The h Bar

12:42 AM

@heather link?

12:57 AM

@EmilioPisanty if you feel like bounty-ifying a really good answer, see the answer by @alarge here.

I'm not giving bounty away quite yet, as I still don't have all the site privileges.

1:12 AM

@alarge you once recommended me some books on stochastic processes. I can't find that list. Can you give me some recommendations? I have VanKampen's book.

I would like to understand the following:
1) Basic rules for stochastic differential equations and at least roughly where the rules come from.
2) How to go from stochastic differential equation to diffusion-type equation to Langevin style equation.
3) A few examples, in particular how to apply these things to problems where I can't get an exact solution.

@ACuriousMind Amazing.

1:24 AM

@0celo7 See this

@DanielSank what's up?

I gave you a link.

That's what's up :-)

yeah, what's up with the link?

It's the kind of post I think you'd answer well.

It's a question. What else do you want to know?

@DanielSank I left a comment

I don't know how to help him visualize it

whatever that means

Vectors are geometric objects. If you understand the trig, you understand this

1:31 AM

@0celo7 uh, often drawing stuff helps people visualize.

whatevz

@DanielSank And the guy just smart assed me.

So screw it

@DanielSank yeah, so he should read the wiki article

I think there's a Khan academy video too

ok thanks for trying

You da man.

no problem

(I'm writing a lab report)

D3075

@BernardoMeurer you called?

@0celo7 good

dmckee

1:53 AM

@vzn We introduce that idea even in the gen-ed survey. Including a brief and non-technical explanation of the main evidence.

New weekly featured post in my chat room

Here's the room

2:12 AM

If you are going to include time travel in your story, make it CONSISTENT!

Terminator did a good job of that.

It's the only time travel story I know of that isn't obviously inconsistent.

My love and curioisity of time travel is really stemmed from one reason: I HATE time travel plot holes, and this is why I spend so much time trying to develop a mathematical model that will unify every single time travel in every known fictional work

Danielsank: Indeed. Classics like The Time Traveler's Wife are also consistent. Most time travel stories that tend to suffers are those with malleable histories

On a more general note, there are many things that I am curious on and even love simply because I hate them so much.
entropy and the second law used to be one (because they often screw up plausibility of some scifi ideas), but the more I learn about it, I respect it and understood it more, and I don't hate them anymore

2:28 AM

"Hate" is such a strong word :P

Q: Want to view deleted answer and comment stream:

I think hate is the correct level to describe it. I have this time travel interest for at least the past 4 years (not counting those years where I develop this interest when I was a child). Time travel is the MAJOR MOTIFICATION of me trying to learn non hausedoff topologies, general relativity and some other relevant physics

I learn all of these in order to better understand how time travel works, so that 1. I can nuke all time travel plotholes with one single mathematical framework and 2. shut up every single person who said time travel is confusing

(To be honest, I don't exactly know why 2. is such a strong motivation to me)

As for the non hate side, I like back to the future style time travel since it preserves freewill (my worldview is known to be so stubborn that unless that is conclusive proof that freewill does not exist, it will not let go of this belief) yet allow all the interesting plotlines of time travel

The issue, however is that it is very hard to make it consistent

This and the branching timeline models, requires quite nonlinear thinking about causality in order to make it work consistently. The issue is easy to solve in a newtonian universe since simultaneity is absolute thus you can e.g. treat many time points sharing the same reference frame and they all move forward in time in a uniform manner

One of the major challenges of getting a mathematical framework for malleable histories is to make it compatible with general relativity, plus trying to come up with a mechanism to allow arbitrary topological changes

Physics Meta

3:02 AM

0

So I asked the question: Thought experiment: ultra-fast outward push inside a black hole And there was a conversation I was having with one of the answerers (it didn't resolve my question but I wanted to read it all as there seemed to be some good substance there). But before I got a chance to...

discussion

4:07 AM

@Mostafa Hmm. In most problems like this we assume that the transistor is in active state. Why would be consider cut-off region or saturation region anyway when amplification is not possible? These problems are based on specific concepts which we need to identify. Here in this case they want to test a students knowledge of the relation between base current and collector current when electrons are injected from common emitter in active state.

Be it a DC(within a small range of values) or an AC (as you said) the concept remains the same: A small base current is amplified into a large collector current when a emitter base is forward biased.

4:18 AM

I also looked up some values. For a typical npn transistor, emitter electron current is 3mA. Emitter hole current is 0.01 mA. Collector electron current is 2.99 mA and collector hole current is 0.001 mA.

So when you say "because 20mA is too low for collector current" it doesn't really make much sense as collector current can have values smaller than that too as I pointed out ^

The data is from this site. Here they have dealt with similar situations. Do have a look. (vle.du.ac.in/mod/book/print.php?id=5321&chapterid=995)

8 hours ago, by Mostafa

@2017 If you apply the wrong voltage, it may not even turn on. Or it can damage the junctions, .... only under very special conditions it enters the useful mode (active forward region). This is why we need to put a lot of bias circuitry around the transistor (or any other IC which doesn't have them inside itself)

I don't know what makes you think that a problem like this could ask about the situation when you apply the wrong voltage! They definitely assume that it is the right voltage which causes active forward state. Or else the problem would make no sense.

Also I would like to point out: Saturation mode is when both emitter and collector junctions get forward biased. In this mode, collector current becomes almost independent of base current. And Cut-off mode is when emitter does not inject carriers into the base region so that no carrier are collected at the collector. The collector current is thus zero, however a very small leakage current flow due to thermally generated minority carrier.

In both of these cases : cut off and saturation they would have never said something like 98 percent of electrons injected in base passes into collector!

4:42 AM

Also, in this case $\beta$ has quite a large value of around $\frac{98}{2}=49$. In saturation state $\beta$ is about $10$ to $15$. So it definitely isn't saturation state. @Mostafa

This is a very nontrivial grandfather paradox that is often missed by various fictional works

We are taught that grandfather paradox involves killing your parents or grandfather or your past self, thus result in you unable to exist and thus the hisotry just end up going yes no yes no yes no ...

But the grandfather paradox actually has a even simpler formulation: It refers to the paradox caused by any event that prevent the time travel from taking place. This one is not obvious because the key event exists in the middle of the timeline and not at the destination.

An action cannot undo the event that inspired said action.

There are many ways to resolve the nontrivial gradnfather paradox, but all of them are in a sense, not very simple"

@DanielSank Van Kampen is a good book, particularly for physicists, but if I remember correctly it does not really directly deal with SDEs much, but instead transforms most the equations to the Fokker-Planck PDE form (using the theorem named after Feynman-Kac)

4:49 AM

1. The trigger that caused them to travel back is a fixed point, thus if the death reason is voided, then there is another reason that ensure they travel back. Therefore even if the history is altered, the predestination loop will remain intact and not cause a gradnfather paradox

What does "contradiction" mean to you?

2. The events are arranged in a sequence such that a consistent predestination loop is formed. E.g. the death of the person is avoided as the past is changed, but at the key moment they still die, thus the reason to travel back is preserved

3. Branching timelines: Saving the person creates a new timeline. since the original timeline is still there, everything is fine

@alarge Ok, I'm back to studying VanKampen.

I'm happy to learn one thing at a time. I can start with Fokker Planck and go from there.

Thank you.

@DanielSank As for the other questions: 1) The rules come from the definition of Ito integral. You can work them out by writing the Riemann sum. 2) The math/physics notation really only differ by a "dt". Where mathematicians would write dW, physicists would \eta dt. Now you might argue that physicists don't actually assume Gaussianity but just something about the first two moments to get to the PDE, but I don't think this is actually important. I believe Van Kampen discusses it briefly.

4. The person is not continously "dead" (Refer to The Time Traveler's Wife for an example). It is possible that the death of that person at the key moment that inspired the time travel is a fixed point, but otherwise he is alive at times outside of this point. At the boundary, somehow the events adjust to make the history progression continous

4:55 AM

How can an action undo the event that inspired the action?

Either you branch off the outcomes (branching timelines), or somehow leave that key event untouched, but alter everything else (malleable histories)

There's one more (but inelegant) solution:

5. ripple effects can only propagate in the forward direction, thus even if the reason of time travel is undone, the undoing cannot travel back in time. The issue of this one is that to anyone else except the time traveler, history becomes inconsistent at that point

Of these, 1 is often the most common solution in most fictional works, some however simply ignore this paradox

@DanielSank To get the O-U result (of the magic twos), just think of it as adding a lot of Gaussians together. The mean of them all is zero, the sum of Gaussians is still a Gaussian which is characterized by its variance.

So you write the definition of variance, then you see that all the dW are independent (in physics the delta correlation thing you have when you write the Langevin eqn), which essentially should derive for you the Ito isometry, and then out it comes.

I'll get you some references if you'd like once I'm not on mobile, later this week.

Ah, that explains it, I think.

I know how Gaussians add. Thanks!

I mean, I know how to add samples drawn from a Gaussian distribution.

5:19 AM

@alarge you're on mobile all week?

@skullpetrol Yes

A: Is it okay to take \$\beta=I_c/I_b=98/2\$ in this problem?

@YashasSamaga Got my answer confirmed by the EE guys. Have a look. (electronics.stackexchange.com/a/289100/96630). So my solution was correct! -_-

2

It sounds like they mean 98% of the emitter current, not the base current. That means you're given \$\alpha = 0.98\$. You can use that to calculate \$\beta\$ and find the base current.

@Mostafa