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The gaussian wave packet. A free particle has the initial wave function

Y(x,0)=Ae-ax2

whereAand are constants ( is real and positive).

(a) NormalizeY(x,0)

(b) Find Y(x,t). Hint: Integrals of the form

-+e-(ax2+bx)dx

Can be handled by 鈥渃ompleting the square鈥: Lety=a[x+bl2a], and note that(ax2+bx)=y2-(b2l4a). Answer:

localid="1658297483210" Y(x,t)=(2a)1/4e-ex2l[1+(2ihatlm)]1+(2ihatlm)

(c) Find . Express your answer in terms of the quantity

localid="1658297497509" =a1+(2ihatlm)2

Sketchlocalid="1658124147567" |Y|2(as a function of x) at t=0, and again for some very large t. Qualitatively, what happens to |Y|2, as time goes on?

(d) Find <x>,<p>,<x2>,<p2>,xand P. Partial answer:localid="1658297458579" <p2>=ah2, but it may take some algebra to reduce it to this simple form.

(e) Does the uncertainty principle hold? At what time tdoes the system come

closest to the uncertainty limit?

Short Answer

Expert verified
  • The normalize wave function is A=2aP1/4.
  • The value ofY(x,t) is role="math" localid="1658125710951" 2aP1/4e-ax2l(1+2t/m)1+(2it/m).
  • The value of Y(x,t)2is 2ape-2ax2l(1+2)1+2.
  • The value of xis 0, pis 0, x2is 14w2p2is 2a, xis 12w, and pis a
  • The uncertainty principle holds and it is closest to the uncertainty limit at t=0, and at that time it is right at the uncertainty limit.

Step by step solution

01

Identification for the given data

The given data is listed below as follows,

The initial wave function for a free particle is,Y(x,0)=Ae-2x2

02

Definition of wave function

The characteristics of a particular wave form is given by a function known as wave function. Also, wave equation is satisfied by this wave function. It is denoted by .

03

(a) Normalize the given wave function

Normalize the given wave function in the following way,

1-A2-e-2ax2dx1-A2p2aA-2ap1/4

Thus, the normalize wave function is A-2ap1/4.

04

(b) Determination of the value of Y(x,t)

Perform the integration.

-e-ax2-bx2dx=-e-y2+(b2/4a)1ady=1aeb2/4a-e-y2dy=paeb2/4a

Find the value of (k).

role="math" localid="1658128927318" (k)=12pA-e-ax2e-ikxdx=12p2a1/4pae-k2/4a=1(2pa)1/4e-k2/4a

Determine the value of .

Y(x,t)=12p1(2pa)1/4-e-k2/4aeikx-k2/2mdk=12p12pa1/4p14a+it2me-x2/4(14+it/2m)=2ap1/4e-ax2l1+2it/m1+2it/m

Thus, the value of Yx,tis 2ap1/4e-ax2/1+2i/m1+2it/m.

05

(c) Determination of the value of |Yx,t|2 and plot |Y|2 on the graph

Assume that=2atm

Substitute for 2atmin Y(x)2ap1/4e-ax2/(1+2it/m)1+2it/m.

Yx,t=2ape-ax2/1+ie-ax2/(1-i)1+i(1-i)

Simplify the exponent.

-ax21+i-ax2(1-i)=-ax21-i+1i1-i(1+)=-2ax21+2

Use the above value and find in Y(x,t)2.

Y(x,t)2=2ape-2ax2l(1+2)1+2

It is known that w=a1+2so, Y(x,t)2=2pwe-2w2x2. Now plot the graph for Yx,t2when t=0

It is known that as the value of tincreases, the graph of Y2flattens out and broadens. So, plot the graph for Y(x,t)2when t>0.

Thus, the value of Y(x,t)2is 2ape-2ax2/(1-e2)1+2.

06

(d) Determination of the value of <x>,<p>,<x2>,<p2>,σx  and σp

Determine the value of xin the following way,

x=-xY2dx

It is the odd integrand.

Determine the value of pin the following way,

p=mdxdt=0

Determine the value of x2in the following way,

x2=2pw-x2e-2w2x2dx=2pw14w2p2w2=14w2

Determine the value of p2in the following way,

p2=-2-y=d2ydx2dx

Write the value of y.

y=Be-bx2

Here, B=2ap1/411+iand b=21+i.

Double differentiate y=Be-bx2with respect to x.

d2ydx2=-2bB(1-2bx2)e-bx2

Multiply y*with the above expression.

y*d2ydx2=-2bB2(1-2bx2)e-(b-b*)x2

It is known that b+b*=2w2B2=2pw.

Substitute all the values in above expression.

y*d2ydx2=-2b2pw(1-2bx2)e-2w2x2

Substitute the above value in p2=-2-y*d2ydx2dx.

p2=2b22pw-(1-2bx2)e-2w2x2dx=2b21-b2w2

It is known that 1-b2w2=a2b, so substitute it in above expression.

p2=2a

Determine the value of xin the following way,

x=12w

Determine the value of pin the following way

p=a

Thus, the value of xis 0,p is 0, x2is 14w2p2is 2a, xis ,12w and pis a

07

(e) Identification of the fact that uncertainty principle holds or not and determination of the value of the system come closest to uncertainty limit

Find the condition that uncertainty principle holds or not.

xp=12wa=21+q2=21+2at/m22

Thus, the uncertainty principle holds and it is closest to the uncertainty limit at t=0, and at that time it is right at the uncertainty limit.

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