Undergrad Can the Lieb-Robinson Bound be Intuited from the Taylor Series of an Operator?

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The discussion centers on understanding the Lieb-Robinson bound through the Taylor series expansion of the operator A(t) = e^{-iHt}Ae^{iHt}, where H is a k-local Hamiltonian. The operator A(t) starts as a single-site operator and grows in size due to its non-zero commutator with local terms in H. The challenge lies in visualizing the "light-cone" effect typically associated with the Lieb-Robinson bound, as contributions from all orders of it complicate the boundary definition. A participant introduces the concept of wave propagation speed, suggesting that finite velocities arise from the absence of wild oscillations in the wave packet. The connection between this intuition and the Lieb-Robinson bound remains uncertain, highlighting the need for further exploration of the topic.
thatboi
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Hi all,
I was wondering if there was a way to intuit the Lieb-Robinson bound from simply looking at the taylor series for an operator ##A(t) = e^{-iHt}Ae^{iHt}## where ##H## is a k-local Hamiltonian and ##A(t)## initially starts off as a single-site operator. The generic idea is that at each order of ##it##, the operator "grows" in size since it will have non-zero commutator with the local terms in ##H##. My issue though, is how to see the "light-cone" that is frequently used in discussions of this topic because for any non-zero ##t##, ##A(t)## will necessarily have contributions from all order of ##it##, so I am not sure where to draw the boundary between operator and the rest of the system that is typically drawn in the Lieb-Robinson bound.
Any advice would be appreciated.
Thanks!
 
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I never heard of Lieb-Robinson bound before, but after a brief googling my intuition is the following. The wave (quantum of classical) propagates with a finite velocity, approximately given by the group velocity
$$v_g=\frac{d\omega}{dk}$$
For example, for a free non-relativistic particle we have ##\omega=k^2/2m## (in units ##\hbar=1##), so
$$v_g=k/m$$
which is finite, provided that ##k## is finite. Thus, if the wave packet has vanishing contributions from infinite ##\omega## and ##k##, i.e. the wave does not have wild oscillations at arbitrarily small time and space scales, then one expects a finite speed of propagation. This is a wave analog of the fact that a classical non-relativistic particle travels with finite speed, provided that its energy and momentum are not infinite.

But that's just my intuition, I'm not certain how much is this related to the actual Lieb-Robinson bound.
 
Time reversal invariant Hamiltonians must satisfy ##[H,\Theta]=0## where ##\Theta## is time reversal operator. However, in some texts (for example see Many-body Quantum Theory in Condensed Matter Physics an introduction, HENRIK BRUUS and KARSTEN FLENSBERG, Corrected version: 14 January 2016, section 7.1.4) the time reversal invariant condition is introduced as ##H=H^*##. How these two conditions are identical?

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