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3:43 AM
At this point I'm writing more greek letters than latin letters
maybe i'll just switch to a greek keyboard instead of typing the entire latex command
 
123
4:22 AM
GooD Morning
 
5:10 AM
any mod online?
 
123
Don't know :-)
 
5:41 AM
 
6:27 AM
COVID19 Vaccination may start from December...
 
123
Is Vaccine developed?
 
Being worked.... testing phase (last one)
 
123
Good or Bad News? Because there are a lot of rumor for vaccination.
not vaccination for vaccine
 
what?
These are rumors.... not to be taken seriously

 देसी अड्डा॥

याहाँ पर सारे बंदे आ कर अपना-अपना ज्ञान बाटते है॥ (People talk...
 
EM waves are produced by oscillating charges. What does that mean? Do the electrons move left and right and produce the EM wave somehow?
 
123
6:32 AM
Hmm.. Yes i know but there are also peoples still not taking it seriously. @Azmuth i can not read hindi language
 
Not Hindi, but Englished version of Hindi
 
123
Aaah i see.
 
@JingleBells They don't need to move, you just need oscillation of fields at all (there are various ways to accomplish that without actual electrons)
 
@JohnRennie is electron a wave ? I am just confused . Please help me.
 
@Ankit an electron is an excitation in a quantum field so it is neither a wave nor a particle. But it can behave like a wave in some circumstances and it can behave like a particle in other circumstances.
 
6:42 AM
@Ankit All quantum particles are intrepreted as waves in space
 
So we know that current is the flow of charges, but the electrons themselves move very little. It's the "pulse energy" that's transferred from electron to electron. But what actually produces the EM wave? The moving of the electrons or the pulse energy of the bumping electrons (like cars in traffic)?
 
@JohnRennie when they are in orbits , how do they behave ?
 
@Ankit it's very important to understand that electrons do not orbit the nucleus in the way that planets orbit a star.
 
I think they are particles and the probability of finding them waves . Am I right ?
 
123
There is wave-particle duality. for smaller particle it is more wave due to smaller mass.
 
6:53 AM
In an atom an electron is like a fuzzy blob. It doesn't have a position in the sense we think of a particle having a position. Instead is is spread out over a region of space, a bit like a cloud in the sky.
 
@JingleBells basically, moving the electron will disturb the electromagnetic field
and that disturbance will travel to different regions
 
@JohnRennie So can a single electron cloud cover the nucleus ?
 
like throwing a rock into a pond, the wave propagates
 
So it isn't a wave like a sine wave, and it isn't a particle like a point mass somewhere inside the atom.
 
123
If you disturb the particle then it shows particle behavior (Quantum Decoherence). Otherwise it is in wave nature
 
6:54 AM
@JohnRennie check this
 
@Ankit yes
 
@123 A particle is always wave (copenhagen interpretation)
 
@JohnRennie then why do we need to define probability of finding them ? What are we finding if we know they are fuzzy ? Are we finding the center of that fuzzy blob ?
 
@Ankit "location" isn't a well defined term for subatomic particles, generally
 
@Ankit You have to define precisely what you mean by "finding the particle".
 
123
6:57 AM
Yes. If one disturbed the system it shows particle behavior
 
@Ankit In chemistry we use concepts of electron cloud, So, yes in case of orbitals we are finding that fuzzy blob, but in physics where high accuracy is required, we are just finding their probability distribution in space
 
123
it depend on the mass of particle
 
@Ankit For example we could fire a high energy electron into the atom. If it strikes the electron in the atom it will knock it out of the atom. Then we can measure the trajectories of the two electrons and trace them back to see where the collision occurred. Yes?
 
123
uncertainty principle. lower the mass higher the wave nature
 
yes
UCP
 
7:00 AM
basically, particles don't have a singular location in space when unobserved. now, if we ever want to observe them, we need to disturb the system (e.g. send a photon that interacts with the particle, unavoidably changing its state, and comes back to our eyes). that lets us learn the particle's location during observation, but tells us nothing about its state before we observed it
this is called the observer effect
In physics, the observer effect is the disturbance of an observed system by the act of observation. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner. A common example is checking the pressure in an automobile tire; this is difficult to do without letting out some of the air, thus changing the pressure. Similarly, it is not possible to see any object without light hitting the object, and causing it to reflect that light. While the effects of observation are often negligible, the object still experiences a change. This effect can be...
 
@JohnRennie if electrons are clouds then what does it mean for knocking out an electron ?
 
123
The beauty i found QM mechanics does not violate the previous experimental results. It shows them as it was proven.
 
electrons aren't clouds!
clouds are visual 3D Representation that represent probability of electrons.
Electrons can be anywhere, we don't know, they don't have fixed trajectrory
 
@Ankit what is happening here is that firing a high energy electron into the atom perturbs the atom and changes the wavefunction of the electron in the atom.
In an undistubed hydrogen atom the electron is lie a cloud, but when we fire in a high energy electron it interacts with the electron in the atom.
 
Also, electrons can pass through barriers, for example that they can pass through walls, surfaces, other materials, this effect is called, tunnelling
Quantum tunnelling or tunneling (US) is the quantum mechanical phenomenon where a wavefunction can propagate through a potential barrier. The transmission through the barrier can be finite and depends exponentially on the barrier height and barrier width. The wavefunction does not disappear on one side and reappear on the other side. The wavefunction and its first derivative are continuous. In steady state the probability flux in the forward direction is spatially uniform. No particle or wave is lost. Some authors also identify the mere penetration of the wavefunction into the barrier,...
 
7:03 AM
@Ankit This interaction changes the wavefunction of the electron in the atom so now it looks more like a point particle.
 
@Ankit Honestly most things in QM will make sense if you get an intuition of the concepts for general waves first
 
The point of all this is that the electron only has a position because we interacted with it i.e. fired a high energy electron at it.
 
tunneling, the uncertainty principle, etc. are all things that exist for waves in general, and translate over to QM
 
Only sensible way to teach quantum Mechanics is to teach them through interpretations, experimental flaws or line integrals.
experimental flaws suits good for beginners!
 
@JohnRennie first of all the electron you used is also a fuzzy blob (right ?) And so when two fuzzy blobs collide (I don't know what collision means here) the first cloud is removed out . Why ? They can interfere and give a dense cloud
 
7:06 AM
The interaction between the two electrons is complicated and I don't think I can explain it in a simple way. But you are correct that initially both electrons are fuzzy, and the collision localises them both to a point.
We can trace back their trajectories after the collision to find out where that point was.
The point I am trying to make is that particles like electrons can exist in lots of states, particle-like, cloud-like, wave-like, and probably others.
 
When we say oscillating charges, we're talking about the moving atoms (that have electrons and protons in them), right? I mean, flying protons and electrons by themselves can produce EM waves but they are usually attached to an atom, right?
 
What state the particle appears in depends on how it is interacting with the rest of the universe around it.
 
@JohnRennie wait a sec. How does that collision localise them to a point ? They can exists as a dense cloud too spreaded to the same space as the one in the atom ?
 
@JingleBells in everyday conditions yes
but more accurately the majority of the universe consists of ionized gas, i.e. electrons not being bound to the atoms
 
@JingleBells see:
68
A: Is there oscillating charge in a hydrogen atom?

Emilio PisantyIn this specific instance you are correct. If you have a hydrogen atom that is completely isolated from the environment, and which has been prepared in a pure quantum state given by a superposition of the $1s$ and $2p$ states, then yes, the charge density of the electron (defined as the electron ...

 
7:10 AM
@JingleBells electromagnetic waves are defined as perpendicular oscillation of electric and magnetic fields
 
@Ankit "How does that collision localise them to a point?" - that's complicated ...
 
well maybe not the majority, but an important amount
 
plasma
There was a theorem that all the wavical solutions of schrodinger's solution are all orthogonal to each other.... I forgot that one :(
 
So in an antenna, the electrons (bounded to atoms I suppose) themselves have to oscillate/move to produce an EM wave, not the energy pulse signal?
 
It starts by taking conjugate wavefunction of the equation and subtracts them to one side and integral vanishes on other....
something like that.
 
7:14 AM
@JingleBells it's the conduction electrons that move, and the conduction electrons are free to move through the metal not bound to any particular atom.
 
123
What is the charm and harm to being a physicist?????
 
But yes, it's the moving conduction electrons that radiate the photons.
 
@123 QM tells us that anything charming is made of particles, called charm quarks
we've yet to find a harm quark
all right i'm going to sleep, 'night
 
123
@SirCumference :-)
:P
 
@JohnRennie Do you remember the proof?
 
7:18 AM
Got it, thanks. So you simply need moving charges (electrons/protons) to produce an EM wave. But how does their movement/oscillation lead to the EM wave (flying photons)?
 
The EM wave and flying photons are the same thing.
@Azmuth what proof?
 
6 mins ago, by Azmuth
There was a theorem that all the wavical solutions of schrodinger's solution are all orthogonal to each other.... I forgot that one :(
 
123
@JingleBells See antenna. When charge is zero on antenna EM fly.
 
@JohnRennie yes, I know :P
 
123
Charge is oscillating. It is zero between shifting dipole.
 
7:20 AM
2
Q: Are eigenfunctions always normed and orthogonal?

Sean BoneI came across this simple proof: We show that Hermitian operators have real eigenvalues. The definition of a Hermitian operator is \begin{equation} \langle \phi_i | \hat A | \phi \rangle = \langle \phi_i | \hat A | \phi \rangle^* \tag{1} \end{equation} Then if $|\psi\rangle$ is a...

 
How does a moving electron give birth to a photon?
 
@Azmuth this is the spectral theorem:
In mathematics, particularly linear algebra and functional analysis, a spectral theorem is a result about when a linear operator or matrix can be diagonalized (that is, represented as a diagonal matrix in some basis). This is extremely useful because computations involving a diagonalizable matrix can often be reduced to much simpler computations involving the corresponding diagonal matrix. The concept of diagonalization is relatively straightforward for operators on finite-dimensional vector spaces but requires some modification for operators on infinite-dimensional spaces. In general, the spectral...
 
123
Answer lies in QFT. May be electron field disturbed the photon field in space
 
@JohnRennie Ah, yes. This one!
@JohnRennie Thanks :)
 
@JingleBells classical EM or quantum mechanics?
 
7:22 AM
@JohnRennie whichever explains what actually happens
 
@JingleBells "actually happens" is one of those phrases that sounds like it means something, but when you look closely no-one knows what, if anything, it does mean.
 
xDD
Alright, classical EM
 
Both classical EM and QM gives descriptions of the process that work well in some circumstances and less well in others. As to what "really happens", who knows?
For the classical treatment see:
 
Okay, afternoon :)
bye :)
 
In electrodynamics, the Larmor formula is used to calculate the total power radiated by a non relativistic point charge as it accelerates. It was first derived by J. J. Larmor in 1897, in the context of the wave theory of light. When any charged particle (such as an electron, a proton, or an ion) accelerates, it radiates away energy in the form of electromagnetic waves. For velocities that are small relative to the speed of light, the total power radiated is given by the Larmor formula: P = 2 3...
 
7:26 AM
@JohnRennie Sorry, I'm not a physicist and these equations mean nothing to me. I need a simple explanation, with words if possible :D
Isn't there a simple explanation as to why a moving electron/proton can produce a photon?
 
@JingleBells yes! :-)
65
Q: How and why do accelerating charges radiate electromagnetic radiation?

clawsLet's consider it case by case: Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property. Case 2: Charged particle started moving (it's accelerating). We were told that it starts radiating EM radiation. Why? What happened to it? What made it do t...

 
Hmm, so it's the propagating electric field that produces a photon? How? I get that as the charge is accelerating, it's electric field also changes and propagates outward, but how is this electric field supposed to represent a photon (EM wave) and I didn't even know that this electric field can actually exist as a real thing and not an abstract representation.
 
Photons don't exist in the classical interpretation. To understand how photons are created we need to move to quantum field theory.
In QFT the objects we see as particles are excited states of a quantum field that fills all of spacetime. So particles can be created by adding energy to the quantum field to excite it to a higher energy state, and particles can be destroyed by removing energy from the quantum field so it moves to a lower energy state.
So the electrons are excited states of an electron quantum field, and the photons are excited states of a photon quantum field.
An accelerating electron creates photons when energy is transferred from the electron quantum field to the photon quantum field.
But, if you're going to ask me how exactly this happens I'm going to say ... erm ... I don't know :-)
 
Got it, thanks! And in the classical explanation, the electron has an electric field and a magnetic field and as the electron accelerates, both fields change and propagate outward?
 
123
Actually we don't know anything about nature. We just performed experiment and it gives us idea how to put this phenomenon in simple mathematics. This is what we are doing so far.
 
7:39 AM
@JingleBells yes
 
@JohnRennie Thanks! Just one last thing, are electric and magnetic fields a real materialistic thing, or are they just an abstract concept that helps us describe how reality works?
 
Again, this is one of those "what really happens" questions.
 
123
There is always unanswered questions and mysterious (It could be GooD news or BaD news). GooD in a sense it tickle to perform more experiment to understand more the research is going. Bad in a sense we don't understand the phenomenon completely.
 
We describe a field using an equation that we get by solving Maxwell's equations. Is this equation what really happens, or is it just a description of what really happens? Who knows?
 
@JohnRennie Got it. Thanks for the help! :--)
 
7:48 AM
:-)
 
 
3 hours later…
10:29 AM
That's definitely a satire website
 
Let's not have the first thing people see when entering the chat room be that.
3 messages deleted
 
 
1 hour later…
11:51 AM
@JohnRennie it is written in my book that volume of gases mean volume of container and then I read avogadro's law that equal volume of gases have equal number of molecules . How can both be true ?
Is it necessary that the number of molecules should be same ?
 
@Ankit Hi :-)
Do you know the ideal gas equation of state?
 
@JohnRennie yes
 
OK, so if we start from PV = nRT we can rearrange this for the volume:
V = nRT/P
 
@JohnRennie what is V here ? Volume of gas or container ?
 
Here n is the number of moles so the number of molecules is n x Na (Na = Avagadro's number).
@Ankit If you put gas molecules into a container those gas molecules are all whizzing around at hundreds of metres per second due to thermal motion. Yes?
 
11:58 AM
@JohnRennie yes
 
So the gas molecules quickly spread out to fill the whole container, where "quickly" means in a fraction of a second.
 
@JohnRennie if there were 10 molecules then they will spread out but if we add 10 more then the 20 molecules will spread quickly and take up the volume of the container . So where am I wrong ?
 
You are not wrong. If we put in any number of molecules they will spread out to fill the whole container. But the final pressure will be different for different numbers of molecules.
The pressure is P = nRT/V
So if we keep the volume constant and the temperature constant the P is proportional to n
 
@JohnRennie if I am not wrong then is the avogadro's law wrong ?
 
Avagadro's law tells us that equal volumes of gases contain equal numbers of molecules if and only if the pressure and temperature are the same.
 
12:07 PM
@JohnRennie the pressure about which you are talking is the one applied by the gas or the one applied on the gas ?
 
The two are equal and opposite.
 
If anyone knows about cross-validation, am I supposed to use this as a way to choose which model I should use? Like, I can do cross-validation on multiple classification algorithms and then choose the one that has the lowest error produced by the cross-validation?
 
Suppose I have a container of gas at a pressure of 1 atm, then it means the container exerts a pressure of 1 atm on the gas, and the gas exerts an equal and opposite pressure of 1 atm on the container.
 
12:24 PM
@JohnRennie the total pressure exerted depends on velocity of each and the number of molecules . Right ?
Assuming time of collision for each the same
 
@Ankit Yes.
 
@JohnRennie so a constant temperature of two different containers means same velocity of each molecules in those container and thus constant pressure applies that the number of molecules will be same . Right ?
 
The pressure depends on the number of molecules hitting the walls of the container per unit area per unit time.
The number of molecules hitting the walls per unit area per unit time depends on the density of the molecules i.e. n/V, as well as the speed of the molecules (because the faster the molecules move the more often they hit the walls of the container).
 
@JohnRennie I meant to say that since they have same temperature (so same speed) and collide with same unit area for same time doesn't equal pressure mean equal number of molecules ?
 
Remember that the number of collisions per second depends on the volume, because molecules have to cross the container to reach the other wall. The large the container the longer is take the molecules to cross it, so the fewer collisions per second.
So the pressure depends on the size of the container as well as the speed of the molecules.
That's why in the equation of state we get P = nRT/V i.e. all three values n, T and V matter.
 
12:37 PM
@JohnRennie so does equal pressure at equal temperature mean equal number of molecules per unit volume ?
 
Yes
 
@JohnRennie and hence equal volume mean equal number of molecules . Right ?
 
Yes
 
Thanks 👍
 
:-)
 
1:05 PM
I don't understand what I'm supposed to do with the cross-validation result. I mean, let's say I choose linear regression as my model and then I compute the cross-validation thingy and I get a number that represents the "generalizability" of the model on unseen data. And then I'm supposed to do this with a few other linera models and go with the one with the highest cross-validation score (accuracy)? I just did that and the cross-validation results of linear regression and lasso algorithms is
almost the same.
I think I get it. When choosing a model, it's better to compute their CVs instead of just checking the accuracy on a single test sample because the CV method gives a better estimate of how generalizable and good the model is. So if I'm wondering which model to use for a certain problem, I can just check their CVs and that's gonna give me a better estimation of their "goodness" than just checking accuracy on a single test batch because CV produces a result that shows how well the specific model
can generalize to unseen data.
Got it, nevermind.
 
1:28 PM
@Ankit If you double the (absolute) temperature, you double the mean energy of the molecules, making their speed SQRT(2) times more. Thus, any molecule hitting the wall, will give to it SQRT(2) times more impulse. Furthermore, SQRT(2) times more molecules will hit the wall (per unit time). Thus, the total impulse given per unit time will be 2 times more. Thus, the pressure will be two times more.
 
 
1 hour later…
2:42 PM
How far is concept of centre of mass applicable in various branches of physics?
 
 
2 hours later…
4:59 PM
Ugh.
 
Life in general, or something specific?
 
I just slept two hours in the middle of the day and I still don't really feel awake, so that 'Ugh.' is just a general representation of my mental state :P
 
It destroys me if I nap during the day. I don't feel fully awake for the rest of the day. Only in extreme circumstances, usually involving over consumption of beer, do I attempt it. Maybe it's something you get used to since it's common in lots of cultures.
 
5:18 PM
I don't usually nap either, but I just couldn't really keep my eyes open
 
5:37 PM
Hello there anyone?
Hello sir
 
6:14 PM
Napping more than 10 minutes absolutely destroys me. If I nap like an hour I wake up feeling terrible and it takes awhile to go away.
 
123
6:25 PM
Yo
 
I need a 15-minute reset every day, otherwise, I feel sleepy
 
123
Reset like computer?
 
yes, my brain is a computer
 
123
;-) Yes
Where other guys are today?
@JingleBells Where you from.
 
@123 Bulgaria
 
123
6:34 PM
@JingleBells what time is it at your place?
 
8:34 PM
 
123
here 11:35p.m
I see you are 3 hours beind
 
123
Are you physicist ?
 
nop, a high school last year "student"
 
123
6:37 PM
Okay. high school means. you are at 13th standard.
 
I'm 12th grade
 
123
We have school till 10th standard. then college education for 11th and 12th sandard. Then university onward
 
how old r u
 
123
33
and you could be 18 ot 19
 
ye, 18
 
123
6:41 PM
nice
How long you are in this room.
 
2 years
 
123
Most of the users of this group are very suppostive like John, CuriousMind, SirCumference, Azmuth
 
yup
 

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