"borrowing" energy

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"borrowing" energy

Postby some_dude » Tue Oct 04, 2011 6:41 pm UTC

I'm taking a course on particle and nuclear physics at the moment and one of the first things my lecturer, who has worked in the field for many years, said was that nature can "cheat" by violating energy conservation for a very short length of time. It can borrow energy (apparently not from anywhere in particular) during an interaction to create a virtual particle then return it in the next "step" of said interaction. My textbook on the subject (Particle Physics by Martin & Shaw) says the same.

Though I thought this was weird I assumed it was just one of those crazy things you're supposed to accept, but then, while reading up on QM, to my surprise I read in Griffith's that the "borrowing energy" interpretation of the energy-time uncertainty relation wasn't valid and was a common misconception. Wikipedia seems somewhat divided on this with the uncertainty principle article stating the same but the virtual particle article (which at least says conservation of energy isn't violated) uses the same interpretation of borrowing energy.

I could understand if this was just one of those numerous misconceptions only found in pop science texts, but this has got to be the widest misconception I've ever come about, evidently common even among people in the field. Is the borrowing energy interpretation just accepted as extremely sloppy language or is this myth so widespread that there are actually Ph.d.s in HEP who doesn't know that it's wrong?
Last edited by some_dude on Fri Oct 07, 2011 9:12 pm UTC, edited 1 time in total.
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Re: "burrowing" energy

Postby Charlie! » Tue Oct 04, 2011 7:42 pm UTC

I think it's like quantum tunneling. So you can do things that, classically, would have required violation of conservation of energy, but the probability decreases exponentially with the amount of energy * the amount of time. "Uncertainty relation" usually refers to the useful fact about fourier transforms, so though it's a real effect, it's doesn't have to do with limits on measurement.
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Re: "burrowing" energy

Postby Technical Ben » Tue Oct 04, 2011 11:16 pm UTC

By any chance were they referring to "nature" as in plants and photosyntheses or "nature" as in just "the natural laws of physics" in general?

As I know there are quantum effects with photosyntheses. http://lmgtfy.com/?q=photosynthesis+use ... vest+light

I've no idea if it's the same system your describing though, it seems to be quantum coherence. Quickly skimming the article suggests it's not using virtual particles. Wiki links to this paper... http://pubs.acs.org/doi/abs/10.1021/jz900062f. Again, not read it myself, sorry. :(
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Re: "burrowing" energy

Postby Winter Man » Tue Oct 04, 2011 11:18 pm UTC

Is burrowed energy measured in moles? :P

But seriously, I remember reading about it possibly in The Little Book of String Theory by... err... I can't remember. Look it up. Short & sweet read.
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Re: "burrowing" energy

Postby Malconstant » Wed Oct 05, 2011 2:15 pm UTC

It's a heuristic which is valid for virtual particles and QED calculations, but feels like quantum cosmology when applied to Schroedinger's cat scales. You'd do best to really understand Griffith's take on it by going over his general derivation of uncertainty relations of quantum operators, because that's where it all comes from. It's not that the heuristic is wrong really. Just, as he says, when someone tries to invoke an uncertainty argument to appeal to something like this it's best to hold onto your wallet. This includes claims like "you can have a universe pop into existence from nothing so long as it's only around for a very small amount of time."

Although the charming thing about that argument and also Boltzmann brains is that they require that this near-instantaneous randomized glimpse of a universe just happens to have derived the theories which predict those phenomena. Just a very tidy self-contained system of unthinkable odds.
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Re: "burrowing" energy

Postby some_dude » Wed Oct 05, 2011 4:50 pm UTC

I can accept it as a heuristic, but it really bothers me that it's presented as a valid interpretation of what's really happening. In general I really don't like virtual particles and all the handwaving surrounding them. Isn't it fundamentally nonsensical to try and describe what happens in between measurements of a QM system, other than the evolution of the wavefunction?
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Re: "burrowing" energy

Postby doogly » Wed Oct 05, 2011 8:38 pm UTC

Oh yes, no one should ever be talking about particles seriously. Of course we all know everything is really a field.
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Re: "burrowing" energy

Postby MHD » Fri Oct 07, 2011 8:29 am UTC

At first glance I thought this was about depositing macroscopic energy reserves in the Earth's crust.

s/burrow/borrow/
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Re: "burrowing" energy

Postby Technical Ben » Fri Oct 07, 2011 9:19 am UTC

Malconstant, interesting that you don't like the "popping into existence" ideas on cosmology. I've missed if you've made those comments before in the other QM threads. Which is a pity, because I've never liked those "thought experiments" or Boltzmann brains*. But never know if there are any fundamental laws or calculations that disallow for them. :(



*The article in New Scientist about 5 years ago on BBs made me stop buying it and rename it "New Age Mysticism". :P
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Re: "burrowing" energy

Postby doogly » Fri Oct 07, 2011 1:41 pm UTC

Nobody *likes* Boltzman brains. If you are proposing a model for cosmology in which the vast majority of observers are BBs, that's a sign that you need to fix your model.
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Re: "burrowing" energy

Postby some_dude » Fri Oct 07, 2011 9:11 pm UTC

MHD wrote:At first glance I thought this was about depositing macroscopic energy reserves in the Earth's crust.

s/burrow/borrow/


Well that is pretty embarrassing, I normally consider myself quite fluent in English even though it isn't my first language, but I didn't even know the word "burrow" and thought that's how you're supposed to spell "borrow". To put an end to the confusion, I've edited my first post.
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Re: "borrowing" energy

Postby Malconstant » Fri Oct 07, 2011 9:20 pm UTC

I appreciate the point where you need to change your model, not because of any measurable reason why it might be wrong, but because its implications are unbearably depressing. And the implication for society becomes too destructive. There must be a preserved element of social utility in our theories on cosmology. Occam's opium.
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Re: "borrowing" energy

Postby Technical Ben » Fri Oct 07, 2011 10:07 pm UTC

That's it? The "ridiculousness" of it would have done it for me. ;)

I'd suggest a failure in the system of describing "complexity" or "brains" if you think BB are more likely to exist than the current universe. IE your models for probability might be weighted in the wrong places.
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Re: "borrowing" energy

Postby Haversine » Mon Oct 10, 2011 11:13 pm UTC

Heisenberg's Uncertainty principle. That allows it and it's consistent. I think that's the basis of all ideas of virtual particles and quantum fluctuations and all that. But the product of uncertainty in time and energy of the particle is greater than or equal to Planck's reduced constant on two. So it's a quantified inequality that allows temporary violations. I have it tattooed on my right arm. Awww yeee.
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Re: "borrowing" energy

Postby doogly » Mon Oct 10, 2011 11:24 pm UTC

Yeah, that is the topic of discussion. Have you been following it at all?
Because t is not an operator, delta t delta H is a completely different beast than every other operator uncertainty inequality.
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Re: "borrowing" energy

Postby PM 2Ring » Tue Oct 11, 2011 2:17 am UTC

Technical Ben wrote:That's it? The "ridiculousness" of it would have done it for me. ;)

Mere "ridiculousness" isn't enough to discount an idea in science, otherwise we would still be using pre-Copernican cosmology.

Technical Ben wrote:I'd suggest a failure in the system of describing "complexity" or "brains" if you think BB are more likely to exist than the current universe. IE your models for probability might be weighted in the wrong places.

One brain is a whole lot less complex than a whole universe, even a universe devoid of brains.


doogly wrote:Nobody *likes* Boltzman brains.
Apart from thermodynamic zombies. :D
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Re: "borrowing" energy

Postby Technical Ben » Tue Oct 11, 2011 8:41 am UTC

PS PM 2Ring, thanks for the corrections. However, the problem still lies "define complexity" ;)
The entropy of a brain may be much greater than the universe etc. However, I'd guess those doing the maths have checked that already.

The idea I heard given for photosynthesis is that the particles take the route that uses the least energy each time. The question "how does the particle know which route is the best" was answered by stating it's in a quantum super position and takes every route, then the least energetic wins out. The reason I was reminded of this, is that each super position is in effect another virtual particle. Like rolling 6 quantum dice till you get the number you need. Then only keeping the one that's correct.
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Re: "borrowing" energy

Postby BlackSails » Tue Oct 11, 2011 2:18 pm UTC

Technical Ben wrote:PS PM 2Ring, thanks for the corrections. However, the problem still lies "define complexity" ;)
The entropy of a brain may be much greater than the universe etc. However, I'd guess those doing the maths have checked that already.


The entropy of a subset of the universe is not going to be more than the whole universe. And there are very well defined mathematical definitions of complexity.
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Re: "borrowing" energy

Postby doogly » Tue Oct 11, 2011 2:19 pm UTC

Kolmogorov is your best bet. See scottaaronson.com/blog for some near-technical discussion of things.
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Re: "borrowing" energy

Postby Technical Ben » Wed Oct 12, 2011 10:27 am UTC

BlackSails wrote:
Technical Ben wrote:PS PM 2Ring, thanks for the corrections. However, the problem still lies "define complexity" ;)
The entropy of a brain may be much greater than the universe etc. However, I'd guess those doing the maths have checked that already.


The entropy of a subset of the universe is not going to be more than the whole universe. And there are very well defined mathematical definitions of complexity.


So is that a mathematical reason why BB are not probable? The system with more entropy is the most probable one right? So a brain has too little entropy to appear "randomly"? So the BB thought experiment is redundant?
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Re: "borrowing" energy

Postby doogly » Wed Oct 12, 2011 3:08 pm UTC

It's actually more the reverse. If you have a model in which BBs are more probable, then you decide that this wasn't a good model.

Of course this is in a really strange subset of physics, eternal inflation with string theory landscape. I would not play with this area of research until full comfort has been reached with things like relativity and quantum mechanics. Then we can get your basics of inflationary cosmology. Then we can speculate on multiverses.
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Re: "borrowing" energy

Postby some_dude » Wed Oct 12, 2011 10:21 pm UTC

I have another question which doesn't seem to require a new thread, since it's kind of in the vain of the general discussion in here. So "phonons" were mentioned today in my particle physics course. Apparently they're "quasiparticles", that is the vibrational energy is exchanged between particles as though it was carried by bosons, but we know it's really not. This made me wonder how come virtual particles aren't classified as quasiparticles if most agree they're just a heuristic as well. Also the book introduced phonons as "quantas of vibrational energy in the same way that photons are quanta of EM energy", which further confuses me as to when and how exactly you can tell if a particle is real, vitual or quasi as clearly photons can be real.
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Re: "borrowing" energy

Postby doogly » Thu Oct 13, 2011 12:37 am UTC

If your particle physics comrades tell you there is a difference, tell them they are just being chauvinists.
You have an effective field theory, you quantize it, boom, you call it a particle.
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Re: "borrowing" energy

Postby Link » Thu Oct 13, 2011 8:42 am UTC

PM 2Ring wrote:
doogly wrote:Nobody *likes* Boltzman brains.
Apart from thermodynamic zombies. :D

We have those now, too‽ Between Maxwell's Demon and thermodynamic zombies, I'm starting to think it's time to call the clergy. And by clergy, I mean battle priests armed with mothereffin' tachyon guns!
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Re: "borrowing" energy

Postby Technical Ben » Thu Oct 13, 2011 2:14 pm UTC

doogly wrote:It's actually more the reverse. If you have a model in which BBs are more probable, then you decide that this wasn't a good model.

Of course this is in a really strange subset of physics, eternal inflation with string theory landscape. I would not play with this area of research until full comfort has been reached with things like relativity and quantum mechanics. Then we can get your basics of inflationary cosmology. Then we can speculate on multiverses.


I thought you were suggesting everyone move onto string theory after studying GR... :shock: I'm glad you meant science should not approach string theory at all until we have done the rest first. ;) :lol:
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Re: "borrowing" energy

Postby Anaphase » Sat Oct 22, 2011 7:22 pm UTC

doogly wrote:If your particle physics comrades tell you there is a difference, tell them they are just being chauvinists.
You have an effective field theory, you quantize it, boom, you call it a particle.

I imagine this is why Feynmann never brought up virtual particles in the famous "magnets" clip--as the inventor of QED, he knew full well that "the magnets exchange virtual photons" is essentially a very roundabout way of saying "the magnets repel", and he was determined not to cop out like that.
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