The h Bar

8:17 AM

this is one hell of a score-over-time graph

points for guessing what post it describes

The end value is close to 750, so my first guess with 711 votes might not be correct. So it must be on a different site.

if it is a question. Else it must be the first answer to the same question as it has 758 votes.

Krishna

8:44 AM

could it be this? physics.stackexchange.com/a/121932/257049

Probably not.

Guru Vishnu

I think that couldn't be a candidate as it was asked only in 2014 whereas the graph starts in 2011.

Krishna

Yesh, I noticed that

Guru Vishnu

I don't know what caused the surge in temperature near the end of 2013 though?

Krishna

The slope tends to infinity

Probably a cold coffee revolution.

user142929

@JohnRennie I'm sorry for this my furtive comment (today I've known this chat room). I would like to know feedback about the second part (not about if my post is suitable for PSE) of the body of the Meta PHYSICS with identificator 12909 that I've asked (May 17 at 21:00) with title Asking if the following post could be suitable for the main site Physics Stack Exchange. I don't know you, but if you or some of your colleagues can do feedback in comment in such Meta or here it is appreciated. Thanks

The link of the question is physics.meta.stackexchange.com/questions/12909/… As was said please add feedback if you can or consider it about the second part of the body of the post. Any case, many thanks and good week,

PM 2Ring

9:01 AM

@DavidZ Why did you close physics.stackexchange.com/q/555344/123208 with the homework close reason? The OP isn't asking for a calculation, they clearly have a conceptual confusion: they don't understand that nuclear binding energy is negative. IMHO, it would be much better to close it as a duplicate.

John Rennie

← 2 messages moved from [Let’s do some Physics ](chat.stackexchange.com/rooms/105599/lets-do-some-physics)

@user142929 hi

@user142929 the Physics SE site is supposed to be for mainstream physics. For example when I write an answer my aim is that you could put what I tell you in the answer to a physics exam question and get full marks.

That means that non-mainstream stuff is not suitable.

You are asking about an idea that involves micro black holes at the edge of the Solar System, and this is just a personal theory that you wouldn't find discussed in any physics textbook, and for that reason it isn't suitable for the main site.

However you would be welcome to ask about it here in the chat room as we're happy to talk about pretty much anything here.

PM 2Ring

9:25 AM

Hello @user142929. Your question is a bit hard to read, but I think I understand it. I think your main point is that you would like to know how would mainstream physics falsify your micro black hole theory. Is that correct? If so, this is similar to the famous "teapot" hypothesis of the great philosopher & mathematician Bertrand Russell.

Russell's teapot

Russell's teapot is an analogy, formulated by the philosopher Bertrand Russell (1872–1970), to illustrate that the philosophic burden of proof lies upon a person making unfalsifiable claims, rather than shifting the burden of disproof to others. Russell specifically applied his analogy in the context of religion. He wrote that if he were to assert, without offering proof, that a teapot, too small to be seen by telescopes, orbits the Sun somewhere in space between the Earth and Mars, he could not expect anyone to believe him solely because his assertion could not be proven wrong. Russell's teapot...

user142929

Many thanks @JohnRennie I'm am a non professional (I'm mathematician) mathematician and I'm interested if the second part, the hypohtesis that I postulate, of the linked post can be falsifiable. I would like to know if it the phenomena that I describe can be just an absurd. What knowledges, experiments or instruments can tell me something about it?

I hope in next hours or days your feedback, feel free to tell/refer it (my hypothesis) to your colleagues (professors that you know in the real life). Many thanks again.

Secret

Does Gravity Require Extra Dimensions?

►

This episode is cool not because of the video

but the cool insightful questions asked in the comments

here are some:

BigB1Lachi
11 hours ago
This reminds me of an idea I once had that gravity wasn't a force itself, but only the probability of a particle being close enough that the strong force would act upon it.

The Ancient Melodies
2 hours ago
Is there a “Reynolds number” for gravity? Above a certain value, gravitational forces dominate. Below it, the other forces dictate the systems behavior. Laminar VS turbulent.

John Rennie

@user142929 you suggest that certain (unknown) masses in a region just outside the Solar System become black holes then evaporate. But this is so vague as to be meaningless. What masses? What is the cause of them becoming a black hole? It's like you are just waving a magic wand.

Secret

鋼の無神論者：The Fullmetal Atheist
10 hours ago
Hey, wonderful episode! I have a question though:

There are expected correction to the Newton potential, both coming from General Relativity and Effective Field Theory. Would these need to be taken into account once we're able to probe the gravitational force at the scales of string theory's compactified extra dimensions, in addition to the Casimir effect? I suspect this largely depends on how large these extra dimensions are expected to be.

John Rennie

@user142929 in any case this is easily falsified since the final stages of evaporation of a micro black hole is expected to be very violent. Something like a nuclear explosion. We'd have noticed by now if this was happening anywhere near the Solar System.

Secret

9:40 AM

Jackbob
1 hour ago
What still haunts me every day is how time and speed works. Basically, time is, how I understand it, a consequence of interactions between fundamental particles and fields. What time actually is for us is the frequency of particles interacting at light speeds, more interactions = faster advancement of time. Which is why time slows down for an accelerated object compared to it's stationary counterparts, because the closer a macro object comes to light speed, fundamental particles within has to slow down compared to the macro object and therefore reduce frequency of interac…

Artur B
42 minutes ago
Question: Couldn't Casimir effect be the result that we are looking for? Isn't it similar in appearance to gravity.

and that is all for now

user142929

10:27 AM

Many thanks for your excellent answer @JohnRennie . I postulate that such a special region has this feature, but I do not provide any explanation, and I refer hypothetical micro black holes evaporating at the edge of our Solar System. With all respect currently physicists can to take a “photo” of a far distant black-hole but there is fog in the edge of our Solar System. Feel free to reply in you want to say more about it.

John Rennie

> there is fog in the edge of our Solar System

@user142929 no there isn't.

We can see stuff at the edge of the Solar System just fine.

The Voyager probes are currently at the edge of the Solar System and we can see them.

PM 2Ring

@user142929 It's not easy to see small objects in the outer parts of the solar system because there isn't a lot of light out there. But it's not foggy. And the density of matter out there is quite low.

user142929

I think that the Solar System (and I know that this can be as a
an outdated thought) is likes as a clock (made of space-time and information) that tell us things about science: for example Newton studied how falls the Moon on the Earth or the movements of other bodies as comet. Similarly I know the Wikipedia's article with title Rømer's determination of the speed of light and the article Arthur Eddington that explains the importance about his observation related to Einstein's theory of general relativity. There is fog, I mean lack of knowleges, at the edge of the Solar System @JohnRennie

Many thanks @PM2Ring the word fog was a metaphor

John Rennie

You are just making things up. I could hypothesise that there are pink elephants at the edge of the Solar System but we can't see them because there is a fog there that hides pink elephants but still allows starlight through.

PM 2Ring

@user142929 It's possible that micro black holes (MBH) were created in the early phases of the Big Bang. But we do not know of any process in the present era that can make MBHs. In fact, as far as we know it's not possible for black holes of less than 2 or 3 solar masses to be created.

user142929

10:38 AM

I didn't ask things as these, for example I was asking if we can detect the
evaporation of a hypothetical MBH from the Earth, this MBH placed at the edge of our Solar System @PM2Ring Many thanks.

John Rennie

@user142929 yes, an evaporating black hole is expected to reach extremely high temperatures in the last seconds of its existence. This would release a burst of gamma rays that would be easily detectable on Earth.

user142929

I believe that you know what is the size of a particle collider with the purpose to discover new physic @JohnRennie This is the kind of fog that I refer, many thanks.

Please then add the word "micro" in your previous comment, because it should be a full answer. Do you think that it is possible to detect the evaporation of a hypothetical MICRO black hole placed at the edge of our Solar System @JohnRennie ? Notice that this is implicit in my question, isn't just insistence.

John Rennie

@user142929 yes, assuming our theories describing black hole evaporation are correct we would easily be able to observe a micro black hole evaporating at the edge f the Solar System.

user142929

Well then I shut up, I have no choice but to accept your answer @JohnRennie If you or your colleagues want add more additional feedback today or in next days or weeks it is appreciated. Many thanks for your responses and invitation to this excellent chat @JohnRennie and @PM2Ring

PM 2Ring

@user142929 We don't know everything about black holes. We need a theory of quantum gravity to fully describe black holes, and we don't yet have a fully working quantum gravity theory. But we can still be very confident about most of the properties of black holes. So there might be some quantum gravity process that stabilizes tiny black holes and stops them from exploding.

However, it is not possible to convert a small amount of normal matter (eg a few kilograms) to a MBH. The required pressure is insanely high. A supernova explosion supplies enough pressure to create stellar mass black holes, but that pressure is far too small to create an Earth mass black hole, and even more pressure is needed for a 1 kg MBH.

@user142929 This site vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator has a great calculator that lets you calculate many properties of black holes. It might not be fully correct for MBHs due to quantum gravity effects. But (as far as I can tell) it does correctly calculate what standard General Relativity and Hawking radiation theory tells us.

user142929

11:00 AM

I've postulated that such magic and special region can to exit at the edge of the Solar System. My belief is that it is absurd (with all respect) try to travel in the cosmos (I say a travel with a space ship) because the cosmics rays affect to the genetic information of the Life. And that Life could have glimpsed a way out (
to face the radiation). I've persuaded myself of this kind
pseudoscience, I accept that aren't science, but really Christopher Columbus or vikings did need in the past in some way to arrive to a new world in their travels three ingredients: gravity since the surface of…

Currently it is also not known exactly what dark matter is, and supposedly there is attached a certain amount of this for our Solar System. Many thanks again to all users.

John Rennie

11:15 AM

@user142929 most physicists I know read and enjoy science fiction, but we know the difference between the two.

Charlie

11:29 AM

In SR/GR, if two different inertial observers are watching a particle move around with coordinates $x^\mu$, if observer $A$ parameterizes the particles coordinates with $\lambda$ so we have $\frac{dx^\mu}{d\lambda}$ and we move into the frame of $B$ where we parameterize the motion with the proper time of $B$, we have $\frac{dx^\mu}{d\tau}=\frac{dx^\mu}{d\lambda}\frac{d\lambda}{d\tau}=\gamma\frac{dx^\mu}{d\lambda}$

Is it ok to use proper time like this when we're talking about two observers looking at the motion of a third body, i.e. taking the "proper time" as the coordinate time of $B$? Or is proper time reserved for the coordinates of the particle they are both watching?

I want to say "proper time" is just the coordinate time of a sort of "privileged" reference frame against which we are comparing other coordinate times

user142929

11:50 AM

Yes, even it seems to me that Life enjoys also with science fiction, the Wikipedia Symbiogenesis explains what is mitochondria, a key improvement in Life: we can to think to travel to Mars or to the stars thanks to this kind of changes @JohnRennie .
If Life has anxiety to get out of its jail, our Solar System, I will not be the one to bet that it does not succeed.

Krishna

endosymbiotic theory does feel like a strange science fiction.

PM 2Ring

@Charlie B is an inertial observer, so they're at the space origin of the B frame, and their proper time is the time shown by a (properly functioning) clock at that origin. If events P & Q are separated by a timelike interval, then that interval is timelike in every inertial frame. And every worldine between P & Q is timelike.

The straight line PQ is the worldline of an inertial observer, R, who is (of course) at rest in their own frame, so the size of the interval PQ is the proper time measured by R between those 2 events. We can assign a proper time to any other worldline from P to Q by treating the worldline as a sequence of straight paths & using standard calculus to find the limit as the sequence of straight paths approaches the desired path.

If we do that, we find that the proper time of R is the maximum proper time of all the worldlines connecting P to Q.

GR gives us considerable freedom of choice for coordinates. It's often convenient to use someone's proper time as a time coordinate, but GR does not force us to do that.

Charlie

12:06 PM

Ok that is helpful thank you

PM 2Ring

No worries.

BTW, that "the proper time of R is the maximum proper time of all the worldlines connecting P to Q" is the solution to the so-called Twin Paradox.

Anshul Sharma

12:53 PM

can anyone explain the origin and need Of time reversal symmetry

i actually asked this question but got zero response

Yuvraj

1:36 PM

@AnshulSharma time reversal?

Yuvraj

1:47 PM

word time reversal is so fancy it makes me feel like some is asking about time machine!

Physics Meta

2:28 PM

0

Q: What's the policy around a jocular comment within a post?

I included a comment that I thought was hilarious (of course, not. I just thought it was a bit humorous) in a recent answer of mine: https://physics.stackexchange.com/a/555094/20427. However, @user76284 removed the comment by editing the answer, documenting that they were "removing irrelevant pol...

M.N.Raia

2:46 PM

Good morning/evening/night for everyone!

Hope everyone here is quite healty.

Well, suppose then Schwarschild black holes. If we take the $r= cte$, $\theta = cte$ and $\phi = cte$ then the metric reduces to:

$d\tau ^{2} = g_{00}dt ^{2}$

Which is the gravitational redshift

Now, this "method" is valid for every metric? I mean

take kerr metric

if we state the same (now boyer-lindnquist coordinates, of course) $r= cte$, $\theta = cte$ and $\phi = cte$

can we say that the gravitational redshif in kerr spacetime is given by:

$d\tau ^{2} = g_{00}dt ^{2}$ ?

(now $g_{00} = -(1-\frac{2Mr}{r^{2}+a^{2}cos^{2}\theta})dt^{2} )

You can disconsideratehere, I will write a proper question.

ACuriousMind

"Hope everyone here is quite healty. Well, suppose then Schwarschild black holes." is one hell of a conversation opener ;)

John Rennie

@M.N.Raia $cte$ means constant?

M.N.Raia

3:16 PM

@JohnRennie yes, it means a number. But please, I wrote down more precisely here

0

Q: Gravitational Redshif in Kerr Spacetime

Well, suppose then Schwarschild black holes. Following the $[1]$, we have the redshift factor: $$d\tau = \sqrt{1-\frac{2M}{r}}dt. \tag{1}$$ This factor have an physical interpretation to be the time a given observer measures on her own clock, i.e., for a stationary observer the proper time re...

general-relativity black-holes spacetime metric-tensor kerr-metric

John Rennie

@M.N.Raia in the Schwarzschild and Kerr metrics the timelike parameter $t$ is the time as measured by an observer at infinity, and yes $\sqrt{g_{00}}$ does give you the time dilation relative to the observer at infinity.

M.N.Raia

But it's valid for any metric?

John Rennie

But some care is necessary as the timelike variable is not always equal to the time as measured by the observer at infinity. For example if we use Kruskal-Szekeres coordinates time timelike variable $v$ is not equal to the time as measured by the observer at infinity and $\sqrt{g_{00}}$ is not the time dilation.

M.N.Raia

Hmmm I see.

John Rennie

The metric can be written in any bizarre coordinates you want.

Actually even in the Kerr metric the cross terms are going to cause you problems, so actually even in these coordinates $\sqrt{g_{00}}$ is not the time dilation.

axk

3:23 PM

Hi all! Looking at doing lemon juice concentration by evaporation, thinking about vacuum evaporation to not heat it up too much. With a small say 100 mL flash would one of these cheap 12V diaphragm pumps from Aliexpress be any good for lowering the boiling point? E.g. to 12kPa to boil at 50C?

John Rennie

You have the cross term $\frac{\sin^2\theta}{\rho^2}\left((r^2+a^2)d\phi - adt\right)^2$

@M.N.Raia when you set $d\phi=0$ you are still left with the $dt$ term.

axk

e.g. one of these aliexpress.com/item/…

M.N.Raia

Right, I see. You're totallr right.

John Rennie

@axk I have no idea, but for the relatively small cost I'd say it's worth a try.

M.N.Raia

Now, can we say that the $g_{00}$ constant, right? because $a$ (angular momentum of the black hole) is constant and also $r$, $\phi$ and $\theta$. So when we integrate the whole $\int d\tau \implies T = \int \sqrt{g_{00}} dt$

can we say that the proper time is then given by:

$ T = \sqrt{g_{00}} \int dt = \sqrt{g_{00}} (t-t_{0})$ ?

I mean, the $\sqrt{g_{00}}$ goes "out of the integral"?

John Rennie

3:28 PM

No, because the cross term $g_{03}$ I mentioned above has to be included.

M.N.Raia

but, to $\phi =$ constant

(also)

John Rennie

Oh wait, OK, yes sorry you're correct.

M.N.Raia

for $(r = a , \theta = b , \phi = c )$ where a,b and c are constants.

user142929

4:01 PM

Notice also that your claim about science fiction (I appreciate again your invitation to this chat room) can be considered for persons as I within the spectrum of those things that could be, maybe, possible in the future). Your claim about MBH sure that is right but not obvious/trivial for me @JohnRennie Many thanks. I want to add this.

David Z

5:48 PM

@PM2Ring hm I suspect it had a lot to do with the phrase "Can u please help out by applying conservation of energy..."

PM 2Ring

6:08 PM

@DavidZ Ah, ok. That does make it look like a "please do my work for me" post.

If instead they said "Can you please help me understand how conservation of energy applies here", or something like that, it would be a lot better. IMHO

David Z

Yeah, that would help

We can definitely revisit it but if the question would be marked as a duplicate anyway, I'm more inclined to just leave it on hold and leave a comment pointing to the proposed duplicate.

BioPhysicist

What would be the correct route to take if I wanted to propose changing the "biophysics" tag to instead be "physics of living systems"?

PM 2Ring

@DavidZ Sure. It should definitely be closed.

David Z

@BioPhysicist I can tell you right now I doubt that's going to get much traction, but I'd suggest making a meta post.

BioPhysicist

6:23 PM

@DavidZ Why do you think it won't get much traction?

David Z

There is a mechanism for proposing tag synonyms and voting on them, but it's kind of an easy thing to miss so not many people know about it.

Biophysics is a pretty standard term for what the word describes

BioPhysicist

oh, I thought that was to unify existing tags, not to relabel ones

David Z

so I would be surprised if people think that there is a need to change it

BioPhysicist

I think it would reflect the direction in which the field is moving as well as highlight the current description of the tag that it should be mainly about physics

David Z

@BioPhysicist Ah, well... I think it can do both, although I haven't looked at it in a while so I don't remember for sure. If we are to rename a tag, we'd probably want to leave a synonym in place so that people who use the old tag get pointed to the new name.

If you have examples of off-topic questions that people asked which you think they wouldn't have asked if the tag had your proposed name, then that would be useful evidence to present in support of your renaming proposal.

BioPhysicist

6:33 PM

@DavidZ Yeah, I'll have to think about it and muster up the energy to try to get something rolling. Was just something that was recently on my mind. Thanks for the info.

David Z

np

No rush, I suppose