Dark theories - opinions needed

[fuelair]

I'm simply curious about how a teacher explains the notion of "energy", athon.

Once we get to a certain level of basic concept we can no longer understand that concept in terms of something simpler. One curse of science orientated TV and pop sci books is that they tend to go straight for these deep issues, bypassing the less sexy stuff on the way.

Very, very true. And partially why lots of students lose interest in science when they realize it is necessary to have the basics before you can do the fun stuff!!

 

[sol invictus]

I guess what I'm curious about is if we find that dark energy is due to a combination of vacuum fluctuations and the interactions of several other understood factors, would we still call the entire mechanism 'dark energy'?

That depends - there are many possible scenarios.

The term "dark energy" refers to any form of energy which is at least roughly homogeneous on large scales, behaves as a nearly perfect fluid, and which has an energy density that decreases with expansion slowly enough that it drives the expansion to accelerate (the cutoff for that is w=pressure/density<-1/3).

If it turns out the observations that led us to believe the expansion of the universe is accelerating were simply wrong (due to systematic error of some kind, or to something like gray dust, or axion-photon oscillations, or something else lie that), then dark energy is out.

If it turns out general relativity is wrong and gets modified in a way important for cosmology, that could also do away with it.

Would we still see it as a moderately discrete concept, as we see electromagnetic radiation as a discrete field?

Probably, yes - because it would still be convenient to describe it that way when discussing cosmology. For the same reason cosmologists might refer to "matter domination" or "dust domination", even though they know the universe contains radiation, plasma, stars, etc. as well as dust (and even dust has some interesting properties).

 

[MattusMaximus]

I can't help but make comparisons between vitalism and our modern theories of dark energy. However, I'm not sure if I'm overthinking this. Dark energy seems to me to be a placeholder for 'we don't know what is causing this observation'. For the most part, from what I understand, dark energy (like historical theories of vitalism) could either be unknown interactions of known physics, or a completely new form of energy.

Just a point of clarification, Athon. "Dark energy" was actually (sort of) predicted by general relativity almost 100 years ago - physicists call this the cosmological constant. Back when the original calculations applying GR to the universe at large were performed, it formed an extra term in the equations. From what I understand, it was largely ignored until recently after the discovery of the acceleration of the universe's expansion. Now, understandably, it is receiving a bunch of attention, and a lot of people call it "dark energy" - I have no idea why this is... I guess it just sounds cooler

ETA: I see others beat me to this point. Ah well, that's what I get for posting before I read the thread

 

[athon]

I'm simply curious about how a teacher explains the notion of "energy", athon.

Oh, ok.

If I'm teaching it in the classroom, it'll depend on the level of the student. Generally, for younger students, I provide a simplified definition, such as 'something that makes stuff happen' or describe the concept of 'work' and go from there. I tend to use the analogy of 'the universe's currency', and discuss how this currency is a way of keeping track of how things change in a system. I'll then describe the different ways energy can manifest.

Once we get to a certain level of basic concept we can no longer understand that concept in terms of something simpler.

No argument here with that point. Which is kind of my point - nobody could describe what mechanism was at work in differentiation cells and making them divide, so they called it a force or a type of energy. How many people would still agree that this observation - the splitting and differentiation of cells - is a type of energy?

One curse of science orientated TV and pop sci books is that they tend to go straight for these deep issues, bypassing the less sexy stuff on the way.

People won't read what isn't interesting. Simple fact of life, unfortunately. The trick is to show how the more shallow things can be just as important and as fascinating.

Thanks again for the feedback. I'll see how I feel in a few weeks, as to whether I think there's an angle in this or not.

Athon

 

[Soapy Sam]

The process of change seems to me to be about as fundamental an idea as my excuse for a brain is comfortable with.
The fact that things change is basic human experience. Material change, or change of state , or of position in spacetime either requires or releases energy.

Not a very rigorous notion, but adequate to make my subconscious stop asking me hard questions.

 

[sol invictus]

And energy really is intimately associated with time passing, just as momentum is associated with spatial displacements (and angular momentum with rotations). The mathematics that makes that precise is not so easy to explain, but it's a fact (and I can try to explain if anyone requests it).

 

[MattusMaximus]

Sol, I believe you're talking about the energy-time version of Heisenberg's uncertainty principle, right?

 

[sol invictus]

Sol, I believe you're talking about the energy-time version of Heisenberg's uncertainty principle, right?

That's closely related, yes. But what I was saying is true also in classical physics.

The reason energy is conserved is that there is a symmetry of the laws of physics - every symmetry has an associated conserved quantity (that's called Noether's theorem). In the case of energy, the symmetry is time translation invariance. For momentum, it's spatial translation invariance, and for angular momentum, rotation invariance.

 

[MattusMaximus]

Ah yes, Noether's theorem - those lectures from grad school are coming back to me now. Ahhh... Lagrangians and the principle of least action!