Energy

[RileyG]

What does it mean "energy is neither created nor destroyed"? Does it persist forever?

Thanks

 

[The IbanezerScrooge]

What does it mean "energy is neither created nor destroyed"? Does it persist forever?

Thanks

yes. we have not observed energy either disappearing or appearing into or out of nothing in our universe. only transforming from one form into another but the equations are always balanced. Probably the 2 questions that come closest to throwing a wrench in this law of physics, and I mean just brushing against it, would have to do with chirality - why do we observe more left-handed molecules than right-handed in our universe and why do we observe almost exclusively matter rather than anti-matter? We're not sure why reality tilts in those directions. Also see virtual particles.

 

[sfs]

What does it mean "energy is neither created nor destroyed"? Does it persist forever?

Energy isn't exactly a thing -- it's a way of characterizing a system. Saying that energy is conserved is equivalent to saying that the physical laws of the universe remain the same across time (see Noether's Theorem).

 

[sfs]

Probably the 2 questions that come closest to throwing a wrench in this law of physics, and I mean just brushing against it, would have to do with chirality - why do we observe more left-handed molecules than right-handed in our universe and why do we observe almost exclusively matter rather than anti-matter? We're not sure why reality tilts in those directions.

I don't see the first question as touching conservation of energy at all. We see molecular chirality only in living things, and that seems readily explainable either as a frozen accident (symmetry breaking) or as a distant result of the second question.

(As for the second question, the usual explanation for the excess of matter over antimatter is Sakharov's, which requires a fundamental asymmetry in the laws of the universe (i.e. CP violation, which is observed, even if the known source in weak quark interactions isn't enough to explain the matter-antimatter asymmetry), baryon number non-conservation (not observed but predicted in grand unified theories), and thermal equilibrium, which might have been sufficiently strong during the very early universe. Whether that rickety construction satisfies as an explanation at present is a matter of taste.)

 

[sjastro]

What does it mean "energy is neither created nor destroyed"? Does it persist forever?

Thanks

In our expanding universe the answer is no.

This is Einstein's field equations and the T term on the right hand side of the equation is the energy matter tensor.
If energy is neither created nor destroyed the time derivative of T is zero, in other words if T is measured over time and remains constant, energy is conserved.
Energy is not conserved in expanding universes such as the Big Bang and Steady State models as their solutions (metrics) result in the time derivative of T not equaling zero.
This is observed in the cosmological redshift of light which is a loss of energy, the energy isn't converted into another form but simply disappears.

In the early 20th century the German mathematician Emmy Noether found if the laws of physics are time invariant (do not change with time) energy is conserved.
Since the universe is expanding with time, it is not time invariant and the conservation of energy does not apply.

A good explanation of this is found in this video.

 

[RileyG]

Thank you for your answers.

 

[ruthiesea]

Energy isn't exactly a thing -- it's a way of characterizing a system. Saying that energy is conserved is equivalent to saying that the physical laws of the universe remain the same across time (see Noether's Theorem).

e=mc^2 means that matter and energy are interchangeable. The are two sides of the same coin.

 

[sfs]

e=mc^2 means that matter and energy are interchangeable. The are two sides of the same coin.

More precisely, it means that rest mass represents some amount of energy. In modern notation, the full expression is e^2 = m^2c^4 + p^2c^2, where p is the momentum of the system and m is its rest mass. E and p are each conserved quantities, so the rest mass is also conserved. That is, the rest mass of a given system never changes. Particles of matter (e.g. electrons, protons) have a rest mass, but systems that don't contain what we normally think of as matter also have rest masses. Whether the rest mass is in the form of matter or not, the energy of the system remains the same.

 

[RileyG]

Thank you for your responses, even though, I'm not sure I comprehend everything.

I last took physics in 2013 as a junior in high school. I didn't retain any of it. It is very much a mystery to me