The h Bar

Sir Cumference

02:43

@RyanUnger woah you're back

how's life

bolbteppa

03:48

pbs.twimg.com/media/F_dssTiXIAAko-5?format=png

naturallyInconsistent

@Mr.Feynman It cannot work out. Simply plotting it out, the shape of the graph is just wrong, and one of them even has a singularity at origin.

PM 2Ring

04:10

@bolbteppa Nothing beats the opening lines of "States of Matter", by D.L. Goodstein:

Mar 25, 2016 at 19:58, by ACuriousMind

Reminds me of this quote: "Ludwig Boltzmann, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics. Perhaps it will be wise to approach the subject cautiously."

bolbteppa

04:21

That guy is great

PM 2Ring

Here's his Wikipedia page: en.wikipedia.org/wiki/David_Goodstein

user400188

06:05

What to do when someone asks for intimidate steps in a question, but the working is several pages long even when it has been simplified down to make it quicker and easier to read?

Do I still include it, or do I leave a lot of it out?

naturallyInconsistent

06:17

You have to make a judgement. Is the question homework-like? Is the difficulty of the question research-level or school-level? Will the solution make sense if you merely post the important milestones?

user400188

@naturallyInconsistent It's a question about an equation in a research level textbook (the bible in my field of study). However the level of math isn't very hard. It's just that there are a lot of constants which need to be defined and rearranged to get it into the final form, and doing so takes a while. Just defining the equation and constants within it, as well as the final answer, has taken over a page already.

I worry the question will become unreadable to most if I include too much more.

naturallyInconsistent

Wait wait, is this a question on the main page? Or are you asked for something in academia? Can you put it as an appendix in a paper? Or link to it?

user400188

Question link is here: physics.stackexchange.com/questions/790458/…

It's nothing special, just asking for an explanation for a few lines that appear in the textbook. I think Gec might have already answered it, although I'm looking at the way the book has been worded and it appears that the quantum pressure is not the kinetic energy of the gas, it's just labeled the same way.

But it's kind of hard to tell.

naturallyInconsistent

06:54

Oh, it is very niche. You are asking for it yourself and you are answering it yourself; feel free to go and collect your own bounty, lol

Ryder Rude

08:06

hi

what r the most interesting theories of quantum gravity

John Rennie

@RyderRude Define interesting :-)

While it's not a quantum theory (yet) Verlinde's entropic gravity idea seems the most interesting to me.

Ryder Rude

@JohnRennie i was looking for something which modifies our understanding of time

@JohnRennie wiki says this makes a prediction that Newton's gravity eqn fails at larger distances

John Rennie

08:22

A number of QG theories suggest that spacetime is two dimensional at the Planck scale. That sounds interesting :-)

(It also sounds like a load of bollocks :-)

Ryder Rude

is that the Holographic principle or something else?

i think this is different

@JohnRennie yes, it's speculations. some others suggest that spacetime becomes euclidean at small distances

John Rennie

@RyderRude Like the Hawking Hartle solution?

Ryder Rude

a lot of people have to throw darts in the dark until something lands and becomes physics @JohnRennie

John Rennie

True, and who knows what crazy idea might prove to be correct?

Ryder Rude

@JohnRennie havent seen it yet. but yes, Hawking was into imaginary time too becuz it made the metric look euclidean

John Rennie

08:27

For the record I have no problem with theorists investigating even the wildest of ideas. The money required to fund theorists is a tiny fraction of what the LHC costs and you never know when some lunatic theorist might be proved right.

@RyderRude Imaginary time is a widely used computational device. Google Wick rotation.

Ryder Rude

i once saw this sentence in an article: 1s = 3x10^8 $\sqrt {i) m$ or something like that :P

@JohnRennie yeah. it's fine as a computational device. Hawking thought of it as something deep

@JohnRennie yeah. people have to throw darts in the dark until something lands by luck

physics is not a one person game. a lot of people have to build it

John Rennie

@RyderRude See here

Ryder Rude

Hawking speculated a lot of things. he may have also abandoned imaginary time later

John Rennie

It's on the Arxiv: The Small Scale Structure of Spacetime

Ryder Rude

@JohnRennie one reason for this speculation cud be that renormalizability is easier in lower dimensions

@JohnRennie thanks

John Rennie

08:36

I don't think the idea went anywhere useful, or at least I haven't heard anything about it recently.

Ryder Rude

they r speculating a dimension collapse to get to 2D

@JohnRennie oh

still, it is great to speculate all possibilities

John Rennie

QG seems to have gone out of fashion just recently. I suspect all the avenues that looked promising have ended up leading nowhere useful and no new ideas have emerged.

Ryder Rude

it's just not its time yet. it will remain niche until experimental discrepancies from standard model come along

John Rennie

For the record my theory is that gravity is emergent so quantising it makes no more sense that quantising the Navier-Stokes equations.

Ryder Rude

lol

i think of it as fundamental because it's very cool that way

since metric is time, do u think time is emergent?

or do u think there is an underlying Minkowski time and it's just that the perturbation time is emergent? @JohnRennie

Rovelli also says that time is temperature

Slereah

08:42

@JohnRennie Hawking Ellis' arch enemy

John Rennie

My theory, which I have absolutely no idea how to write in a concrete way, is that there is some form of super space that contains all possible states of the universe, and what we think of as time is just the parameterisation of some trajectory through this space.

In principle any trajectory can be followed, but only certain trajectories give rise to the phenomenon we call conscious thought.

Ryder Rude

oh

John Rennie

So we observe certain correlations that we interpret as laws of physics because only if those correlations exist can we exist as conscious beings.

@Slereah Ah, I didn't know :-)

@RyderRude I call this anthropic principle++ :-)

Slereah

@JohnRennie just because of the title :p

John Rennie

Ah, of course! doh!

Slereah

08:47

idk who was Hawking's arch nemesis

Probably someone, emotions run high in theoretical physics

Ryder Rude

so ur theory shud derive the known laws of physics by taking the superpace and the conditions under which a trajectory can have consciousness

John Rennie

Yes

Ryder Rude

this addresses both ontology and epistemology

John Rennie

My theory is of course a prime example of the complete bollocks approach to theoretical physics, but it amuses me to speculate about it.

Ryder Rude

the superspace is the ontology and the consciousness condition is the epistemology

John Rennie

08:50

And who knows, some day it might turn out to be right. I should patent it just in case.

Ryder Rude

:)

i personally dont believe in an ontology anymore

but it still has a good chance of being true

@JohnRennie is this superspace the classical space or the quantum space?

u can talk bout trajectories in the former but harder in the latter

i guess u also plan to quantise this later if it's classical

John Rennie

Don't know. I have yet to work on that aspect of the theory :-)

Ryder Rude

is consciousness fundamental in this theory or just some emergent field/particle configuration that can be called intelligent life? @JohnRennie

John Rennie

I think consciousness is emergent.

Ryder Rude

this choice would severely affect the conditions of consciousness

@JohnRennie oh

the conditions would be easy to formulate in the former choice, but u've got a lot of work to do in the latter ;)

i dont advocate either choice myself

but the former choice would surely be rejected as crackpottery, so good u r on the latter :P

@JohnRennie i also had a similar idea a while ago. the idea was that time is in general undefined in quantum gravity, and consciousness can exist in situations where it becomes well defined

cuz the metric becomes probabilistic, so no time

in this idea, we shud b able to derive the known quantum theory in the conditions where time exists

John Rennie

09:09

@RyderRude It is very unlikely that only a single trajectory is consistent with consciousness.

There will be many such paths, and we can find the expectation value of the conscious observer by integrating over them.

We could call this ... hmm, let's see ... a path integral!

I wonder if you could formulate QFT using such an idea ...

Ryder Rude

this is pretty wild

it's bordering on panpsychism :P

@JohnRennie what r ur views on qualia?

i mean the hard problem, sorry

John Rennie

In general I am uninterested in anything I cannot write down as a mathematical model. Occasionally I surf the philosophy SE out of idle curiosity but I invariably leave shaking my head.

Ryder Rude

lol

John Rennie

This is a personal view not an opinion on the usefulness or otherwise of philosophy as an occupation.

Ryder Rude

yes, i understood

John Rennie

09:24

That's the disclaimer you understand, I actually think most philosophers should get a proper job :-)

PM 2Ring

IMHO, John's idea kind of needs some degree of panpsychism. When actual conscious entities finally evolve they find themselves in a universe that has momentum in a consciousness-friendly direction. But that's because it was also friendly for the baseline minimal consciousness of raw matter.

John Rennie

One day they will be building statues of me!

Ryder Rude

@JohnRennie they would rather debate about what's a proper job :P

(just a joke). no hate on philosophy

PM 2Ring

Perhaps big collections of matter have more consciousness. But to get high level consciousness you need structure. And some kinds of structures are more supportive of consciousness than others.

Ryder Rude

Chalmers also speaks of this idea @PM2Ring

PM 2Ring

09:30

Perhaps a star can think "Let there be light". But maybe it can't have deep & meaningful conversations. ;)

William Martens

Just a kind message, I hope everyone is all well! WIshes from Sweden ^_^

Slereah

why do people always think that if objects have consciousness that they will automatically think that the main thing that we see them do will be what they think about

or what they are useful for

Do you think about how your body is metabolizing sugar

PM 2Ring

Fair call

@Slereah Depends how hungry I am.

2

Slereah

Toys come to life always wants to be played with which sounds like a very optimistic way of seeing it

Which I guess is a more pleasant outcome than toy slavery

Basically this

Maybe all objects are constantly in torment

PM 2Ring

I really enjoyed that classic Olaf Stapledon novel (Star Maker?) where all the stars are conscious. He did it so well that not many other writers have attempted to explore that theme.

Slereah

09:37

Didn't they

Usually the trope is more conscious planets but it is the same principle

PM 2Ring

I suppose so.

John Rennie

I read First and Last Men as a boy and absolutely loved it for its amazing scope. I have to say I found Star Maker hard work.

PM 2Ring

The core theme is that the stars are the dominant consciousnesses, the rest of us are just tagging along in the environment they've created.

Sir Cumference

that a sci-fi or fantasy book?

Slereah

It's a historical book

Sir Cumference

09:41

guess i still have a lot to learn about stars

John Rennie

@SirCumference SF, though it's somewhat fantastic SF.

Sir Cumference

i don't think I've ever actually read a sci-fi book. guess i could give that a try

PM 2Ring

Well, they're sci-fi fantasy. They're from the early 20th century. They don't get into details of technology, and the travel & communication are more like teleportation / astral travel / telepathy.

Slereah

@PM2Ring was that the 60's

John Rennie

@SirCumference I wouldn't start your adventure into SF with Star Maker!

Slereah

09:45

1937

Pretty early

But I guess that was also basically the plot of van Vogt's books

Sir Cumference

@JohnRennie I'm open to suggestions

I think the only sci-fi story I've really given a try is star wars, and that was only recently

good movies though

John Rennie

Try Ancillary Justice by Ann Leckie.

A good example of how great modern SF can be!

Sir Cumference

oh neat, it's a shared universe

I'm into those kinds of stories

PM 2Ring

@JohnRennie Well, he was a philosopher. His novels were a way of tricking people into reading his ideas. ;)

I guess thst description applies to a lot of authors...

Slereah

@PM2Ring what do you think the myth of Atlantis was

PM 2Ring

09:57

I think it was about a real place, but the story got garbled and expanded over time.

Most likely a volcanic island in the Mediterranean that exploded. Like Santorini.

Slereah

Maybe, or just made up

PM 2Ring

Santorini

Santorini (Greek: Σαντορίνη, pronounced [sadoˈrini]), officially Thira (Greek: Θήρα Greek pronunciation: [ˈθira]) and Classical Greek Thera (English pronunciation ), is an island in the southern Aegean Sea, about 200 km (120 mi) southeast from the Greek mainland. It is the largest island of a small circular archipelago, which bears the same name and is the remnant of a caldera. It forms the southernmost member of the Cyclades group of islands, with an area of approximately 73 km2 (28 sq mi) and a 2011 census population of 15,550. The municipality of Santorini includes the inhabited islands ...

Slereah

It was just a place for Plato to say that his ideas were cool and good

John Rennie

I suspect sunken cities is a trope that has existed in myths for as long as cities have existed.

It's just that most of the stories were never written down, or were written down then lost.

Slereah

People are somehow very willing to believe that stories of the ancient world weren't entirely made up for some reason

Which was an attitude people already mocked back then

PM 2Ring

10:01

Maybe. But that sort of thing works better if you attach it to a place that people have already heard of, rather than something you fabricate from scratch.

Slereah

ie. en.wikipedia.org/wiki/A_True_Story

PM 2Ring

In the southern hemisphere, we have the (mostly) sunken continent of Zealandia. en.wikipedia.org/wiki/Zealandia

Slereah

Islands do disappear, but that's a pretty far cry from saying that Atlantis is in some specific way based on a true story

Unless you go by the definition of "based on a true story" that horror movies do

The Babadook was born in Atlantis

PM 2Ring

Oh, I wasn't claiming that the civilisation on Atlantis was anything like the stories. Just that there was an island that blew up.

Slereah

> Iambulus also wrote many strange miracles of the great sea, which all men knew to be lies and fictions, yet so composed that they want not their delight: and many others have made choice of the like argument, of which some have published their own travels and peregrinations, wherein they have described the greatness of beasts, the fierce condition of men, with their strange and uncouth manner of life
but the first father and founder of all this foolery was Homer's Ulysses, who tells a long tale to Alcinous of the servitude of the winds, and of wild men with one eye in their foreheads that…

He knew it was all bogus

Mr. Feynman

11:11

@naturallyInconsistent thanks for fixing the link :P

naturallyInconsistent

12:54

np, Mr Feynman

Mr. Feynman

15:48

In the Ising model the classical variable $\sigma_z=\pm1$ is replaced by the Pauli matrix $\hat{\sigma}_z$. Why is that in the case of the quantum rotor the classical dipole $\vec{n}$ remains $\hat{\vec{n}}$ instead of $\vec{\sigma}\cdot\vec{n}$?

Ryder Rude

16:32

@Mr.Feynman hi. in the latter model the $n$ is analogous to the position degree of freedom. they have a separate angular momentum observable there

ACuriousMind

@Mr.Feynman The quantum Ising model is not the quantization of the classical Ising model, while the quantum rotor is the quantization of the classical rotor.

Ryder Rude

i think the latter model is doing something like representing the $[\theta, L]=i$ algebra, while the former model merely represents the rotation algebra @Mr.Feynman

so there exists no position degree of freedom in the former model

one important thing to note here is that the state of a classical rotor is not fully specified by its angular momentum. because a rotor has a spatial extent, u can also measure angular position wrt the three axes

but the state of a spinning point particle is fully specified by its angular momentum. so u only need to represent the SO(3) algebra

Ryder Rude

16:54

one can wonder "what if elementary particles have an internal angular position degree of freedom but we just dont know how to measure it?"

however, either of field quantisation or Wigner's classification does not give that degree of freedom

but molecules do have this degree of freedom, as they can be modelled as rotors

so if were to model molecules using an effective QFT, the field would need to have this internal degree of freedom?

i think this case is just of an infinite dimensional internal degree of SO(3) freedom, which is a reducible representation. so Wigner's classification technically does give this

Mr. Feynman

17:10

@RyderRude yes, I know that

@ACuriousMind ok, the second sentence is very helpful. In the former I think you refer to the fact that the quantum Ising model is related to the classical model by that dimensional crossing stuff and so on

But then, I have some questions: what would be the quantization of the classical Ising model then? I guess it would make no sense

ACuriousMind

@Mr.Feynman The classical Ising model is not really a "classical mechanics" system to which you can apply canonical quantization - classical mechanics doesn't really have discrete variables

while the quantum rotor is just the canonical quantization of a classical rotor

Mr. Feynman

@ACuriousMind As I imagined. Now, what if I consider a rotor on a $N$-sphere. $N=1,2$ gives the traditional rotors. What about $N=0$, isn't that Ising?

ACuriousMind

In some sense, yes

Mr. Feynman

So it makes sense to think of the rotor as a generalization of Ising

Although one has to be careful

So to wrap it all up: from which classical system, if any, does the quantum Ising chain come?

By that I mean which system you quantize

ACuriousMind

none

there's no position variables here, it's just a bunch of qubits

qubits are not the quantization of any classical system

Ryder Rude

17:23

u can arrive at qubits by quantising the S^2 system

Mr. Feynman

@RyderRude that's a rotor isn't it?

Ryder Rude

it is the Poisson bracket algebra $[l_x,l_y]=l_z$

@Mr.Feynman no it's just a rotation algebra. no position variable

fqq

Also note that unless you add a transverse field, quantum Ising and "classical" Ising are exactly the same model

Mr. Feynman

Oh yes you're right, because only $z$ eigenstates pop up in that case

@ACuriousMind I'm confused as to whether I should know this from QM

ACuriousMind

@Mr.Feynman I mean more or less by definition a classical Hamiltonian system always has variables $[x_i,p_j] = \delta_{ij}$

so a quantization of such a system can never have a finite-dimensional Hilbert space

Ryder Rude

17:30

yes, but the S^2 phase space does not qualify that @ACuriousMind

Mr. Feynman

@ACuriousMind Ok, that's what I was thinking. It's not about qubits specifically, it's an issue of finite dimensional spaces

Alright, I understand now

Ryder Rude

18:00

there is this cool thing that the Heisenberg algebra does have finite dimensional reps but these r irrelevant for QM en.m.wikipedia.org/wiki/Heisenberg_group

these matrices r not symmetric

this is y the SvN theorem has to specify unitarity

Sanjana

18:47

A friend just shared a question where the Lagrangian of a harmonic oscillator is written in a mysterious way in terms of error integrals $$L=e^{-x^2}(e^{-\dot{x}^2}+2 \dot{x} \int_0^\dot{x} e^{-y^2}dy)$$
While it is straightforward to verify directly by EL eqns, can this be seen in a different way---intuitive/qualitative...you name it...also, is this form well known/useful or "natural" in some context or is it just some crazy question-setter playin' with symbols and coming up with a beauty?

Giorgi Lagidze

20:02

hey all. out of curiosity, i was playing with lagrangian and imagine as $L = 5\dot x^3 + 4x$ and i want to check it in translated frame(i.e x = -x') so $L' = -5\dot x'^3 - 4x'$. If you use EL on each of them, you get the same EOMs. but if you do $L' - L$, you get $-10\dot x^3 - 8x$. Since we got the same eoms for $L'$ and $L$, doesn't that mean that $L'$ and $L$ must differ with $\frac{d}{dt}f(x, t)$ ? If so, I can't find a function whose derivative is $-10\dot x^3 - 8x$.

ACuriousMind

@GiorgiLagidze No, there's two options: Lagrangians with the same E-L differ by a total derivative or they're multiples of each other, $L' = \lambda L$. You have the latter case, $\lambda = -1$.

Giorgi Lagidze

@ACuriousMind awesome. that's why I get the same eoms then. it's the latter case of your 2 options. Thank you <3

btw, I might ask this: $L = 5\dot x^3 + 4x$ imagine such lagrangian. this obeys homogeneity and isotropy both, right ? @ACuriousMind to me it does

ACuriousMind

I don't know what you mean, neither $\dot{x}^3$ nor $x$ is invariant under rotations

in fact those aren't even scalars in more than 1d

Giorgi Lagidze

it's not generalized coordinates, it's 1D (i.e. one particle)

ACuriousMind

I don't know what "isotropy" is supposed to mean in this case

why do we have to use these silly words instead of just stating the symmetry groups we're talking about?

Giorgi Lagidze

20:09

ah, eoms being the same under rotation and under translation

ACuriousMind

@GiorgiLagidze ...and what is a rotation supposed to be in 1d?

Giorgi Lagidze

180 degrees

ACuriousMind

so what's your question?

Giorgi Lagidze

$L' = -5\dot x^3 - 4x$ , so this got the same eom as well as if you translate, $L' = 5\dot x^3 + 4(x'+k)$, the same thing

just wanted to double check that eoms are the same under rotation and translation. that's all

ACuriousMind

you literally just told me that you showed you got the same e.o.m. for $L$ and the 180° flipped version

Giorgi Lagidze

20:11

yes, i am doubting myself sometimes :D

Thank you <3

Obliv

I am clueless when it comes to bio/chem but while reading about hadean rocks containing traces of carbon, I wonder what are the prevailing theories on the origin of life not only on earth but on planets in general.

Is it just something that emerges from specific conditions

i.e having a certain composition of elements, temperature, processes, and time it is almost like a statistical result

ACuriousMind

@Obliv the buzzword is abiogenesis

Obliv

Surely there are planet formations around us that we can observe, to better understand the conditions of early earth?

I guess that's the best we can do, that and observing old rocks..

ACuriousMind

@Obliv ...where do you think those are?

Obliv

sigh the fermi paradox makes it seem like observing the planets around us won't do any good

In nearby star-systems?

Or far-away

ACuriousMind

20:16

@Obliv so what do you think we can see here over those distances?

It took us until 1992 to even have the first confirmed detection of an exoplanet

Obliv

I'm not entirely sure but can't we discern composition/temperature etc from interferometry?

ACuriousMind

like, before that no one could even definitively prove there were any other planets in other star systems

Obliv

Oh..

Well theoretically if we were able to visit other planets during their formation, and gather samples, we could see just how common early life is.

ACuriousMind

@Obliv You probably mean spectroscopy, not interferometry, but sure, we can make a good guess at chemical composition and surface temperature but not much else

Obliv

I imagine it's pretty common but the evolution aspect is very rare.

ACuriousMind

20:19

We literally have no evidence at all for life outside of earth

Obliv

Wait didn't we find fossils on mars?

ACuriousMind

???

Obliv

i guess not lol

Well, if life did exist on mars, would fossils even be able to be preserved

ACuriousMind

Fossils on mars would've been the discovery of the century

Obliv

Well there is so much trapped water that I just assumed it should have existed at some point. If it could exist on earth, that is.

but that's an assumption that it was abiogenesis through chemistry

I don't see any reason to assume otherwise

ACuriousMind

20:22

why?

naturallyInconsistent

@ACuriousMind did we not find some tardigrades in space? Or did they only just survived space conditions? yawn

ACuriousMind

we literally have no real idea how likely it is that life evolves, even under Earth-like conditions

@naturallyInconsistent pretty sure we shot them into space ourselves

Obliv

because I'm not particularly religious so I'd think of life being something that happens "naturally"

ACuriousMind

the sample size for "planets on which we know life evolved" we have is 1, but the sample size for "planets with similar conditions" we can look at is also 1

there's just no way to infer anything about the likelihood of life either way

Obliv

it just boggles my mind that fossils of single cell organisms via traces of carbon existed in rocks ~4 billion years ago.

ACuriousMind

20:24

maybe every earth-like planet has life, maybe we're the only one - if we're being honest we just don't know, and it's unscientific to pretend otherwise

Obliv

Perhaps even that form of life is super rare, so evolving beyond it would be even rarer still.

Mr. Feynman

@ACuriousMind what about the Covenant? Not to mention the few Forerunner survivors after the Halos were fired

I hope they are still alive in some galaxy out there

Obliv

I wonder what a scientist feels like, when they find traces of life in 4 billion year old rocks lol.

I mean, without radiometric dating, we didn't even know how old anything was.

Mr. Feynman

If there had been any civilization as advanced as us on Earth, gone extinct - say 5 billion years ago - would we be able to tell?

ACuriousMind

@Mr.Feynman that would be very impressive, given that the earth is just 4.5 billion years old

and it was a molten rock for like the first few hundred million

Obliv

20:40

Apparently we don't know exactly how the moon was formed, but a theory is that some giant object hit earth and knocked off the chunk that is the moon. This would explain why moon rock is so similar to earth, and why it's made mostly of high silica surfacey rock. Perhaps the rock that hit earth might still be in the lower mantle, slowly melting.

Old oceanic lithosphere that subducts into the mantle takes a long time to melt so it could be the case that the rock is still there.

Mr. Feynman

@ACuriousMind Oh right, I didn't take into account the age of the solar system. :P

Let's say two billion then

Obliv

we'd have fossil records.. @Mr.Feynman

unless they cleaned up after themselves and launched off into space

ACuriousMind

@Mr.Feynman that's still long before the first evidence we have for multicellular life

Obliv

but I'm a complete newbie in geology, I'm sure it's theoretically possible if the fossils weren't preserved. Fossils are quite delicate

ACuriousMind

like it's just a bunch of bacteria and algae back then

Charlie

20:43

At some point someone hanging around in a deep enough cave would accidentally connect to their wifi

ACuriousMind

the earliest plants and animals we know of are less than 1 billion years old

Obliv

Oh wow.. wonder why it took so long

Mr. Feynman

@ACuriousMind I'm not sure if with this you are excluding it or saying we can't tell

ACuriousMind

@Mr.Feynman I'm saying there would need to be a large part of development of advanced life forms that left no trace at all in the fossil record

which isn't impossible but it's pretty unlikely

Obliv

I wonder if the earth's magnetic field has anything to do with life forming, besides deflecting solar radiation.

Like if it inherently affects the necessary conditions for formation & evolution

Mr. Feynman

20:47

I agree it is unlikely. It was cool sci fi material though :P

Obliv

Some animals apparently use it for guidance, but I'm not sure how..

Does finding carbon on a planet suffice to show life existed? Isn't it just an element.. that could be formed through nuclear processes?

ACuriousMind

@Obliv the thing is that all life we know is based on the kinds of molecules you can form with carbon

it's not that carbon is evidence for life, it's that absence of carbon is evidence for the absence of life (as we know it)

naturallyInconsistent

What do you mean, you are not sure how? We have discovered quite a lot of biological mechanisms that exploit quantum mechanics to detect stuff. Even smelling has something to do with resonances in quantum theory. It is very fun. Nature had simply co-opt-ed some small detectors into biological processes and use them for detecting magnetism.

Obliv

Until of course, we create artificial computer life lul

@naturallyInconsistent wow, did not know that..

naturallyInconsistent

Ah, it is about not knowing? There are many exciting viral videos on animals detecting magnetism. An example is that dogs seem to align to the North-South line when pooping. It is the kind of "WTF are these scientists drunk" kind of experiments that makes us do a double-take.

But things like flight of birds and insects, etc, kinda make it obvious that magnetic guidance could be evolutionarily selected for.

Obliv

21:11

@Mr.Feynman this video is a really nice brief introduction to the geologic time scale if you're interested.

Sanjana

21:48

I have a feeling that all proofs of any theorem are equivalent at a formal level if the same axioms are assumed. Is there a "formalization" of some sort of this thought formal logic?

ACuriousMind

@Sanjana They are not; there is a subfield of reverse mathematics concerned with figuring out which axioms are actually necessary for the proof of a theorem and which are just convenient

Sanjana

Okay, so given one has proved somehow that we are starting with a minimal set of axioms, then is it true that all proofs are sort of equivalent or something?

ACuriousMind

what do you mean by "equivalent" in that case?

Sanjana

I mean, that although when we write them at the level of rigor of a mathematician who is not a logician also, then there can exist two or more proofs that seem different superficially but if we write those proofs using formal logic (you know something like what Whitehead and Russell did...), then we can see that they are exactly the same, not just because they are proving the same thing but the "logic" is also the same...

I apologise that I am handwaving because simply I am new to this stuff and I don't know anything. But please enlighten me if you get even a bit what I am trying to hint a…

ACuriousMind

there is no general notion of two proofs being "the same"

see mathoverflow.net/q/3776/157071 for discussions of this concept

Sanjana

22:00

This was the kind of thing I was looking for...thanks. Is there any other buzzword/wiki page/book/paper you would add to this...introductory stuff...

homotopy turns up mysteriously!

ACuriousMind

I mean, not mysteriously :P

the homotopy theorists more or less have an explicit programme to put every possible notion of equivalence/"sameness" in homotopy theoretic terms

Sanjana

Oh...You mean to say that it's not just a thing of curves and spaces but more general stuff?

ACuriousMind

yes

this "homotopy" is not the topological notion you're thinking of

Sanjana

Oh, that's why I said mysteriously perhaps...

This is another world...I saw that video by Frenkel bolbteppa pointed out a few days ago. It was full of crazy content I wasn't aware of before...I mean I have rarely seen this type of stuff appearing in physics.

ACuriousMind

there's a lot of math you generally won't see in "normal" physics

Sanjana

22:09

Hmm...In that video Frenkel says that mathematics is subjective and there's a measurement/observer dependency in math---the math depends on the mathematician's choice of axioms and that in turn depends on his own choices...

I didn't find any reference on this either. I guess it wasn't about ZFC or axiom of choice stuff cz he talks about it kinda separately and he says that he realised this "recently". Pretty sure a mathematician of his stature would know about that from an early age...

@ACuriousMind Did you see that video?

ACuriousMind

I have no idea what video you're talking about :P

Sanjana

Edward Frenkel: Infinity, Ai, String Theory, Death, The Self

►

This one

bolbteppa

His comments about string theory are pretty ridiculous

ACuriousMind

I'm not watching a 3 hour video with a title that pretentious :P

Sanjana

oukay...btw the measurement problem thing starts in about 33 mins and goes about 7 mins. Have a peep if u wish...

bolbteppa

22:12

I didn't listen to the rest, but he is basically using one comment by one guy as a vehicle to vent his feelings about the whole field

Sanjana

@bolbteppa What I gather is that he is saying that string theory promised us a clean derivation of standard model but it didn't deliver that in time, and instead moved on to holographic stuff and big people act like there's still hope which misguides younger souls!

naturallyInconsistent

you should note that some of the people you might be talking to are die-hard proponents (or detractors) of string theory.

PM 2Ring

@Sanjana Take a look at Kolmogorov complexity en.wikipedia.org/wiki/Kolmogorov_complexity Also,

Berry paradox

The Berry paradox is a self-referential paradox arising from an expression like "The smallest positive integer not definable in under sixty letters" (a phrase with fifty-seven letters). Bertrand Russell, the first to discuss the paradox in print, attributed it to G. G. Berry (1867–1928), a junior librarian at Oxford's Bodleian Library. Russell called Berry "the only person in Oxford who understood mathematical logic". The paradox was called "Richard's paradox" by Jean-Yves Girard. == Overview == Consider the expression: "The smallest positive integer not definable in under sixty letters."Since...

naturallyInconsistent

I suppose we should all be grateful that the religious wars in science are so peaceful; we might wish for each other to die, but we will always be happy to leave it to waiting.

Maybe some (radio-)chemical concortions, but no abuse of physical implements.

Obliv

by religious wars in science are you referring to proponents and detractors of a theory? Or religion vs science in general? (The latter was pretty bloody)

Sanjana

22:22

@naturallyInconsistent that's the point :p To know their views. tbh I am not a sadist but I feel really thrilled when experts start quarreling about some stuff. Since they are experts, they won't say total bogus and it's a both way win for someone ignorant like me...I gain knowledge!

@PM2Ring Thanks

Obliv

how much chem/bio would I need to know to understand articles in areas like paleontology? I'm guessing not much, perhaps just organic chemistry 1 & 2?

ACuriousMind

@Obliv why are you asking this to a bunch of physicists :P

Obliv

I figured maybe some of you have a side hobby in related fields :P

but yeah I should probably ask people in the geological sciences building at my school

naturallyInconsistent

@Obliv obviously not the actual religions.

Obliv

but I can't imagine a physicist doesn't know any other sciences.. I know we've become super specialized over the years but yeah compulsory education + natural curiosity makes me think it isn't that uncommon

naturallyInconsistent

22:26

@Sanjana scary sadist confirmed

Sanjana

@naturallyInconsistent I once wished for a long break from everything and COVID happened :(

naturallyInconsistent

@Obliv your assumption here is warranted, but we tend to venture into chem and CS, maybe a little bio; paleontology is a bit rarer for physicists to be venturing into

@Sanjana sloowwwllyy backs away

into comfy slumber. sneeppuuuu

Sanjana

but sometimes I really feel angry and sad when Neil deGrasse Tyson almost "insults" other physicists, especially Brian Greene and other string theorists---mocking that their program has failed etc. For a moment, I even want some string theorist equivalent of NdgT to be there...but then I control myself because wrestling is not the purpose of physicists...

One day somebody will reply by answering all the questions and then NdGT won't be able to say a thing...One day somebody will stabilize all moduli upto all ordersand find our universe in the landscape and write a bulk effective non perturbative proof of FRLW/CFT and get the Page curve fixed and get the initial conditions of the universe and...oops that's a lot of work :)

Obliv

22:41

Dang I just realized.. you're an english teacher yet you're quite knowledgeable about physics @Sanjana

PM 2Ring

NdeGT often makes minor errors and occasionally says some really dumb stuff, eg

9

Q: Neil de Grasse Tyson on Nukes and Radiation - What the Heck Is He Talking About?

Last week Neil de Grasse Tyson was on Bill Maher's show and the topic of nuclear weapons came up. Tyson said, "modern nukes don't have the radiation problem.... They're a different kind of weapon than Hiroshima and Nagasaki." When pressed by Maher, he said, no, there is no radiation issue "if it'...

Obliv

That seems very unlikely :P but then again I hear most physics students get jobs in other fields other than physics

PM 2Ring

@naturallyInconsistent Mostly peaceful. But there are numerous examples of people succumbing to morbid depression whose work was rejected by the establishment.

From the link here: chat.stackexchange.com/transcript/message/28518021#28518021

> in 1909, only three years after Boltzmann hung himself for having the scientific community, namely: Ernst Mach, Gustav Jaumann, Ernst Zermelo, and Wilhelm Ostwald, reject his belief of the existence of atoms and his interpretation of thermodynamics as an atomic and molecular phenomenon, French chemist Jean Perrin proved that atoms exist, experimentally, by calculating and determined the number of atoms in one mole of substance.

Obliv

Wait they didn't think atoms existed in 1906?

ACuriousMind

not definitively, no

PM 2Ring

22:56

Well, Mach certainly didn't. And he never changed his mind. Ostwald eventually admitted that atoms were more than just a convenient fiction for doing calculations. But Boltzmann was dead by then.

Obliv

wasn't the basis of earlier mechanics the idea of point-like particles that made up the notion of mass/inertia

PM 2Ring

Sep 22 at 8:12, by PM 2Ring

There'd been a strong battle between particle vs wave theories regarding light, since Newton. The Arago spot phenomenon seemed to show rather conclusively that light is waves. And the success of Maxwell's electromagnetism seemed to indicate that everything could be explained in terms of waves.

Obliv

oh right the corpuscular theory lost favor, but it was still a viable theory and the photoelectric effect & quantization of light kind of brought it back to relevance?

but atom is derived from greek and I thought the idea that atoms existed was a popular position

ACuriousMind

@Obliv There's a difference between some people believing in atomism and it being accepted as scientific fact

Obliv

I guess I'll get more context when I take thermo next semester.. but I would have thought his theory would gain a lot of favor since it had good predictive power?

ACuriousMind

23:03

you will not get more context, physics courses generally aren't history lessons :P

Obliv

wow i just realized that link was to a convo I was a part of in 2016... @Pm2Ring

ACuriousMind

nowadays we just teach particle mechanics and atomism as obviously true

PM 2Ring

Einstein got a Nobel prize "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect". Also, his work on Brownian motion had a big impact. Both of those things were major points for the Atomists.

Obliv

Well at least if I could see what the theory was about, I could see why it might be so controversial

but yeah it's probably not going to be taught the same way as boltzmann taught would have back in the day

Mr. Feynman

23:17

@ACuriousMind what makes a physicist, though?

What is the turning point after which you can consider yourself one?

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