lol! real book? havent seen it before. if you enjoyed that, try this too, or his earlier one :)

@SirCumference Big chapter 7 test coming up?

I find String Theory pretty crap....

The worst thing is that you can always choose the solutions you want to appear (wiki criticism)

If any real theory exists on unifying all the physics, then it must start with assumptions different from QM, GR (not strings....) and should be able to explain all the constants of the universe, (not take those constants as assumptions.... I mean it is not the way it is now.

I think you should be cautious about criticising a theory unless you understand it. If you don't understand it you are not in a position to judge the assumptions upon which it is based.

@JohnRennie Wikipedia criticisms...

@JohnRennie former is mine...

The criticism is justified by taking account of Bohr's Model (before QM) and it was found that it was wrong because of wrong assumptions, the same happened with Newton which was corrected by Einstein... So, that's the same thing. Although, I don't know String Theory/M Theory myself.

$\langle . \rangle$

I wonder what is the record time for a question being closed after it is posted? I am sure this is not the fastest, but wow haha.

If you also consider it was done by 5 users instead of a mod, that was fast

@KyleKanos Yes that too. There were 3 votes to close and then when I went to click to close it said there were 4. Very fast

This one is somewhat unfortunate to me because I feel like the OP could have asked the question in s different way to make it on topic

Like the information before the final two questions wasn't horrible. But then they ended it with a subjective question and then a broad question.

Not sure, could be on topic, but might be a dupe off another, depends on phrasing

Hey everyone.

@AaronStevens Funny enough, the same thing happened to me with the 4th vote. It said 2, then 3 by the time I clicked it. You must have been right behind me, as I was right behind Giorgio.

@AaronStevens And don't worry about closing it too early, it's "On Hold" to give OP a chance to fix it, that's how the system is supposed to work.

So, in a rotating frame of reference, objects at rest experience a fictitious force; that fictitious force is called centrifugal force. Likewise, in a linearly accelerating frame of reference, objects experience a fictitious force. Is there a common name for that force, or is it just called "the fictitious force in a linearly accelerating frame of reference"?

@JMac Yeah I know that. I was more commenting on my thoughts reading it. "Ah this isn't horrible...oh nevermind"

@TerranSwett Maybe "The trivial one?" Many texts kind of treat it as a trivial case and just focus on rotating frames. I don't think I have heard a name for it (or at least I cannot recall one)

It's basically worked directly into newton's laws. I don't think I've ever heard it given a name either.

@JMac I talked to that one teacher who was thinking of putting a question on a test that had an incorrect solution and asked the students to find an error, if any

When we say a CEO is worth 5 billion for example, does this mean he has the amount of money right away to spend and invest, or that 5 billion is simply the large percentage he owns of his company and therefore would have to sacrifice a part of his ownership to buy let's say a yacht?

@NovaliumCompany Hello Brooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

@NovaliumCompany The assets - liabilities

@NovaliumCompany see dis - moneycrashers.com/calculate-personal-net-worth

So is a CEO's share in his company an asset? And therefore part of his net worth?

Also, if he wants to buy a yacht, does he have to give up a part of his ownership?

@Slereah uva.nl/shared-content/uva/en/vacancies/2019/09/…

@NovaliumCompany If he wanted to pay cash he might have to do that; but I doubt many people buy yacht's straight up if they didn't already have the cash to cover it. There might not be many situations where selling your shares so you can pay cash makes sense; when instead you could finance it some other way.

@JMac What other way?

@NovaliumCompany The same way most people buy things they don't have the cash to buy outright. You get some sort of loan.

@JMac How do I pay it out. My salary as a CEO is 1$/year cuz my company shares are worth 5 bil.

So most billionares actually cannot spend a lot of money because they'll be loosing a part ownership of their company?

@NovaliumCompany Presumably your CEO still has some form of income, or else he wouldn't buy the yacht. Perhaps by selling some company shares they are given, or from company "bonuses" which are not reflected in the salary. Regardless, if they had no income, and no cash; just a bunch of shares, and they weren't going to sell them, I don't think they could "afford" a yacht. All their net worth would be tied up in assets that they would need to start selling (or they could take loans and manage debt)

Maybe a CEO could sell a small part of his shares and then invest it in something that will make him money...?

I mean, those company billionares don't actually have the billions but have the potential to withdraw them out for the expense of ownership?

@NovaliumCompany Presumably the CEO expects their shares to increase in value, or else they aren't doing a very good job, and will have to answer to shareholders. Selling off some of your shares to invest in "something that will make him money" is a delicate topic; though selling a small amount of shares probably isn't abnormal or too frowned upon.

@NovaliumCompany They have a "net worth" of billions. If they converted asset they had to cash and paid off everything owed, that's the estimate of how much money they would have. Realistically, you're going to have a hard time converting all your assets back to cash for their full value most likely; and there's no real reason to have billions in cash when you have nothing to spend it on, and it increases value over time if you invest it.

Hmm

What can a CEO do if his company shares are going up to increase his money or something? I mean, so it turns out billionares are actually just millionares.

If I wanted to buy an island for 2.5 billion, and my shares are 5 billion, does that mean I'll have to give up 50% of my company?

(ignoring taxes)

@NovaliumCompany If you wanted to pay upfront cash and had nothing else, sure. But most people wouldn't do that AFAIK. You could probably work out some sort of loan instead (though maybe not a loan worth as much as half your shares).

@JMac My final question. What is it that people would do in that scenario instead of giving up half of their ownership over the company?

hey I so if black holes have infinite gravity, do they have 0 volume?

so some mass into 0 volume?

@MartianCactus Black holes don't have infinite gravity. If you replaced the Sun with a black hole of the same mass, the planets' orbits wouldn't change.

the notable thing about black holes is that their gravity becomes infinite at their center of mass

the same cannot be said about e.g. the sun, by the shell theorem

Ok, but that's only because you can get so much closer to their centre of mass.

but if its infinite at center of mass, won't it be infinite everywhere according to gravity equation?

And to be honest, we can't really say what happens at the exact centre of a BH until we have a theory of quantum gravity.

ya that makes sense..

@MartianCactus gravity drops with 1/r^2, so although it will go to infinity as you get closer to the BH (i.e. as r goes to zero), it is not infinite if you're at a nonzero distance

i'm using newtonian gravity here but it should give a decent intuition

wait but if its zero, it would become non defined not infinite?

that's a good point. in math, we call points that are undefined "singularities"

the idea is that if we start at a finite distance and get closer to r=0 (i.e. the singularity), then the gravity will rise up to infinity

we can't say what happens exactly at the singularity, i.e. r=0, without a better theory of physics

@MartianCactus As r approaches zero, the gravitational force approaches infinity. It's a limit thing, as is usual in calculus.

@MartianCactus if you're familiar with asymptotes, then what i'm saying is that gravity asymptotes at r=0. so the gravity can become ever stronger as you get close enough

but at 0 distance it is indeed undefined, so we can't make any statements about that with current physics

OTOH, we're pretty confident that quantum gravity effects only become significant around the Planck length scale, give or take a few orders of magnitude. So a semi-classical theory of standard GR augmented with some quantum stuff should be fairly accurate down to the size of a proton, and probably even a few bit smaller. The Planck length is really tiny.

IOW, we can say that the core of a BH is at least as small as a proton, and possibly smaller. In pure GR, you get a mathematical singularity of zero size, but that seems unphysical, and quantum effects probably stop that from happening.

What does a 3D Penrose diagram look like?

Quick, someone teach me something cool.

@DanielSank quasi stars

basically hypothetical stars in the early universe, thousands of times bigger and brighter than the sun

they're so massive that their centers contain black holes, which eventually eat the star, possibly explaining where supermassive black holes come from

that black hole's accretion disk powers the star, instead of nuclear fusion

comparison with the biggest stars observed today

woah

Whenever I'm at a mcdonalds, I turn into a black hole :p

Ain't no marginal utility decrease in that Big Mac.

Hola Amigos! My professor today asked us to think about applying Poynting theorem to static fields. I am not sure whether it makes sense or not. Hoping someone here can explain if we can indeed apply Poynting theorem to static fields.

Intuitively I feel there should be no power transfer

But I am not sure :P

ohh ok thx!

It feels like most of the emails I send nowadays are "please remove me from your mailing list"

I need to calculate for the force in newtons for a magnetic field on an arc of plasma under vaccuume. How do I do this?

I'm generating an arc under lower atmospheric pressure. Then,using a medium of helium gas instead of air.

I'm putting a hallback array of neo magnets to control the arc of plasma and direct it. I need to calculate the force the magnetic field will have on the electrons. This will help me determine the degree of deflection.

I've looked at Lorentz force law and I thought it would give me the best straight foward shot into solving it. But I'm not getting numbers and values that are possible.

Could someone help me with this? I'm completely lost.