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Show that the energy-time uncertainty principle reduces to the "your name" uncertainty principle (Problem 3.14), when the observable in question is x.

Short Answer

Expert verified

The energy-time uncertainty principle reduces to σH2σx2=ħ24m2p2.

Step by step solution

01

The generalized uncertainty principle for two operators.

The generalized uncertainty principle for two operators is:

δA2δB2(12i<[A,B]>)2

02

Show that the energy-time uncertainty principle reduces to the "your name" uncertainty principle.

From the energy-time uncertainty principle, an operator Q satisfies the equation:

ddtQ=iħH,Q+Qtforthepositionoperator,Q=x,thetimederivativeofthepositioniszero,sincexandtareindependentvariables,ddtx=iħH,xletA=HandB=xanduse(2),toget;σH2σx2-ħ2dxdt2=ħ24m2mdxdt2=ħ24m2p2σH2σx2=ħ24m2p2Thus,theenergy-timeuncertaintyprinciplereducestoσH2σx2=ħ24m2p2.

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Most popular questions from this chapter

(a) Check that the eigenvalues of the hermitian operator in Example 3.1 are real. Show that the eigenfunctions (for distinct eigenvalues) are orthogonal.

(b) Do the same for the operator in Problem 3.6.

(a) Write down the time-dependent "Schrödinger equation" in momentum space, for a free particle, and solve it. Answer: exp(-ip2t/2mh)Φ(p,0).

(b) Find role="math" localid="1656051039815" Φ(p,0)for the traveling gaussian wave packet (Problem 2.43), and construct Φ(p,t)for this case. Also construct |Φ(p,t)|2, and note that it is independent of time.

(c) Calculaterole="math" localid="1656051188971" pandrole="math" localid="1656051181044" p2by evaluating the appropriate integrals involvingΦ, and compare your answers to Problem 2.43.

(d) Show thatrole="math" localid="1656051421703" <H>=<p>2/2m+<H>0(where the subscript denotes the stationary gaussian), and comment on this result.

Consider the wave functionΨ(x,0)={12ei2πx/λ,-<x<0,

wherenis some positive integer. This function is purely sinusoidal (with wavelengthλ)on the interval-<x<, but it still carries a range of momenta, because the oscillations do not continue out to infinity. Find the momentum space wave functionΦ(p,0). Sketch the graphs of|Ψ(x,0)|2and|Φ(p,0)|2, and determine their widths,wxandwp(the distance between zeros on either side of the main peak). Note what happens to each width asn. Usingwxandwpas estimates ofxandp, check that the uncertainty principle is satisfied. Warning: If you try calculatingσp, you're in for a rude surprise. Can you diagnose the problem?

Is the ground state of the infinite square well an eigenfunction of momentum? If so, what is its momentum? If not, why not?

Prove the famous "(your name) uncertainty principle," relating the uncertainty in positionA=x to the uncertainty in energyB=p2/2m+v:

σxσHh2m|p|

For stationary states this doesn't tell you much-why not?

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