The h Bar

GPhys

12:46 AM

@DavidZ the point being to try to stop decay

Dawood ibn Kareem

12:59 AM

I suspect it would decay whenever it wants to decay, regardless of your efforts to measure it.

Forgive me being anthropomorphic there.

GPhys

for sure such measurements can stop it, the question is moreso about experimental limits

the measurement I gave rate is just from the energy difference of the states as scale for how fast you'd have to measure before these things matter

Dawood ibn Kareem

Yeah, first you gotta catch it! :-)

GPhys

after that time dependent perturbation theory is valid again and you get back exponential decay

Dawood ibn Kareem

OK, you obviously know something I don't. Surely any measurement occurs at just one point in time; and you have exponential decay both before and after that time, right? So any effects due to the measurement would just integrate away to zero.

Unless you're talking about making an uncountably infinite number of measurements; in which case you'll have difficulty finding a database that will store them all.

@Abcd I think the point of knowing the length of the motorbike is that to clear the gap safely, it actually has to travel 16.7 feet, rather than 11.7 feet. In other words, treat the motorbike as a point mass at the position of the front of the bike before the jump, and the back of the bike after the jump. To do anything more complicated would require all sorts of assumptions which are almost certainly invalid, as @JohnRennie implied.

GPhys

1:20 AM

@DawoodibnKareem the exponential decay comes via fermi's golden rule. When you derive it via time dependent perturbation theory you need the time evolution to be longer than the energy difference to make an approximation with the oscillatory integral. The idea is if you measure faster than this, then you can slow down the decay

the wiki page on Fermi's golden rule has a derivation, but they just say "large t", but the actual timescale there is the energy difference between the final and initial state

or really at least that fast (if not faster?)

so I was asking really if it was already a no-go for the decay of, say, a muon

so the idea was then that taking a muon for the energy difference, one would need to repeatedly measure it at least 10^23 times a second for ~a halflife to be able to see if you're making a difference, but I think this is already not possible

(?)

Dawood ibn Kareem

1:36 AM

I'm not sure what you'd be comparing it to. So you make a large number of measurements and it decays; or you make a large number of measurements and it doesn't decay. Either way, you can't be sure whether you've changed what it was going to do anyway.

You'd have to repeat the experiment an extremely large number of times, calculate a new half-life for muons that you "mess with" in this way, and decide whether you're within experimental error of the known muon half-life.

GPhys

@DawoodibnKareem yes

Dawood ibn Kareem

So is there any fundamental principle that you think implies that it would be impossible to measure a muon something like $10^{17}$ times in quick succession?

GPhys

@DawoodibnKareem well, you need to do it 10^23 Hz, and you need to do it long enough to conduct an experiment, and I don't know of any technology for anything at all operating that fast

Dawood ibn Kareem

So you're saying it's about current technology, not about some fundamental principle.

GPhys

no I"m asking

Dawood ibn Kareem

1:50 AM

I certainly agree that it would be impossible with the tools we have today, but I can't see any reason for it to be impossible in principle.

GPhys

the current technology is what I'm not so sure about

Dawood ibn Kareem

If anything, I have a nagging doubt over whether a measurement truly "counts" if you don't record its value somewhere; but maybe I'm overthinking it.

GPhys

@DawoodibnKareem oh yes by impossible earlier I just meant impossible with any experiment you could conduct today

I mean, maybe it's also impossible ever, but I only really care about the former right now

Dawood ibn Kareem

The chief difficulty, the way I see it, would be keeping track of your muon for a couple of microseconds. Those things move really fast, and there's no way you're going to keep it within range of your apparatus.

I guess you could shove it in a magnetic field, and make it run in circles. But I don't know how to calculate the required size of the track you want to run it in.

John Rennie

3:16 AM

@DawoodibnKareem quantum zeno effect

dmckee

Couple of comment relevant to @DawoodibnKareem and @GPhys's recent conversation. First the quantum Zeno effect is experimentally accessible in some system: we know it works for some classes of measurements. Secondly Emilo's post here is a must-read about the limits of claims that decay is strictly exponential (and is actually related to the qunatum Zeno effect).

@JohnRennie Bzzzzzsttthhhht!

John Rennie

Ah I hadn't seen that post by Emilio. Yes, that looks and interesting read.

Las Vegas Raiders

Hi chat

dmckee

@DawoodibnKareem The classical equation for the radius of curvature for a charged mass in a uniform field works correctly for relativistic particles too if you write it $r = p/(qB)$ where $p$ is the relativistic momentum.

John Rennie

@LasVegasRaiders Morning :-)

Las Vegas Raiders

3:20 AM

How are you? @JohnRennie

Dawood ibn Kareem

@dmckee OK, so are we talking LHC or 2p coin?

John Rennie

@Kaumudi.H for some reason the Add friend feature has suddenly started working for me on Steam so I've sent you a friend invite.

@LasVegasRaiders tired! It's 04:21 here in the UK. I woke early and couldn't get back to sleep so I decided I might as well get up.

dmckee

@DawoodibnKareem What kind of field can you make and how energetic are your muons? But you're basically talking a small warehouse or larger.

Look up "muon g-2" for an experiment that uses a muon storage ring. It takes a up a big bay but has actually been moved cross-country with the ring intact.

Dawood ibn Kareem

So quite a reasonable thing to be doing. The next issue, of course, is measuring it very quickly. I can't imagine what sort of circuitry you'd need.

Las Vegas Raiders

Sounds like a day for coffee @JohnRennie or do you nap?

dmckee

3:23 AM

@DawoodibnKareem One of the problems I noticed above is that you keep saying "measuring it", but I didn't notice what you propose to measure.

Dawood ibn Kareem

@dmckee This is not my experiment, but GPhys's. I wondered that myself.

dmckee

Gotta measure something. I'm an experimenter and I insist on it.

Las Vegas Raiders

How do you define "measurement?"

Dawood ibn Kareem

@LasVegasRaiders Again, a question for GPhys.

dmckee

Possibly he wants to test for the current existence of a muon state (as opposed to some compound state with similar quantum numbers), but I'm at a loss for how one would do that.

Las Vegas Raiders

3:26 AM

@DawoodibnKareem I meant in general :-)

The question was for @dmckee

dmckee

@LasVegasRaiders An instrument reads something and the DAQ digitizes it and writes it to disk. Is there another way?

And despite the naive way that comes off sounding it corresponds closely to what quantum fundamentals people say if you try to pin them down on the topic.

A 'measurement' leaves a record.

2

John Rennie

18

Q: What constitutes an observation/measurement in QM?

Fundamental notions of QM have to do with observation, a major example being The Uncertainty Principle. What is the technical definition of an observation/measurement? If I look at a QM system, it will collapse. But how is that any different from a bunch of matter "looking" at the same system...

0celo7

3:48 AM

@JohnRennie I'm trying to benchmark my computer but the PassMark software keep crashing

John Rennie

@0celo7 It would be interesting to see how it compares with the published figures for the 1080 card.

0celo7

third time's the charm

computers are strange

@JohnRennie running the tests now.

@JohnRennie

why is my memory so bad

John Rennie

I don't know what the results mean. Are there test results for the B350M mobo around anywhere?

0celo7

4:03 AM

@JohnRennie my GPU is on fire though

it's better than the old Titan

just short of the 1080 Ti

@JohnRennie I dunno

John Rennie

So the new PC has worked out well. Good :-)

0celo7

I'll ask Bernardo

@JohnRennie no, my overall score is in the 75th percentile

who are these hundreds of millions of people with monster rigs?

John Rennie

The memory and disk will be pulling you down. Fit an SSD and figure out why the memory score is low and you'd be up with the leaders.

0celo7

@JohnRennie memory latency is in the 6th percentile

David Z

4:17 AM

@JohnRennie you're on Steam? I wouldn't have thought

John Rennie

@DavidZ :-) I haven't actually bought any games. It was more a matter of being curious.

Las Vegas Raiders

@dmckee thank you.

John Rennie

4:45 AM

@0celo7 just for laughs, this is my laptop ...

0celo7

5:02 AM

@JohnRennie your ram is a lot better than mine...

I found someone else complaining about latency issues with my mobo

but they seem to think it's a CPU issue

what do I do?

John Rennie

I wouldn't worry about it.

Your PC is plenty fast. Worrying about the details of benchmarks seems pretty pointless to me.

user228700

@JohnRennie And I've accepted it! Now I'm going to try buying that game again...

John Rennie

The big difference is in the disk speeds. That's where the SSD gives you a big win.

@Kaumudi.H Let's see if it works this time ...

user228700

Dammit, I don't think that there is any other option but to pay via credit card!

Dawood ibn Kareem

They want you to put 58 rupees on a credit card? Wild!

user228700

5:10 AM

Say, when you added £5 to ur wallet, did u use your credit card or were you able to do it via net banking?

John Rennie

You can owe me the 58 rupees ...

user228700

@JohnRennie It's only Rs. 36 here!

John Rennie

I think with Steam you have to first transfer money to your wallet then you can use the wallet to buy games. But you have to transfer the money in chunks i.e. you can't just transfer 36 rupees.

user228700

And Portals 2 is available for Rs. 56.

John Rennie

Which is a bit of a swizz really. But then most games are a lot more than the minimum transfer.

user228700

5:13 AM

Ah, hmm, OK...

John Rennie

I'm one click away from buying Portal. I won't press, but you might seriously consider owing me the 36 rupees ...

You couldn't even buy a coffee for that in the UK

Dawood ibn Kareem

The cheapest coffee you could get in my country would be about 200 rupees.

user228700

Actually, I did go ahead and try to do it via netbanking and it worked! The problem, however, is that the OTP hasn't been messaged to my mum yet and it's already been 2 minutes.

John Rennie

What is Netbanking. Is that an Indian thing? That option doesn't exist in the UK.

user228700

Actually, no, it's done! Hang on...

user228700

5:19 AM

@JohnRennie Oh, really? It is a portal for paying online.

John Rennie

This?

user228700

Yes!

Dawood ibn Kareem

@JohnRennie Yes, it's an Indian service.

user228700

Transaction was successful and now I'm in the process of installing Steam to see if it all worked!

user228700

I'm going to buy Portals 2 as well! Only Rs. 56 wtf!

user228700

5:23 AM

@JohnRennie Whoa, really? I could buy juice and a small doughnut here for that much.

user228700

...and I know this because I've spent exactly that much on exactly those items in the past week :-P

user228700

Hey hey!

user228700

... nothing?

John Rennie

Odd. It shows a download speed, so presumably the game is downloading ...

user228700

5:30 AM

It is, yes. 23% now.

John Rennie

@Kaumudi.H I'd guess it's a 4th July offer i.e. a special for the holiday. I note the offer ends on 5th July.

Incidentally, you can just delete that Games folder that the Doom unzip created. There's no uninstall.

user228700

@JohnRennie Yes, yes, noted!

user228700

Thanks so much for letting me know about this offer! I'd looked it up before, but on Amazon because I am an idiot.

John Rennie

It was just pure chance. I Googled Portal because I'd never heard of it and the top link was the Steam site.

user228700

Wow, I see.

John Rennie

5:36 AM

I didn't even know the Steam site existed, though now I think about I had vaguely heard about Steam in relation to gaming.

user228700

Wow, it's taking quite a while to download...

user228700

@JohnRennie Me neither!

John Rennie

That 480KB/sec download speed seems quite low.

Maybe the Steam servers are on a go slow at the moment.

user228700

Hmm, maybe...

Dawood ibn Kareem

Maybe everyone's cashing in on the 4 July special.

user228700

5:46 AM

Still downloading!

John Rennie

What percentage now?

user228700

All done now! :-D

user228700

YES YES YES it wooorks!!

user228700

Now all I need is a mouse. And breakfast. And probably a shower...

Dawood ibn Kareem

@Kaumudi.H Good to know you've got your priorities sorted.

John Rennie

5:59 AM

The HD4000 GPU in your laptop doesn't have the greatest 3D speed, but it should be plenty fast enough for Portal.

user228700

Wokay! :-) I'll reverse that order and come back to this game asap!

Las Vegas Raiders

6:24 AM

do you think acm was justified in kicking me out of the chat room? @JohnRennie

Slereah

100% justified

John Rennie

@LasVegasRaiders I thought you were being a bit of a troll. I probably wouldn't have kicked you, but bear in mind that a short kick is just a warning. If the mods were really annoyed you'd have got a 24 hour suspension.

Slereah

let's kick him again!

John Rennie

@LasVegasRaiders The SE is currently taking a harder line on the chat rooms. Bear in mind the chat rooms are provided for free by the SE as a perk. If the SE find it's taking a lot of resource to keep the chat rooms in check they will simply remove them.

Las Vegas Raiders

ok

John Rennie

6:27 AM

This is not a theoretical threat - it has already happened to the SciFi SE chat room

Las Vegas Raiders

indeed

Dawood ibn Kareem

@JohnRennie What happened there?

How do you explain to someone that dates in a database aren't stored in a particular format?

Slereah

@DawoodibnKareem Aren't they

Alex K Chen

I have a simple mechanics problem doubt. May I ask it here ?

Slereah

I'm pretty sure they're stored as timestamps

Dawood ibn Kareem

6:31 AM

OK, one person on SO is anxious to make sure his dates are stored in month/day/year format.

John Rennie

@AlexKChen yes, of course

Slereah

Tell him to sit on it

Dawood ibn Kareem

And we're talking about a date column here, not a string/varchar.

I guess I need to learn to walk away from a question when the querent starts annoying me.

John Rennie

29

Q: How come our main chat, Mos Eisley, got frozen this time?

Mos Eisley is currently frozen#, apparently indefinitely. I assume it has something to do with the users mentioned in @Shog9's banner, but is it possible that we be told - at least in concept - what happened in our room to make it get frozen again? If the rest of you want to create a new room...

discussion chat

Dawood ibn Kareem

@JohnRennie Wow, I've never seen Josh use that kind of language before.

Alex K Chen

6:37 AM

You have a rotating handle with a stand like ^

The stand is fixed on a table.

Now if you push the ball forward (Yea yeah very dumb question I told you before) it goes round, even when no force is applied later, instead of going in a straight line as per newtons first law. I don't understand it - why it does so ?

Dawood ibn Kareem

The shaft of the handle applies a force on the wooden part. Is that what you're asking?

John Rennie

@AlexKChen do you mean that when you start the handle rotating it will carry on rotating after you've let go (ignoring friction)?

Alex K Chen

@JohnRennie Yeah, exactly that.

John Rennie

It's the same reason the Moon keeps going round the Earth, or the Earth keeps going round the Sun.

Alex K Chen

Sorry I don't understand (I am a newbie) can you explain it ?

John Rennie

6:42 AM

Let me draw a quick diagram ...

Dawood ibn Kareem

The shaft of the handle pulls the wooden part inwards. But because this force is perpendicular to the direction that the wooden part is already moving, it doesn't cause the wooden part to speed up or slow down - it only changes the direction of its motion. It's still a force.

A force speeds something up if the angle between the force and the direction of motion is less than 90 degrees. It slows something down if the angle between the force and the direction of motion is more than 90 degrees. But if it's exactly 90 degrees, the motion just changes direction.

Alex K Chen

So the key to understand is the angle 'tween the shaft and the handle is exactly 90 degrees?

Dawood ibn Kareem

No.

Between the shaft and the direction that the handle's moving.

John Rennie

That shows the rotating handle seen from above.

Alex K Chen

@JohnRennie Yeah, so how it's changing the direction then ?

Dawood ibn Kareem

6:47 AM

So John's diagram is end-on (not from above as he stated). Imagine the wooden part of the handle sticking out of the page.

John Rennie

At any moment in time the shaft exerts a force at right angles to the direction the handle is moving

Alex K Chen

@DawoodibnKareem OK, then ?

John Rennie

@DawoodibnKareem that's what I meant by above. I guess if the crank is oriented sideways I should have said a side on view

Dawood ibn Kareem

Look at the two arrows in John's diagram. There's exactly 90 degrees between them. And when the handle moves a little bit around the circle, there will still be exactly 90 degrees between the shaft and the direction of the handle's motion.

Alex K Chen

Ah, so then I should view it as a limiting process when $\delta \text{ Straight movement } \rightarrow 0$

Dawood ibn Kareem

6:49 AM

So ignoring friction, what you have is a handle that will maintain a constant speed, but keep on changing direction, so that it moves in a circle.

No, I don't think it helps to view it as a limiting process.

Alex K Chen

> And when the handle moves a little bit around the circle...

Dawood ibn Kareem

OK, then, a big bit.

Alex K Chen

I mean is imagining like this correct: The red handle goes straight for $\delta$, then turns because the force towards the blue, and the proces repeats. Now $\delta \rightarrow 0$ ?

Dawood ibn Kareem

Well, I suppose if you wanted to do calculus with it, you could, but it seems overly complicated to think of it that way.

Alex K Chen

I will think about it and ping you two later if I have any further problem regarding this. Thanks for the help, though.

Dawood ibn Kareem

6:58 AM

You're welcome. I hope it helped.

DanielSank

7:10 AM

@ACuriousMind I think the comments here use an unnecessarily confrontational tone. No big deal, just a note.

Slereah

7:35 AM

@0celo7 How to prove that a manifold admits a simply connected atlas

My first thought is

Decompose the subset of $\Bbb R^n$ into $n$-balls, those are simply connected

But that's a bit crude and not countable

Hm, I thought that Lee would help me, but

Exercise 1.1. Show that equivalent definitions of manifolds are obtained if instead of allowing U to be homeomorphic to any open subset of Rn, we require it to be homeomorphic to an open ball in Rn, or to Rn itself.

shakes fist

I'd like to show that there's a countable atlas, ideally

user228700

@JohnR: So. Portals is awesome.

John Rennie

7:50 AM

@Kaumudi.H :-) Glad it was worth all the hassle.

user228700

Definitely was! I'm on level 13 now; 6 more to go...

user228700

This level is a bit harder than usual so I'm going to take a quick break before I get back to it.

Mithrandir24601

The cake is a lie

Just in case you're not aware of that

user228700

...I wasn't. Dang.

Mithrandir24601

What? It has it written all over the place...

user228700

7:55 AM

What? In the game?

Mithrandir24601

steamcommunity.com/sharedfiles/filedetails/?id=210926717

user228700

Dude, I haven't gotten to level 19 yet.

user228700

...or any other level in which the walls have things written on them.

Mithrandir24601

That's just one example - it's hidden around in various places

Secret

In response to first answer:

19

Q: What constitutes an observation/measurement in QM?

Fundamental notions of QM have to do with observation, a major example being The Uncertainty Principle. What is the technical definition of an observation/measurement? If I look at a QM system, it will collapse. But how is that any different from a bunch of matter "looking" at the same system...

quantum-mechanics quantum-interpretations uncertainty-principle measurement-problem wavefunction-collapse

How will lubo explain:

Slereah

7:57 AM

@0celo7 Oh wait I guess that's just the assumption of secound countability

nvm

Mithrandir24601

It's now a meme (or at it has been). I would give you wiki articles, but they do actually contain spoilers

Secret

Jun 28 at 4:52, by John Rennie

@SirCumference So you have a hallucination that cars are moving very slowly. Does that make it safe to step out onto the highway in busy traffic?

Sometimes, I felt like quantum mechanics is a cheater. It seemly allow a lot of counterintuitive things to happen, but they happen in such specific confines that we never observe the truly counterintuitive outcomes we expect

1. For example, you can have correlations established that is not confined by the light speed barrier (which depends on which interpretation you take, may have or don't have an objective explanation) yet because of the probabilistic nature, it is protected from violating causality

2. And then you have measurement, which for psi-epstemic models, it is a subjective process, yet because only sufficiently decohered states we can obtain information from, the subjectivity is never allowed to actually results in inconsistencies like a physics law happens in a way depends on how it is observed

And this is counterintuitive because of how it taught us there are such witty ways to prevent a truly counter intuitive, law breaking thing to affect the bigger environment while somehow allowing it to exist

Mithrandir24601

@Secret it all depends on the interpretation used

Secret

For me, I am psi-ontic, objective collapse models, except agnostic about the actual projection mechanism itself. That is, to me, wavefunction is something as physical as a field in spacetime

John Rennie

@Kaumudi.H: you've completed two thirds of the game in an hour?

Secret

8:10 AM

However, I do understood why psi-epstemics and (to a small extent) Qbist thinks and why they also make sense

user228700

@JohnRennie Yep :-P

John Rennie

Good job it was only 36 rupees.

Still, I suppose there's always Portal 2 to come

user228700

Lol, yes, but this game is worth much more than Rs. 36 for things other than its duration alone.

user228700

@JohnRennie There is, yeah, and I'm hoping I can persuade someone to play it with me; there is a multiplayer mode in that one.

user228700

Stupid level 12. I have no idea how to solve this one.

John Rennie

8:13 AM

Well don't look at me :-)

Secret

Previous night dream:

/.../ There's a trailer which depict a woman (straw colored wavy hair western) and a man (black short hair western) standing on a metallic surface of some futuristic building complex. They are two of a group of humans who are born with the ability to phase through solid objects. After a black screen transition, the pair discovered that the metallic panels that tiled all over the city are the only thing they cannot phase through.
They then looked each other for a second before are forced to flee the scene as a blue laser from above chase after them. I then wondered whether this is Blade Runn…

user228700

@JohnRennie :-P Is that a response to "I have no idea to solve this one" or "I'm hoping I can persuade someone to play it with me"?

John Rennie

Either. Both.

Secret

Last night dream= forgot except I got killed (screen turns black) by a man carrying something that look like cookie cutters and there's a mall where customers fire paintballs at shelves at 2 storeys lower to buy things

user228700

@JohnRennie :-) OK.

Mithrandir24601

8:17 AM

@Kaumudi.H That's one thing I haven't done yet

user228700

Oh, BTW, we've found a hostel for me and everything.

user228700

@Mithrandir24601 Same here :-/ I've never played any multiplayer game.

John Rennie

The most interesting game I've played recently is Monument Valley. However it's a phone/tablet game and isn't available on Windows.

@Kaumudi.H is that a hostel run by the college? Or just a hostel?

user228700

@JohnRennie It isn't run by the college (because, as I said before, the campus doesn't have a hostel for women in it) but it's right outside (about 1 km away) and it's a private one. YMCA. You know YMCA, don't you?

Dawood ibn Kareem

I know the song!

John Rennie

8:19 AM

Yes, the YMCA exists in the UK as well, though it seems to have a low profile these days.

Dawood ibn Kareem

And the actions!

user228700

@DawoodibnKareem x'D

Las Vegas Raiders

lol

John Rennie

Presumably the hostel will have lots of students staying? That should mean the social life will be good.

user228700

Yes, yes, it's affiliated with the college so most prefer to stay there.

user228700

8:21 AM

YMCA but they have a building for women.

John Rennie

It's all coming together then! Scary!

user228700

Definitely. My parents have gone out to get me stuff to take! An iron, hangers, a table lamp, some laundry stuff etc.

Dawood ibn Kareem

You should have a look and see what they provide first. It's quite likely that coat hangers and table lamps are included with the room.

user228700

...hmm, I don't know about the table lamp but maybe hangers, yeah. We'll call and ask about that stuff later today, then.

user228700

I still dunno when I gotta leave tho.

Dawood ibn Kareem

8:28 AM

Better just to go there, and have a look round. Concentrate on the kitchen - for example, is there a toaster, a microwave, a kettle, cutlery, plates, pots and pans, and so on.

user228700

Ah, lol, they don't have a kitchen for us to use!

user228700

They prepare all the meals for us. We do have a plug in the room so I might be able to take an induction stove with me to make maggi or something once in a while but I dunno if using one will be permitted.

Mithrandir24601

:o No kitchen? :o

Ah, OK - at least it's catered. But still

Dawood ibn Kareem

@Kaumudi.H Right. Those are the important things to find out.

user228700

...I suspect my parents aren't too keen on buying another induction stove just yet; they said that if conditions are too dire about a month in, only then will they consider buying one and asking for special permission to use it.

user228700

8:31 AM

All the meals are s'posed to be OK. We called and asked...

John Rennie

When I was an undergrad only very basic cooking facilities were provided.

user228700

@DawoodibnKareem It's a 15 hour journey from home :-/

user228700

...wow, none of the college hostels here provide cooking facilities!

Dawood ibn Kareem

@Kaumudi.H LOL! "Hello, this is Kaumudi. Are your meals OK?"

user228700

@DawoodibnKareem Dude, no, we called to ask what type of meals they provide and they sounded OK although there is no way to know if they really are.

Mithrandir24601

8:35 AM

@JohnRennie Same here (although possibly not as basic), but we still had a kitchen with kettle/toaster/microwave-oven, so we could at least make basic things and tea

Dawood ibn Kareem

OK, if the distance is prohibitive, then have a good think about these things, and try to make a comprehensive list of questions that you can ask them over the phone. I don't mind helping you if you like.

John Rennie

"Are your meals OK?"
"No, we always heat them"

user228700

@DawoodibnKareem Hmm, questions like what? We already asked them about the meals and timings...

user228700

(Timings being 6 AM to 6:45 PM)

Dawood ibn Kareem

Like what kitchen facilities are available in the individual rooms. One day, you just might crave something that's not on their standard menu, and want to make it yourself.

user228700

8:42 AM

...there are no kitchen facilities available in any of their rooms.

Dawood ibn Kareem

And things like whether you're allowed to plug in your own fan, or personal air conditioner in your room.

So you can't even make tea or coffee?

user228700

@DawoodibnKareem Nope.

Dawood ibn Kareem

What about a fridge to keep your soft drinks and other personal food items in?

Or is there a communal fridge where you have to name things and hope nobody steals them?

user228700

I am hoping that there is enough space for a desk and chair, let alone a fridge and all :-P

Dawood ibn Kareem

And that's another good question to ask.

user228700

8:44 AM

And I'm pretty sure that there is no communal fridge, no...

Dawood ibn Kareem

And if you have your own special pillow, for example, whether you can bring it and put their pillowcases on it.

user228700

None of these things are common to any hostels here so I wasn't preparing accordingly...

user228700

@DawoodibnKareem Aww x'D Um, no, I don't know about that. I think the plan is to check on those things when we visit the college sometime next week to verify my documents etc.

Dawood ibn Kareem

Do you have your own bathroom or is it communal? And if it's communal, can you keep your own shampoos, and toothpaste, and stuff like that there?

user228700

Thank God for we do have an attached bathroom and I will be sharing it with the one other person with whom I will also be sharing my room.

Dawood ibn Kareem

8:56 AM

OK. Anyway, these are the kinds of questions to think about. I'm sure after a day or two, you'll have a good list to ask them.

user228700

Brb!

Dawood ibn Kareem

Then, your parents can shop for you based on the answers you get.

Las Vegas Raiders

9:27 AM

What does "ibn" mean in your username? @DawoodibnKareem

Son

"son of"

thanks

My father's preferred first name doesn't map particularly nicely to Arabic, but "Kareem" is fairly close, as well as being a perfectly cromulent Arabic name.

Secret

11:07 AM

Vectors of all 1s are so useful that I am annoyed they don't have special symbols for them

Dawood ibn Kareem

Interesting. What do you use them for?

Secret

Well, you can write the criteria for a matrix being stochastic (each columns sum to 1, or each row sum to 1) equivalently as the matrix, regardless of basis set, has the column (resp row) vector of all 1 as an eigenvector with eigenvalue 1

because when you summing the rows or columns of a matrix, it is the same as multiplying this matrix by the vector of all 1s

in Mathematics, 25 mins ago, by Secret

$$SPS^T$$

Let $S^T=U$ and $(1)_{xy}$ be the components of the matrix of all 1s. Now write everything in terms of indices, i.e.:

$(S)_{ij}=S_{ij}$, $(P)_{kl}=P_{kl}$, $(U)_{mn}=U_{mn}$

Now

$(PU)_{kn}=\sum_{l}P_{kl}U_{ln}$, $(SP)_{il}=\sum_{j}S_{ij}P_{jl}$

$(SPU)_{in} = \sum_{j}S_{ij}(\sum_{l}P_{jl}U_{ln})$, $(SPU)_{in} = \sum_{l}(\sum_{j}S_{ij}P_{jl})U_{ln}$

Given $P$ is doubly stochastic, i.e.

$\sum_{k}P_{kl}=(1)_{1l},\sum_{l}P_{kl}=(1)_{k1}$

Now let $w,v$ be the row and column vectors of all 1s

and then, you can show that given a doubly stochastic matrix $P$ and rectangular matrix $S$ which is block diagonal with vectors of all 1s, $SPS^T$ is also doubly stochastic

Dawood ibn Kareem

I think $(1)$ is perfectly OK, if you explain what it means (which you did).

ACuriousMind

@Secret I don't think that is true. In fact, it cannot be true because it would mean that for any stochastic matrix the vector that represents a uniform distribution (which is what you get when you divide that vector of 1s by the dimension of the space) would be a stationary distribution, but clearly the uniform distribution is not a stationary distribution for all stochastic processes.

The two-state process with transition probability 1 from the first to the second and 0 from the second to the first state being an easy counterexample.

Secret

11:23 AM

This guy?
$$\begin{pmatrix}1 & 0 \\ 0 & 0\end{pmatrix}$$

ACuriousMind

That's not a stochastic matrix.

Secret

o sorry

uh...

> The two-state process with transition probability 1 from the first to the second and 0 from the second to the first state being an easy counterexample.

Ok I must be missing something, I don't know how to translate that into a matrix

I keep getting 2x2 matrices of only 1 entry being 1

The lines reads to me that the off diagonal term from state 1 to state 2 is 1, while that of state 2 to state 1 is 0

ACuriousMind

@Secret I only specified two of the four probabilities, of course you have to deduce the two missing probabilities from the constraint that you have to have a total probability of 1 to end up somewhere when leaving a state.

Secret

ok, let me try again...

If I recall, total probability of a density matrix is its trace, and that has to sum to 1. Next I want it to be left or right stochastic, thus:

ACuriousMind

This isn't a density matrix.

Las Vegas Raiders

11:33 AM

What do you mean by "density" in this context?

Secret

sorry, thinking too much quantum mechanics

ACuriousMind

@Secret It's just a matrix of transition probabilities: The $ij$-th entry is the probability to go from state $i$ to state $j$. Depending on how you label rows and columns, you get $\begin{pmatrix} 0 & 1 \\ 0 & 1 \end{pmatrix}$ or its transpose, since the probability to go from the first to the first state must be zero, and the one to go from the second to the second must be 1.

Secret

ok right I see, that's a left or right stochastic matrix, but it is not doubly stochastic

but isn't suming each row of this matrix I get $\begin{pmatrix}1 \\ 1\end{pmatrix}$?

which is exactly the same as I multiply this matrix to $\begin{pmatrix}1 \\ 1\end{pmatrix}$

since $\begin{pmatrix}0& 1 \\0 & 1\end{pmatrix}\begin{pmatrix}1 \\ 1\end{pmatrix}=\begin{pmatrix}0 \\ 0\end{pmatrix}1+\begin{pmatrix}1 \\ 1\end{pmatrix}1=\begin{pmatrix}1 \\ 1\end{pmatrix}$

ACuriousMind

Note that it is row stochastic, but only the column vector $\begin{pmatrix} 1 \\ 1 \end{pmatrix}$ is an eigenvector.

You claimed that a row stochastic matrix has the row vector $(1\quad 1)$ as an eigenvector.

Secret

ah, that's what I tried to say originally, I must have mix up the row and column labels

that is, column stochastic has row vector of all 1s as eigenvector
row stochastic hs column vector of all 1s as eigenvector
and doubly stochastic has both

Dawood ibn Kareem

11:40 AM

16

Q: Notation for the all-ones vector

What's the most common way of writing the all-ones vector, that is, the vector, when projected onto each standard basis vector of a given vector space, having length one? The zero vector is frequently written $\vec{0}$, so I'm partial to writing the all-ones vector as $\vec{1}$, but I don't know ...

notation co.combinatorics

Secret

Right, in that case, the claim about the eigenvectors (modulo careless mistake) still holds

ACuriousMind

@Secret Yes, that is true, because multiplying a matrix with that column/row vector is the same as summing up all the rows/columns.

Secret

Hmm, this eigenvector also seemed quite special for these matrix. You can do a basis change on the original matrix and not only you still have the same eigenvalue, the eigenvector is also exactly the same, meaning this is one of the few eigenvectors that are left unchanged by a basis transformation

ACuriousMind

@Secret You'll have to restrict your notion of "basis transformation" considerably for that to hold true.

Emilio Pisanty

Thanks, I have to think about something really carefully, I'm dizzy now. — Bettertomo 9 mins ago

at least they're honest =P

Yashas

11:48 AM

0

A: entropy change during heat loss

Entropy is a name given to a universal process of elemental capture and bonding determined by localised environments and time,(the random meeting of random dispersal), heat transfer is the friction of interaction and heat loss is decay, what remains is minus heat(fuel) minus time(a measured rate ...

:|

ACuriousMind

@Secret For instance, my example above can be diagonalized to $\mathrm{diag}(0,1)$, and diagonalization is a basis change and that matrix clearly does not have the vector $\begin{pmatrix}1\\1\end{pmatrix}$ as an eigenvector.

Yashas

VLQ Review Queue Link

Secret

Hmm... Let $P$ be a row stochastic matrix and $\vec{1}$ be the column vector of all 1s. Let $S,S^{-1}$ be a pair of basis transformation matrices, and let primed variables be the matrix and vector under the new basis. Then:

$$P\vec{1}=\vec{1}$$

$$P'\vec{1}'=(S^{-1}PS)S^{-1}\vec{1}=S^{-1}P\vec{1}=S^{-1}\vec{1}$$

= fail unless $S^{-1}$ has $\vec{1}$ as eigenvector with eigenvalue $1$

Right, and yours is a nice counterexample of the general result here

ACuriousMind

@Yashas Hmmm?

Yashas

@ACuriousMind nvm... I thought it would get deleted faster if I had posted the link here :p

ACuriousMind

11:52 AM

Just let the queue do its job, it's not as if that post is so terrible it needs to be instantly purged from the face of the earth :P

Yashas

it has been 1 hour since it was posted :P

Jim

@ACuriousMind speaking of which, if you find any good ones of those, you're remembering to let me know, right?

Las Vegas Raiders

long time no message from Jim

ACuriousMind

@Yashas 1 hour is far below the average flag handling time, just be patient

Jim

I've been averaging about one a day

I'm sorry I've been neglecting you all

ACuriousMind

11:54 AM

@Jim Huh? I purge them from the face of the earth all on my own, of course! Why would I let you have the fun?

Jim

@ACuriousMind I just wanna read 'em at least

Las Vegas Raiders

Have you considered doing an AMA? @Jim

ACuriousMind

@Jim Your best bet for that would probably be watching the "recently deleted" tab in the 10k tools

But I can't recall any particularly egregious and/or funny posts lately.

Jim

@LasVegasRaiders I've been offered many times and I've said every time that I could do one, but don't make any promises about the quality. Then nobody talks to me about scheduling and it never happens

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