The h Bar

frogeyedpeas

01:34

i got a hard question

what is the least effort way for strong mathbackground to get into QFT

kotozna

03:19

Try the 6 core theories of modern physics by charles stevens

very concise and readable

Novalium Company

09:50

Hey guys! I built the voltage multiplier with alternating square wave from a 555 timer as a source (which is measured 4.5V by my multimeter) but the voltage multiplier doesn't seem to work. I tried first making a voltage doubler and it showed 9V (which is correct I suppose) but when I try a quadrupler for example and the voltage starts from like 6V and starts to go down around 0.1V per second.

Oh! I found a mistake in my wiring and fixed it. Now it seems to show 12V and instantly starts to go down by 0.1V per sec.

It stops at about 7.3V

PM 2Ring

10:17

Aug 16 at 9:54, by PM 2Ring

But you really should ask the people in Electrical Engineering. I just had a quick peek, and there was a recent conversation about voltage multipliers. I assume there are people there who've made high voltage stuff, like rail guns, which need a lot of current, so a low current circuit like yours should be simple for them.

So what did the guys in the EE chat say...

The voltage multiplier should be ok on a capacitive load. It will drop the voltage on a resistive load, as mentioned in various Electrical Engineering links on the topic. I assume you have thoroughly explored the links I have been posting for you...

A multimeter is basically an ammeter. To measure voltage, it puts a stable resistor into the circuit and measures the current running through it.

Slereah

11:04

hey hey

Rudi_Birnbaum

11:16

Hi all! There is theorem that links the imaginary and the real part in a time dependent analytic function. I forgot its name. Its named after some dutch(?) scientist and is used in solid state physics, who can help?

Sorry I got it Kramer-Kronings....

Slereah

Errr the Wick rotation?

Rudi_Birnbaum

no its really kramer-kronings

Kramers–Kronig relations

The Kramers–Kronig relations are bidirectional mathematical relations, connecting the real and imaginary parts of any complex function that is analytic in the upper half-plane. These relations are often used to calculate the real part from the imaginary part (or vice versa) of response functions in physical systems, because for stable systems, causality implies the analyticity condition, and conversely, analyticity implies causality of the corresponding stable physical system. The relation is named in honor of Ralph Kronig and Hans Kramers. In mathematics these relations are known under the names...

Novalium Company

12:27

@PM2Ring electronics.stackexchange.com/questions/453772/… :P

Novalium Company

13:52

I have a weird question: The output on an astable multivibrator will be shown on a multimeter as half the input voltage (for example we have 9V-0V-9V-0V...and the multimeter averages it out and displays 4.5V). But then if I put that output to a voltage doubler, the voltage should be 18V, not 9V right? Since the voltage doubler will output in DC.

Novalium Company

14:28

I've tried hooking up a transformer (9V to 230V, 0.5A) to an astable multivibrator (which operates at 671Hz) but something starts to smell burnt and the components of the astable multivibrator get hot. How do I fix this? I check it after that and the astable multivibrator works.

enumaris

16:13

bow chicka wow wow

ACuriousMind

what's up?

enumaris

my AC won't turn off so I'm waiting to see if I can turn it off before I go to work lol...

ACuriousMind

have you tried pushing the "on/off" button? :P

enumaris

indeed...

the controller makes a click but the AC is still on

hmmm...

I submitted a maintenance request and they said they fixed it but I guess they didn't really do anything

ACuriousMind

dunno...just trip the breaker? :P

enumaris

16:24

yeah I guess I could try that

oh it finally turned off lol

time to go to work...

JMac

logs into hacked IoT and turns AC back on again

That's the kind of trolling I'm worried about, the future with too many "smart" devices with no security

Semiclassical

16:48

@JMac at some point that goes from 'trolling' to cyber-terrorism/warfare/crime

JMac

@Semiclassical And I'm sure that point varies quite a bit between who the target is, who you ask, and how maliciously you're messing with stuff

Semiclassical

right

plus there's a lot of room for unintended consequences of such

Aaron Stevens

17:15

@JMac But what if I want to be able to start my microwave while sitting on my couch?

JMac

@AaronStevens That's totally fine. You just have to accept that I might also be able to start your microwave while sitting on my couch.

totally a rational tradeoff. You can do whatever you want with my appliances as long as I can sit down for an extra 15 seconds.

Aaron Stevens

@JMac BTW This chat about the propulsion system that violates physics laws is hilarious

Semiclassical

EM drive stuff?

Novalium Company

17:33

I searched the whole god damn internet, asked every god damn forum and I can't find a single schematic that converts 9V DC to 1500V DC without using giant transformers and power stage devices that weight 1 billion tons....

something so "simple" turns out to be hard as duck

Student404Mus

In peskin book of QFT the sum over zero point energy modes is an infinite c-number, fortunately, it's experimental evidence doesn't appear, since experimentalists measure the difference in energy from the ground state. According to my understanding the zro pt energy is the same as the ground state, isn't it?

If so, always it is possible to substract a finite number (higher exited state for e.g.) from this zero point enrgy (which is infinte), it follows that, experimentally we always obtain infinte spectrum.

Is it a misunderstaning statement?

JMac

@AaronStevens Yeah, I had a good laugh to myself when he responded back with "Yeah, maybe they considered it and it was just too complicated". I can't even be mad at people like that. They are clearly fairly new to physics and don't quite grasp yet that most "novel" ideas have been thought of to death by someone; likely 100+ years ago if it's classical physics

Semiclassical

Which nonsense idea are we talking about?

JMac

-4

Q: Is it possible that this electromagnetic field propulsion system would not violate any conservation laws?

I have recently come up with a design of a conceptual electromagntic field propulsion system which should not violate any conservation laws, particularly the Law of Conservation of Momentum and the Law of Conservation of Energy. In fact, this system should work in conjunction with these two laws ...

Novalium Company

17:54

@JMac Thanks for helping me in the forums. I decided to try reddit for a specific circuit, see what happens.

Semiclassical

the sheer exasperation in comments of this question: math.stackexchange.com/q/3328970/137524

JMac

18:06

@NovaliumCompany Reddit is pretty good for the type of stuff we would never touch here, since it's more discussion friendly. The quality may suffer, but there are tradeoffs everywhere

enumaris

hmmm

Novalium Company

@enumaris \o

enumaris

sup

Novalium Company

My head hurts, I've been working on the electronics all day, literally.

A simple question: I have this 2 mm sheet of plexiglass. How should I go about making a $1 cm^3$ container out of it?

enumaris

maybe the EM waves are messing with u

2mm probably won't be enough material bro

I don't think plexiglass can be flattened like gold...hmmm

Novalium Company

18:17

xDD it's 2mm thin, it's 25x25 cm

enumaris

cut out six 1 $\text{cm}^2$ faces and glue them together?

JMac

@NovaliumCompany Cut it into 6 1 cm x 1 cm sides...

damnit enumaris like 1 second before me

enumaris

sniped

Mine is even latex'd

JMac

ikr, i don't even have a latex converter here and i could still tell i was shown up

ACuriousMind

@enumaris but you didn't latex the 1 so it looks odd

enumaris

18:19

$1~\text{cm}^2$

ACuriousMind

Actually, there's lots of ways to cut out 6 faces with that size and not end up with a closed container after gluing them :P

So @JMac was slightly more correct than you

enumaris

1cm x 1cm can also describe a rhombus

or...if you are more free...any quadrilateral with 2 sides of 1cm

But I suppose his description is slightly more constrained than mine :P

Novalium Company

@enumaris Yep, thought of that. I was just wondering if there was a better way. Luckily, I have a hot glue gun.

enumaris

Mine could also be a circle of $1/\sqrt{\pi}~\text{cm}^2$ radius

wtf why is the squareroot so ugly

@NovaliumCompany a better way might be to hire someone to do it for you

then you don't even need to do the work

less work is strictly better than more work

Semiclassical

18:39

pet peeve: I make a comment about how to improve a question and they reply "but i still don't know what to do"

enumaris

o.o

Semiclassical

(in the sense of "I still don't know how to solve the problem" not "I don't know how to improve it")

enumaris

My pet peeve is people who think hot dogs count as sandwiches

Semiclassical

lol

my standard smbc reference applies here

smbc-comics.com/comic/know-your-linguistic-philosophies

Novalium Company

reddit.com/r/diyelectronics/comments/ct2zl3/…

Slereah

19:20

you know

I rememeber that Gordon Freeman's thesis was "Observation of Einstein-Podolsky-Rosen Entanglement on Supraquantum Structures by Induction Through Nonlinear Transuranic Crystal of Extremely Long Wavelength (ELW) Pulse from Mode-Locked Source Array "

Half Life predicted ER = EPR

ACuriousMind

Half Life 3 is taking so long because they first need to finish the Theory of Everything

Novalium Company

19:40

@ACuriousMind Did you watch the Kerbal Space Program 2 trailer?

ACuriousMind

not yet

Novalium Company

The top comment is mine

and is about Half-Life 3

lucky me got 600 likes ;D

Rudi_Birnbaum

Hi all, anyone there with experience in applying the Kramers-Kroning theorem?

Student404Mus

In peskin book of QFT the sum over zero point energy modes is an infinite c-number, fortunately, it's experimental evidence doesn't appear, since experimentalists measure the difference in energy from the ground state. According to my understanding the zro pt energy is the same as the ground state, isn't it?
If so, always it is possible to substract a finite number (higher exited state for e.g.) from this zero point enrgy (which is infinte), it follows that, experimentally we always obtain infinte spectrum.

ACuriousMind

@Student404Mus What do you mean by "infinite spectrum"?

In any case, in QFT without gravity, the vacuum energy is a renormalization parameter and has no physical meaning.

Student404Mus

19:50

@ACuriousMind the difference between each two levels of energy

ACuriousMind

That's not a spectrum, and I don't know how that difference could be infinite.

Do you mean that there are infinitely many states above the ground state?

If so, that is true, but has nothing to do with vacuum energy.

Student404Mus

Because of the hamiltonian expression in terms of creation and annihilation operators

I will snapshot the expression

since I am missing mathpix tool I will spend lot of time

ACuriousMind

You can type TeX in chat, see the room description for how to display it.

Student404Mus

This is the Hamiltonian

the second term is proportional to $\delta (0)$

anyway, which is infnite

ACuriousMind

yes

Student404Mus

19:56

By analogy to the simple harmonic oscillator, the second term equals to unity times1 /2

Rudi_Birnbaum

@Student404Mus You have misunderstanding, between a mathematcian and some physicist here.

for a mathematician the spectrum are the eigenvalues when I 'm not wrong

ACuriousMind

@Student404Mus No. It's infinite, it's not equal to unity.

Rudi_Birnbaum

while some physicists they call spectrum only what they can measure. And this is differences between eigenvalues

Student404Mus

@ACuriousMind It's infnite. So, this is the zero point energy mode

right?

ACuriousMind

It would be the energy if it weren't infinite

As I said, in the end the energy of the ground state is a renormalization parameter without physical meaning

Student404Mus

20:00

Alright

ACuriousMind

For a QFT beginner, I can't really improve on the discussion in chapter 1.8 of Weigand's notes

Student404Mus

@ACuriousMind What confuses me is the interpretation of Peskin to this infinite c-number and the experimental fact

He said, the second term is the sum over zero point energy modes which is infnite as you mentioned. He added," fortunately, this energy cannot be detected experm., since the experiments measure only the difference between from the ground state of H".

ACuriousMind

Sure - there's no experiment that would detect the "energy of the ground state".

This isn't really anything quantum: In classical physics, the zero points of potentials are also arbitrary, meaning there's no way to measure "absolute energy" - only energy differences

Student404Mus

20:45

@ACuriousMind Thank you, I understood your explanations clearly. However, regarding what Peskin mentioned in his book, there is a contradiction between what he said about the infinity of the zero point energy/ground state energy, and the fact that this energy is not detectable experimentally because the measurable quantity is the difference in energy between the ground state (which is infinite and this is the confusion) and a higher level.

bolbteppa

This is an example of the incompleteness of qft

ACuriousMind

@Student404Mus where's the contradiction?

Student404Mus

@bolbteppa indeed

ACuriousMind

It's just the first encounter with something that needs to be renormalized. Renormalizable theories are not "incomplete", even though you can take the Wilsonian standpoint that renormalized QFTs are effective theories cut off at a scale.

enumaris

what about re-re-normalization

or re-re-re-normalization...hmmm...

Student404Mus

20:51

according to the author, the energy differenc is always infinite according to two fact. the first is, the ground state energy is infnite, secondly, the energy differenc is defined by substituting a higher level energy from the ground state one.

bolbteppa

Which is a cut-and-paste way of simply ignoring the fact that these problems arise from the point-particle model of relativistic physics, something motivating going beyond particles to strings

Student404Mus

@enumaris Just I am focusing on what the author said in his book

ACuriousMind

@Student404Mus No. If you keep the ground state energy $\Delta_H$ as a variable, it drops out of all energy differences, which are then finite.

bolbteppa

In classical electromagnetism, similar infinities arise

enumaris

if you close your eyes hard enough the inconsistency goes away

and if you wave your hands the infinities look finite

ACuriousMind

20:58

@enumaris That is an unfairly pithy way of putting it. There are finite, rigorous frameworks for renormalized perturbation theories following the work of Epstein and Glaser (buzzword: Causal perturbation theory). Just like in many other areas, the physicist's math sweeps a lot of subtlety under the rug, but that is far from unique to QFT or renormalization

enumaris

True. I did not mean my statements to be unique to QFT.

bolbteppa

The classical electrostatics formula $H = \int \frac{\mathbf{E}^2}{8 \pi} dV = \frac{1}{2} \sum_a e_a \phi(\mathbf{r}_a)$ with $\phi_a = \sum_b \frac{e_b}{R_{ab}}$ allows for $R_{aa} = 0$ terms i.e. dividing by zero to get infinities also, the problem stems from the fact that $R_{aa}$ can be zero due to using point particles, overall it's an infinite constant added to the particle that we throw away just as in QFT

enumaris

yeah...which is why my prof always said..."to work with self forces, use energy methods"

Student404Mus

@bolbteppa I understand the idea that we need to drop such terms to be in consistency with experiments. But i cannot understand why the experiment didn't predict such infinities that arose in the theory?

bolbteppa

These $e_a/R_{aa}$ terms in the big sum are called self-energy terms, and are infinite, which means a relativistic electron would also have to have infinite mass if taken seriously, and relativity forbids the notion of a rigid body so we have to model them as point particles and can't avoid these $R_{aa} = 0$ values.

ACuriousMind

21:10

@Student404Mus I don't understand what you're saying. Experiments don't predict anything.

Student404Mus

Sorry, I mean the experiments didn't confirm such infinities that arose in the theory

ACuriousMind

How could you ever measure "infinity"?

Student404Mus

lol

I am far away from what I asked before.

ACuriousMind

It's not a joke - this is why any prediction from a theory that is "infinity" is seen as a problem with the theory

And this is why it is important that the "ground state energy" is not actually something you can measure, so it is not a fatal problem that it's infinite

bolbteppa

Basically, the same formula giving an infinity in the classical and quantum case is a good sign for the quantum theory since the classical theory already predicted it

Student404Mus

21:54

I am stuck in this calculation

I didn't find a way to inject this commutator

bolbteppa

It's the same idea as on p. 30 here www.damtp.cam.ac.uk/user/tong/qft/qft.pdf#page=30

Student404Mus

@bolbteppa Thank you for your help. It is exciting.

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