Quantum Physics

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Re: Quantum Physics

Post by Pettytyrant101 on Wed Jul 04, 2012 9:38 pm

Very Happy

When you say things pop in and out of existence- where exactly are they popping in and out from? Suspect

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Re: Quantum Physics

Post by David H on Wed Jul 04, 2012 9:59 pm

There is no "from". This is where wave analogies work well intuitively. Think about flat water.
Now think about a snapshot just after a splash has caused a wave.

Where did the peak come from?
Where did the valley come from?


Nowhere really. They're just our names for certain shapes that the water can take.

A more interesting question might be "where did the energy to make the wave come from?"
Then you can start hypothesizing about pebbles thrown into pools and such.
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Re: Quantum Physics

Post by halfwise on Wed Jul 04, 2012 10:04 pm

David has it right, particles are just ripples on the surface, which makes their ability to appear less mysterious; but of course ripples require energy.

The uncertainty principle allows fluctuations of energy within a short time; the larger the energy the shorter the time. So things really can appear out of nowhere so long as they vanish again shortly. To conserve charge and other quantum numbers they appears as matter-antimatter pairs, and typically annihilate each other soon after (but since they came from nothing they don't emit a flash of energy when they 'annihilate'). This popping of things in and out of existence is called the vacuum energy. It has been measured and is very real. Think of waves happening and then canceling themselves out.

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Re: Quantum Physics

Post by David H on Wed Jul 04, 2012 10:05 pm

Halfwise, I'm afraid I lost you at "I can draw parallels to the properties of the electron and other particles...". I think I understood the properties, but I'm still shaky on the parallels.
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Re: Quantum Physics

Post by halfwise on Wed Jul 04, 2012 10:49 pm

The clustering of vacuum energy particles around electrons and such cause them to have polarity and moment of inertia, the clustering of Higgs particles around everything else causes them to have mass.

At least that's the way I'm reading it. I'm no field theorist, not by a long shot.

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Re: Quantum Physics

Post by David H on Wed Jul 04, 2012 11:03 pm

OK, I get the analogy now. Thanks. I'm curious what the math on all this looks like, but I'm terrified to look. Shocked
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Re: Quantum Physics

Post by halfwise on Wed Jul 04, 2012 11:15 pm

I've seen scraps of it, there's enough stuff they're not telling you about to make it more complicated than it should be. No, beyond a certain point I've never been able to follow it. But read Feynman's QED to get an idea. Many graduate students sat in on these public lectures just so they could get an idea of where all this math was going.

but I can tell you there are lots of infinite series (which sometimes can be reduced algebraically the way 1/(1-x) = 1 + x^2 + x^3 + ....) to account for all the possible occurrences of particle interactions. It's these infinite series which can't be reduced algebraically that hold things up...assuming they even are open minded enough to take into account all the possibilities.

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Re: Quantum Physics

Post by David H on Thu Jul 05, 2012 12:10 am

I went to the Amazon reviews for Feynman's QED, and I wasn't reassured. If the math still looks like a patch on a tire, then for me the theory isn't really ready to roll. I still may give it a try after harvest when the nights are longer though.

He briefly discusses the process of renormalization (that he admits is not mathematically legitimate), which is required to get answers that agreed with experimental data and the difficulties in determining the coupling constants that are also required. In the end, he admits that there is no mathematically rigorous support for QED. Its virtue lies in the fact that it provides the correct answers, even if the approach to getting them involve a bit of hocus-pocus (again his words)........

As far as the deeper questions of why photons and electrons obey the ruled of QED, he does not care, so long as he can get the right answer. This may therefore not be the book for you if you are interested in this deepest WHY, but it definitely is if you want to know more about Feynman's powerful approach to quantum mechanics.
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Re: Quantum Physics

Post by halfwise on Thu Jul 05, 2012 12:29 am

the thing is, there is no deeper why...the Feynman approach is at the base of the standard model (Quantum Chromodynamics). At present we don't have anything deeper that is currently producing predictions consistent with observation. Yes, it looks like a patch on a tire, but if you are waiting for something better, it's been 70 years (for QED, QCD is 40 years)...I wouldn't try to wait it out. And the weak points are only in the math, the concepts are very solid.

the book is purely conceptual, from the consumate master of field theory. It's 4 lectures that can be read in 4 consecutive nights - a no lose situation. There's a lot left out: he doesn't talk about vacuum energy or quarks, but puts the mechanisms in place. Nobody else has made an attempt to describe this stuff at this level, possibly because Feynman did it so simply it was pointless to try to improve.

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Re: Quantum Physics

Post by David H on Thu Jul 05, 2012 1:58 am

Like I said, I'll probably give Feynman a try this winter. But let me explain my prejudice. (Apologies in advance!)

At one point in my life I was in a PhD track in math (number theory, algebraic topology and all that). You might have called me a purist. I made a lot of money tutoring physics and engineering undergrads in the finer points of math, but I was constantly appalled by their common habit of using certain precision mathematical tools as if they were sledgehammers without fully understanding them just to get the supposedly "right" answer, and then throwing them away when they broke Evil or Very Mad

Since those days I have let my own mathematical toolkit get rusty and I'm no longer really in a position to criticize, but I still live in hope that someday a brilliant mathematician/physicist will come along and clean up all the patches, clutter and garbage that's been strewn about Physics so freely for the last half century (sort of like Petty did for Jackson's LotR).

But it sounds like I still have a long wait ahead of me.....Sad
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Re: Quantum Physics

Post by halfwise on Thu Jul 05, 2012 3:15 am

yeah, physicists use math the way engineers use physics. So I get the part about plowing through the beauty like a hedgehog through a flowerbed.

But sometimes a path is blazed that the purists wouldn't see, and then they have to come along later and clean it up, but they never would have gotten there themselves. Feynman was particularly famous for coming up with mathematical tricks that appalled mathematicians, but the fact is they worked very efficiently (differentiating under the integral sign is his most famous trick - it still makes me chuckle when I use it).

In this case you may enjoy QED simply because he doesn't show any of the math that would give you the heebie-jeebies, yet is conceptually rigorous. The 'dippy process' seems to be about taking a series expansion that would not converge, stopping short after a number of terms, then reweighting it so that it sums to one (as it does before you expand: the leading term is 1). This is called renormalization, and the word 'dippy' may seem apropos, but it's really just saying that you've taken account of more things, and are reweighting the probabilities of all things to sum to 1. Given that you know the next term must be smaller but don't know how small, this is just being somewhat practical: what else are you gonna do?

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Re: Quantum Physics

Post by David H on Thu Jul 05, 2012 3:42 am

halfwise wrote:
But sometimes a path is blazed that the purists wouldn't see, and then they have to come along later and clean it up, but they never would have gotten there themselves.

Absolutely! Even within mathematics there are people who are famous for their ability to blaze trails. But these trails are labeled "conjectures". The first rigorous proof will often be only of certain cases, and the first full proof will consist of several such partial proofs patched together in such a way as to show completeness. An elegant proof may take decades to follow.

But the big diffence is in mathematics the person who finally presents the elegant proof is showered in glory. I get the sense that in physics the glory belongs to the trailblazer alone, and there's no glory left for the person who tidies up behind.

The article on Miller's questions of Michelson-Morley is an example where it's just surprising to me that nobody ever bothered to do further research, one way or another. Everybody just accepts what would still be considered conjecture in mathematics and moves on.

I mean, by comparison, Fermat's last "Theorem" was universally accepted, but for 3 hundred years mathematicians lost sleep over the proving of it, until it fell to a herculean effort and people became international celebrities. Can you see that happening in physics? Evil or Very Mad

Rigor and elegance. Rigor and elegance. That's the mantra of the mathematician.

And just to be clear, I think Feynman is brilliant and I'd never expect a mind like his to dot all the i's and cross all the t's. I just wish there were some good math geeks following along behind him connecting all the dots. (But if there aren't, I guess it's probably more the fault of the math geeks than the physics geeks.)
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Re: Quantum Physics

Post by halfwise on Thu Jul 05, 2012 11:19 am

Actually it was Freeman Dyson who went through the trouble of cleaning up QED - I gather the situation was much worse before he came along sweeping up behind Feynman. He used to invert the classic phrase to say "God is in the details".

but a physicist's rigor is different from a mathematician's rigor. That's not to say physicists weren't worried by renormalization - Feynman (and others) clearly say they were. I gather from the wikipedia article on renormalization under "attitudes and interpretation" that the problems go away with the different types of forces seen in quarks (quantum chromodynamics) and many theorists assume things change on the small scale for electrons and photons as well - hence the interest in string theory and other things like it.

But I wouldn't wait for it. right now it seems the basic conceptual mechanisms of QED are good, but they go mathematically haywire if applied to ideal smooth fields and point particles. As of now we have no firm replacement for point particles and the fields they produce, but it's one of the motivations for all the wild ass theories you hear about (that and quantum gravity).

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Re: Quantum Physics

Post by Mrs Figg on Thu Jul 05, 2012 2:12 pm

PhD in Maths.. gulp.. Shocked
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Re: Quantum Physics

Post by David H on Thu Jul 05, 2012 3:06 pm

Relax Mrs Figg. I'm truly just a humble farmer-hobbit who happens to have dabbled in other things from time to time. :carrot:
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Re: Quantum Physics

Post by Mrs Figg on Thu Jul 05, 2012 5:44 pm

I think you are more like Strider than a Hobbit. Cool
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Re: Quantum Physics

Post by Norc on Thu Jul 05, 2012 7:00 pm

halfwise wrote:Things with funny names bunch around other things with funny names, and everything would be simpler if they didn't.

How's that?

And speaking of things with funny names, you might say that as a result Benedict Cumberbatch has a nice mass. cheers

he definitely has a nice mass Smile (I thought you were commenting on my post all from the beginning xD ) well, cumberbitches and sherlockians and benaddicts are all funny names, so you have that right xD
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Re: Quantum Physics

Post by Norc on Thu Jul 05, 2012 7:01 pm

David H wrote:There is no "from". This is where wave analogies work well intuitively. Think about flat water.
Now think about a snapshot just after a splash has caused a wave.

Where did the peak come from?
Where did the valley come from?


Nowhere really. They're just our names for certain shapes that the water can take.

A more interesting question might be "where did the energy to make the wave come from?"
Then you can start hypothesizing about pebbles thrown into pools and such.

wow.. that's an interesting thought! (why haven't I bothered looking at this thread before? this is right up my street!)
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Re: Quantum Physics

Post by David H on Thu Jul 05, 2012 7:13 pm

Mrs Figg wrote:I think you are more like Strider than a Hobbit. Cool

As you wish, Mrs Figg.
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Re: Quantum Physics

Post by halfwise on Thu Jul 05, 2012 8:04 pm

David H wrote:Relax Mrs Figg. I'm truly just a humble farmer-hobbit who happens to have dabbled in other things from time to time. :carrot:

From your posts here and elsewhere it was clear you knew what was what, but farmers are typically damn clever people. Got to be to keep a tractor going for 50 years on baling wire and crisco.

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Re: Quantum Physics

Post by Mrs Figg on Thu Jul 05, 2012 10:07 pm

:carrot:
Indeed hidden talents like a Dunedain.
Maybe in a parallel Universe Strider goes train-riding.
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Re: Quantum Physics

Post by Norc on Wed Sep 05, 2012 8:34 pm

dunno if this is quantum physics, but I need some help with this:

a car has to break down and the acceleration is -3,2 m/s^2 and it has this acceleration in 2.5 seconds. In this time the car drives 45 m. Speed is mesured in metres per second. the acceleration is constant. what is the car's speed before and after the break down?

I have a number of different "formulas", but I dunno which one to use for this kind of problem. please help me Smile
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Re: Quantum Physics

Post by Norc on Wed Sep 05, 2012 8:36 pm

edit: i know the answers, but I have to know how I get to them.
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Re: Quantum Physics

Post by Pettytyrant101 on Wed Sep 05, 2012 8:55 pm

Oh this is one of those speed distance time formula- um I used to know this one, um, drunken do you need imaginary numbers (like umpty-seven) to work this out? Suspect drunken

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Re: Quantum Physics

Post by Amarië on Wed Sep 05, 2012 8:58 pm

Hm, I really should be able to do that. Perhaps if I had the formula book in front of me. Shrugging

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