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/sci/ - Science & Math

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6507333 No.6507333[DELETED]  [Reply] [Original]

So there is quantum mechanics and classical mechanics, one is for big shit and the other is for small shit, but are they mutually exclusive? one must be wrong? why do we have two mechanics to describe the same world?

Explain this stuff to me like i'm 10, pic unrelated

>> No.6507334

QM morphs into CM as the scale gets larger

>> No.6507337

It's all quantum mechanics. Classical mechanics is just what quantum mechanics looks like when things get big.

>> No.6507345

>>6507333
> Explain this stuff to me like i'm 10
/r/explainlikeimfive/

>> No.6507386

>>6507333
>are they mutually exclusive? one must be wrong?
Classical mechanics is wrong.

>one is for big shit and the other is for small shit
Actually, the universe behaves according to quantum mechanics all the time. But on macroscopic (large) scales, the "quantumness" seems to go away.
But in fact, we see quantum mechanical phenomena all the time. Permanent magnets, light radiated from a light bulb (and infrared light from your body), and even the colors of objects (or transparency) are all examples of phenomena that can only be explained by quantum mechanics.

>why do we have two mechanics to describe the same world?
We still use Classical mechanics to describe the macroscopic world, even though it's wrong.
Why? Because for macroscopic scales, the difference between answers predicted by QM and the answers predicted by Classical mechanics are so close, that the differences are smaller than the margin of error of most of our measuring tools.
It really doesn't make a measurable difference whether we use QM or Classical mechanics, so we use Classical because it's much, much easier.

>> No.6507416

Ehrenfests theorem (a result of QM) contains all of classical mechanics. In other words, everything that we describe clasically can be derived from QM by simply applying QM to a large quantity of particles and letting the number of particles approach infinite.

Take a look at this for a derivation of Newtons 2nd law of motion from QM:

http://farside.ph.utexas.edu/teaching/qmech/lectures/node35.html

>> No.6507418

>>6507333
>So there is quantum mechanics and classical mechanics
*general relativity

>>6507386
>Actually, the universe behaves according to quantum mechanics all the time.

That's wrong though faggot. At very small scales with incredibly large masses, general relativity and quantum mechanics are mutually exclusive.

>> No.6507423

>>6507418
>general relativity
that is classical mechanics

>> No.6507432
File: 38 KB, 415x604, 1397665463778.jpg [View same] [iqdb] [saucenao] [google]
6507432

>>6507333
>Explain this stuff to me like i'm 10, pic unrelated

Impossible to truly explain without a lot of math. All non-math explanations will just do more damage to your understanding of QM.

>> No.6507433

>>6507418
>At very small scales with incredibly large masses, general relativity and quantum mechanics are mutually exclusive.
Yes I know that nonrelativisticQM and QFT don't describe the universe properly at high energy scales.
I was taking a bunch of theories—nonrelativistic QM, QFT, ST, LQG, and whatever the TOE may turn out to be—and putting them together under the name "quantum mechanics," to make it simple for OP.

But thanks for your marvelous display of autism, bro.

>> No.6507435

>>6507432
explain to me like I'm a STEM college graduate.

>> No.6507439

>>6507435
>explain to me like I'm a STEM college graduate.
Then you already understand it.

>> No.6507443

>>6507433
stay mad retard

>> No.6507445

>>6507439
Generic STEM. Not physics.
My one week of quantum mechanics was just some shitty algebraic equations we were forced to use.

>> No.6507448

Neither is wrong, at least not at the moment. If you want to get into extreme detail a vast majority of classical mechanics is wrong, but we know this and ignore it. Classical mechanics doesn't need to be exact, it's just for estimations. Quantum physics is the opposite.

>> No.6507449

>>6507443
>stay mad retard
nice rebuttal bro

>> No.6507453

>>6507445
How much math do you know?

>> No.6507455

>>6507435
>explain to me like I'm a STEM college graduate.

There's a reason why QM is a 4th year course. And there's a reason why only in grad school you get to see all the neat QM stuff.

Go pick up a QM book, say Shankar's excellent textbook, and go through it if you really want to understand things.

>> No.6507457

>>6507453
pretend multivar calc and diff eq.

>> No.6507460

>>6507455
My university offers Quantum physics as an undergrad course, one of the few if not the only.

>> No.6507464

>>6507457
>pretend multivar calc and diff eq.
Then the only remaining thing is pretend Linear algebra.
Take a look at Griffiths's Introduction to Quantum Mechanics. It's the most plebeian QM book there is, but it's good at explaining everything in a way that beginners can understand.

>> No.6507468

>>6507460
>My university offers Quantum physics as an undergrad course, one of the few if not the only.

They all do (unless you go to some shit diploma mill uni). But even in those shitty ones, you need to know a lot of math to understand QM.

>> No.6507477

>>6507468
>you need to know a lot of math to understand QM.
No you don't. Maybe for QFT you do, but for elementary QM all you need is Linear algebra.

>> No.6507499

>>6507477
⇒for elementary QM all you need is Linear algebra.

Do you define "Linear algebra" to include functional analysis, spectral theory and representation theory of Lie groups?

>> No.6507504

>>6507499
No, I mean like a Linear algebra course. I said elementary QM, as in <span class="math">basic[/spoiler] QM.

>> No.6507519

>>6507504
Even the simplest QM cannot be understood with only linear algebra. Do you even know the postulates of QM?

>> No.6507526

Not him, but most of the fundamental QM formulation relies on basic calculus + inner product spaces (& functional spaces), thus anyone who has gone through a Linear Algebra course should have all of the prerequisites to attack the formulations used in Schrodingers equation, and the rest of the basic formulation of QM in any number of dimensions.

Knowing more will only be an advantage, so I can only recommend continued advancement in Mathematics, past Linear Algebra

>> No.6507531

>>6507477
>>you need to know a lot of math to understand QM.
>No you don't.

Idiot who never took QM detected.

>> No.6507543

>>6507526
Tell me what you believe the "quantum mechanical Hilbert space" is. What kind of "functions" are allowed to be kets?

>> No.6507545

>>6507531
for fuck's sake, I said <span class="math">basic[/spoiler] QM.
And besides, Lie groups and representation theory isn't really "difficult math".

>> No.6507559

Functions which are square-integrable.

Even kets are not necessary to work with the basic 1 dimensional schrodinger equation. You will not get very far when it comes to understanding QM without them, but you will be able to solve the 1 dimensional QM box potential, harmonic osclilator and so on.

I think you are overestimating the requirements for a basic understanding of QM. Once you've seen how the basic math works you can start all over after the student has worked a little more with vector spaces, lagrangian mechanics and so on.

>> No.6507567

>>6507559
⇒Functions which are square-integrable.

So you're saying the position operator and the momentum operator have no eigenkets?

>> No.6507591

>>6507567
>So you're saying the position operator and the momentum operator have no eigenkets?
use a "rigged" Hilbert space bro.

>> No.6507609

>>6507591
Rigged Hilbert spaces aren't known in "Linear algebra".

>> No.6507751

You can use quantum mechanics to analyse macroscopic phenomena. Plainly speaking, your answers will be infinitesimally "more correct" than classical mechanics.

You cannot use classical mechanics to analyse quantum phenomena. Insofar as this I guess you could say QM is "right" and CM is "wrong."

inb4 someone brings up QFT as being more right, OP doesn't give a shit.

>> No.6507772

>>6507432
"If it makes you happy to repeat words that don't mean anything, which is in fact what unscientific people want when they ask for an explanation, then you may say that we work by exploiting the less observed properties of solar radiation."

>> No.6507783

>Ctrl+F "stat"
>No results

I thought more of /sci/ would have had Statistical Mechanics. It's the bridge between the quantum world and the macroscopic world, usually taught after Quantum.

https://en.wikipedia.org/wiki/Statistical_mechanics
https://en.wikipedia.org/wiki/Partition_function_%28statistical_mechanics%29

>> No.6507792

>Explain this stuff to me like i'm 10

Got it.

Remember that guy who used to clean the pool when you were 5?

He is your dad.

>> No.6507793

Because using QM to explain big stuff would be a huge pain.

>> No.6507903

>>6507345
>knowing reddit boards by name

>> No.6507908
File: 114 KB, 500x750, animorphs-propane.jpg [View same] [iqdb] [saucenao] [google]
6507908

>>6507334
this

>> No.6507911

>>6507903
actually I had to google it
you should check out /r/askscience
you might learn something

>> No.6507917
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6507917

>>6507911
>redditard scum

>> No.6507921

>>6507917
lol so I have to choose if I like reddit or 4chan?

>why_not_both.jpg

>> No.6507925

>>6507921
>lol so I have to choose if I like reddit or 4chan?

All you're doing is spamming that shit site. Fuck you. And read the fucking rules about spam.

>> No.6507930

>>6507917
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>> No.6507932

>>6507925
no just making fun of you.

>> No.6507941

>>6507930
fuck.
also
> yoursel

>> No.6507971

>>6507921
reddit is pop-culture,
4chan is (was originally/kind of still is) counterculture.

At this point much of the counterculture has become the culture (e.g. being a "nerd" is now cool), so it doesn't mean much, but some of us remain here who reject pop-culture entirely.