Quantum physics tells us that energy cannot be created nor lost, but what if this was wrong and both could occur.
Is it actually possible or would the universe as we know it just not work
Cymek said:
Quantum physics tells us that energy cannot be created nor lost, but what if this was wrong and both could occur.
Is it actually possible or would the universe as we know it just not work
I’m not sure if the world would necessarily be a different place without conservation of energy, but discovery of a violation to CoE would certainly lead to a fundamental change in the way we view the physics on the universe.
The problem is that CoE is readily observable so any violations would have to be tiny.
diddly-squat said:
Cymek said:
Quantum physics tells us that energy cannot be created nor lost, but what if this was wrong and both could occur.
Is it actually possible or would the universe as we know it just not work
I’m not sure if the world would necessarily be a different place without conservation of energy, but discovery of a violation to CoE would certainly lead to a fundamental change in the way we view the physics on the universe.
The problem is that CoE is readily observable so any violations would have to be tiny.
Or rare.
dv said:
diddly-squat said:
Cymek said:
Quantum physics tells us that energy cannot be created nor lost, but what if this was wrong and both could occur.
Is it actually possible or would the universe as we know it just not work
I’m not sure if the world would necessarily be a different place without conservation of energy, but discovery of a violation to CoE would certainly lead to a fundamental change in the way we view the physics on the universe.
The problem is that CoE is readily observable so any violations would have to be tiny.
Or rare.
fair point
Conservation of energy is a consequence of Noether’s Theorem.
diddly-squat said:
dv said:
diddly-squat said:I’m not sure if the world would necessarily be a different place without conservation of energy, but discovery of a violation to CoE would certainly lead to a fundamental change in the way we view the physics on the universe.
The problem is that CoE is readily observable so any violations would have to be tiny.
Or rare.
fair point
Like the big bang.
btm said:
Conservation of energy is a consequence of Noether’s Theorem.
So, as a consequence, failure of conservation of energy would result if the universe is not invariant under time transformations.
The Planck time is 10^-43, so how about time transformations shorter than 10^43 seconds? Would they violate conservation of energy? I think they would. And that helps to understand how all the mass/energy of the universe came into being.
What about other events where the universe is not time-invariant? Such as the singularity at the centre of a black hole?
There may be more subtle examples where time transformations are not invariant in the universe. Unruh radiation perhaps?
Cosmic Variance
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Energy Is Not Conserved
By Sean Carroll | February 22, 2010 8:57 am
I’ve been meaning to link to this post at the arXiv blog, which is a great source of quirky and interesting new papers. In this case they are pointing to a speculative but interesting paper by Martin Perl and Holger Mueller, which suggests an experimental search for gradients in dark energy by way of atom interferometry.
But I’m unable to get past this part of the blog post:
The notion of dark energy is peculiar, even by cosmological standards. Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10-10 joules per cubic metre. What’s strange about this idea is that as space expands, so too does the amount of energy. If you’ve spotted the flaw in this argument, you’re not alone. Forgetting the law of conservation of energy is no small oversight.I like to think that, if I were not a professional cosmologist, I would still find it hard to believe that hundreds of cosmologists around the world have latched on to an idea that violates a bedrock principle of physics, simply because they “forgot” it. If the idea of dark energy were in conflict with some other much more fundamental principle, I suspect the theory would be a lot less popular.
But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)
The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.
It’s not that all hell has broken loose; it’s just that we’re considering a more general context than was necessary under Newtonian rules. There is still a single important equation, which is indeed often called “energy-momentum conservation.” It looks like this:
$latex nabla_mu T^{munu} = 0,.$
The details aren’t important, but the meaning of this equation is straightforward enough: energy and momentum evolve in a precisely specified way in response to the behavior of spacetime around them. If that spacetime is standing completely still, the total energy is constant; if it’s evolving, the energy changes in a completely unambiguous way.
In the case of dark energy, that evolution is pretty simple: the density of vacuum energy in empty space is absolute constant, even as the volume of a region of space (comoving along with galaxies and other particles) grows as the universe expands. So the total energy, density times volume, goes up.
This bothers some people, but it’s nothing newfangled that has been pushed in our face by the idea of dark energy. It’s just as true for “radiation” — particles like photons that move at or near the speed of light. The thing about photons is that they redshift, losing energy as space expands. If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases. A decrease in energy is just as much a “violation of energy conservation” as an increase in energy, but it doesn’t seem to bother people as much.
<<<<<<<< At the end of the day it doesn’t matter how bothersome it is, of course — it’s a crystal-clear prediction of general relativity>>>>>>>.
And one that has been experimentally verified! The success of Big Bang Nucleosynthesis depends on the fact that we understand how fast the universe was expanding in the first three minutes, which in turn depends on how fast the energy density is changing. And that energy density is almost all radiation, so the fact that energy is not conserved in an expanding universe is absolutely central to getting the predictions of primordial nucleosynthesis correct. (Some of us have even explored the very tight constraints on other possibilities.)
Having said all that, it would be irresponsible of me not to mention that plenty of experts in cosmology or GR would not put it in these terms. We all agree on the science; there are just divergent views on what words to attach to the science. In particular, a lot of folks would want to say “energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.” Which seems pretty sensible at face value.
There’s nothing incorrect about that way of thinking about it; it’s a choice that one can make or not, as long as you’re clear on what your definitions are. I personally think it’s better to forget about the so-called “energy of the gravitational field” and just admit that energy is not conserved, for two reasons.
First, unlike with ordinary matter fields, there is no such thing as the density of gravitational energy. The thing you would like to define as the energy associated with the curvature of spacetime is not uniquely defined at every point in space. So the best you can rigorously do is define the energy of the whole universe all at once, rather than talking about the energy of each separate piece. (You can sometimes talk approximately about the energy of different pieces, by imagining that they are isolated from the rest of the universe.) Even if you can define such a quantity, it’s much less useful than the notion of energy we have for matter fields.
The second reason is that the entire point of this exercise is to explain what’s going on in GR to people who aren’t familiar with the mathematical details of the theory. All of the experts agree on what’s happening; this is an issue of translation, not of physics. And in my experience, saying “there’s energy in the gravitational field, but it’s negative, so it exactly cancels the energy you think is being gained in the matter fields” does not actually increase anyone’s understanding — it just quiets them down. Whereas if you say “in general relativity spacetime can give energy to matter, or absorb it from matter, so that the total energy simply isn’t conserved,” they might be surprised but I think most people do actually gain some understanding thereby.
Energy isn’t conserved; it changes because spacetime does. See, that wasn’t so hard, was it?
http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/
:)
macx
Is Energy Conserved in General Relativity?
In special cases, yes. In general — it depends on what you mean by “energy”, and what you mean by “conserved”.
In flat spacetime (the backdrop for special relativity) you can phrase energy conservation in two ways: as a differential equation, or as an equation involving integrals (gory details below). The two formulations are mathematically equivalent. But when you try to generalize this to curved spacetimes (the arena for general relativity) this equivalence breaks down. The differential form extends with nary a hiccup; not so the integral form.
The differential form says, loosely speaking, that no energy is created in any infinitesimal piece of spacetime. The integral form says the same for a finite-sized piece. (This may remind you of the “divergence” and “flux” forms of Gauss’s law in electrostatics, or the equation of continuity in fluid dynamics. Hold on to that thought!)
An infinitesimal piece of spacetime “looks flat”, while the effects of curvature become evident in a finite piece. (The same holds for curved surfaces in space, of course). GR relates curvature to gravity. Now, even in newtonian physics, you must include gravitational potential energy to get energy conservation. And GR introduces the new phenomenon of gravitational waves; perhaps these carry energy as well? Perhaps we need to include gravitational energy in some fashion, to arrive at a law of energy conservation for finite pieces of spacetime?
Casting about for a mathematical expression of these ideas, physicists came up with something called an energy pseudo-tensor. (In fact, several of ‘em!) Now, GR takes pride in treating all coordinate systems equally. Mathematicians invented tensors precisely to meet this sort of demand — if a tensor equation holds in one coordinate system, it holds in all. Pseudo-tensors are not tensors (surprise!), and this alone raises eyebrows in some circles. In GR, one must always guard against mistaking artifacts of a particular coordinate system for real physical effects. (See the FAQ entry on black holes for some examples.)
These pseudo-tensors have some rather strange properties. If you choose the “wrong” coordinates, they are non-zero even in flat empty spacetime. By another choice of coordinates, they can be made zero at any chosen point, even in a spacetime full of gravitational radiation. For these reasons, most physicists who work in general relativity do not believe the pseudo-tensors give a good local definition of energy density, although their integrals are sometimes useful as a measure of total energy.
contd
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html
The notion of dark energy is peculiar, even by cosmological standards. Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10-10 joules per cubic metre. What’s strange about this idea is that as space expands, so too does the amount of energy. If you’ve spotted the flaw in this argument, you’re not alone. Forgetting the law of conservation of energy is no small oversight.
——
I wouldn’t fret it. Dark energy is just another one of those placeholder terms that physicists use for stuff they can’t explain.. Dark matter, time, gravity, the big bang, entangled particles etc etc
I really don’t have time to start discussing this, so I’ll just say:
Hi macx, good to see you.