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6:00 AM
That's an, erm, interesting point of view :-)
 
In the end, vacuum fluctuations is code for "the wave function of the ground state of a harmonic oscillator has nonzero width".
As my colleague and I like to remind each other: "quantum noise is not noise".
:D
IMHO, half of the time when someone says "vacuum fluctuations" they have no idea what they're talking about.
 
The SHO ground state is an energy eigenfunction so there are no fluctuations in the energy of the SHO ground state.
 
This is unfortunately true in astrophysics, but don't tell anyone I said that.
@JohnRennie Who said anything about fluctuations in energy?
 
For an interacting quantum field the ground state is not an eigen function of any energy operator.
So the energy of the vacuum is definable only as an expectation value.
 
Aye! caramba. We're doing interacting fields now?
@JohnRennie Uh, wait really? Why isn't the ground state an eigenvector of the total interacting Hamiltonian?
 
6:05 AM
(I think) free fields don't have vacuum fluctuations. The energy of their ground state is precisely defined.
 
If the answer is "relativity" then I give up.
 
Interacting fields are only accessible via a perturbative approach. We know surprisingly little about them, not even how to construct their equivalent of a Fock space.
Outsiders (like me) tend to assume that QFT is all done and dusted but as far as I can see that is far from the case.
 
I never understood what vacuum fluctuations were supposed to be... if there is a vacuum, then there is nothing to measure it, if there is something to measure, then it's not a vacuum.
 
@CuriousOne in principle it's very simple, and Daniel's example of the SHO is an excellent way in.
 
@CuriousOne For heaven's sake, remove the literalist devil whispering in your ear for a moment and try to actually understand what might be meant.
 
6:07 AM
The SHO ground state isn't nothing
 
@JohnRennie What about fields in a bounded space?
 
But we are happy to say there is a well defined zero energy so we can say the SHO ground state energy is $\tfrac{1}{2}h\nu$ and sit back.
 
I have a feeling that you've been trained to think about a mathematically pathological case whereas you can understand what's going on in a much simpler, bounded, non-relativistic system.
 
I get it if any measurement on a field has to go trough some non-trivial perturbation states. That's OK... but that's still the measurement exciting the field, isn't it?
 
But in QFT the fields are the vacuum. There isn't any zero reference we can point to and feel smug. So understanding exactly what the QFT ground state means is hard.
@DanielSank I don't know enough about the subject to comment :-(
That may change one day, but QFT has such a scary barrier to entry.
 
6:11 AM
@JohnRennie Well, in my lab I have a strip of coplanar waveguide.
This thing is a quantum field is a very real sense.
It's like a vibrating string, but instead of a string we have the charge of the superconducting condensate.
It has modes, just like a vibrating string. Each mode can be excited into the various Fock states.
We can make superposition states etc. We know this because we couple the waveguide to a superconducting qubit, which is a zero-dimensional, slightly non-harmonic quantum thingy.
 
@DanielSank: The field inside your waveguide is nowhere close to the ground state. What you got there is half a lbs. of mass-energy surrounding a space of insignificant volume.
 
We use the qubit as a probe, because we can very accurately control and measure the qubit.
@CuriousOne I am not interested in having this discussion with you again.
 
@DanielSank God I wish I was still a graduate student and working at Google. It sounds ridiculously exciting :-)
 
Sorry. It's a good discussion, but we've done it, and you ignored everything I said.
@JohnRennie Yeah, our field is awesome.
 
@DanielSank: Then don't, but don't ignore that what you got there is not the ground state. It's the ground state after you discount everything that you don't like to think about.
 
6:14 AM
@CuriousOne Give me an example of something inside my sample box which isn't in the ground state. Keep in mind the whole thing is at 20 mK.
@JohnRennie I have an unusual view of quantum mechanics because of my experience. Namely, that quantum mechanics applies to collective degrees of freedom just as well as it does to microscopic degrees of freedom. This is something that e.g. @CuriousOne argues with me about all the time.
 
@DanielSank: It doesn't matter what the temperature is, you got the equivalent of a medium size nuclear weapon of non-ground state matter in your experiment. You are making a small perturbative measurement on top of a giant excited state.
 
@CuriousOne: leaving aside specifics, I'm a bit surprised you find the QFT vacuum obscure. While it's certainly mathematically obscure it seems qualitatively straightforward.
 
@CuriousOne I'm going to say this one more time, and for the love of science please pay attention, try to actually absorb the information I'm trying to convey to you, and wait at least one minute before responding:
Those e.g. nuclear degrees of freedom are completely adiabatic throughout every experiment we do, and as such, that are completely irrelevant to the outcome of any and all experiments we do.
I have never once done an experiment where I couldn't predict the answer with basic quantum mechanics applied to the collective degrees of freedom I engineered in my system (and a few I didn't engineer but were there anyway). The degrees of freedom at energy scales orders of magnitude above the GHz range where we operate are completely irrelevant.
You know, I apologize for coming off as an arrogant arse.
It's just that every time we talk about this I really feel like you completely ignore 90% of what I say.
 
@JohnRennie: I find it obscure in the sense that I doubt that we will have any chance of actually measuring it. The em rest mass is tiny, but it is zero? That is a question that I can't even imagine we will be able to attack. We are like the beings living on the surface of a neutron start who are wondering if He can exist and if 3He might be a superfluid.
 
And downloadable as PDF :-)
 
6:24 AM
@JohnRennie Figures and all!
Lemme know if you need help with the circuit stuff, or in bridging from circuits to quantum mechanics.
 
@DanielSank: You are simply renaming the ground state to something you can implement on the bench top without even thinking about how far away from the ground state the bench top is. That is something I have seen over and over again by people who don't want to think about the full theory.
 
@CuriousOne As I keep saying, ignoring the nuclear stuff gives the right *@#*$%& answer.
 
That's not your fault. I didn't think about it this way, either, as long as I was surrounded by low energy physics.
 
@CuriousOne yes, the QFT vacuum is detectable via the Lamb shift. There's nothing philosophical or metaphysical about it. The effects are quite real.
 
@CuriousOne Goodbye.
 
6:26 AM
@DanielSank: You haven't even been asking the right question.
 
@JohnRennie Nice talking to you. Looking forward to hearing about your opinion of the paper.
 
@DanielSank Thanks for the info. Much entertainment ahead I feel :-)
 
@JohnRennie :D Physics ought to be entertaining.
 
Why else would we subject ourselves to so much pain? :-)
 
@JohnRennie: That's what I meant: we do a measurement and we have to go trough some perturbative states. No argument with that. It doesn't mean that those states are the ground state. They are the relevant perturbative states at our scale.
 
6:28 AM
If you imagine a huge number $N$ of coupled quantum SHOs then there is a well defined ground state that is easy to understand.
And that's effectively what we have in a non-interacting QFT, so that's easy to understand too.
 
I do, by the way, not know why Daniel blows up when I say that there may be space at the bottom that we can't see... it's not like we saw superconductivity coming, right?
 
But when we turn on interactions that's like making our SHOs non-simply harmonic so there aren't any normal modes any more. Now the ground state is much harder to understand. And that's why an interacting QFT is hard.
 
Isn't that the idea behind conformal field theory and eternal inflation, anyway? At least the way I understand it... if rescaling is non-trivial, then all kinds of stuff can happen that we can not see, no matter how hard we try.
 
I don't see any obvious connection to CFTs.
 
I guess I might need @SirCumference help to translate all of this into English
https://www.mediawiki.org/wiki/Manual:Extensions

I don't even know where all these folders are given that Mediawiki already installed my wiki when it created it
 
6:31 AM
I have to go offline for a few minutes. The first of my daily tasks (checking servers) is now due. Back in about half an hour ...
 
Maybe there isn't, I wouldn't know better. All I am saying is that the SM doesn't help to predict the critical temperature of a superconductor from first principles. For that we need to go into the lab, measure it and construct an effective theory, which is what tons of solid state folks are doing for a living.
Now tell me with a straight face that you have a theory that works twenty orders of magnitude below what you already can't predict with the usual approach.
All I am saying is that I don't know where the bottom is and that Daniel is not asking the right questions to establish it if he thinks its inside a metal tube that is made of 50GeV nuclei.
 
6:47 AM
@CuriousOne I do not blow up; I lose interest.
 
6:59 AM
@DanielSank: That's hard to understand. I am not asking you to talk about something trivial, but about something that is highly non-trivial. You have one real big problem, by the way: "I have never once done an experiment where I couldn't predict the answer with basic quantum mechanics..."
You know what that means, right? It means that you have never actually tried to discover something new.
The folks who are looking for the "real vacuum" are the guys who are trying to measure proton decay and such. Try predicting that with basic quantum mechanics... or measuring it.
 
@CuriousOne Let's go right past the part where that comment is ignorant and insulting, and move onto the part where I discovered something new despite the fact that you can understand it with the Schrodinger equation.
I am very tempted to break the "be nice" rule on this site right now.
 
Gosh that Koch paper is 1981. It's practically archeology :-)
 
@JohnRennie :-)
 
The Courier font is a dead giveaway :-)
 
@CuriousOne I also invented a useful device that enabled a new level of precision in quantum mechanics experiments and quantum computing. I guess since I didn't discover a new term in the standard model Lagrangian I wasn't "asking the right questions" though, according to you.
@JohnRennie What happens if I hurl insults at another user? Do I get banned?
 
7:14 AM
@JohnRennie I heard that you are a computer nerd. Maybe you can help me to understand the alien language that is wiki markup and mediawiki installing extensions?
 
@DanielSank: I wasn't trying to insult you. You do know what the biggest problem of LHC science is? They merely confirmed what they already knew and so far they haven't discovered anything fabulously new. Nada. Zip. Sucks to be them. Seriously. That's not what they are praying for, over there, to detect what has been predicted in the 1960s.
I did not say that you can't invent useful stuff. Of course you can. Plenty of it. I am still amazed how many new toys they optics folks are coming up with by playing around with nothing else than geometry and dielectric constants. Hats up!
 
@CuriousOne I guess this is a demo of the skills learned in that oral exam...
 
@DanielSank I suspect so. Breathe deeply and think calming thoughts about awesomely cool physics. It works for me whenever he who shall not be named rears his head :-)
 
@JohnRennie o_O
JD?
 
Voldefield
 
7:18 AM
Is that a Harry Potter reference?
 
'Fraid so
 
lol
@JohnRennie I'm listening to a techo remix of the music of an 8-bit video game I used to play as a kid. Feeling good about that, anyway.
 
@DanielSank: The cool thing is that you may, one day, stumble upon something that you don't expect using simple QM, and that will be the actual discovery of your lifetime. I wish you do.
 
@CuriousOne Unlikely.
And I'm ok with that.
 
@DanielSank: Why not?
 
7:19 AM
@CuriousOne that's a caricature of the work done at the LHC, as I'm sure you're aware. It's true there haven't been any new fundamental particles found, but there is still much to be understood about the SM and the LHC is instrumental in advancing this.
 
@JohnRennie Specifically, because the Mediawiki is distributed by wikia, how to locate the localsettings.php?
 
@CuriousOne Because quantum mechanics works very, very well and I'm not doing anything that's particularly likely to break it.
 
@JohnRennie: Is any of that going to get another Nobel? Should any of it get another one? Would you want what they can still do be your legacy?
 
@Secret can't help I'm afraid. It's true I'm (employed as) a computer nerd but that doesn't mean I have a God-like knowledge of all things computational.
 
If I were still there, I would be dreaming of more. Just saying.
 
7:21 AM
ok nvm...
 
@DanielSank: QM is one thing, your idea of what the vacuum is is another. There are more unusual ideas than yours and you actually may have a chance of going after them with the very technique that you are comfortable perfecting on the known.
It just doesn't seem to me that you have any interest in thinking about that. That's OK. All I am saying is that it's out there.
 
@CuriousOne there is lots of fascinating work being done at the LHC, where in this context fascinating means there are physicists fascinated by it. That group doesn't include you. OK.
 
@JohnRennie: That sounds a little bit like a copout. I actually read the LHC/ATLAS/CMS technical reports. The physics section contained a lot more than the Higgs... we didn't build it for "the Higgs and nothing else". Sorry, but that's not what the hope was or still is, no matter how you may think about that. I was there and I was among the least hopeful, even back then, when the paper wasn't even dry and the machine wasn't anything but imagination what one could do with a full tunnel.
I am still disappointed and that is not a function of me not being there.
Neither is proton decay a side show. At the moment that and the neutrinos and dark matter are the real show.
 
@CuriousOne The hell does any of this have to do with whether or not the presence of (adiabatic) nuclear degrees of freedom in my experiment?
 
@DanielSank: When you ask about the real ground state, everything. When all you want is the Bohr scale ground state, absolutely nothing.
 
7:36 AM
@CuriousOne I care 0 units of caring about whatever you call the "real" ground state.
It has absolute nothing whatsoever to do with the issue of vacuum fluctuations.
 
I am sorry to hear that and I apologize if I upset you. That wasn't why I asked the question. Let's leave it at that.
 
7:53 AM
So... this spin thingo... It's a thing, what thing is it?
 
What spin are you referring to?
 
Electron spin
 
@DisplayName Hi, have a look at:
57
Q: What is spin as it relates to subatomic particles?

NickI often hear about subatomic particles having a property called "spin" but also that it doesn't actually relate to spinning about an axis like you would think. Which particles have spin? What does spin mean if not an actual spinning motion?

 
@JohnRennie Why is the result differing by a single planck length? This is the minimal length that can be measured right? So what if it were sub a planck length?
Result being the z axis angular momentum
 
8:10 AM
hello
 
olleh
I guess taking it for granted everything else seems to make sense
 
@DisplayName: That's an interesting way of looking at it. From a historical perspective the spin must have gotten a few physicists to rub their eyes, swallow a couple of times and then question the experiments that suggested its existence and eventually discovered it directly. en.wikipedia.org/wiki/Spin_(physics)#History certainly suggests that there must have been quite a few discussions about it.
 
8:25 AM
I meant everything else I read on that page made sense to me other than that for the record
(and the Lie algebra part)
Isn't this a current discussion funnily enough:

I agree. This comment speaks to my confusion (see my Physics Meta question) over how this Physics site is conceived. Namely, what is the level (high school, undergrad, grad) of the intended audience? – Eric Zaslow Nov 2 '10 at 22:05
1

Yeah, it's a point worth discussing. I think all three of those levels you point out should be acceptable. (You are clearly aiming at lower graduate level plus in this case.) Personally I would not like to see this site dominated by too many basic high school questions nor by many research-level ones. – Noldori
What does he mean by transform in that Lie algebra post. I am happy with SO(3) and SU(2)
 
The spin vector of the electron in hilbert space (which consists of spin up and spin down base states) transforms unitarily under the action of the rotational group (or more generally the Poncaire Group). Fermions transforms under SU(2) while bosons transforms under SO(3)
by transform it is similar to how you rotate something, except such rotation took place in hilbert space
 
Can you mathify that?
Do they otherwise stay invariant? Transform means change to the other state?
 
8:50 AM
sort of, so it will look like it is in some different state in some rotating and boosted frame.

As for the maths details, I guess you might need to wait for e.g. Acuriousmind because I am not terribly confident with lie groups
But the idea is similar to getting a representation of the group elements by expressing it as matrices, then multiply the state vector with that matrix, and then you see the vector changed as a result
(Or more accurately, you cannot see the vector changed experimentally in a direct fashion, but the probabilities of spin up and spin down you measure for n replicates will change in a way that reflect the change in the state vector)
Slereah, I have some stuff for you to check shortly, see if it makes sense to you. More details later
 
9:41 AM
@Qmechanic are you currently free to provide some comments on a scifi calculation I did?
 
10:08 AM
@DanielSank : "What happens if I hurl insults at another user? Do I get banned?" Apparently not. Not by the guy who also hurls insults at another user: "It works for me whenever he who shall not be named rears his head".
 
@JohnDuffield Remember when you said time travel is science fiction? I currently have some really crazy science fiction model on time travel and I would like you to give me some comments on it to improve or it something

http://secretuniverse.wikia.com/wiki/Ripple_dynamics

Because I am currently only doing newtonian calculations so far, there should not be issues with things like space vs spacetime...yet
 
10:33 AM
JohnRennie, are you currently free to provide some comments on a model I made for my scifi to see if anything else can be done (or directions to be taken) to improve it or something?
 
@Secret yes ...
Ripple dynamics!
 
0
Q: Account suspensions

NumrokI've noticed some account suspensions recently. I thought this feature was meant for people who do not contribute to the site in a valuable way and do not follow the rules of asking/answering questions repeatedly. However the two accounts that were affected recently were two high reputation users...

 
10:54 AM
@Numrok: Moderators traditionally don't comment on suspensions, however voting irregularities almost always means sockpuppetry.
I've removed my comment from the meta site since there is a certain sensitivity about these things. In fact you might want to delete your question as well.
@Numrok: yes, sockpuppetry is when a user sets up two accounts. However if two users are colluding in upvoting each other's posts that also counts.
 
@JohnRennie
 
@PhysicsGuy Hi
 
@John Rennie Do you remember me ? I was the guy with the string theory question.
 
@PhysicsGuy I don't remember I'm afraid, which proves nothing other than I have an aging brain :-)
 
@John Rennie It doesnt matter then. It was about quantizing a field and quantizing a string. You helped me there. Just wanted to thank you for it.
 
10:59 AM
Your're welcome. Was it the discussion about how particles emerge from quantisation of strings?
 
Yes.
 
That was good fun to research, though I know very little about string theory and I wouldn't have said anything without people like ACuriousMind around to check I wasn't saying anything completely wrong :-)
 
Since then I worked on string theory, too.
 
11:16 AM
[Poem without rhymes]
The 3 theorists: Acuriousmind, Slereah and 0celo7
Their jokes far and wide
The relaxation generated by them
permeated all corners of h bar
In this small quantum realm
Lies the heart of insanity
The joke of the 3, shields visitors from the heart
Buying perhaps just a milisecond
For the heart's plan to be served
 
11:26 AM
WTF
 
It talks about how the 3 users in the poem when they are together at the same time, h bar is a lot more lively during the afternoons
 
I know.
 
btw what's your physics background. I heard you worked on string theory?
 
I am active at mathematical physics (means (axiomatic) quantum field theory, string theory, loop quantum gravity, general relativity and other mathematical stuff related to physics). But I am not doing it professionaly.
 
I see. Mine is mostly undergrad stuff. I have a very strong interest in quantum mechanics and was reading text on that to further my studies
 
11:35 AM
Thats interesting.
But quantum mechanics are not undergraduated stuff (I think).
 
yes, the full scope of it is graduate stuff. For the undergrad stuff is like calculating hydrogen atom, harmonic oscillators, angular momentum those kind of things in undergraduate courses
quantum has a lot of weird phenomenon (at least to our common sense) that is fascinating when you find that hilbert spaces and operators can explain why they are so
entanglement in particular is very interesting, both fundementally and also in quantum information theory
 
Yeah, the real beauty of QM lies in the mathematical formalism
 
How to think about entanglement. From the formalism of tensor products (and that these can superimpose like vectors in hilbert spaces) is entanglement some kind of correlated information between the subsystems?
 
kind of.
 
When I do the entangled 2 qubits exercises in susskind's theoretical minimum book, I noticed the probabilities of the outcomes become correlated
 
11:48 AM
If you have two systems wih two hilbert spaces A and B, then the tensor product A x B is the hilbert space of the connected system. This becomes important if you want to define a state as a pure state (or not).
The connected system is a pure state in this case.
and its separable.
 
@Secret Hey, what do you need?
 
@SirCumference I am trying to find out how to reach the localsettings.php file. Firstly, I am not sure if media wiki is already installed in the sense described in the article because when I first made that wiki, Wikia already provided all the media wiki and packages along with it
 
Wikia is based on mediawiki
What do you need to edit the local.php file for?
 
https://www.mediawiki.org/wiki/Extension:MathJax
Iam trying to install a mathjax extension because currently the way I type formulae it requires a very long piece of code that is repeated many times. Because the code is used inside the <math> tags, it does not work well with templates
alternately, I am trying to see if I can change the default text color of the png math item in the localsetting.php so I don't need to add the same bunbh of code every itme Iwrote an edquation
 
You need to email Wikia and ask them to install it
 
11:58 AM
Currently to type a formula, I need to type the following mass of code to get it to not be hard to see in a dark background <div style="text-align: center; direction: ltr;"><math>\definecolor{GGG}{RGB}{128,128,128}\definecolor{Spectralviolet}{RGB}{202,175,255}\pagecolor{GGG}\color{Spectralviolet} (some latex expression) </div>

Ok I'll try
 
@Secret : I'm sorry secret, I just can't enthuse about time travel. I'm not the guy to help you with this.
 
@JohnDuffield ok nvm then
 
@JohnDuffield Er, do you know GR?
 
@SirCumference : like no other! But not everybody agree with me on that.
 
Great
So you know how time dilation increases as you approach a black hole?
 
12:02 PM
@JohnDuffield Why not ?
 
@SirCumference : yes, like DavidZ was saying earlier, it goes infinite at the event horizon. And as regards what you were asking earlier, IMHO you should look at the last line of this.
 
@JohnDuffield Well, my question is: does it go infinite at the absolute horizon, or apparent horizon?
 
there's 0celo7, now where's acuriousmind in order to materialise Slereah?
 
@PhysicsGuy : some people disagree with some of the things I say. I don't have a physics degree. I'm self-taught.
 
Me, too.
that self-taught thing.
 
12:07 PM
@SirCumference : what's the absolute horizon and apparent horizon? The event horizon is where time dilation goes infinite.
 
In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again. An absolute horizon is the boundary of a black hole by definition. In the context of black holes, the absolute horizon is almost exclusively referred to as an event horizon, though this is often used as a more general term for all types of horizons. The absolute horizon is...
In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward. Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from event horizons. Within an apparent horizon, light is not moving away from the black hole, whereas in an event horizon, light cannot escape from the black hole. It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in...
This line sums it up
> It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in the future will not be able to escape (e.g. because the mass of the black hole is growing) and therefore inside the event horizon.
 
I actually have an undergrad degree in physics and chemistry. However my enthusiasum in back to the future and time travel result in me risking of robbing too many potential future expected response from others everytime I speak about that topic

In fact, at least one user think I am an insane person
and to top that, most of the time I have thoughts, but they are almost always arranged in an incoherent fashion thus peopel often have trouble understand my queries and etc.
 
@SirCumference : IMHO the absolute horizon is right, and the apparent horizon is wrong. If a light ray is directed outward, it isn't moving inward.
 
"The event horizon is where time dilation goes infinite." Thats not true, though. You have to differentiate the inertial frame of the falling system and the observing system. This sentence is questionable.
 
@JohnDuffield Really? Then why would the apparent horizon be the boundary between light moving outward and light moving inward when directed outward?
 
12:21 PM
(I have never seen 0celo7 this quiet, where are you, acuriousmind, we need you to restore the balance of h bar!)
 
@SirCumference : I don't know. I can only reiterate : if light is directed outward, it isn't moving inward. Do you know about the "coordinate" speed of light? That's the speed of light at some location as measured by distant observers. See this: "For example, at the event horizon of a black hole the coordinate speed of light is zero".
 
Isn't the speed of light constant regardless of reference frame?
 
yes.
 
@PhysicsGuy So...?
"That's the speed of light at some location as measured by distant observers."
 
So what ?
 
12:25 PM
@SirCumference : no.
 
Shouldn't the speed of light at all locations measured by anyone be the same?
 
Yeah, it is.
 
It isn't.
 
Er...
I'm getting conflicting answers
 
12:27 PM
"Does the speed of light change in air or water?

Yes."
This is not what I'm talking about
The speed of light doesn't change depending on medium, but its path will change
Making it take longer to get through a medium
 
Coordinate speed is something else.
in this case.
 
It depends on how the question is meant, so you can't just say "no" without qualification. (Nor "yes")
 
@SirCumference : not so. See this.
"so there is no other way to interpret the velocity of light in this usage except as a variable scalar speed".
 
Okay, hold on
I was led to believe that massless particles have only one speed — lightspeed
That's what makes them massless – they have no resistance to approaching lightspeed
 
@SirCumference: the word speed tends to be used to mean coordinate speed i.e. dx/dt as measured using some particular coordinate system.
The problem with this is that it doesn't behave well in GR. Instead we use four-velocity.
 
12:33 PM
@JohnRennie Oh yeah
 
This is a four-vector and well defined even in curved spacetime.
But a four vector is not a physcial observable. You have to choose a coordinate system to observe it, and the obvious coordinate system to choose is the observers rest frame.
 
@SirCumference : that's right. Massless particles move at the speed of light. But look at this:
 
So my coordinate system would have it's origin here in my chair. It's the coordinates in which I am rest.
Now if I measure the speed of light, or any massless particle, at the origin of my coordinate syste I will always get the value $c$.
And that's true regardless of where I am or what I'm doing. Whether I'm floating freely in space or plunging into a black hole I always measure the speed of light to be $c$.
This isn't too hard to show mathematically, though the details are probably only of interest to us nerds.
But ...
 
You always measure the local speed of light to be c because of the tautology wherein we define the second and the metre using the local motion of light. And we then use them to measure... the local motion of light.
 
If I use my coordinates to measure the speed of light at some non-zero distance from me then I will in general get a value different from $c$. The only way I will measure the speed of light to be $c$ everywhere is if spacetime is flat everywhere.
@SirCumference Right, I think that's it. Have I made sense to you?
 
12:39 PM
@SirCumference see this answer by John Rennie: "The obvious example of this is a black hole, where the speed of light falls as it approaches the event horizon and indeed slows to zero at the event horizon".
What that means, is this: the outward-directed light beam speeds up.
Ain't physics fun?
 
Now I have to go shopping, which is less fun than doing GR but rather more necessary. I will be back in a few hours if you want to pick this up.
 
The constance of the speed of light can be derived from maxwells equations.
 
@PhysicsGuy : that's what people say. But I'm afraid it isn't what Einstein said.
 
@JohnDuffield That guy from 100 years ago?
 
(Don't worry, shopping is more fun than time travel. I do notice it is robbing h bar activity every time it is mentioned, even more so in terms of impact than that sacred cow whatever that vzn rant about)
 
12:44 PM
Pretty sure there've been a lot of advancements in physics since him
 
In my opinion, the bizarre probably comes from misinterpretation... since probability only makes sense with many ensembles, it is meaningless to ask the "odd" of an individual whether will increase or not.
anyone any ideas?
 
@SirCumference : yep. The guy who came up with general relativity.
 
@JohnDuffield As far as I know, there are people nowadays who understand GR better than Einstein
 
the constance of the speed of light is the most fundamental and important thing in relativity.
 
@SirCumference : that's what they'll tell you, but they don't.
 
12:46 PM
@Shing My understanding is that whatever door the host reveal will not be one that contains the prize, thus that's one door down out of 3 doors, thus the probability of winning if switched becomes 2/3
 
@JohnDuffield [citation needed]
 
@PhysicsGuy : I'm afraid it isn't. See things like this: "the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned."
 
Anyway
 
@Shing The probability of choosing the goat is 2/3, so the probability of choosing a car if you change the door is also 2/3.
@JohnDuffield You do know that effects like time dilation, etc. occur because of the constance of c ?
 
Joke time: What did one photon say to the other?
 
12:50 PM
What ?
 
Nothing. Photons don't talk.
 
Do lie groups help against sleeping disorders ?
 
I don't think so. Moreover, you get different outcomes if you listen to A's advice followed by B, rather than listen to B's followed by A
 
@DavidZ : I can cite Einstein. Is that what you meant? Here's another example.
@PhysicsGuy : it isn't true I'm afraid.
 
How do they occur then ?
in your opinion ?
Why am I doing this ?
 
12:54 PM
They LIE as a group, and they are not even consistent in their lying
 
@Secret ;)
 
(This chain of jokes will be even better if one can condense everything we know about lie groups into layman terms)
 
You know what I saw last week?
 
What ?
 
Light. In fact, no one has ever seen anything other than light.
 
12:57 PM
To be more precise, light that is some time ago in the past
 
Then the baarkeeper said: Sorry, we dont serve neutrinos travelling backwards in time. A neutrino walks into a bar.
 
thanks... I understand what you meant. I am thinking if we can interpretate that way. In other words, does one single outcome counts as a prediction of probability? Probably not (since probability only deals with many ensembles). but we would be silly if we join a game with 99% probability to lose. I am getting confused.
 
@PhysicsGuy I got one better
A neutrino walks into a bar...and keeps on going...
 
Thats good ;)
Haha
 
@PhysicsGuy : gravitational time dilation occurs because a concentration of energy in the guise of a massive star "conditions" the surrounding space, affecting its properties, this effect diminishing with distance. See Einstein talking about it here.
 
12:58 PM
Physics jokes are best jokes
 
Haha
 
haha
 
I think probability is easily visualised when you have an ensemble (or many trials), but it does not mean a single outcome is not subjected to a probability distribution. However if you only ever have a single outcome and nothing else, then you don have enough information to work out the probability distribution of that thing
(That's the frequentist probaility)
 
1:00 PM
 
@JohnDuffield You do know why an ether doesnt exist, right ? You do also know, what relativity of simultaneity is, right ?
 
As for bayenesian probaility, it depends on how much information you know and hence these infromation will tell you how confident you think the outcome is
 
Hehe Schrödingers cat.
 
Actually, if you look closely at the abstract algebraic thing known as an ideal, it does kinda like a black hole. You have some r in I subset S and x in S such that rx in I.

Once you roll into there you never dreamt of getting out
 
HAhahah
 
1:03 PM
@PhysicsGuy : see this and this. Yes I know what relativity of simultaneity is.
 
@Secret thanks, I will think about it
:D
 
@JohnDuffield Good, if you know, why are we talking anymore ?
Then you should know how relativistic effects occur in respect to that.
special (!) relativistic effects
 
(The only other time travel guy in h bar is only stable if acuriousmind is nearby. Otherwise he decays in seconds into something else)
 
I have to go I'm afraid. Bye for now.
 
Bye.
 
1:09 PM
New joke
Why did the photon want to lose weight?
 
He has no weight.
 
He wanted to stay light!
Oh crap I forgot...
 
That kind of ruins my joke. Thanks.
:D
 
Its still funny.
Yo Mama is so fat, she cant be embedded in $R^n$
 
1:12 PM
What did the gluon say to the particle? You have a strange quark about you
 
@JohnDuffield no, not at all
 
Yeah.
 
(David Z, are you interested in giving me comments/suggestions on my scifi project?)
 
Where does bad light go? In a prism.
 
What is ypur scifi project ?
@SirCumference Dont lose your flow. ;)
 
1:14 PM
(Physics Guy secretuniverse.wikia.com/wiki/Ripple_dynamics . Warning obviously not physics because GR does not allow changing history)
(Talk about why I am so interested in it will destablise h bar's atmosphere, thus I am going to leave the motivation in the air for now)
I am an electron, the more you confine me, the harder to catch me
O and I sprung round and round, but you won't see me spinning other than a bunch of ups and downs
 
BAck to the Future fan ?
 
Oh my god, not physics jokes again
 
@PhysicsGuy The insane type, so insane that I am actually doing maths on it
 
Keep on going, thats very interesting.
It is definetely worth it.
 
@PhysicsGuy Which direction should I calculate next that will be of importance in mutable time travel story perspective?
 
1:20 PM
Let me read that article to get into this topic.
 
@0celo7 einsteinandtheevidence.wordpress.com haven't updated recently. Btw you read anything interesting in your GR studies?
 
@Secret Whats the matter with this paper ?
 
O, that's 0celo7's blog. I often visited his to read about some GR stuff. He self studies GR to the point that his knowledge is on par with slereah and other GR people in h bar
thus a reliable source of GR discussion
 
Interesting blog....
 
1:41 PM
I've been too busy with other things to write things there
 
1:52 PM
like Ricci-flow ;) ?
 
screw that
It's way too hard
 
I know. But it was the main tool for the proof of poincare conjecture, thats why I am a bit interested in that.
 
It's the tool for $n=3$
For $n\ge 5$, it's $h$-cobordism and for $n=4$, it's false (I think).
 
[Ignore this message] (If we assume what those user said is true and that I am insane, I wonder if in the history of h bar there is someone who is more insane than me...?)
 

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