The h Bar

John Rennie

4:00 PM

That addresses your question (3) directly, and indirectly addresses (2).

ManishEarth

o/

John Rennie

We can't be in a black hole because there are no stable orbits inside the event horizon.

Kyle Kanos

@ManishEarth \o

Richard

Ah, now that's interesting; "you either hit the singularity or are ejected" - From inside the event horizon?

Danu

@JohnRennie The chat session is the only time we can lure ole John to the chat :D

Why don't you hang around here more often, John?

John Rennie

4:03 PM

@Danu I'd completely forgotten the chat was happening. I'm sitting happily digesting a large lunch (of vegetable biryani) and the site nagged me the chat was starting.

Danu

@JohnRennie Duty calling, also for @TerryBollinger

Terry Bollinger

Hi folks, black holes and John Rennie, cool!

Kyle Kanos

@ACuriousMind Not that I can recollect

John Rennie

I'm normally working during the day so i dip in and out of the SE. It isn't practical to monitor the chat while trying to write software. It kind of ruins the concentration :-)

Danu

It appears we have seen a major resurgence of interest in quantum foundations in chat over the past few weeks

Notably... Bohmian mechanics! :P

Terry Bollinger

4:04 PM

Though the reality of wave packets via Ringbauer et al's papers are on my mind, amazing work...

Danu

@JohnRennie Agreed, agreed

Terry Bollinger

Ringbauer!

John Rennie

@Richard Yes, there are geodesics that pass through an event horizon and back out. I answered a question on this a while ago and I'm just searching for the question ...

ManishEarth

The snake fight portion of your thesis

John Rennie

Aha!

8

Q: Entering a black hole, jumping into another universe---with questions

I'm quite familiar with SR, but I have very limited understanding in GR, singularities, and black holes. My friend, which is well-read and is interested in general physics, said that we can "jump" into another universe by entering a black hole. Suppose that we and our equipments can withstand th...

Terry Bollinger

4:06 PM

Bohmian? I must be tuned into a different channel (or drummer?)

Richard

@JohnRennie - I was under the impression that the event horizon was a finite boundary, requiring FTL speeds to escape.

Danu

@JohnRennie You're referring to Kerr, right?

John Rennie

@Danu or Reissner-Nordstrom

Danu

@Richard This stuff is all very speculative

But if GR would hold inside, it would predict that rotating black holes are very different from the 'standard' Schwartzschild black holes

John Rennie

@Richard Life is more complicated than that. Which is just as well, as it's what makes GR so interesting.

Danu

4:08 PM

...including stuff like going into different universes where the black hole is actually repulsive

John Rennie

I should add that this conclusion is based upon lots of assumptions. The string theorists currently believe black holes have no interior.

At least I think that's one idea.

Richard

@Danu - That actually puts my first question on the front foot. Can I use gravity (for example firing smaller black holes) to tear a black hole into pieces that are too small to create a black hole?

John Rennie

@Richard No.

Jimdalf the Grey

@JohnRennie perfectly reasonably idea too.

Danu

@Richard This is quite an ill-defined question

You cannot 'fire something through' a black hole

Terry Bollinger

4:11 PM

Q: This is going to be badly phrased, I know: For a amazingly large black hole with an asymptotically flat horizon, does that horizon remain infinitely "sharp" in its apparent location in space?

John Rennie

@JimdalftheGrey Hmm, for a subset of definitions of the word reasonable

Richard

@Danu - But I could (even with current technology) fire a stream of somethings past the event horizon.

ACuriousMind

@Danu Well, the stuff that missed the singularity and comes out in a disconnected region has technically been fired through the hole, hasn't it?

Jimdalf the Grey

@JohnRennie Well, more reasonable than other ideas string theorists have had

Danu

@ManishEarth lol, what the f...?

John Rennie

4:11 PM

@TerryBollinger Ignoring quantum gravity then yes it does.

Danu

@ACuriousMind Like I said, this Kerr stuff is very speculative at best, and the matter will be in a different universe so I don't think it qualifies

John Rennie

What quantum gravity may have to say on the subject I don't know.

Richard

So, if a very heavy object passes a black hole, what happens to the black hole?

Can I stretch it?

ACuriousMind

@JohnRennie I'm not sure if quantum gravity knows, either :)

John Rennie

@Danu Nooooooooooooooooooooooooooooooo. It's not a different universe.

Danu

4:12 PM

@JohnRennie Even semiclassical methods seem to be primarily confusing at the moment ;)

John Rennie

It's a causally disconnected region of the same universe.

Danu

@JohnRennie Wait a sec

Was my GR TA horribly inaccurate?

...this would annoy me

John Rennie

I suppose it depends on what you mean by universe. Did you read my answer to the question I linked above?

Danu

@JohnRennie Reference, maybe? You're talking about matter passing through the disk-shape in the middle, yes?

@JohnRennie Ah, clear. I think that qualifies as different universe as far as I'm concerned :P

John Rennie

@Danu I only really understand the Reissner-Nordstrom metric. I've never really got to grips with the Kerr metric. In the RN metric the process is basically straightforward.

David Z

4:15 PM

ah, I made it this time

for now anyway

Danu

@JohnRennie In my GR course we skipped RN, only did Kerr

Hi @DavidZ

John Rennie

@Danu Probably wise as the RN metric isn't likely to be more than a mathematical curiousity.

Danu

@JohnRennie Ron's answer there is good

John Rennie

It's hard to see how you'd charge a black hole enough to make any real difference to the geometry.

Danu

@JohnRennie Charge, yeah...

Jimdalf the Grey

4:17 PM

In my GR course, we did RN and Kerr and Kerr-Newman

Kyle Kanos

In my GR course, we did not cover any metric outside the Minkowski

Jimdalf the Grey

how is that possible?

ah

Danu

@JimdalftheGrey Neat! We do have a separate course on black holes, and one on cosmology too, so this won't be the end of it ;)

Kyle Kanos

Which made it bad :(

Danu

@KyleKanos Literally stuck in SR? ;)

ACuriousMind

4:18 PM

In my GR course, we did Schwarzschild. And some stuff about quadrupole tensors. That's all I remember.

Kyle Kanos

Who teaches GR w/o getting into any of the cool metrics

Danu

@ACuriousMind You mean gravitational waves?

My GR course's part on waves was funny

Terry Bollinger

@JohnRennie Misc tangential Q again: I've tended to assume that for a very large BH there can be non-trivial, enduring dynamic structure going on inside the event horizon, e.g. objects orbiting each other. Am I wrong on this?

Jimdalf the Grey

Who teaches GR without touching the FLRW metric?

ACuriousMind

@Danu Yes. Never understood them, the course was horrible. I passed the exam with pure math. I couldn't do a single physics problem.

Danu

4:19 PM

It was taught my Mukhanov who, in addition to being just a totally epic guy, has the most amazingly stereotypical Russian accent ever.

Kyle Kanos

@JimdalftheGrey Mine did!

ACuriousMind

@Danu Ah, you linked a video of him here once, I think

Danu

For a sample:

Slava Mukhanov: The quantum origins of the Universe structure

►

ACuriousMind

That one, yes

Danu

So anyways, at some point he's motivated that the energy loss due to waves should be proportional to the square of the third derivative of the quadrupole moment, right

John Rennie

4:20 PM

@TerryBollinger The trouble is that all the metrics for which we have analytic expressions are time independant i.e. they describe a black hole that has existed unchanging for ever.

Danu

And now comes the point where you really have to calculate what it is.

John Rennie

Amusingly, it takes an infinite time for a true horizon to form as measured by observers outside it.

Danu

After 10 seconds of contemplation he said: Okay, we want to avoid cheadache (Russian spelling ;) )

and just did dimensional analysis instead :D

John Rennie

The best we would ever see is an apparent horizon.

ACuriousMind

@Danu One exam question was like: Consider the earth with a long rope extending from it. Calculate the quadrupole tensor of this thing. They didn't give you the formula, if you didn't know it, you were basically done with that exercise already

John Rennie

4:21 PM

However, if we gloss over this, there cannot be any permanent orbits inside the event horizon as all geodesics inside it lead to the singularity.

Danu

@ACuriousMind ^^

John Rennie

However make the black hole big enough and it can take a long time to reach the singularity.

Danu

@ACuriousMind Did you not learn about multipole expansions in your classical electrodynamics course though?

ACuriousMind

@Danu Yeah. Hated them there, too :D

Danu

It's a running gag here at LMU that, whenever a professor expects something unreasonable in terms of previous knowledge, he will say: But didn't you already learn this in classical electrodynamics?!

ACuriousMind

4:23 PM

My ED course was not much better than my GR course, lecturer-wise

Danu

Apparently classical ED covers representation theory here

(lol)

ACuriousMind

@Danu Wat

Not even I would expect that there

Danu

@ACuriousMind LMU... :)

John Rennie

In fact I just answered a question on this. The time to reach the singularity from the horizon is roughly is approx 6.57 \frac{M}{M_{Sun}} microseconds.

Danu

@JohnRennie Hey, I remember that

John Rennie

4:24 PM

Where M is the mass of the big black hole and M_Sun is a solar mass.

That's the time measured by the infalling observer i.e. the proper time.

Danu

This would be measured in proper time of the falling guy, or what?

Terry Bollinger

@JohnRennie A black hole that has existed unchanging forever is a non-physical statement. How can the mathematical model be correct if it reflects a non-physical situation?

Danu

Yeah okay

Kyle Kanos

@JohnRennie So for a million solar mass black hole, that's 1 second?

Danu

@TerryBollinger It works ;)

Kyle Kanos

4:25 PM

That's insane

Danu

@KyleKanos yeah

Kyle Kanos

@JohnRennie What question was that one?

Danu

@TerryBollinger Many things can be approximated as static very nicely

...especially considering we have set $c=1$ so things are slower than you think

JiK

The main page of physics.SE just says " Physics chat session ends in 17 minutes" and links to this chat page without any explanation. What is the purpose (and rules) of this chat?

John Rennie

@TerryBollinger the Schwarzschild metrics etc are approximations to real black holes. We expect calculations made using the to give answers very close to what would actually happen.

Terry Bollinger

4:26 PM

@Danu Approximations become very dangerous as you approach extreme limits.

That is the computer scientist in me talkikng!

Danu

@JiK It's a bi-weekly session where you can bring up any 'important issues'

Kyle Kanos

@JiK The purpose of the chat sessions is to discuss issues with the site.

Danu

but we just talk :P

ACuriousMind

@JiK Not much more than chatting, really, except if there's something urgent to discuss about the site

John Rennie

4

Q: A Hollow Black Hole

I was just reading a question about the gravity inside a hollow neutron star. It was a trivial question, obviously there is no force felt. But then it got me thinking. Suppose you had a hollow sphere that was massive enough that outside of it, it became a black hole. It's not hard to imagine a s...

general-relativity gravity black-holes

Kyle Kanos

4:27 PM

But this is the generic physics chatroom, so we tend to talk about whatever

Danu

@TerryBollinger I'm pretty sure there are many people working on it numerically that understand how to control errors etc

Terry Bollinger

@JohnRennie John, thanks, that's where I was hoping to head: Configuration where the central singularity is not a pre-given...

John Rennie

@JiK if you want to hazard a question that you think might get closed on the main site this is a good place to start.

Kyle Kanos

@JohnRennie Aha, thanks!

John Rennie

@TerryBollinger Hmm, I think the Hawking Penrose theorems apply to real black holes, so the singularity is inevitable.

@Richard: we've kind of wandered away from your question. Did you want to chase it further?

Terry Bollinger

4:30 PM

@JohnRennie Yep, just wondering (probably wrongly) if it can be delayed to varying degrees, and if so, how longl

John Rennie

Delayed according to whose clock? For external observers the singularity never forms i.e. takes an infinite time to form.

ACuriousMind

And people complain quantum mechanics is unintuitive...

John Rennie

@ACuriousMind QM has no simple elegant framework like GR does. That's what makes QM unintuitive.

Terry Bollinger

@JohnRennie Internal. One presumably could watch one's world collapse in diverse ways.

Rats, I find QM more intuitive than this stuff, I'm just weird.

Danu

I think there are a bunch of funny things like this that can be said in the context of Kerr metrics

like... someone falling in seeing the entire history of the universe unfold or something

@TerryBollinger The nice thing in QM is that one can just totally give up on intuition while GR is much more 'physical' in feeling

ACuriousMind

4:34 PM

@JohnRennie Wait. What about Hilbert space + algebra of observables is less elegant than the classical phase space + observables?

John Rennie

@JimdalftheGrey: have you settled on the new name yet?

Terry Bollinger

@Danu approximation errors are more than just controlling accuracy. Approximations can also leave out conceptual issues that become critical at the extrema. Ask any plasma fusion person...

Danu

@JohnRennie ...he asks, with a big smile

Richard

@JohnRennie - I'm going to have a read through the references and raise it as a proper question. I'm reasonably sure it's not a dupe though.

Terry Bollinger

@Danu :)

Richard

4:35 PM

Cheers for assistance. I will return

John Rennie

@Danu actually I was going to offer to withdraw my suggestion on the grounds it was only ever a really bad pun.

Danu

@JohnRennie Don't do it!

all of them are bad puns

ACuriousMind

@JohnRennie Isn't the whole point of that thread making bad puns with Jim in them? :D

John Rennie

@ACuriousMind it's a long and tired discussion, but GR follows from the minimum of basically simple starting positions. Basically just the principle of general covariance. QM seems to have no such simple starting point.

Danu

@JohnRennie It's true that the fundamental simplicity in ideas is unrivaled, imo

ACuriousMind

4:39 PM

I see

Terry Bollinger

(still wishing I knew what the pun was?... :)

ACuriousMind

Alright, can't (and don't want to) argue with that

@TerryBollinger Are you aware of this thread?

Terry Bollinger

If you accept space and time and (wow) rotation as simple and fundamental, many things become intuitive. My genuine fear (actually, delight) is that I think a powerful argument can be made that these are all very complicated concepts that emerge from rules that are more quantum than classical. Classical is emergent...

@ACuriousMind Hi Curious, and no, I was not, looking... and wow, did your icon image have a really bad hair day??

ACuriousMind

@TerryBollinger Heh, yeah. The dark arts are not without their price ;)

(The thread contains the pun you wanted to know about, that's why I linked it)

John Rennie

If the conversation has dried up momentarily ... is Aron right to criticise my answer to:

4

Q: Where does information go if thermodynamic death?

Often read that until Hawking the black holes were introducing a problem with information. Allegedly information should be unable disappear, while this was happening in classic black holes. What about death? When information, contained in brain or device, just irreversible dissipate in the envir...

thermodynamics black-holes entropy information

Terry Bollinger

4:47 PM

@ACuriousMind Jim? Jim? Jerry? Well, the chimney sweeps are happy at least...

Danu

@TerryBollinger :D

Ell

Hi all

Danu

@JohnRennie He mostly appears to be incoherent, to me

hi @Ell

@ManishEarth that sit is pretty funny! mcsweeneys.net/articles/…

Terry Bollinger

@JohnRennie If every classical decision branches us into a specific Deutsch universe, then the binary expression of the concatenation of all of those decisions defines an integer that captures the entire history of that particular universe. Ignoring black holes (!!), that means nothing is lost in that universe.

Ell

I don't see any rules of this chat room so I'm going to go ahead and ask my question...

Jimdalf the Grey

4:50 PM

@JohnRennie It's not up to me. On March 13 the most popular one becomes my new name

Ell

What is the smallest unit of information?

Danu

mcsweeneys.net/articles/lady-gagas-creepy-borderline-ghoulish

John Rennie

@JimdalftheGrey You don't have to call yourself Jimnosperm. ACuriousJim is a far better anme and also accurate! :-)

Danu

@Ell The standard is a bit

Kyle Kanos

Qubit

In quantum computing, a qubit (/ˈkjuːbɪt/) or quantum bit is a unit of quantum information—the quantum analogue of the classical bit. A qubit is a two-state quantum-mechanical system, such as the polarization of a single photon: here the two states are vertical polarization and horizontal polarization. In a classical system, a bit would have to be in one state or the other. However quantum mechanics allows the qubit to be in a superposition of both states at the same time, a property which is fundamental to quantum computing. == Origin of the concept and name == The concept of the qubit...

Danu

4:51 PM

@TerryBollinger A specific German universe ^^

Terry Bollinger

@Ell Picture a wave function with two equally likely branches. Poke it with say an EM photon to force selection of one or the other. You have just created exactly one bit of information. Do it right and sometime you can erase it, take it back, but usually it becomes too disseminated very quickly.

Jimdalf the Grey

@JohnRennie Are you saying I'm not a conifer?

Ell

Meh. I was hoping for something less than a bit, somehow

Terry Bollinger

@JimdalftheGrey Fir shur?

Danu

Yes, explain yourself!

ACuriousMind

4:52 PM

@JohnRennie The Wikipedia article and Hawking's article seem to indicate that indeed it is now believed that all information is eventually radiated back into the environment. Aron got the details of the bet wrong, but it seems he is essentially right.

Ell

It feels too coincidental that the unit our computers use is the smallest unit

though I guess cpus use words, not bits, but still

ACuriousMind

@Ell What less than a simple storage of Yes/No do you want?

Danu

@ACuriousMind ...like John said, it's not settled

John Rennie

@Ell A qubit is not the same as the bits used in your computer

Terry Bollinger

@KyleKanos A qubit is interesting, but in some way the classical bit is more unusual. It requires the creation of history and the loss of quantum symmetry.

Ell

4:53 PM

I thought a qubit can store more than a regular bit?

Hmm nope. > A qubit is a two state ....

Danu

I have spoken to some researchers about this at the time when it (the firewall paradox etc) was really hot (most notably one of the Verlinde twins) and it was not even clear whether we are in a position to answer questions like this yet.

ACuriousMind

Come on now, @Jimdalf, really? :D

5

Danu

LOL

Jimdalf the Grey

@ACuriousMind I know he's young, but at some point they have to learn the truth

5

Martin

@JohnRennie About Information loss, I mostly agree with your last statement. Whether or not information is deleted ultimately seems to come down to the question whether the time evolution of the universe is unitary or not. Locally, however, we have an increase in entropy, which does indeed mean that information is lost (locally, not when we gather it throughout the universe).

Terry Bollinger

4:55 PM

@Ell Not exactly -- it's a more symmetric state, like a pencil balanced on its tip (and even harder to do). You can process more options with such states, but eventually you have to get back to a solid state -- a fallen pencil.

Ell

@TerryBollinger This is a good analogy

Terry Bollinger

@Ell Thanks, I am actually surprise that the symmetry analogy of e.g. the forces isn't used more to describe the symmetry of quantum states. Just habit I think.

John Rennie

@ACuriousMind so is someone going to give the QED description of a photon then?

Ell

Yeah

Danu

@TerryBollinger ...I guess because it's essentially identical to using a sphere

which is the standard visualization, as far as I know

@JohnRennie it must be a dupe

ACuriousMind

4:57 PM

@JohnRennie I am typing already ;)

Danu

@ACuriousMind Not that that'll help the asker ;)

ACuriousMind

@Danu Lol, no formulae, I promise

Danu

...and NO COHOMOLOGY!

:)

Jiminion

Are photons even 'real'? They don't even seem like particles to me, just an energy transfer mechanism.

John Rennie

12

Q: What exactly is a quantum of light?

I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a photon as a quantum of light, which it further explains as some kind of a wave-packet. What exactly is a quantum of light? More precisely, is a quantum of light meant to be just a certai...

quantum-mechanics visible-light photons

Danu

4:59 PM

@Jiminion LOOK AROUND!

@JohnRennie there we go

Ell

Let's say I have a bit, and I may or may not toggle it - after I have (maybe) toggled it I can tell you that I've either toggled it, or that I've maybe toggled it, isn't this less than a bit of information?

ACuriousMind

Okay, stopped typing :)

Ell

meh, idk vOv

Terry Bollinger

I actually am becoming more attracted to the idea of the history of the universe being one of an increasingly broken wave function, with each new Deutsch universe "bit decision" (entropy, if you look at it rightly) being part of the breaking process...

Jiminion

@Danu Obi Wan Kenobi told me not to trust my senses.

ACuriousMind

5:00 PM

Though this question is a bit different - it's asking how photons are produced, not what they are (though you have to tell the latter to answer the former)

Danu

@Jiminion Do you want me to force you? ;)

ACuriousMind

@Jiminion Define real

John Rennie

@Danu yes, though actually I'm not that keen on the answers to the previous question.

Terry Bollinger

Photons, ah! Feynman advanced, Feynman retarded, Feynman QED: He copped out a bit by simply saying "both ends happen at the same time" as far as the photon is concerned. But embed that in classical time, and it still looks advanced/retarded...

Danu

@JohnRennie Agreed

Terry Bollinger

5:01 PM

@ACuriousMind Really?

Danu

@TerryBollinger By the way, did you ever get 'closure' on that question of yours?

I didn't really get your problem

Terry Bollinger

@Danu Eh? wow, we've been around: The idea of a completely stable gigantic black hole surface... disturbs me. It feels incomplete, and weird, and overly precise, like something magically saying "right here" across a light year of distance.

Jiminion

Real: inside of time, having mass, able to remain stationary (within heisenberg) you know, the normal stuff.

ACuriousMind

@TerryBollinger I was a bit flippant, but also a bit serious

John Rennie

@TerryBollinger it's stable in the limit of infinite time. In the short term the system will radiate gravitational waves and dissipate any asymmetries in the process.

Terry Bollinger

5:04 PM

I'm a heretic on reality, I think folks like Ringbauer are experimentally moving us towards a more singular concept of reality, one with a Smolin-style "now" to it.

@JohnRennie Is it... I dunno, thick, thin, what? Oddly, I have a lot easier time coping with very small black holes and severely curved, inherently "hot" event horizons.

glance

@Jiminion is there something outside of time?

DanielSank

@ACuriousMind Maybe, I dunno. Perhaps everyone gets to submit one play per some amount of time.

John Rennie

@TerryBollinger horizons are curious beasts. To define them requires knowing the complete future evolution of your universe to infinite times.

Terry Bollinger

@glance Here's a thought for you: Simulator time. Even if it's naturally occurring, some aspects of "retro causality" in interpreting wave function collapse are most easily resolved by invoking an outside simulator time that is always forward moving.

Danu

mcsweeneys.net/articles/the-elements-of-spam

Sorry for spamming this site, but it's so funny IMO

Jiminion

5:09 PM

@glance Yes, photons are outside of time. From their standpoint, like Feynman indicated, they are emitted at the same instant they are absorbed.

Terry Bollinger

BTW retro time is an idea I may have accidentally come up with (I doubt for the first time) in 2004 to explain how "instant" entanglement works. It's easier to go back in time and change the original event, which quantum rules guarantee have left no outside information signature on the universe, and so cannot violate causality.

ACuriousMind

@Jiminion They have no standpoint. There is no frame moving at the speed of light

glance

@TerryBollinger interesting but... what does that mean?? :) With "outside simulator time" you mean some other kind of parameter describing evolution, which you call time, or what?

@Jiminion apart from the point of @ACuriousMind, I could also argue that if they are emitted/absorbed at some instant, then they are inside time... whatever that means

ACuriousMind

Much talk in this room lately about reality and truth. Did someone catch a philosophical virus? ;)

2

Kyle Kanos

I kinda want to format annav's answer

glance

5:13 PM

@Jiminion by the way, if having a mass is a requirement for your definition of "real", then photons are not

Terry Bollinger

@glance I'm actually trying to write a paper on that point, since it came up with Ringbauer's recent mention of retro time in his Jan 2015 paper. But the idea is not that hard: If self-consistent QM requires things that seem to reach back in time, you just make reality into a simulation, and place it in a box that only goes forward in time. Your "backwards in time" stuff just becomes unresolved state info in that case. But all that requires a real, Smolin-style "now."

@ACuriousMind The philo virus, aieeee!

@KyleKanos Is Anna here? I didn't see her...

glance

@ACuriousMind I think it's transmitted genetically. Someone must have caught it a few thousand years ago

Kyle Kanos

@TerryBollinger No, her answer to the "how is a photon made" question

She's got paragraph breaks where it really should be bullet points

Terry Bollinger

@glance Just now reading a scifi "Starships of Eden" by Howard Marsh, he seems to have come up with something roughly like that, or at least "guiding" folks down paths...

Philovirally speaking, wow, this session has covered a lot of topics...

Jiminion

it just seems that force mediators are lumped into the group we call "particles", and in many ways, they aren't. The are subject to wave/particle duality, but they are massless and travel at c. Not typical of basically every other type of particle.

glance

5:17 PM

@TerryBollinger what do you mean with "simulation" in this context?

Terry Bollinger

@Jiminion Spin! Spin is so, so amazing, so weird, so fundamental, fermions and bosons and baryons, oh my!

glance

@Jiminion not all of them are massless

John Rennie

@glance above the EW transition everything is massless ...

Terry Bollinger

@glance Only that it gives a framework for resolution that seems to work surprisingly well. You make the universe into a set of state variables -- there are SR/GR issues there BTW -- and then, well, you run the state machine. Both space (as we know it) and time (as we know it) become emergent aspects of how the state changes, and so can do funny thinks like go retro in ways that don't violate causality.

@glance Interestingly, Dirac (at least in one random comment) did not seem to view massive bosons as "truly" fundamental.

Jiminion

Bah, the weak interaction. I don't dignify it as a force.

Emilio Pisanty

5:22 PM

Hey folks

ACuriousMind

@TerryBollinger They, in a sense, aren't, because they can (if they mediate a force) acquire mass only through spontaneous symmetry breaking below some energy scale. At GUT scales, all force bosons (and thus all fundamental vector bosons we know) are massless

Emilio Pisanty

is anyone going to the APS March meeting next week?

(sorry to barge in, btw)

Terry Bollinger

@Jiminion It is kind of a floppy flippy thing, ain't it?

Jiminion

@EmilioPisanty Hey! Ask a question, they are ganging up on me!

@TerryBollinger The weak force? I think so.

ACuriousMind

It's wrong for scalar bosons though, the Higgs is actually a tachyon above the breaking scale

Kyle Kanos

5:24 PM

@EmilioPisanty It's a chatroom, there's no such thing as "barging" in here, so no apologies needed! That said, I'll not be going.

glance

@JohnRennie what do you mean? Last time I checked they were told to be massive, EW symmetry breaking and all...

Terry Bollinger

@ACuriousMind Here's an odd question: Does gravity truly have to be unified with the other forces to develop a complete picture of the universe? Or is it just a faith premise of quantum types that it must be? I argue for at least looking at "well, maybe not... maybe gravity really is different."

John Rennie

@TerryBollinger Yes, because otherwise superpositions aren't possible.

Terry Bollinger

@KyleKanos Yes indeed, I define barge!

John Rennie

Though I must admit that I don't understand the details.

Terry Bollinger

5:25 PM

@JohnRennie Intriguing, elaborate?

ACuriousMind

@TerryBollinger QFT does not obey the principle of general covariance, and GR does not obey the principle of interference/superposition. They just don't fit together well.

John Rennie

@glance that's the point, they're only massive at energies below EW unification. At higher energies everuything is massless.

glance

@TerryBollinger so basically you say that what we call time is not really the best suited parameter for describing causal evolution, so we invent another "real time", of which the usual one is just an approximation?

Emilio Pisanty

@ACuriousMind That's actually the clearest and most succinct answer to the classic question "why don't those two fit together" I think I've ever seen.

Terry Bollinger

@ACuriousMind So you are... of similar mind?

ACuriousMind

5:28 PM

@TerryBollinger That it could be they're just really different? Well. Yeah. That'd be really unsatisfying though, because both theories are kinda built on the premise that the respective principles apply universally.

John Rennie

@TerryBollinger see ...

backreaction.blogspot.co.uk/2012/01/…

Terry Bollinger

@glance Pretty much. Keep in mind my background is computer science, so I just don't think of these issues in quite the same way, I guess. I just like to say, "what leads to the simplest structure?", and having out time as one-off from "real" time seems to have some nice features for explaining certain QM anomalies.

Jiminion

@glance I once thought that there is only a single point in the universe that actually has the correct time.

Terry Bollinger

@EmilioPisanty Concur! Nice answer, that.

John Rennie

and probably lots of other blogs. Google semiclassical gravity and superposition for lots more.

Terry Bollinger

5:30 PM

@JohnRennie thanks, will look!

Jiminion

Star that b..., um answer!

ACuriousMind

@TerryBollinger: Also, what use would GR be, if it really can't be made quantum? The whole point of geodesics is they are the trajectories particles follow, but QM tells you (regardless of interpretation) that there isn't a uniquely (and in advance) determined trajectory anything will follow.

Terry Bollinger

@Jiminion That's the SR/GR issue with any singular "now". Fascinating, fascinating issue, and one in which you must be very careful not to violate SR/GR when defining state.

Jiminion

@TerryBollinger Put of fact: I'm WAYYY out of my league here....

Terry Bollinger

@ACuriousMind Good points, not easily resolved...

John Rennie

5:33 PM

Wow, 17:30, I have to go. Bye all.

Terry Bollinger

@ACuriousMind ... though actually ... if wave packets are real (Ringbauer again, can you tell I'm plugging his papers?), then it's the wave packets that follow geodesics... there are interesting implications to that...

@JohnRennie Hmm, same here, I'm burned out anyway. Great session, bye all!

ACuriousMind

@TerryBollinger I can't tell because I have no idea who Ringbauer is :)

Alas, bye!

Kyle Kanos

Martin Ringbauer?

glance

@TerryBollinger that's a pretty cool idea. It really rises the problem of defining what "time" is though. In a way, it's simply a parameter summing up our knowledge of the way physical states evolve isn't it? Allowing "retroactions" in time isn't that bad if we just admit that that parameter $t$ we've always used to describe the changing of states, i.e. the EOM themselves that we know up to now, are incomplete

Jiminion

Yep. He looks like he's about 12.
http://www.researchgate.net/publication/269877163_Measurements_on_the_reality_of_the_wavefunction

Danu

5:59 PM

@ACuriousMind I have such a hard time imagining the fact that general covariance used to not be a vital requirement for any real theory

JiK

@TerryBollinger I wish this question was addressed more often when talking about QM and GR.

Danu

General covariance is such an incredibly beautiful idea, philosophically.

ACuriousMind

@Danu It's an infinte-dimensional symmetry, so it is really a strong constraint.

The only quantum theory with infinite-dimensional symmetry I know is 2D CFT, and it is essentially solved by just thinking about the symmetry

Danu

Yeah, but just on philosophical grounds!

ACuriousMind

@Danu With that attitude, you should become a string theorist ;)

Danu

6:03 PM

I've said before that if string theory is as nice geometrically as some say it is, it may be worth investigating purely for the mathematics.

Jiminion

6:13 PM

@Danu Woit thinks ST is inelegant, I think because of the infinite universes. So I guess elegant is as elegant does.....

ACuriousMind

Ontology again! That's nothing to do with mathematical elegance

Jiminion

6:31 PM

elegance != beautiful?? I thought they were equally subjective terms used to further an viewpoint in lieu of tangible evidence or reasoning.

ACuriousMind

@Jiminion Nah, but the "universe" complaint is physical, and about the consequences (and I don't really understand what it's supposed to mean, either, because there are also infinitely many possible QFTs, and infinitely many possible GR universes). That doesn't change that the premise of the theory is beautifully and elegantly simple

Stan Shunpike

Why do there exist theories in ST, QFT, etc that posit fewer than four dimensions. I have seed this where people consider 1+1 dimensional theories and so on. Doesn't that seem silly if we think four exist already?

Danu

@Jiminion No, this is totally physical, and I don't care all too much about that

@StanShunpike Because you can get more done in them, sometimes?

ACuriousMind

@StanShunpike Because a) these theories are easier to solve and hence good for "training" (even if you're a researcher and not a student) and b) some systems are effectively described by less than three spatial dimensions

E.g. many topological ideas are easier to see in lower dimensions because you don't have as many particle excitations contributing.

Or, the continuum limit of the Ising model is described by a 2D CFT

Stan Shunpike

Ah, makes sense. I suspected something like that. But I just wanted to be sure.

@Danu What is the funky looking object in your profile picture?

Danu

6:46 PM

@StanShunpike Some manifold

ACuriousMind

@Danu Very specific ;)

Jiminion

@Danu It looks like one of those plastic coin purses from long ago. I wonder if they were manifolds....

Kyle Kanos

Everything is a manifold

Jiminion

walletgear.com/plastic-squeeze-coin-purse.html#.VOzIAvnF-oc

If everything is a manifold, then the term has no meaning.

user54412

@Qmechanic Ooguri taught me my very first, low-level intro to QM, which used Liboff. Later on he taught advanced mathematical methods of physics, which used some of Nakahara and a few other references, but didn't follow any book too closely.

Danu

6:53 PM

@ACuriousMind I don't know more either :\

user54412

I also remember not following Liboff much either. We never managed to convince Ooguri that in fact we did not already know Lagrangian/Hamiltonian classical mechanics, as he "reviewed" that entire subject in 1 lecture, and then drew on those topics for the rest of the term.

user54412

It was quite fun :)

user54412

@Danu Yes, we have had this conversation before. You worked with Daan Meerburg, right?

vzn

re ringbauer, thx for the ref, interesting. hes working with cavalcanti, who is also in the news lately.

ran into this other interesting paper that cites the new ringbauer paper.

Danu

@ChrisWhite Yush

vzn

6:57 PM

No Return to Classical Reality jennings/ leifer

Danu

@ChrisWhite Is he still working there?

user54412

I haven't seen him in a while. Postdocs tend to cycle quicker than grad students, or to be gone for months at a time giving job talks.

Jiminion

@ChrisWhite Thank you! I couldn't remember my QM book, but Liboff jogged my mind. It must have been the 1st edition (Grayish blue). I did horrible in the class and couldn't figure much out.....

user54412

@Jiminion You're the only other person I know who used Liboff. Everyone else talks about Griffiths.

Danu

@ChrisWhite Never even heard of Liboff, haha

Jiminion

7:04 PM

He didn't follow the book much either. I can't remember the prof.'s name.

vzn

7:23 PM

wrt complaints about "realism" here, a notable sentence from ringbauer/ cavalcanti abstract

> Assuming that some underlying
reality exists, our results strengthen the view that the entire wavefunction
should be real.

Jiminion

@vzn We think, therefore we are.

Mark Mitchison

@Jiminion The question is whether there is anything real outside your mind.

ACuriousMind

@MarkMitchison The question could also be if that even matters ;)

Mark Mitchison

Sounds like a question for somebody else :)

But I was just pointing out that Descartes has nothing to say about modern debates over realism

user54412

@Jiminion Yes, but does the wavefunction think?

ACuriousMind

7:34 PM

@MarkMitchison Well, that's certainly true

vzn

more "philosophy virus" :( :p

2 hours ago, by Terry Bollinger

@ACuriousMind The philo virus, aieeee!

2 hours ago, by ACuriousMind

Much talk in this room lately about reality and truth. Did someone catch a philosophical virus? ;)

maybe philosophy is the last refuge of scoundrels :\

user54412

@ACuriousMind Something I've idly pondered for a long time, and found surprisingly difficult to get direct answers to: Is QFT on a general, dynamical spacetime self-consistent?

user54412

I can imagine three answers: yes, always; no, not at all; or yes, but only if you ignore the backreaction of the QFT stress-energy on altering spacetime

Danu

@ChrisWhite It has not been resolved yet. I wrote my thesis on semiclassical gravity

No one is able to take into account backreaction

but QFT on a fixed background 'works'

user54412

But can any of my alternatives be ruled out?

Danu

7:48 PM

not at all is ruled out

user54412

Ok, that makes sense. Otherwise even the Unruh effect and Bogoliubov transformations would be suspect, I guess.

Danu

It is widely agreed upon that results such as Hawking radiation are genuine

and yes, Unrun effet (basically the same thing as Hawking radiation anyways)

user54412

I wonder if a person's choice of Hawking/Unruh says something about whether they're the quantum sort or the GR sort

Danu

Hmmm perhaps

Unruh being quantum, I assume

ACuriousMind

@ChrisWhite Are you thinking Hawking=quantum, Unruh=GR?

lol

user54412

7:52 PM

To me, Hawking radiation is this weird thing where you start with some quantum field doing its thing, and you just happen to have an event horizon that discriminates between two classes of future timelike infinity trajectories

user54412

While Unruh is simple for us GR folk: just take what QFT people say is vacuum, frame transform it, and give it back to the QFT people, who interpret the result as a thermal bath of particles

Danu

Oh, okay

To me though, Hawking as a person is famous for GR

Mark Mitchison

Dunno, I'm not a GR guy and I find the Unruh effect much easier to understand!

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