The h Bar

12:01 AM

Could please someone help me with this little problem?
I want to calculate the lenght of a photon of $780 \text{nm}$ if the observer is travelling $v = 0.99\text{c}$.
I am reading the Doopler Effect of wikipedia but there are a lot of different calculations and I am not sure which I have to use.

Mithrandir24601

@heather Yeah... He's super legit...

(although it's still only on arXiv, so there could be errors and there may be little things here and there. I'm very, very interested in where this goes, but I'm not squealing in excitement yet)

Also, if it is right, it's not surprising - a lot of people have assumed this to be the case. Still an amazing achievement if true though

ACuriousMind

@EnderLook Do you mean wavelength by "length"? Also, do you mean the observer is travelling with that speed with respect to the photon source, and that the 780nm are the wavelength of the photons in the rest frame of the source?

Mithrandir24601

Quoting from a professor of theoretical computer science: "Corollary 1 Let $P$ be the set of languages accepted in polynomial time by a deterministic Turing machine and let $NP$ be the set of languages accepted in polynomial time by a nondeterministic Turing machine. Then $P \neq NP$." Thanks to @heather for finding this :D

heather

NP =P

4

Mithrandir24601

I see what you did there

heather

12:09 AM

=D

Ender Look

Lenght = wavelenght, sorry for my english.
The photon is travelling at light speed to the observer, and the observer is travelling to the 99% of light speed to the photon, so the speed is 199% light speed. The 780nm is the real wavelengh of the photon, I want to know how the observer would see it.

ACuriousMind

@EnderLook It sounds you need to review relativity. Photons travel with light speed in all frames. In what frame is the observer travelling with 0.99c?

0celóñe7

@ACuriousMind making Mohnstrudel

@EnderLook the word is spelled length

ACuriousMind

@0celóñe7 Nice. Not in the opium variant though, I hope?

dmckee

@EnderLook In particular you should probably review the relativistic velocity composition rule.

You are used to the Galilean rule $v' = u + v$, but in a Minkowski space the rule is

0celóñe7

12:14 AM

@ACuriousMind nope, but I hear eating copious amounts of the stuff triggers drug tests

There's like a kilo of the stuff in there

dmckee

$$ v' = \frac{u + v}{1 + \frac{uv}{c^2}} \;, $$ or in terms of beta (fraction of the speed of light) $$ \beta_v' = \frac{\beta_v + \beta_u}{1 + \beta_v \beta_u} \;.$$

Ender Look

Photon 780 nm

ACuriousMind

@EnderLook I'm afraid unless you specify with respect to what the observer is travelling with 0.99c, that still doesn't mean anything. That is, what is the object that is at rest in the frame where you say the observer has 0.99c?

David Z

@Mithrandir24601 I might actually not be there for this AMA. (Which is fine.) That's our chat session time slot, right?

Secret

High energy light by light scattering results in ATLAS

Mithrandir24601

12:18 AM

@DavidZ It's a few hours (4?) after the regular chat session time - it's 9pm in the UK

ACuriousMind

@DavidZ No, the chat session is 4pm UTC, it's four hours after that

David Z

@Mithrandir24601 oh right. I mixed up my time zones.

ACuriousMind

I'll be there at that time almost certainly though, so don't worry about moderation issues

Mithrandir24601

Anyway - I'm off to bed - night all!

Ender Look

mmm, but... I think... that... I am missing something.
An inmobile person see the photon at 780 nm and also see a second person travelling at 99%C. I want to calculate how the second person would see the same photon, Is it possible?

ACuriousMind

12:22 AM

@EnderLook Ah, yes. Then your observer is moving at 0.99c with respect to the photon source, and you have to use the relativistic doppler effect.

dmckee

@EnderLook All it took was you saying what you meant.

Ender Look

What??? There is a Doopler Effect and a Relativistic Doopler Effect??? Oh my god...

ACuriousMind

Yes, the two are different. The doppler effect is for sound and every other wave carried by a true medium, the relativistic doppler effect is for light

Ender Look

ok, I'll read it

heather

@Mithrandir24601 good night =)

kind of a broad/dumb question:

how do you get from a physical gate, like a beamsplitter, to the matrix that represents the action it performs?

(in quantum computing)

ACuriousMind

12:36 AM

@heather The one rule to get matrices: The columns of the matrix are the images of the basis.

(Sounds much better when I imagine my old math teacher saying that to me in German, but it's true)

heather

the basis vectors wrt to the old basis? that's how i learned it

ACuriousMind

So you pick a basis, look what your device does to them, and then write the results into a matrix, the n-th column being the result of applying the device to the n-th basis vector. Tah-dah, there's your matrix representing the device

heather

::groans:: okay

so...in my example, i'd have to shoot a photon at a beamsplitter, measure its state before and after, and use that?

ACuriousMind

Well, ideally if you have a beam splitter, someone else should have done that for you

heather

okay, true.

is there some index of what matrices correspond to what optical components?

ACuriousMind

12:41 AM

Remember that you can't really "measure a state" - you can only measure observables and if the state that came out was not an eigenstate of the observable you're measuring, you're not really gaining much information about the state that came out

heather

right...

ACuriousMind

@heather I think that people doing that sort of experiments would know what the components do but I suspect that no one would write that up into a list because everyone "should already know" what the components do.

heather

there's a few examples on wikipedia that cover all but one thing i'd like to know, so that's good

(i don't know what i'd do without wikipedia)

i think i'll ask about that on the main site.

thanks @ACuriousMind

0celóñe7

Watching a dramatic biopic on Einstein

ACuriousMind

There's a movie about your cat?

0celóñe7

12:50 AM

@ACuriousMind no

There should be though

Emilio Pisanty

1:10 AM

Hey @ACuriousMind

::grin::

0celóñe7

Hilbert was a savage

The Raiders of Las Vegas

2:54 AM

@dmckee please do :-)

Razor

5:09 AM

@Qmechanic I feel that you are a Goldstein expert. Can you take a look at this question physics.stackexchange.com/questions/351116/…

Anonymous

@Razor "Goldstein expert" That's a strange compliment =P

Anonymous

But yeah, he seems to be awesome at classical mech

vzn

@heather its an impressive effort with high degree of sophistication however these types of efforts are not unheard of. see a page maintained by woeginger for a current list of attempts. win.tue.nl/~gwoegi/P-versus-NP.htm an example of similar work by Hauptmann from last year thats generated very little online reaction so far. wp.me/p2OLTI-1xN more on general topic vzn1.wordpress.com/category/p-vs-np

Anonymous

6:27 AM

@JohnRennie I need a bit of help with this algorithm question in Java. According to me, the outer loop runs $n$ times, and for each of those $n$ times, the inner loops runs $(n-i-1)(n-1)$. So the number of array accesses should be $\sum (n-i-1)(n-1)=\frac{n(n-1)^2}{2}-n(n-1)$. Also, each time the array is accessed thrice. So the answer should be an order $3n^3$. But that is wrong. I am not being able to figure out why :/

Anonymous

Oh, I think they are taking the $\frac{1}{2}$ also into account

Anonymous

That's strange

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