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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<[AÁåœ,BÁåœ]>)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+∂Q∂tforthepositionoperator,Q=x,thetimederivativeofthepositioniszero,sincexandtareindependentvariables,ddtx=iħH,xletAÁåœ=HÁåœandBÁåœ=xÁåœanduse(2),toget;σH2σx2≥-ħ2dxdt2=ħ24m2mdxdt2=ħ24m2p2σH2σx2=ħ24m2p2Thus,theenergy-timeuncertaintyprinciplereducestoσH2σx2=ħ24m2p2.

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Most popular questions from this chapter

(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.

An anti-Hermitian (or skew-Hermitian) operator is equal to minus its Hermitian conjugate:

QÁåœt=-QÁåœ

(a) Show that the expectation value of an anti-Hermitian operator is imaginary. (b) Show that the commutator of two Hermitian operators is anti-Hermitian. How about the commutator of two anti-Hermitian operators?

Let Q^be an operator with a complete set of orthonormal eigenvectors:localid="1658131083682" Q^en>=qnen(n=1,2,3,....) Show thatQ^can be written in terms of its spectral decomposition:Q^=∑nqnen><en|

Hint: An operator is characterized by its action on all possible vectors, so what you must show is thatQ^={∑nqnen><en|} for any vector α>.

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

Extended uncertainty principle.The generalized uncertainty principle (Equation 3.62) states that

σA2σB2≥14<C>2

whereC^≡-i[A^,B^̂]..

(a) Show that it can be strengthened to read

σA2σB2≥14(<C>2+<D>2) [3.99]

whereD^≡A^B^+B^A^-2⟨A⟩⟨B⟩.. Hint: Keep the term in Equation 3.60

(b) Check equation 3.99 for the caseB=A(the standard uncertainty principle is trivial, in this case, sinceC^=0; unfortunately, the extended uncertainty principle doesn't help much either).

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