The h Bar

DanielSank

4:10 AM

@EmilioPisanty you around?

Emilio Pisanty

5:56 AM

@DanielSank vaguely

Why?

Yashas

7:16 AM

Is there a way to modify the close vote reason? I accidentally clicked on X in an attempt to mark Y.

John Rennie

@Yashas no there isn't. I've found myself in exactly the same position in the past.

John Rennie

7:54 AM

If you have the free time I strongly recommend Graham Farmelo's new book The Universe Speaks in Numbers. My review is here.

I'd guess for most of us only the last chapter will be new, but the book is well written and entertaining even when it's covering ancient history like Dirac and Einstein :-)

pss 1

@JohnRennie sir may I know in which field of physics you worked..

John Rennie

@pss1 colloid science

Interface and colloid science

Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and 1000 nm dispersed in a continuous medium. A colloidal solution is a heterogeneous mixture in which the particle size of the substance is intermediate between a true solution and a suspension, i.e. between 1–1000 nm. Smoke from a fire is an example of a colloidal system in which tiny particles of solid float in air. Just like true solutions, colloidal particles...

Slereah

morning

The East Wind

@JohnRennie Hey

@Slereah Morning

John Rennie

@TheEastWind hi :-)

The East Wind

8:01 AM

Just a quick doubt. What would happen if we perform the double slit expermient but instead of the screen used to obtain the interference patter we used a metal instead

John Rennie

@TheEastWind Do you mean a metal plate with the two slits cut into it?

The East Wind

I mean would we observe the photoelectric effect (provided the light is of sufficient energy) or interference patterns?

No I'm talking about the final screen where we obtain the pattern

John Rennie

I guess there would be photoelectrons, but they wouldn't affect the diffraction pattern. Light would still go through the slits and interfere to form the pattern on the screen.

The East Wind

But I read that at any given instant in time light can only show one type of nature?Either wave or particle.

Wouldn't what you said imply that light behaves both as a wave and as particles at the point where it strikes the screen?

John Rennie

Each individual photon can either pass through the slits or it can strike the metal and produce a photoelectron.

The photons that pass through the slits behave like waves, while the photons that eject photoelectrons behave like particles.

The East Wind

8:06 AM

I am talking about the screen where we obtain the pattern. After the light has gone through the slits.

John Rennie

As a general rule light behaves as a wave when it's propagating through space and it behaves like a particle when it is exchanging energy with something.

The East Wind

So that would mean no interference pattern?

John Rennie

A photon that passes through the slits behaves like a wave when it is travelling between the slits ad the screen. Then when it hits the screen it transfers its energy to the screen and behaves like a particle.

The interference pattern is created in the region between the slits and screen where the photon is behaving like a wave.

Slereah

why must we spend all our life discussing the double slit experiment

7

Death to QM I say

Let's all discuss metrology instead

The East Wind

In the region b/w the slits and the screen? In thin air?

Isn't it created on the screen which is placed past the slits?

John Rennie

8:12 AM

Suppose the screen is a photographic plate. Photons interact with the plate like a particle, so one photon will create one spot on the plate i.e. it behaves like a tiny particle hitting the point at the plate and leaving a mark at the impact point.

But when you send lots of photons you find the dots build up to form the diffraction pattern.

There is a nice illustration of this here:

50

A: Is it wrong to say that an electron can be a wave?

What is a wave? From sound and water waves we come to an association with sine and cosine variational behavior. Wave equations are differential equations whose elementary solutions are sinusoidal . In water waves and sound waves and even electromagnetic waves what is "waving", i.e. has a sinuso...

That's for electrons not photons, but the principle is the same.

The East Wind

Now those are interesting images..

"There is obviously interference, yet each particle also collapses to a single point when it hits the film. Which means that when the interference occurs each particle is travelling alone thus not being interfered with by anything else. Each particle, in essence, interferes with itself.

Every so often people would come up with new interpretations of this phenomena. It used to be said that each electron's probability distribution act as a wave thus interfere with itself. Now it is said that electrons are just excitations in the electric field "

That's something definitely weird..

John Rennie

@TheEastWind it's less weird than you think.

The East Wind

Dont we sometimes stretch models/assumptions too far, at the point wehre they seem(at least to me) intuituvely wrong?

John Rennie

Particles are described by quantum field theory, where they are excitations in a field. In some circumstances it is a good approximation to describe as particles and in other circumstances it is a good approximation to describe them as waves. But both of these are just approximations.

It is important to understand that an electron or photon isn't sometimes a particle and sometimes a wave. It is a quantum field that sometimes behaves like a particle and sometimes behaves like a wave.

The East Wind

Dont we abuse the meaning of the word "behave"?

John Rennie

8:25 AM

The theory that describes electrons and photons is quantum electrodynamics and this is around 80 years old so it's a well tried and tested theory.

The East Wind

I mean a LOT of chemistry is explained by thinking of electrons as particles(basicity of compounds,electronegativity and so on). How would we able to explain these effects by thinking of electrons as waves(even approximately)

John Rennie

@TheEastWind a lot of molecules have delocalised electrons. How do you explain that if the electron is a particle?

The East Wind

I mean that the lone pair of electrons are particles, which can form a pi bond between two atoms.

John Rennie

@TheEastWind particles have a well defined position. Waves don't. So if an electron is delocalised isn't it more like a wave than a particle?

The East Wind

dont particles also only have a probability of being somewhere at any point in time. So that randomness in position can be explained via this?

Mikhail

8:48 AM

I'd say the novelty was that these objects were discrete. Your detector could catch/eat/consume a discrete quantum of the field, it would also seemingly "disappear" from the other detector, something that classical fields don't do. Hence the "mechanics" of "quanta".

Also the vacuum noise, which leads to a lot of fun things like the Casmir effect, or the less well known tendency for light in a vacuum to experience chromatic dispersion because it hits stuff.

0

A: Polarization in high energy vacuum non-linearity?

You need Quantum Electrodynamics (QED) to understand vacuum polarization. In a nut shell, imagine that the vacuum is filled with virtual electron-positron pairs, and as the high-energy light propagates it interacts with these pairs and they separate creating a polarization-like effect. In some ...

Physics Meta

11:41 AM

0

Q: How do I retrieve the starred questions?

I hope this is the right place to ask. If not I will delete and ask somexhere else (where though?). I noticed there is a star I can click next to vote arrows close to questions title. So I thought I could bookmark them this way, but I cannot see any effect, nor can I find a list where those marke...

support

Secret

12:16 PM

I really like to think quantum fields are neither waves nor particles. It just when given a correct circumstances, it interacts in a way that is similar to how particle or waves interact with something, but it is always a quanutm field, never suddenly becoming something else

The thing that makes QFT much less straightforward is there isn't really much place one can build an ontology from the mathematical formulism without risk overinterpreting with classical intuition

e.g. as Acuriousmind once reminded me, that the quantum field is really an infinite dimensional generalisation of e.g. momentum operator, thus nothing is actually physically produced when we operate a quantum field onto a quantum state in the mathematics. It is only the expectation value that has physical meaning

JMac

I mean, a computer program which to the question of the form "Does x exist?" or "Does x not exist," returns the answer "x both exists and does not exist." is computable. That "phrase" or answer can be computed. Even though the answer itself is computable, it expresses something that is not computable (we cannot simulate a universe where x both is and is not). @Wolphramjonny — nielsbohrden 13 mins ago

This reminds me a lot of the person trying to push the idea that if they could get a computer program to print X = X and X =/= X it proved we were a simulation or something

Secret

The rarest thing in our universe is not antimatter

It's dialetheia

In fact, I will bet that given unbounded amount of time, the probability of having a physical infinity is much much closer to certainty than the probability of obtaining a dialetheia object

Secret

12:39 PM

List of human thinking habits:

1. Love hierarchies

2. Love categories

3. Love ontology

4. Love asking why

5. Fear of the unknown and the uncontrollable

etc.

List of nature thinking habits:

1. Really, really, really have issues with letting diletheia to exist
2. Really love ensemble averages

FIN

Captain Bohemian

1:05 PM

I like to explore the unknown but I really hate the uncontrollable.

the latter may feel like a quagmire where you are making Sisyphean effort constantly.

Captain Bohemian

1:25 PM

the exploration of the unknown which leads to unveil something new to me usually delights me very much.

user351417

3:38 PM

Any 10k rep users about?

user351417

Can someone look at physics.stackexchange.com/questions/478964?

user351417

Is it similar to physics.stackexchange.com/q/478991?

user351417

Ah, spectacular. Qmechanic posts a comment saying that the existing one is a duplicate of the deleted one just as I cast that flag.

user351417

Ah never mind. The post was undeleted : P

Kyle Kanos

4:02 PM

@JMac not sure how a device built under the principle that the LNC is absolute could violate said LNC...not sure how a potential violation would also indicate anything more than a broke computer...

JMac

@KyleKanos That was what all the comments on the question had tried to explain, but the OP kept insisting that it "represented something physical" and therefore for some convoluted reason had to be describing something physical because he could make a program output seemingly contradictory statements

the worst part IMO was he basically just made it print out "A = True; A = False" and conclude that it necessarily meant the program considered it both true and false. From my limited understanding of code, it seemed more like they were changing A from true to false and printing both results

Semiclassical

4:17 PM

@JMac seems like that's the equivalent of treating whether a coin is heads/tails as an immutable fact of the universe, and then acting shocked when it changes

Kyle Kanos

Yeah, print("True" if A else "False") can't print both answers, unless the underlying language is broken.

But you can probably make a computer language that doesn't care about LNC & output both. But it'd take special handling of the syntax

Semiclassical

LNC?

Kyle Kanos

And even then, I quite imagine the language bring basically useless

@Semiclassical law of non-contradiction

Semiclassical

ah

ACuriousMind

@JMac I can make a program print that by just telling it to print the string "A = True; A = False" :P

JMac

4:30 PM

@KyleKanos It was programmed in C or C++ IIRC, maybe Python. I'm pretty sure it was at least a common language

Kyle Kanos

Okay, then it'd never work.

Though maybe one could do it in C with some clever, evil macros....

JMac

@ACuriousMind Yeah, they basically did that, but added in a step where it printed based off the status of a variable, and then just changed that variable between calls so that it was true, then false. I might be remembering wrong, but I'm pretty sure it was that trivial

Kyle Kanos

395

A: Loopholes that are forbidden by default

Interpreting the challenge too literally That is, if the challenge says "write a function that, given a number n, returns the n-th prime", posting something equivalent to: function f($n) { return "the $n-th prime"; }

ACuriousMind

@KyleKanos You didn't say "Standard loopholes apply", so I'm fine!

Kyle Kanos

But they're forbidden by default!

ACuriousMind

4:38 PM

If true can be false, "by default" can be "not by default" :P

Kyle Kanos

My brain hurts reading that

ACuriousMind

That means it's working!

Transcript for