joigus

Another way of saying it: Enthalpy is a function of state that allows you to express heat balances. Heat is not a function of state, but you can relate it to a function of state.  Some kind of "heat potential". Isn't that nice? You must keep pressure constant if you want it to do its job as "heat potential".

Does that help?

Thank you for your comments. +1.

It sounds funny that a small contamination of heavy elements would lead to completely different "phases" for stars that scatter all over the place in the diagram. But I can't see any other major reason why the diagram would look so "multi-phase" so to speak.

6 hours ago, Eise said:

I am not sure if the lives of stars (like the sun!) that are second-generation stars (i.e. mixed with debris from first-generation stars) also live the biggest part of their lives on the main sequence.

Sure, but I agree. My intuition is precisely that because these second or third-generation stars are composed of matter that has collapsed, blown up, and re-collapsed again, that could be a good "in principle" reason why they get out of the main sequence. Suppose that at first, they are mostly protons fusing in the nucleus, giving off their protons and helium-nuclei exhaust. Then the fact that they get depleted of hydrogen would be the reason why this small contamination of heavy elements would start showing up as more significant in relation to the remaining hydrogen than for a star in its first generation.

Whether this is covered by standard astrophysics, I don't know, to tell you the truth.

4 hours ago, Sensei said:

..what is minimum velocity of dust from a supernova explosion? (i.e. which won't get back to it due to gravity and being sucked by the newly forming black hole)...

..what is maximum velocity of dust from a supernova explosion? (can it exceed escape velocity of the entire galaxy? analyze per galaxy size and mass)..

In the case of Crab Nebula velocity measurements x time, can give the moment in which supernova exploded.

According to Wikipedia it might be 1054 year A.D.

https://en.wikipedia.org/wiki/SN_1054

Enjoy Betelgeuse until you have it!

The same extrapolation can be done with any other supernova, or supernova-to-be. i.e. when (and in what quantity) remnants of Betelgeuse will reach Earth in the future (and any other close to the Solar System supernova-explosion-to-be)

The same extrapolation can be done with the Solar System.

I think the best is to write a computer simulation which calculates, estimates, predicts where and when it happened (or predicts where any other historical supernova was in the past billions of years ago).

Where is remaining black hole after supernova explosion, which created the all heavier elements which you have here on the Earth, and in the Solar System?

Thanks, Sensei. Very interesting comments. +1. Yes, debris from supernova explosions that get ejected out of SN attraction "sphere". That makes a lot of sense. So do you suggest tracking BH as candidates for previously existing SN that gave rise to our solar system is (or could be) accomplished by some kind of signature method? If that's not what you're suggesting, can you think of ways that it could be done or is being done?

Give you an example: Accretion disks of BH's having same isotopic signature than ours, therefore likely that we emerged from that particular BH? Also kinematics of "us" with respect with particular BH signaling more likely that we running away from them. Although if we came out with just escape velocity we would be considerably slowed down by now, so difficult to detect.

Now that you mention Betelgeuse. I remember some 6 years ago going out late in the night to watch Orion in the small village where is was living. In the Summer in Spain it only comes out really very late (about 5AM). Once the police (the rural police is the "Guardia Civil") stopped me and asked me for ID. They asked what I was doing. I told them the truth: I was looking at the stars. But I didn't tell them that I was waiting for a supernova to go off, which is what I secretly was hoping for. They looked at me funny. But there were no more questions. 😌

PD: I have to read your wiki entry yet.

https://medicalxpress.com/news/2020-07-rna-reversing-mutations-underlying-neurological.html?fbclid=IwAR38JE4SGPC-ODBqRzTmdpenmcsLHHiSbfUa63rZhH42UFk0Lfxg-qPd5vI

This sounds to me like a piece of very interesting news. Possibilities of editing RNA is a topic that fascinates me, although I still have to learn a lot about it.  

On 7/13/2020 at 5:27 PM, studiot said:

This is a genuine sub sub branch of Pure Mathematics which is very obscure.

Try reading this

Wikipedia has a simple offering for once

https://en.wikipedia.org/wiki/Limited_principle_of_omniscience

 

Then read a full blooded paper from Birmingham University

 

https://www.cs.bham.ac.uk/~mhe/papers/omniscient-journal-revised.pdf

 

Avoid the religious books by Paul Tranter

They are not connected.

 

https://books.google.co.uk/books?id=I8bKDwAAQBAJ&pg=PA69&source=gbs_selected_pages&cad=2#v=onepage&q&f=false

 

I wasn't aware of this branch of maths. Thanks a lot, Studiot. +1

I've found this other one:

https://plato.stanford.edu/entries/mathematics-constructive/

Stanford's Encyclopedia of Philosophy has helped me a lot in the past (to understand the Kochen-Specker argument in QM, for example).

Hi again. I hope everybody is well.

Without further ado, is there any appreciable difference between matter that has gravitationally collapsed from a primeval cluster made up of mostly hydrogen and matter that has collapsed several times within a certain galactic region? I suppose matter that collapses again and again in regions where many supernova explosions have taken place before would be richer in heavy elements.

Could the wild variation in the types of stars as reflected in the Hertzsprung-Russell diagram reflect this variation in the "degree of collapse" that there is in the universe?

My intuition tells me that, if all stars had started up from a universal prototype cloud of mostly pure hydrogen (only varying in clustering size) the kinds of stars that would give us would nicely group into a 1-parameter curve in the Hertzsprung-Russell diagram. I have no mathematical proof for that, but it seems right (angular momentum, temperature, etc. are there too, so I'm aware that it may be an oversimplification). The fact that they don't, strongly suggests that matter in different parts of the universe collapses from very different samples of stellar debris. Some of them loaded with heavy elements, which would reflect in a very different nature of star formation.

Does that make sense?

Is there any hint of an answer that you know of or can point to?

Thank you very much.

Edit: By "collapsing" I don't mean black holes, I mean stellar formation. Sorry for possible confusion.

Now it seems to give a "bad gateway" error.

It worked the other day. Thank you. +1

3 hours ago, Sensei said:

..it would be a much more interesting discussion if you would stop a few words earlier..

It would be discussion about location of supernova explosion which gave birth to the all heavier elements in the Solar System..

+1. This is a very interesting re-focusing of the question. Maybe the OP is interested in it?

I don't think it can be done with our solar system because AFAIK remains of supernova explosions are seen as halos of dust (e.g., Crab Nebula).

I surmise that our Solar System is much older than the Crab Nebula... 

18 hours ago, Dave Enamu said:

I’ve only recently joined this forum, and I’m not ashamed to say that a lot of this goes over the top of my head. I’m just an ordinary guy who sees things in a simple way and not all this gobbledygook I’m not clued up with all these equations. Let’s bring it down a level so us ordinary guys can join in. Another thing what’s with the names are they to make you sound mystical and magic - why not use your given names? Come on lads if you let the ordinary guy in it might open up a new world. So what d’ya say.

I had a teacher of classical field theory many years ago who, answering to a student who complained for his constant use of tensor analysis, replied:

"If you want to understand Chinese poetry, it's a good idea to learn some Chinese."

I'm sorry for the use of overly technical gobbledygook as AxB = C.

You want to understand cosmology, the nature of time and what not, that's fair enough. Why don't you meet me halfway (or a tenth of the way), and remind yourself of what multiplication and division of simple numbers is? You do need some mathematics if you want to understand anything at all about time and the big bang. You can't just declare simple maths as off-limits.

Ordinary guy is OK with me. An "ordinary conversation about the origin of time" is a different matter. There is no such a thing.

Let me give you an example: Would you ask an economist to explain to you about the economy, without using concepts as interest rate, inflation, GDP or the like?

I think it's surface chemistry we're talking about here, rather than wax changing phase. Otherwise it would be incompatible with the principles of thermodynamics, I think.

Waxes and rubbers have some surprising properties. Rubbers, e.g., cool down when stretched. When long molecules cool down under situations that would normally induce a temperature increase, that's because the very long molecules get more ordered when stretched, instead of disordered. Not same case as OP but, as I said, "surprising" properties.

3 hours ago, studiot said:

So why is the substance to be liquid at a lower temperature than when it is a solid ?

I share the puzzlement. +1

It can't be just the temperature that does it. It must be a combination of temperature, moisture, and most importantly, polar bonding with molecules in the skin cells. Otherwise, I'm clueless about what goes on here.

1 hour ago, Dord said:

I don't think the melting point is relevant here.

+1. I agree. Again, it must be polar bondings and their effect, having to do with introducing molecular ordering in the very long wax chains.

Interesting topic. Yes, please, provide more info.

2 hours ago, motlan said:

Negative invisibility

In all my study of physics [...]

 

Is there a positive invisibility?

Totally concur with @Janus &  @Endy0816.

I'd like to know who said that too, @Strange.

(+1)³

Let me offer you a complementary picture of why everything running away from one point doesn't work.

If everything in the universe were running away from one point, we would look at the night sky and see something very special at that point. That would be the point we're running away from.

Instead, what we see is a series of spherical layers older and older in every direction the farther away from us we look. Until we hit the very feeble, very dilute image of a primeval plasma state of the universe (this is called the surface of last scattering). A picture of the universe when it was opaque to radiation, because all the particles were ionized (plasma) so it didn't let radiation through. That's a picture of a pretty early universe. And it appears more or less the same in every direction. So, where is the original point?

I hope that helps.

On 1/29/2020 at 8:50 AM, lucien216 said:

Hhhhmm. This is what I wanted to hear. So you saying that the universe could be, say 30 billion years old, for some observers? So the age of the universe is only relative to us then?

Very good question IMO. +1.

Not really. The key to this is what @Mordred suggested when he mentioned the key words "FLRW":

On 1/29/2020 at 2:08 PM, Mordred said:

Cosmology using the FLRW uses what is called a fundamental observer which is an observer whose time would be based on the mean average mass density of the universe. The universe we refer to is our Observable universe.

Then he went into a very interesting argument that really this co-moving time extracted from FRWL model (exact solution of GR) is actually an average and it would be affected by corrections due to fluctuations in density (+1). That's my understanding of what he said, at least. So there would be local underestimations or overestimations of the Hubble parameter (see below.)

<question for @Mordred>

Would an underdensity lead to an overestimation of H (and thus an underestimation of the age of the universe), or the other way around? I'm feeling a little confused right now (I think it depends on the global balance of omegas for DE, DM,...).

</question for @Mordred>

If the cosmological principle were exact, the FRWL solution would be exactly how the universe evolves, and the age of the universe would be (proportional to) the inverse Hubble expansion parameter.

\[\frac{a}{\dot{a}}=H^{-1}\]

Where

\[\dot{a}\]

represents the time change rate of the expansion parameter a, which in turn represents "how far away from each other typical galaxies are", and is dimensionless.

The Hubble parameter is the proportionality factor that tells us how fast a galaxy is moving away from us as a linear (directly proportional) function of its separation from us:

\[\dot{a}=Ha\]

Now, don't ask me why (ask @Mordred perhaps), but Einstein's GR allows you to re-scale time and expansion parameter at the same time for the whole timeline of cosmic events, if you want, but it doesn't allow you to mix both in this re-scaling.

So all you would be allowed to do is a re-scaling of both for the universe as a whole. Something like this:

\[dt'=\tau\left(t\right)dt\]

\[da'=\alpha\left(a\right)da\]

Where the primes indicate new variables and the d's indicate small increments.

So all observers would agree on a universal time that would be possible to re-define by re-scalings. Mind you, this is not your familiar wristwatch time. It's to do with the expansion rate of the universe. As others have pointed out, there are many kinds of time you can define, depending on what standard or physical process you use to provide you with the clock, so to speak.  When defining such standard clocks, you generally find observer-dependence. But as long as the FRWL metric is valid, it allows you to define a standard clock for all observers in the universe that are co-moving with the galaxies in the common expansion. You may re-scale such clocks, but all these galactic observers would essentially agree on it, because it's just the ratio of a and its time rate of change.

And t=0 goes to t=0 under any re-scaling. The origin of time, within the model, is absolute. The timeline scale is not. The deep underlying reason is that there is a singularity at the origin.

I hope that helps and I haven't made any gross mistakes.

10 hours ago, Eise said:

Because physicist don't bother, as long a they have perfect operational definitions of time. The question 'What is time?' is, as already remarked, a philosophical question. Not a question about physics. I think even you have no problem to understand what the traffic sign '50 km/h' means. Really, physicists have no problem with their 'dx/dt', or whatever changes according to t. And there are already several threads about time, and I think most of them in the philosophy-forum, where it belongs.

I was about to talk about operationalism and how important it is and how many speculators forget that. Connection with what you would do in a laboratory is essential. You saved me some work. +1.

21 hours ago, MigL said:

The 'bowling ball on a rubber sheet' is a two dimensional  reduction of a 4 dimensional configuration.
It has multiple problems, one of which is that you can observe it from an embedding third dimension.
[...]

A three dimensional representation would already get rid of some problems, but not all.
Picture a three dimensional grid, where x, y, and z axis divide up the space into cubic elements.
A mass placed in this space would curve the x, y, and z lines such that the elements are moreskewed, and smaller, as you get closer to the mass.
[...]

Brilliant explanation IMO. +1

Talk about Brexit.

I think "informed decision" are the key words for tackling this problem. The boxer should be able to decide given as much of the objective information as it's technologically possible.

Also, we tend to think in terms of banning or just plain allowing; yes or no solutions for problems that admit graded solutions. If you think about it long enough, it's not difficult to see that there are ways to tackle this that take care of the risks without just banning the activity. Banning was good enough for pre-industrial societies. We can, and should, I think, do better than that.

"Case studies" is another bundle of key words. The case of Anthony Joshua that @mistermack proposes, strongly suggests that not everybody would suffer the same effects, so proper monitoring of every sportsperson would be in order.

Gamma can never be zero.

Ok. Everybody gave such good and precise answers (+1,+1,+1) that I didn't know of anything else to say. Except give the complementary mathematical focus, which is my favourite. Mathematically, a reading scale normally involves something like (at least for most measures within a certain range for both T and X:

T=T₀+kX

Your apparatus is sensitive to X, while your theory connects it to the readings T. One example could be temperature as a function of the position of the mercury column. The zero error would be the error in T₀, whether T₀ be actually zero or not.

I think something that may help most people reading this post is to provide simple examples of what you mean just after you've introduced some of your definitions.

Great philosophers (especially philosophers of science, like, e.g., Bertrand Russell) always set up explanatory examples after an abstract notion was introduced. Examples are like the "laboratory" of philosophy. Help your potential readers know that you mean business.

On the whole, I don't think for a second that getting an idea of what a TOE will look like will be helped along by philosophical thinking alone. I'm pessimistic if you want.

On 7/6/2020 at 12:05 AM, cogujada said:

I'm here because I have a question (not a doubt ) 

👍 I have no doubt you're getting better.

4 hours ago, dimreepr said:

It says that our natural cooperation is a double edged sword, and can be more destructive than beneficial, if we don't have a good leader.

I'd choose good (and generalised) education above a good leader any time. Leaders I see as a necessary evil.

4 hours ago, dimreepr said:

It also suggests that politics and religion are bad bedfellow's, when the magic (or understandable message) is gone.

I think we all know (deep down) that God is a fiction, yet he/she/it is a convenient method to lord it over the "non believer", whatever side of the fence we find ourselves.

Religion is never a good bedfellow of anybody. 😆

It's always worked for itself pretty well, though.

Just one more thing. When you say "understandable message", do you mean politics or religion?

On 7/6/2020 at 9:22 AM, Ken123456 said:

Could God use a scientist to help save the world from a devastating destruction but the scientist was not a believer in God?

Maybe. But I don't think he would be interested in saving the world from a devastating destruction. He seems to be quite happy with us having a fair amount of devastating destructions every now and then.

So the thing you suggest sounds to me something like:

God: Let's send another devastating destruction and see if this person who doesn't believe in me can thwart me (God whispers the solution to the scientist's ear).

Scientist: I don't know who's talking to me, but OK.

It's possible. It's certainly compatible with the schizophrenic type that I can see depicted on the Bible. What terrifies me is the kind of psychotic God that our culture seems to have devised. What does it say about us as a species?

On 7/2/2020 at 8:26 PM, JGNLBCA said:

I can relate with every word here.  Bravo!  I'm 49 and didn't even know I was on the spectrum until about 3 years ago.  

Back to OP.  I don't know how far you have gotten with your student but I know when I was a kid in high school and after I had mastered trig and and trig based physics I had a giant shock, a trauma really.  At that time, calculus and that progression in physics felt like someone telling me that "everything you have just learned isn't correct". I thought I knew precisely how most things worked, but then I was told to  start over from scratch where everything is much messier.  I crashed and burned.  My academic career never really recovered until a a few years ago. 

Break the news to the gently and when you change from one way of thinking to another, make sure they understand why.  Also they should know that they do not need to forget or disregard the previous way of thinking because it is still useful. I hope this helps.

 

Yes, it helps. Thank you. Every bit of information that all of you are giving me helps. People on the spectrum, as well as people who've had experience with it. It's helped me anticipate many things and assess the emotional breakdowns when they've come.

In the case of A, only once we've had an emotional breakdown. It was due to an obsessive series of thoughts in relation to something a classmate told him. Today we've had a similar episode, but it's been so much easier to control. Tomorrow he's doing his maths and physics exam. Everything seems to be going very satisfactorily. His family are doing a great work, I must say.

I'm amazed that you discovered it so late... I'm sure there are many people out there in their 40's + that weren't properly diagnosed.