The h Bar

3:55 AM

-2

A: Can gravity cause a Big Crunch, if the cosmological principle holds?

Can gravity cause a Big Crunch, if the cosmological principle holds? No. That's a myth that grew out of the Friedmann equations. Alexander Friedmann was a ballistics instructor on the Austrian front. He modelled the expanding universe as something like a cannonball fired aloft. See John Peac...

My question is old and riddled with misconceptions, but jesus

> We have no evidence that the universe is closed, or that the cosmological principle is scientific.

No one is talking about evidence there

NormalsNotFar

6:24 AM

Does anyone know of a reference where I can find how one canonically quantizes the EM field via the Dirac procedure (through Dirac poisson brackets and constraint analysis etc)?

I found a reference (Textbook of ashok Das, if anyone is interested)

PM 2Ring

6:43 AM

@JohnRennie See physics.stackexchange.com/questions/469279/…

John Rennie

@PM2Ring yes?

@PM2Ring ah, the OP has just stated the equipment is evacuated so you wouldn't get scattering by air molecules.

PM 2Ring

@JohnRennie Exactly.

John Rennie

7:45 AM

Back-in-Time and Faster-than-Light Travel in General Relativity. Looks interesting. Expensive though.

Nick

hi guys, anyone interested in particle physics?

John Rennie

@Nick hi Nick. I guess it depends on what you mean by particle physics - that's an awfully large area :-)

Nick

I have a stochastic algorithm from swarm intelligence. I'll share it here.

@JohnRennie sites.google.com/view/rxys-log/algorithm

PM 2Ring

The misconceptions in this question make my brain hurt. physics.stackexchange.com/questions/469313/… But I figured I'd answer it anyway. :)

John Rennie

@Nick ah, OK, I assumed you mean subatomic particle physics, as in CERN etc

Nick

7:58 AM

@JohnRennie lol, I thought that doubt will be there. Though I do wonder if I can make my own particle zoo from such experiments :D

some will be fireflies, some will be ants, some will be bees, birds, flying seeds, pinballs, bouncy balls, dragon balls, giant baby heads...

et cetera.

John Rennie

@PM2Ring I thought that was quite an interesting question. I did a quick calculation for the evaporation rate due to sunlight at the orbit of Pluto and got 1 gram per square meter per 2600 seconds. But that's actually quite high e.g. a typical comet would last only a million years out there so I've made a mistake somewhere.

@PM2Ring I have to say I don't think your answer is a great one. You need to provide some calculations to make it concrete.

PM 2Ring

@JohnRennie Fair point. But I'm feeling too lazy ATM. It was actually written as a comment, but I decided it was too answer-ish. :)

If you (or someone else) want to post a more quantitative answer, I'll delete mine.

Nick

8:20 AM

besides dan schiffman's nature of code, there doesn't seem to much on my topic that's understandable and visualizeable. Do you guys all use Matlab for your simulations?

Captain Bohemian

8:36 AM

Does chiral symmetry refer to the discrete symmetry regarding left-hand and right-hand coordinate systems? But isn't this symmetry called parity?

PM 2Ring

@CaptainBohemian Does this help? physics.stackexchange.com/questions/232591/…

Captain Bohemian

oh, maybe parity refers to $x\rightarrow -x$ let me think.

PM 2Ring

Wikipedia says "Invariance under parity transformation by a Dirac fermion is called chiral symmetry".

Captain Bohemian

@PM2Ring yea, I feel parity symmetry and chiral symmetry seem to be equivalent.

JD_PM

@JohnRennie morning Professor. I'd like to ask you for advice on good books in General Relativity. I'm interested in a book that also includes solved problems. I've found one convincing book on special relativity (it has 200 solved problems, the author is Michael Tsamparlis), but not in GR. Sean Carroll's one is nice, but doesn't include solved problems. Thanks

John Rennie

8:45 AM

← 1 message moved from Problem Solving Strategies

Captain Bohemian

only the parity in Dirac spinor is called chiral. In my thesis, I always called it parity.

John Rennie

@JD_PM hi, we should discuss this here rather than the problem solving room.

JD_PM

@JohnRennie I agree

PM 2Ring

Note that "parity" gets used a few different ways, and people don't often bother being specific unless it's important. Eg, parity is often explained in pop-sci as a mirror image transformation. But if you look into a mirror in the XY plane, the mirror image inverts the Z dimension, i.e., swaps front for back. But strictly speaking a parity transform swaps all axes, so (x,y,z) maps to (-x,-y,-z).

John Rennie

@JD_PM to be honest I'm not that familiar with the literature so I can't really help. I guess you want something with the solutions o you know whether you've done the problems correctly.

Schutz has solutions, but I suspect it's too elementary for you as it's only an introduction.

JD_PM

8:51 AM

@JohnRennie yeah that's the idea. I read you learned GR for fun so I thought you'd have like favorite books on it or something like that. If you have books that aren't problem solving style in mind, please feel free to comment about them as well

John Rennie

@JD_PM I did learn GR for fun, but I was mainly interested in understanding how to work with metrics so I'm missing a lot of the more mathematical aspects. If you look at my GR related answers they are mostly explaining how to use the various metric to compute things. I wouldn't know how to obtain a metric by solving the field equations, and my grasp of differential geometry is sketchy.

2

If you've read Carroll you know a lot more about GR than I do :-)

PM 2Ring

FWIW, John Baez has a nice GR tutorial.

JD_PM

@JohnRennie Thank you :) I am most interested in understanding so thought that studying solved problems would also help

@PM2Ring Thanks :)

John Rennie

@JD_PM not that I'm condoning piracy, but Bernard F. Schutz's book and the solutions manual can allegedly be found with excessive Googling.

JD_PM

nice

John Rennie

9:04 AM

Every now and then someone posts a question from Schutz. Having the solutions manual is quite helpful in these cases :-)

Actually I've just glanced through my (paper) copy of Schutz and the questions are quite interesting. Have a look and if you find them too simple move on.

JD_PM

Thanks, it will be helpful

Physics Meta

12:01 PM

0

Q: Can anyone advice the real name of "Qmechanic"?

I am a big fan of the rigidity, depth and straightforwardness of "Qmechanic" answers, I wonder if he\she has a book\notes in some fields of physics, I believe we can learn much faster by using them. So I wonder if we can know his real name (if that allowed by StackExchange or he\she has willingn...

discussion

GodotMisogi

@JohnRennie His style is very nice indeed. He covers a lot of the mathematics as a major chunk of the material in most chapters, and leaves the physics to the exercises as applications of the maths

Secret

@JohnRennie I don't think GR really provide a way to make a time machine. It can only be discovered in the form of CTCs

John Rennie

@GodotMisogi I learned GR from Schutz's book and I do quite like it. However I did find he skips over the details sometimes and left me confused. I even had to ask here what he meant at one point:

3

Q: Why is $p_\phi$ conserved in a Schwarzschild orbit?

This arises from the question What is the relationship between $a$ and $m$, which I'm afraid I answered just by looking it up in Schutz's book. However Schutz (as he frequently does) glosses over details he thinks are irrelevant or too simple to be worth explaining, and I have realised I don't un...

general-relativity black-holes conservation-laws symmetry geodesics

Secret

What I am not sure, though is whether CTCs are frame independent, it seems unthinkable there exists noninertial frames where the worldline does not loop back and some where it does

That is, all observer should agree some region of spacetime, there is a CTC. I don't see how one can get away from this

PM 2Ring

12:20 PM

@JohnRennie Jeremiah has written some more comments for you. Although he writes eloquently, they don't make much sense to me.

John Rennie

@PM2Ring it turns out this is all related to his personal theory. I think we can safely put him in the ~~crackpot~~ unconventional thinker bin.

knzhou

Jeez, what an essay.

It's another example: knowing how to write is good, but only knowing how to write is worse than knowing nothing.

PM 2Ring

@JohnRennie Agreed, although he's quite polite & respectful, unlike a lot of the crackpots we see, so I prefer to think of him as misguided. OTOH, I get the feeling that there's no way to convince him that his personal theory is wrong.

Secret

he will find his way out eventually

misguided are easier to handle than crackpots

Our world is simply not idealistic enough to have many pet theories to describe reality

Also I think I have found the answer myself: CTCs are worldlines that closed onto itself. Since all curves in spacetime are geometric objects, they have to be frame independent, thus if there is a closed curve in spacetime, then it means it will remain closed in all reference frames. It may be CTC, spacelike or a mix of the two depending on the reference frame

PM 2Ring

Here is a page selling the "textbook" he mentions. I don't think I'll be investing in a copy. ;)

Secret

12:33 PM

I am bored, I am going to spend some time indulging that essay lol

PM 2Ring

@Secret Correct. If a path is timelike in 1 frame it remains timelike under any Lorentz transformation. And if it's closed, a Lorentz transformation cannot change that.

Secret

I see

Secret

12:47 PM

So I took the plunge. Basically, Jeremiah's pet theory is postulating there exists a macroscopic 4th nonspatiotemporal dimension where every physical matter is held together in spacetime by some strange, uniform "field like" structure that always tries to hold the universe together and that generates oscillations between two 3D spatial regions that is separated by this nonspatiotemporal dimension. Like almost every misguided pet theory in physics, it has this common theme of "unity" that is

prevalent in spiritual and religious communities

Basically he is postulating there is The Force from star wars

Ba'lroc Demos

@Qmechanic Some of the duplicate flags cite Questions asked years ago. Either you are historical savant of Physics SE or you have a tool. How do you find such ancients qurations to flag Dups?

Secret

I think they mostly do a keyword search, and it just happens very old questions matches the criteria

Ba'lroc Demos

Do most folks do such diligence in asking questions?

PM 2Ring

@Secret Or brahman from Hinduism.

Secret

yeah, other than "oscillation" probably makes no sense to something like Brahman

Buddhism, Hindulism and New Age are the three most common inspirations for all crackpot and misguided pet theories

PM 2Ring

1:01 PM

@Ba'lrocDemos Not as many as we'd like. It's expected that you do some searching before submitting your question. But we understand that it can be hard to search when you don't know the answer, so you don't know the right keywords that will narrow down your search.

Secret

this might be because they share a structural and hierarchical description of concepts, thus similar to how physics models are build up

unlike the abrahamic religions which mostly surrounds a divine authority

PM 2Ring

@Secret Or how the mind's modelling process works, in general. I guess meditation can give you deep insights into that modelling process, but unfortunately people tend to lose the distinction between the model and the thing being modelled. And to think the model they've discovered must be the True Secret of the Universe.

Secret

Yeah, I actually don't think any "ultimate reality" will be perceptible in any form at all to us. One thing I noticed the more I chatted with people in this spritual communties, and then look at how problems are approached in different knowledge domains, is how we as a species, seemed to be heavily fixated on binning things, finding patterns and making hierarchies

Ba'lroc Demos

@PM2Ring But do they read the queatiins they cite. I got flagged for a dup and when I read the cited question, My question was not answered. Like I was flagged in title alone.

PM 2Ring

@Ba'lrocDemos Most questions get closed by ordinary users with sufficient rep, not elected mods. Yes, mistakes are made, and if you believe that your question was closed incorrectly you are encouraged to give feedback, either in a comment, by editing your question to make it clear why the nominated dupe(s) doesn't apply, or by writing a question on the Physics Meta site. Or mentioning it in this chat room.

@Ba'lrocDemos When I dupe-close, I often use dupe target questions I'm familiar with. Otherwise, I read the dupe question & its answers to make sure it's a good match for the new question. But it's certainly possible to make mistakes if you're in a rush, and you haven't read the new question thoroughly.

Ba'lroc Demos

1:17 PM

Oh I just assumed Qmech is a mod.

PM 2Ring

@Ba'lrocDemos Yes, Qmechanic is a diamond mod, and can close any question single-handedly.

It takes 5 regular users to close a question, except that duplicates can be closed by one person if they have a gold badge in any of the question's tags.

Also, a duplicate target question doesn't have to match the new question exactly, but the info in its answers do need to answer the new question.

Ba'lroc Demos

Ok Ill keep working with @Qmechanic to remove.the flag. I am certain my question is not answered or addressed in the cited post. But perhaps by fixing this one I can learn how to avoid flags in the future.

PM 2Ring

There's nothing wrong with asking a dupe, as long as you've made some effort to search for existing questions. Dupe questions aren't bad questions, and they can act as good signposts to the dupe target.

Dupe answers are bad, because we want all the answers to a question to be in the one place so they can be easily found, and so that people can easily compare them. And so those answers can compete with each other for votes. If the answers are in several separate voting pools, that undermines the voting process.

23 hours ago, by PM 2Ring

FWIW, I agree that your question is different to the dupe target. The target asks what are the incompatibilities between GR & QM. You ask why do we even need to unify them, if I understand correctly.

Ba'lroc Demos

1:34 PM

ok, but unless the original question is placed.in the community wiki the tendency will be for answets to get buried.

I edited the question to clarify what.I am getting at.

Slereah

Any basic question about QM and GR has been asked

Ba'lroc Demos

This is not a basic question. I propose that perhap the reason GR and QM are simultaneously correct and yet incompatible is that they really are two disparate yet interacting aspects of reality.

Slereah

I can tell you that this has definately been answered before

I know because I answered that question

The theory that we don't need to unify GR and QM has been floated around before and it's unlikely to be the case

Ba'lroc Demos

Can you point me to the correct question?

Slereah

4

A: Is it possible that there is no theory of quantum gravity?

The theory was put forward that semiclassical gravity (a classical gravitational field generated by quantum matter) was indeed the correct theory, something of the form $G_{ab} = \langle T_{ab}\rangle$ With $G$ the Einstein tensor and $\langle T\rangle$ the expectation value of the stress ener...

Jake Rose

1:47 PM

Anybody get why the equation before 5.2 is less than or equal?

Slereah

I mean it's not a bad theory, but it's kind of weird and doesn't really work out experimentally

Jake Rose

just sort of comes out of nowhere

Jake Rose

1:58 PM

It says it comes from kelvin’s statement if the second law but I’m not exactly seeing it

Ba'lroc Demos

Except that you don't really address the observation that Spacetime and the theories that describe it provide a frame of reference. While QM describes the particles and interplay that occur within it.

Closer than the present flag though.

Slereah

@Ba'lrocDemos You forget that matter curves spacetime in GR though!

And matter is quantum fields

if spacetime was curved but static it wouldn't be an issue

Ba'lroc Demos

2:19 PM

I never say there is no interplay between the two, in fact it is central to why as I propose, the two theoriea are at once consistent and seemingly incompatible.

Slereah

You're a bit short on the explaining how they are part

The simplest union of QM and GR is, as I said, semiclassical gravity

and it doesn't work great

Ba'lroc Demos

GR describes how matter deforms Spacetime. How matter interacts with its frame of reference. QM describes particles and the interplay between them.

Slereah

Sure, but I mean more mathematically :p

Ba'lroc Demos

Trying to describe interactions between particles with out.providing a frame of reference is meaningless. And our perceptions change as the frame changea. But it is perfectly acceptable to describe.the FOR by itself.

Now what makes up the frame which is spacetime, that is a whole other conversation.

There is certainly interplay between the FOR and the particles described in QM. I think Q Relativity will describe.that mathematically but actually describing Space time itself as a quantum phenomenon IMO wont work because because the frame itself and the managerie with in it are truly unique.

Akash. B

3:24 PM

@EmilioPisanty Can you please elaborate?

John Rennie

@Akash.B it was a joke.

In English the word super can mean especially good, so superposition could mean an especially good position.

Superposition is actually a technical term meaning that two or more wavefunctions can be added together to give a new wavefunction.

user8718165

@JohnRennie hello

John Rennie

3:48 PM

@user8718165 hi

Akash. B

4:05 PM

@JohnRennie Wave funtions?

John Rennie

@Akash.B I suspect this may be a bit advanced for you right now. You need some basic knowledge of quantum mechanics and that usually only starts in your first year of a physics degree.

Avantgarde

hey

amanuel2

I keep getting this question wrong since Im not supposed to be using 11m/s for the speed of the wrench since thats relative to the astronaut. Do you know how to get the velocity to use for the wrench for question #1. THanks!

John Rennie

@amanuel2 let's call the speed of the astronaut after throwing the wrench $v_1$ and the speed of the wrench $v_2$. Then the question tells us that $v_1 + v_2 = 11$. OK so far?

amanuel2

Yeah @JohnRennie , so how would i get v1 in order to plug it in my momentum equation

John Rennie

4:15 PM

Before throwing the wrench both the atsronaut and the wrench are stationary so the total momentum is zero. Conservation of mometum means that the momentum after throwing the wrench must also be zero i.e. the momenta of the astronaut and wrench are equal and opposite.

That means $Mv_1 =mv_2$, where $M$ is the mass of the astronaut and $m$ is the mass of the wrench.

amanuel2

Ohhh, thanks a lot!

John Rennie

So now you have two simultaneous equations for $v_1$ and $v_2$ and you can solve them to calculate the values of $v_1$ and $v_2$

amanuel2

Oh now i get 0 :/

Isnt this the setup for question #1: v1(Ma+Mw) - Mw*v1 = 0

I ommited the division by total mass since it will multiply with 0

@JohnRennie

John Rennie

4:44 PM

@amanuel2 your equations are $v_1 + v_2 = 11$ and $Mav_1 = Mwv_2$. From the second equation we get $v_2 = v_1 Ma/Mw$ and we can substitute in the first equation to get:

$$ v_1 + v_1Ma/Mw = 11 $$

amanuel2

Yeah my bad I was wrong. I got Va = 11Mw/(Ma+Mw)

Which lead to -1.055m/s , which is wrong answer :/ (added negative since astronaut going -x direction)

John Rennie

@amanuel2 yes, I've been careless with the signs. If we take right to be positive then the astronaut's velocity is a negative value.

amanuel2

Va being the velocity of astronaut. I first substitued Vw for 11-Va (Vw being velocity of wrench)

Do you get -1.055m/s for the velocity of astronaut as well? @JohnRennie

John Rennie

Yes

amanuel2

@JohnRennie

John Rennie

4:56 PM

@amanuel2 Facepalm! The astronaut is still holding one of the 10.3 kg wrenches so for Ma we need to use 97 + 10.3kg

amanuel2

Ohhh i keep forgetting about that. Major Facepalm

John Rennie

@amanuel2 you and me both! :-)

Qmechanic

@Ba'lrocDemos : A previous version of your question was rejected by reviewers. But now it is under review again.

PM 2Ring

You also have to be careful how you throw the object, otherwise you'll give yourself too much angular momentum. — PM 2Ring Mar 21 at 13:20

You don't want to get into a spin. It wastes kinetic energy, it's disorienting, and it makes it harder to grab a handhold when you approach the ship.

amanuel2

@JohnRennie Indeed. Now i got the right answer ;)

Thanks for the help!

PM 2Ring

5:03 PM

Hey @Qmechanic Would you like to move these comments to chat? It's getting messy... physics.stackexchange.com/a/469358/123208

John Rennie

@amanuel2 Cool :-)

PM 2Ring

I guess having 2 wrenches to throw helps a bit, since the 2nd one can be used to counteract the spin induced by the 1st one.

amanuel2

@JohnRennie If your available here i got one last question. Here what do you use for the Vw. I tried setting it up the same way Vw=11-Va , and then I tried Vw = (11+0.9634)-Va . And yes im aware this time he isnt holding any wrench so its directly his mass

bolbteppa

@Ba'lrocDemos string theory is a way to bypass the issues with QM and GR, so they are compatible in one known way

The Case for String Theory - Sixty Symbols

►

John Rennie

@amanuel2 in the rest frame of the astronaut+wrench their velocity changes by $-1.056$ m/s when they throw the second wrench. Yes?

amanuel2

5:14 PM

Yes

John Rennie

And that rest frame is already travelling at -0.9634 m/s as a result of throwing the first wrench. So the total velocity of the astronaut after throwing both wrenches is -1.056 - 0.9634.

I get -2.019 after rounding to 3 decimal places

amanuel2

That makes a lot of sense. Again Thanks a lot @JohnRennie!

M.N.Raia

5:46 PM

Good morning/evening/night for you guys!

Hmm I know that this is not a standard procedure (post questions here),but I'm struggling with a particular silly point and I don't have satisfatory answers.

https://physics.stackexchange.com/questions/469349/the-chain-rule-and-velocity-transformation-in-relativity/469369#469369

I appreciate.

amanuel2

Any ideas how to do #5 @JohnRennie , I promise its the last question now :}

The general idea is CM = M1X1+ M2X2/ M1+M2 . M1 mass of being wrench 1 and M2 being mass of wrench 2

M1 and M2 we already know. For X1 I used (11-Va(from wrench #1))t which equaled 321. Am i correct so far?

Ba'lroc Demos

8:02 PM

@Qmechanic I have extensively edited my post. The the essense hasn't changed but I believe the format is now in compliance. Can you please review it. Thanks

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