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Does the conservation law prove that energy is eternal? 
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#1
Aug1909, 05:14 PM

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I've heard many people claim that the first law of thermodynamics proves that the energy/matter must always exist in some form or another, since it cannot be created or destroyed. This would mean that energy/matter had no beginning, and will have no end. Is this a valid claim? Or is there something I'm misunderstand about the conservation law?
And while I'm on the subject, I'd like to ask a few semirelated questions about the Many World's Interpretation of Quantum Mechanics. From what I understand, the MWI says that the random events of quantum mechanics "splits" the universe into different realities, one with each respective outcome of the random quantum event. My questions are: 1. Do physicists take the MWI seriously? Or is it just an imaginative hypothesis that probably isn't true? 2. Would the "universe splitting" of the MWI violate the energy conservation? Does it really create a new universe for each random outcome? 3. If the MWI is in fact true, could we then conclude that there exists an infinite amount of universes? Would it be true infinity, or just a really large number? Thanks. 


#2
Aug1909, 08:54 PM

Astronomy
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PF Gold
P: 23,122

If you can isolate a subsystem, enclose something in a box, and give meaning to all the relevant types of energy in the box, then you can apply the law. But on large scale in cosmology there are processes which apparently to not obey the local restricted law. Yet they are accepted by the community because they agree with observation and because there is no logical reason to suppose that energy conservation is true for the system as a whole. There is an FAQ about this that people recommend sometimes. Maybe someone will offer URLs. I realize this is a vague unsatisfactory answer. There are plenty of unanswered questions in science and maybe this is one. Wish I could be more helpful. Good luck pursuing this! 


#3
Aug2009, 04:41 AM

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P: 4,782

This essay is good on the subject:
http://www.math.ucr.edu/home/baez/ph...energy_gr.html 


#4
Aug2009, 05:23 AM

P: 1,162

Does the conservation law prove that energy is eternal?
If Is this in regard to the apparent motion or related to the energy loss through expansion and photon frequency reduction?? Other??? Thanks 


#5
Aug2009, 05:46 AM

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P: 4,782

This is a problematic statement in GR, however, as a spacetime geometry that is independent of time for one observer will not be independent of time for some other observers. But in any case energy won't be conserved in an expanding universe because of the expansion. 


#6
Aug2009, 05:34 PM

PF Gold
P: 2,432

The old style inertial expansion assumption was a closed system description wasn't it? ***** On the general issue raised by 8lak, MWI is a surprisingly popular interpretation of QM among professionals. There are those who say it is the only choice left to them. But its radical violation of energy conservation is only one of its many obvious holes. I have always taken MWI as evidence that it is faulty metaphysical principles which have led to such a patently unnatural view of quantum cosmology. 


#7
Aug2009, 07:23 PM

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Edit: I should mention that this does depend upon what you mean by "energy". If you include gravitational potential energy into the mix, then energy is always conserved in GR by definition. With dark energy and inflation the increase in energy in the dark energy/inflaton field is understood as coming from an increase in negative gravitational potential energy, just as with the perhaps more understandable case of two rocks falling towards one another (if they start at rest far away, then their kinetic energy is zero and potential energy nearly so. As they fall towards one another, their kinetic energy increases, and their potential energy also increases in the negative direction, leaving the sum constant). 


#8
Aug2009, 07:46 PM

PF Gold
P: 2,432

But this would the view that would come from taking QM as essentially an open thermodynamic story? Uncertainty would be something that can be endlessly borrowed from? 


#9
Aug2009, 07:57 PM

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Because the number of components of the wavefunction depends upon what you take the wavefunction in terms of, it wouldn't ever make any sense for there to be conservation of a sum of some physical quantity over the many wavefunction components: the sum would vary just by changing the representation! Instead, there are two operations in quantum mechanics that correspond to determining the energy. One is to ask the question, "What is the expected value of the energy?" and the other is to ask, "If I make a measurement of energy, what value will the measurement produce?" In the first case, you take a weighted average over the many components, not a sum. In the second, the outcome of your measurement will be one of the components of the wavefunction with a probability given by the square of its amplitude. So instead of thinking of it as a 'whole new world full of stuff,' perhaps a better way of thinking of it is as just a different configuration of our world coexisting with ours, rather like how an electron can be in two states at once. 


#10
Aug2009, 08:21 PM

PF Gold
P: 2,432

This is either bogus or subtle. I guess I am reacting against it because I take the enlarged thermodynamic view where there is topdown causality, global constraints, etc. So that is the framework I'm trying to fit this into. In this light, I would still feel that the QM uncertainty is like a bottomless well. Then the questions you say are asked  "What is the expected value of the energy?", "If I make a measurement of energy, what value will the measurement produce?"  are the kind of topdown constraints which closes matters to produce the coherence of a "GR system". Not asking you to agree but you have helped sharpen up the questions for me. 


#11
Aug2009, 08:49 PM

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#12
Aug2009, 09:32 PM

PF Gold
P: 2,432

In this line of thought, many worlds seem a violation of conservation as macrostates are being freely created (even if microstates are conserved). It is like if we were talking about the usual ideal gas model. Conventional approach is to take the flask that contains the gas as read. The thermodynamics is the collection of microstates inside. But in a wider view  as would have to be taken in cosmology  we then have to factor in the issue of who made and paid for the flask, the boundary conditions. MWI would be like saying you can make new and differently shaped flasks freely. 


#13
Aug2009, 09:55 PM

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#14
Aug2009, 10:12 PM

PF Gold
P: 2,432

Regular notions of infinity would rely on the image of infinite extension  keep on adding forever. But I am thinking more in terms of the magic pudding  the infinitely divisible. http://en.wikipedia.org/wiki/The_Magic_Pudding I can see that you are arguing that all that is happening is a multiplication of the complexity of the superposition state to become an ever more massive MWI agglomeration. And it would be correct in so far as the formalisms of QM superposition are concerned. But the big QM issue here is the placing of the epistemic cut, the collapse of superposed states. And the MWI just ducks the issue by saying it never has to happen. There is no place the critical questions you mention actually get asked. Edit: Although the escape clause may be that, as you say, the presumption a lack of interference and so a quasiseparation of some kind. Yet this too must be vulnerable to the criticism it is unrealistic when it looks collapse does generally happen at quite small scales. 


#15
Aug2009, 11:20 PM

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P: 4,782

As for it not being legitimate to treat it as a macrostate in the thermodynamic sense, well, that's fine. A macrostate in thermodynamics is the set of macroscopic variables that are needed to fullydescribe the macroscopic behavior of the system. Because different components of the wavefunction aren't observable in macroscopic systems, they don't belong in a consideration of thermodynamic macrostates. This doesn't mean that the MWI is wrong, just that the word "macrostate" is a poor word to use to describe the entire ensemble of configurations of the system. 


#16
Aug2109, 12:45 PM

P: 871

Of course, assuming conservation laws, something will always exist and something has always existed. But this argument is circular. Welcome to philosophy . It's all about the assumptions, and sometimes the higher level theories (QM, thermodynamics) can just be obfuscating noise. 


#17
Aug2109, 02:20 PM

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P: 4,782

Of course, one cannot prove in a strict, mathematical sense that a conservation law always holds. But one can place reasonable limits upon them. 


#18
Aug2109, 02:24 PM

P: 871




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