The h Bar

user54412

5:21 AM

remember the Oort cloud is rather hypothetical

user54412

it's treated as a definitive, well-known thing in popular science books for some reason, but it's existence is only inferred

Sir Cumference

Of course, but I want to know why theorists believe that there is such a large space between it and the heliopause

user54412

so I wouldn't lose too much sleep over its inner edge

Sir Cumference

Surely there's a reason, right?

Otherwise it wouldn't be a single region, but instead a bunch of planetesimals simply past the heliopause

It would be laughable if we had no reason.

user54412

it's not clear to what extent they really do believe that

Sir Cumference

5:25 AM

I guess enough to define the Oort cloud as a region

Instead of just calling it "all the planetesimals beyond heliopause, randomly dispersed"

They then wouldn't believe it had an inner-edge thousands of AU away

user54412

also, keep in mind that diagrams of the Oort cloud are just like diagrams of satellites around Earth -- they are extremely misrepresentative of the density of objects, since each one is expanded to one pixel, which is millions or billions of times its actual size

Sir Cumference

Of course

But you can clearly see how packed the Oort cloud is relative to the "gap" I was talking about

dmckee

One of the comments under the question indicates that there are more known non-periodic comets with major axes beyond 1000 AU than those with axes between 100 and 1000. But the volume element may contribute.

There certainly seems to be a puapacy (sp?) of hard data.

user54412

that diagram is also just a cartoon

Sir Cumference

And? It's a diagram.

It's supposed to represent something.

user54412

5:29 AM

a thing that might not even exist ;)

Sir Cumference

I know, haha

But still wondering why the Oort cloud is thought to be in a region, rather than spread about all beyond the heliopause

dmckee

@Chris Any notion why the Kuiper belt is assigned a 100ish AU outer limit? I mean, that's a largely hypothetical structure too (though we can at least see a few objects in that region), but surely the outer edge is even more hypothetical.

Sir Cumference

It shouldn't be random guesswork. It's a model.

user54412

I would trace the wiki citation about the inner edge, but alas it's a book, not an article

user54412

astronomy citations to anything other than freely available online articles ought to be banned

dmckee

5:31 AM

@SirCumference "Model" can means as little as "wild ass guess" if the data is sparse.

user54412

@dmckee No idea.

Sir Cumference

Yeah, true, but it would be laughable if it were

user54412

perhaps because much further and it would start interfering with the Oort cloud :p

Sir Cumference

Nobody laughs at the Oort cloud like they do string theory

dmckee

@SirCumference Word to the wise: all science reporting and primary or secondary texts should be assumed laughable until proven otherwise.

Sir Cumference

5:32 AM

Hmm...you make a good point

But I assume there's some reason why it isn't a joke by now

dmckee

@SirCumference The Oort cloud people don't claim to be the smartest people in the room in front of TV cameras. That makes a difference.

Sir Cumference

I'm too lazy to see what the data is actually like

But it may be pretty logical. If so, there's surely a reason for the gap

Astronomers must have ruled out the possibility of tons of objects being there, right?

user54412

One thing to note is that comet-hunting used to be at least 50% of astronomy (back a couple hundred years ago). Now there might not be 50 people in the world who research comets.

Sir Cumference

Haha

True

user54412

If comets were as exciting as exoplanets these days, you can be there'd be a dozen competing theories as to where comets come from.

dmckee

5:38 AM

Ah! The edges of the Kuiper belt are assigned by putting a little wiggle room around the 3:2 and 1:2 resonance orbits outside of Neptune.

Sir Cumference

I'm sorry?

dmckee

But to this utter ignoramous that doesn't seem like a strong argument that there is nothing further out. Just that it should be lower density.

Sir Cumference

We're talking about the 900-4900 AU space cleared up

Er, no need to insult anyone...

We're just discussing space

dmckee

@SirCumference You might be. I'm still puzzling over the edges assigned to the Kuiper belt.

Sir Cumference

Why?

Anthony

5:40 AM

Heyo

dmckee

That's a puzzle that (a) has some data and (b) is about my speed.

Sir Cumference

Dammit, ya got me at (a)

Anthony

Can someone here give me any direction on how I'd approximate the change in entropy from dropping my pencil on the floor?

dmckee

@SirCumference Because they seemed arbitrary.

user54412

@dmckee cute

dmckee

5:41 AM

@Anthony The gravitational energy get turned (eventually) into heat. Check your thermometer and apply the thermodynamic definition of entropy?

Anthony

Oh. That's easy, isn't it.

Sir Cumference

So we still haven't figured out why there's so much space. And yes, that doesn't seem like a strong argument that there is nothing further out

Call me an ignoramus

dmckee

@Anthony If it's right. I'm guessing.

Anthony

I mean that makes sense.

Sir Cumference

Wait a minute

in about a billion years, the Sun will get too hot for life on Earth...

wait, nvm, my question is stupid

user54412

5:52 AM

One of the early extensions of Oort's work, suggesting an inner edge of 10,000-20,000 AU

dmckee

Ooh ... more. Extra solar observing has revealed Kuiper belt like structures around a few stars other and some of them have sharp cut-offs. Interesting. Plus we see a drop outside the 1:2 resonance.

Anthony

@dmckee Doesn't that formula depend on something like a reversible process, or something?

Quasi equilibrium? Something? The entropy one that is.

Sir Cumference

adsabs.harvard.edu/full/1978AJ.....83...64M

dmckee

Yeah. Sets a lower limit.

Sir Cumference

tl;dr

What's his reasoning for the lower limit?

user54412

5:54 AM

Followup paper: "we shall show that the abrupt apparent termination of the Oort cloud at a critical inner semi-major axis $a = a_c \sim 2\times10^4$ AU is due to observational selection."

Sir Cumference

Huh, okay

dmckee

Hm. Volume element or what?

Anthony

Thanks @dmckee.

user54412

"We show that about one percent of the comets initially present in such an inner comet cloud, where the comets have $a$ semimajor axes on the order of $3\times10^3$ AU, would have been ejected by the perturbations of Jupiter and Saturn [...] Self-consistency then requires that the present-day mass of the inner-comet cloud be about two orders of magnitude greater than that of the Oort cloud."

dmckee

::boggle::

user54412

5:57 AM

Much more recently: "The detached object Sedna is likely at the inner edge of the Oort cloud [...] all objects with perihelion distance q > 45 au and semimajor axis a > 250 au belong to the IOC."

user54412

I'll leave you with this gem from my literature search. No Death Star--For Now: "A star passing within ~$10^4$ AU of the Sun would trigger a comet shower that would reach the inner solar system about 0.18 Myr later. We calculate an a priori probability of ~0.4% that a star has passed this close to the Sun but that the comet shower has not yet reached the Earth."

Slereah

10:20 AM

How am I supposed to learn anything when most of my memory is just ad jingles from the 90's

3

Danu

11:04 AM

Small terminology question: Is the conformal gauge (in string theory) the one where $h_{\alpha\beta}=\Omega^2 \eta_{\alpha\beta}$ or is the name reserved for $\Omega\equiv 1$?

I guess it's the case $\Omega^2\equiv 1$, based on Wikipedia and Lüst's book, but I find that a but of a misnomer!

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