On Tuesday, February 13, 2018 at 7:45:59 PM UTC-6, Pierz wrote:
>
> Quantum physics tells us that anything that commutes with the hamiltonian 
> is preserved (doesn't change), the hamiltonian being the measure of energy 
> in a system. This has led me to understand energy as a measure of change 
> over time in a physical system. That might be obvious, except I've never 
> heard anybody say it quite like that - with the result that many people 
> tend to reify energy as some kind of physical "thing". The fact that energy 
> and matter are interconvertible has led me to the summary that change 
> across space is matter, change across time is energy. The only problem in 
> this picture is potential energy, which you could simply call "deferred 
> change", but that does beg the question as to how it is deferred. I'm 
> trying to think about this in relation to chemical energy - the potential 
> energy held in chemical bonds. When I studied chemistry I was simply told 
> that certain bonds are more stable and have lower energy than other bonds 
> which are less stable and have higher energy. So energy is released when a 
> molecule reacts with another to form a more stable compound. The reason for 
> and nature of the stability wasn't explained. So I'm wondering, is the 
> "potential energy" in the chemical bond actually a kind of very localised 
> motion, with more motion occurring in high energy bonds than in lower 
> energy ones? In other words, the energy (motion/change) is temporarily 
> contained in the small area of the bond, thus hiding the energy it as it 
> were from the environment? If so, then this form of potential energy is not 
> really different in kind from other types of energy, it's just relatively 
> isolated. If this is valid, perhaps a similar analysis of other forms of 
> potential energy such as gravitational potential might be possible too? Can 
> a physicist/physical chemist perhaps shed light on whether my speculation 
> here regarding chemical energy is valid? 
>

The Hamiltonian is the generator of time development. A quantum wave 
function  ψ(t) is pushed to the time t' > t by the operator exp(-iH(t' - 
t)) so that ψ(t') =  exp(-iH(t' - t))ψ(t).

LC

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