Conversation

Mar 21, 2021

I was on a quest to figure out what “energy” is a couple years ago, and got very frustrated. Thermal energy= molecules moving faster Electrical energy= electrons moving Sonic energy= molecules of a medium moving in waves twitter.com/liminal_warmth

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in the wilderless

@the_wilderless

Mar 21, 2021

Every type of energy was just “x moving,” Which seemed to make it clear that energy was whatever was *causing* these various things to move, But I couldn’t find anything on what it was that caused this.

in the wilderless

@the_wilderless

Mar 21, 2021

Then I started thinking more on the role of Time, is energy anything that can exist without time? Are time and energy the same thing?

in the wilderless

@the_wilderless

Mar 21, 2021

I finally found some physics prof giving a class where he just admitted freely “we have no idea what energy is, it’s just a word to describe some stuff we can observe and use”

in the wilderless

@the_wilderless

Mar 21, 2021

Which seems like an easy thing to admit. Why was it so hard to find someone who didn’t just talk in circles to act like they could explain it?

QC (in Berlin 7/29-???)

@QiaochuYuan

Mar 22, 2021

are you familiar with the hamiltonian approach to classical (and quantum) mechanics? from that pov "energy" is anything that shows up as a term in a special observable called the hamiltonian, and the hamiltonian is the thing that makes classical systems change over time

QC (in Berlin 7/29-???)

@QiaochuYuan

Mar 22, 2021

to be more specific, it's differences in energy between different states that make change possible. if the energy of every state is the same then no change occurs. so... i guess it wouldn't be too inaccurate to say that "energy (difference) is the capacity for change"

QC (in Berlin 7/29-???)

@QiaochuYuan

Replying to

@QiaochuYuan

and

@the_wilderless

this means energy is deeply tied up with time as you note, and in fact in relativity they're "conjugate," in the same way that position and momentum are conjugate. time becomes the fourth component of "4-position" and energy becomes the fourth component of "4-momentum"

12:17 AM · Mar 22, 2021Twitter Web App

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QC (in Berlin 7/29-???)

Replying to

and

what that "means" in a fundamental way i couldn't really say, i can't claim to understand relativity all that much

bucket of kets

@bucketofkets

Mar 22, 2021

yeah this is still probably unsatisfying from a “why” angle, but I think I’ve personally gotten to the point that “why” questions aren’t entirely well-posed