that isn't GA needs attention. So what to do when confronted with a banner that says "needs immediate attention," and it's not obvious that the page is actually on fire? PianoDan (talk) 17:30, 11 January 2022 (UTC)

Just remove it. I took a look at Talk:Helium atom, where the flag was added in 2009; it was likely for concerns that have long been resolved. Actually, I think we should deprecate the parameter since it can be added without any obvious reason. –LaundryPizza03 (dc̄) 17:50, 11 January 2022 (UTC)

For reference, the category is Category:Physics articles needing attention. –LaundryPizza03 (dc̄) 17:55, 11 January 2022 (UTC)

Oh, yeah, thanks. It's interesting - the category is "needing attention", but the text in the template is "needs immediate attention.". PianoDan (talk) 18:05, 11 January 2022 (UTC)