/sci/ - Science & Math

[Physics Guy No.3276035]

4chan tries to ban my CERN proxies, fails miserably.

LMFAO.

Physics questions anyone?

[Anonymous No.3276040]

Can you explain quantum physics in an easy-to-understand way?

[Physics Guy No.3276042]

>>3276040
Quantum Physics is a big field. What in particular are you interested in?

[Anonymous No.3276043]

Can you explain quantum physics with anything else than fancy mathematics models? Like real life experiment? And no double slit experiment doesn't count due to the lack of consideration of the electron spin.

[Anonymous No.3276045]

>>3276042
I don't even know what quantum means, so can you start with that?

[Anonymous No.3276047]

What's the meaning of life?

[Anonymous No.3276055]

>>3276043

> And no double slit experiment doesn't count due to the lack of consideration of the electron spin.

do elaborate please

[Mr. Swede No.3276057]

Uneducated fag reporting in, here to become educated.
>>3276045
Also, this

[Physics Guy No.3276061]

>>3276043
see >>3276042

Are you fucking retarded? The double-slit experiment is done with photons, not electrons. WTF are you talking about?

[Anonymous No.3276062]

>>3276057
uneducatedbro <3
>mfw OP ignores my question because he doesn't even know what quantum means either

[JamesBond !!JAU/DZkp95n No.3276064]

>>3276057
Means in discrete pieces.

[Not OP No.3276066]

hey guys, i don't wanna hijack OP's thread if he's typing out an intro to QM now, but if he's gone i'd love to try teaching something about QM.

lemme know if you're interested, i'll check back in a few to see if OP's typed anything though.

this thread has potential

[Anonymous No.3276067]

>>3276061

idiot

[Not OP No.3276076]

>>3276061
>>3276067

double slit experiment can be done with photons, electrons, or anything really.
i'm just curious to know why electron spin fucks it up.

still don't care to elaborate?

[Anonymous No.3276077]

http://www.youtube.com/watch?v=JX0wtC1LEPY

[Anonymous No.3276080]

>>3276061

The double slit experiment has been done with carbon-60 molecules.. Are you fucking retarded? Don't know shit about anything and call yourself physics guy? Fuck, I mad.

[Mr. Swede No.3276081]

>>3276066
Yeah, i'd love too! OP seems a little bit.. passive right now..
>>3276062
Hai thar!

[Physics Guy No.3276090]

>>3276045
>>3276057
Quantum comes from the idea of quantization, a set of discrete property values. Measured properties can only have certain allowed "values".

You can think of a quantum system as analagous to the positive integer system. I can get a value of "1", "2", "3", "4", etc, but not "1.5". A classical system, would be analagous to the real numbers, 1.5 would be allowed.

>>3276062
>impatient much

[Physics Guy No.3276094]

>>3276076
>>3276080
Yeah, ok, you could do it with a shit ton of things, if yall really wanted to get into that.

That is besides the point though.
It doenst matter though, >>3276043 is still fuckin nonsense. Spin doesn't invalidate the double-slit experiment whatsofuckin ever.

[Anonymous No.3276098]

>>3276090
In said system what happens if you divide 3 by 2 if you can't have in-betweens?

[Anonymous No.3276105]

>>3276055
Ok. So as we all know it, we get an interference pattern in the case of the "unobserved" shooting of electrons. But when they are "observed", or to put is simply they are put through an electromagnetic field, they suddenly start to behave like particles.

But what the electromagnetic field actually does it homogenize the electronic spin of the particles. For example technically the MRI works by homogenizing the protons and then sending a radio wave through the tissue and as such you easily measure the different density of tissues.

Now if I make a relevant correlation, though not completely correct, is in the case of soccer, when the rotation of the ball gives the ball a curvy trajectory instead of a straight one. Now presuming that the electrons have the same energy but different spins, they would travel at constant but differently oriented parabolas and the slits only function only as a filter to predetermined parabolas and thus obtaining an interference pattern.

[Anonymous No.3276111]

Physics guy is an obnoxious teenager who has never even been to university.

[Physics Guy No.3276114]

>>3276098
The number 1.5 was just an example by analogy. That value may be allowed, it may not, it really depends on the actual system and property you are working with.

I think you are grasping the "concept" though. Yes, there will be no "inbetweens". You will have a discreet set of allowed values only. This is the only set of numbers that your values can take.

These are all values you will be measuring, and you will never measure an "inbetween" value, no matter how much fucking measurement you do.

[Not OP No.3276118]

>>3276105

yeah, i'm gonna call bullshit on that.

it sounds nice, but it's called an "interference" pattern for a reason. it can only be formed when interference occurs. i'd like to see some math or proof that a bunch of parabolic paths can create the interference pattern.

i'll be waiting for that response

[Anonymous No.3276120]

>>3276090

Do you think the dark energy can be explained with help of quantum physics? Or is there room for a whole new theory?

[JamesBond !!JAU/DZkp95n No.3276138]

guys!Guys!

What if... you know...

Physics is not an interpretative science at all???

What then?

[Physics Guy No.3276140]

>>3276105
Ok, so you have a spin selector, how very Stern-Gerlach of you. This is very very very old news though, what you are describing is not the double-slit experiment.

The standard double-slit experiment does not require you to subject your particles to a polarizing EM field.

Nice try though

[Physics Guy No.3276149]

>>3276120
>Do you think the dark energy can be explained with help of quantum physics?

Yes
The only thing we need now is more data.

[Not OP No.3276153]

>>3276120
here's how i look at it: if it could, it would have been already. it requires a whole new way of thinking, we really have no idea what's going on with dark matter/energy.

but just to elaborate on the meaning of *Quantum* Mechanics:

it works like this: suppose that you are trying to fit a sinusoidal function in a box. well it's not really a box, but just think of it as bounded on both sides. so, for example, you're trying to fit y = sin(k*x) between x = 0 and x = pi, where k can be any number. in addition, you want to impose the condition that the sine *must be 0 at x = 0 and x = pi* (i can elaborate on why we need this later, if you want to know). if you think about this for a little bit - and maybe write some math or draw some pictures - you'll see that this "quantizes" the values that k can have!

i've already done this, you can see it in these graphs:
http://www.wolframalpha.com/input/?i=Plot%5B{Sin%5Bx%5D%2C+Sin%5B2+x%5D%2C+Sin%5B3+x%5D}%2C+{x%2C+0%
2C+Pi}%5D

http://www.wolframalpha.com/input/?i=Plot%5B{Sin%5B0.5+x%5D%2C+Sin%5B1.5+x%5D}%2C+{x%2C+0%2C+Pi}%5D

in the first link, you'll see values of k that are all integers. and notice that the sine is nicely 0 at x = 0 and x = pi (we say that it "fits" there nicely).

but in the second link, you'll see half-integer values of k. again, notice that the sine doesn't fit our restrictions at all!

so we see that - if our conditions are to be satisfied - k can only have integer values! thus, k is said to be "quantized". additionally, anything that depends on k is said to be quantized as well (can you see why this would be true?)


in reality, this problem is known as the "particle-in-a-box" problem, and you may have even heard of it sometime. and in this problem, the wave represents a particle, and the energy of the particle can be shown to depend on k! thus, the energy of the particle is quantized!

i hope this helped, do you guys have anymore questions? (of course, i'm happy to answer)

[Physics Guy No.3276168]

>>3276153
> requires a whole new way of thinking

You are jumping the gun there kid. We are not at the point yet. Right now the only thing we need is a sufficient data. You cant expect us to sufficiently explain dark matter with the shit data we have at the moment.


Nice example of "quantization", although it may be a bit too complicated for most of those here.

[Not OP No.3276190]

>>3276168

you're probably right, it could just need more data. it just doesn't look too hopeful at the moment!
i'm not following those discussions as closely as i'd like, but the general consensus seems to be "oh fuck we have no clue what the shit is going on"
have any theories been presented that are doing pretty well to explain them yet?

[Anonymous No.3276191]

>>3276153

Thanks for the example.

What does the 'box' represent then? Quantum states?
What is the

[Anonymous No.3276197]

>>3276153
You're not really describing quantization though. Quantization is when you go from a classical description with c-numbers and points in phasespace, to a picture with Hilbert spaces and hermitian operators. Or if you want to be more fancy, path integrals. The essence of quantum mechanics really is that a state isn't a point in phasespace anymore, instead it is a vector member of a Hilbert space (a ray really). This means that states can be added, giving us interference, superpositions and so on.

[Anonymous No.3276204]

>>3276168
Dark matter isn't really a problem, SUSY solves it in a reasonable fashion and we have other models that also works out as well. We only have to find low-energy susy or something else, which LHC will do, probably. The small cosmological constant is a much worse problem.

[Physics Guy No.3276207]

>>3276190
>>3276190
>have any theories been presented that are doing pretty well to explain them yet?

FUCKING Tons! Including MACHOs, RAMBOs, WIMPs, axions, Holeums, even certain types of neutrinos. The one thing that all these theories lack is more fuckin data.

>oh fuck we have no clue what the shit is going on

Not really. You should take a basic course in cosmology. Dark matter and shit like that is already incorperated into standard cosmological models (and general rev), and gives fucking great results (consistant with observations).

[Physics Guy No.3276222]

>>3276207
Dark Matter = a particle that interacts very very very very very very very very very very fucking weakly through all forces (except Grav).

We don't even have sufficient data to really detect grav of individual particle yet, how the fuck are we supposed to detect what dark matter actually is (what particle/properties).

WE need more data, that is all there is too it. We don't face a lack of theory or cleverness, we fucking face a lack of data. Until we have more data we really cant do much. It is likely that questions concerning dark matter wont be fully resolved until we explore space.

[Anonymous No.3276271]

>>3276035
Expain the ideas behind and difficulties regarding the Navier-Stokes equations, kthx.

[Not OP No.3276276]

>>3276191

great question.

the "box" is actually a potential well. it would help if you've had some good physics courses, but i'll try to explain it assuming you don't.

think of a potential well much like a physical well (like the thing you'd lift water out of). if you were at the bottom of a very deep well, you'd have a hard time getting out. this is because you don't have enough energy to overcome the potential energy surrounding you.

as another example, think of a ball trapped in a huge valley with 2 mountains on either side. if the ball had enough kinetic energy, it could roll out. but it doesn't, so it's stuck!

this is why we call it a potential well, because the particle (or ball, or person) is stuck inside, and doesn't have enough energy to get out.
this is what the box represents. if we graph potential versus position (potential on the y-axis and position on the x-axis), then - for the example that i gave before - potential would be something very large (possible infinity) to the left of x = 0 and the same thing to the right of x = pi. and in between, 0 < x < pi, it would be 0 (that is where you are. potential energy is set arbitrarily to 0 there). it might look something like this:

http://www.wolframalpha.com/input/?i=plot+Piecewise%5B{{1000%2Cx%3C0}%2C{0%2C0%3C%3Dx%3C%3Dpi}%2C{10
00%2Cx%3Epi}}%5D+from+-2+to+5

(continued in next post)

[Not OP No.3276281]

(continued from previous)

so then, let's ask "what's the likelihood oh the ball (or person) being anywhere outside of it's little valley?" well if it doesn't have the energy to overcome it (like the wall is infinitely high), then it's impossible to escape!
this "likelihood" or probability to be somewhere can be represented by the "wave-function" of the particle (this is where it gets a little complicated - but don't worry about it for now).

so the wave-function of the particle is 0 outside of the potential well. we also require (for other reasons) that the wave-function be continuous - that is, it doesn't have any jumps, it's a smooth curve. so, if it's 0 for x < 0 and x > pi, then it must be 0 at x = 0 and x = pi in order for it to be continuous. in other words, it couldn't be 0 at x = -0.001 and then all of a sudden it's 12 at x = 0.

and if you recall, this is why we had to force the sine wave to be 0 at x = 0 and x = pi!

then, we can show (although it's somewhat difficult) that the wave-function (remember, this just represents the likelihood of finding the particle at any given position) must be a sine wave inside the potential well.

thus, it has to follow the rules and restriction that i detailed in the first post. and from there we derived that k and the energy of the particle is quantized

[Anonymous No.3276316]

>>3276281
Thanks for the explanation.

I'm going to have a stupid question. Few of them, even
The probability function must be always greater than or equal to zero, or?
And the sum of all probabilities a one?
How does quantum tunneling come in here? Wasn't that supposed to mean a probability of finding particle outside the well was not zero?

[Anonymous No.3276333]

Is gravity also Time?

[Anonymous No.3276342]

Is there really good evidence suggesting dark matter/energy exists, or is dark matter/energy just a good theory to fit the evidence?

By that I mean that if it was leaning more towards the former, it would predict new phenomenon more so than it explains explains pre-existing ones.

[Anonymous No.3276393]

How can I deliver a few gigajoules to a point on the Earth's surface within an accuracy of 5 m for less than $500?

[Not OP No.3276395]

>>3276316

nice, those are all good questions!

>The probability function must be always greater than or equal to zero, or?
there's actually a distinction here that i haven't been to clear about. the function i was talking about, the "wave-function" (written as ψ(x), the Greek letter "psi"), is not actually the probability. the square of it's magnitude is actually the probability: |ψ(x)|². we take the magnitude because ψ can sometimes be complex, so the most general probability function is: ψ(x)ψ*(x), where ψ* is the complex conjugate of ψ. (note that this reduces to |ψ(x)|² if ψ is entirely real).

so then, although ψ(x) can be negative at times, the probability function, |ψ(x)|², will always be positive.


>And the sum of all probabilities a one?
yes! this is actually a very important result, and it is known as the "normalization equation":

∫ |ψ(x)|² dx = 1

where the limits of the integral are -infinity to +infinity (or, more generally, "over all space". in the 1-dimensional case, "all space" is -infinity to infinity).

so then let's say that ψ(x) is a sinusoidal function. we can use sine or cosine, but it is more general - and very interesting! - to use to complex exponential function, e^(ix). we can do this because we know, from Euler's formula, that e^(ix) = cos(x) + i*sin(x).

so let's let ψ(x) = A*e^(ikx), where A is the amplitude (unknown, at the moment) of the wave.

now also note that ψ(x) = 0 for x < 0 and x > pi. so we can break up the integral from -infinity to +infinity to only ψ(x) = A*e^(ikx) from 0 to pi:

∫ A² e^(2ikx) dx = 1, where the limits of the integral are 0 to pi.
i don't want to run through it all now, but you can see that once we pull A² out, the rest of the integral will just be a number (assuming we can find k, and indeed we can, using another equation - the Shroedinger Equation). so we can solve for the amplitude of the wave-function in order to make the sum of the probabilities 1.

[Not OP No.3276399]

(continued from last post)

>How does quantum tunneling come in here? Wasn't that supposed to mean a probability of finding particle outside the well was not zero?
tunneling is a very interesting and important phenomenon, and i did a good job to intentionally avoid it here.
i purposely set up the problem so that there would be no tunneling. you can actually show that tunneling does not occur when there is an infinite potential wall, or when there is no break later on in the potential wall.

for example, the following set-up would have tunneling:
the potential, V(x), (instead of being infinite, then 0, then infinite) is:

V(x) = 0 ( x < 0 ), V1 ( 0 < x < a ), 0 ( x > a )

so then we have a potential wall with height V1 for a distance a.

in this case, there will be some probability of the particle finding itself on the other side of the potential wall even if it's energy is less than V1 (like 0.8*V1 for example).

the mathematics of this are quite easily worked out using reflections and transmissions of the wave-function at the boundary of the potential wall.

[Anonymous No.3276436]

>>3276395
>>3276399

That's... pretty fascinating, thanks Not_OP!

[Anonymous No.3276442]

helo im 14 and i want to build a fusor is this legal

[Not OP No.3276459]

>>3276436
no problem!
if you (or anyone else) have any other questions about how this works, i'd be happy to answer them!

>>3276442
yup.

[Anonymous No.3276461]

>>3276197
Go on...
Or ignore me if I'm late to the request. I've been reading and not refreshing

[Anonymous No.3276516]

What is entropy?

[Anonymous No.3276539]

>>3276062

A quantum is the minimum amount of any physical entity involved in an interaction.

Straight from wikipedia, bro.