Post on 27-Dec-2015
Can Spacetime curvature induced corrections to Lamb shift be observable?
Hongwei Yu
Ningbo University and Hunan Normal University
Collaborator: Wenting Zhou (Hunan Normal)
OUTLINE
Why-- Test of Quantum effects
How -- DDC formalism
Curvature induced correction to Lamb shift
Conclusion
Quantum effects unique to curved space
Hawking radiation
Gibbons-Hawking effect
Why
Unruh effect
Challenge: Experimental test.
Q: How about curvature induced corrections to those already existing in flat spacetimes?
Particle creation by GR field
What is Lamb shift?
Theoretical result:
Experimental discovery:
In 1947, Lamb and Rutherford show that the level 2s1/2 lies about 1000MHz, or 0.030cm-1 above the level 2p1/2. Then a more accurate value 1058MHz.
The Dirac theory in Quantum Mechanics shows: the states, 2s1/2 and 2p1/2 of hydrogen atom are degenerate.
The Lamb shift
Important meanings
Physical interpretation
The Lamb shift results from the coupling of the atomic electron to the vacuum electromagnetic field which was ignored in Dirac theory.
In the words of Dirac (1984), “ No progress was made for 20 years. Then a development came, initiated by Lamb’s discovery and explanation of the Lamb shift, which fundamentally changed the character of theoretical physics. It involved setting up rules for discarding … infinities…”
The Lamb shift and its explanation marked the beginning of modern quantum electromagnetic field theory.
Q: What happens when the vacuum fluctuations which result in the Lamb shift
are modified?
What happens if vacuum fluctuations are modified?
How spacetime curvature affects the Lamb shift? Observable?
If modes are modified, what would happen?
2. Casimir-Polder force1. Casimir effect
How
Bethe’s approach, Mass Renormalization (1947) A neutral atom
fluctuating electromagnetic fieldsPAH I
Relativistic Renormalization approach (1948)
Propose “renormalization” for the first time in history! (non-relativistic approach)
The work is done by N. M. Kroll and W. E. Lamb;
Their result is in close agreement with the non-relativistic calculation by Bethe.
Interpret the Lamb shift as a Stark shift A neutral atom
fluctuating electromagnetic fieldsEdH I
Feynman’s interpretation (1961) It is the result of emission and re-absorption from the vacuum of virtual photons.
Welton’s interpretation (1948)
The electron is bounded by the Coulomb force and driven by the fluctuating vacuum electromagnetic fields — a type of constrained Brownian motion.
J. Dalibard J. Dupont-Roc C. Cohen-Tannoudji 1997 Nobel Prize Winner
DDC formalism (1980s)
a neutral atom
Reservoir of vacuum fluctuations
)(IH
)(N)()1()()(N ttAtAt
)()(N tAt
Atomic variable
Field’s variable
)(N)( ttA
0≤λ ≤ 1
)()()( tAtAtA sf
Free field Source field
Vacuum fluctuations
Radiation reaction
Vacuum fluctuations
Radiation reaction
Model: a two-level atom coupled with vacuum scalar field fluctuations.
Atomic operator)()( 30 RH A
))(()()( 2 xRH I
d
dtaakdH
kkkF
3)(
How to separate the contributions of vacuum fluctuations and radiation reaction?
Heisenberg equations for the field
Heisenberg equations for the atom
The dynamical equation of HA
Integrationsf EEE
Atom + field Hamiltonian
IFAsystem HHHH
—— corresponding to the effect of vacuum fluctuationsfE—— corresponding to the effect of radiation reaction
sE
uncertain? Symmetric operator ordering
For the contributions of vacuum fluctuations and radiation reaction to the atomic level , b
with
Application:
1. Explain the stability of the ground state of the atom;
2. Explain the phenomenon of spontaneous excitation;3. Provide underlying mechanism for the Unruh effect;
…
4. Study the atomic Lamb shift in various backgrounds
Waves outside a Massive body
22222122 )/21()/21( dSindrdrrMdtrMds
A complete set of modes functions satisfying the Klein-Gordon equation:
outgoing
ingoing
Spherical harmonics Radial functions
,0)|()(22
2
rRrVdr
dl
),12/ln(2* MrMrr
and the Regge-Wheeler Tortoise coordinate:
with the effective potential
.2)1(2
1)(32
r
M
r
ll
r
MrV
)()( ll AA�
222)()(1)(1 lll BAA
�
The field operator is expanded in terms of these basic modes, then we can define the vacuum state and calculate the statistical functions.
It describes the state of a spherical massive body.
Positive frequency modes → the Schwarzschild time t.Boulware vacuum:
D. G. Boulware, Phys. Rev. D 11, 1404 (1975)
reflection coefficienttransmission coefficient
0)(
dr
rdV Mr 3
0)(
3
2
2
Mr
dr
rVd
2
2
max 27
2/1)(
M
lrV
Is the atomic energy mostly shifted near r=3M?
For the effective potential:
32
2)1(21)(
r
M
r
ll
r
MrV
For a static two-level atom fixed in the exterior region of the spacetime with a radial distance (Boulware vacuum),
B
2
2
64
with
Lamb shift induced by spacetime curvature
rrvf
In the asymptotic regions:
P. Candelas, Phys. Rev. D 21, 2185 (1980).
Analytical results
The Lamb shift of a static one in Minkowski spacetime with no boundaries.
M —
It is logarithmically divergent , but the divergence can be removed by exploiting a relativistic treatment or introducing a cut-off factor.
M
The revision caused by spacetime curvature.
The grey-body factor
Consider the geometrical approximation:
3Mr
2M
Vl(r)
,max2 V ;1~lB
,max2 V .0~lB
The effect of backscattering of field modes off the curved geometry.
2. Near r~3M, f(r)~1/4, the revision is positive and is about 25%! It is potentially observable.
1. In the asymptotic regions, i.e., and , f(r)~0, the revision is negligible!
Mr 2 r
Discussion:
The spacetime curvature amplifies the Lamb shift!
Problematic!
Mr 2
r
position
sum
Candelas’s result keeps only the leading order for both the outgoing and ingoing modes in the asymptotic regions.
1.
The summations of the outgoing and ingoing modes are not of the same order in the asymptotic regions. So, problem arises when we add the two. We need approximations which are of the same order!
2.
??
??
Numerical computation reveals that near the horizon, the revisions are negative with their absolute values larger than .
3.2
02
)()12(1
l
lBlr
Numerical computation
Target:
Key problem:
How to solve the differential equation of the radial function?
In the asymptotic regions, the analytical formalism of the radial functions:
Mrs 2
Set:
with
The recursion relation of ak(l,ω) is determined by the differential of
the radial functions and a0(l,ω)=1, ak(l, ω)=0 for k<0,
with
Similarly,
They are evaluated at large r!
For the outgoing modes, r
The dashed lines represents and the solid represents .2
)(lA 2
)(lB
4M2gs(ω|r) as function of ω and r.
For the summation of the outgoing and ingoing modes:
The relative Lamb shift F(r) for the static atom at different position.
For the relative Lamb shift of a static atom at position r,
Conclusion:
F(r) is usually smaller than 1, i.e., the Lamb shift of the atom at an arbitrary r is usually smaller than that in a flat spacetime. The spacetime curvature weakens the atomic Lamb shift as opposed to that in Minkowski spacetime!
2.
The relative Lamb shift decreases from near the horizon until the position r~4M where the correction is about 25%, then it grows very fast but flattens up at about 40M where the correction is still about 4.8%.
1.
What about the relationship between the signal emitted from the
static atom and that observed by a remote observer?
It is red-shifted by gravity.
F(r): observed by a static observer at the position of the atom
F′(r): observed by a distant observer at the spatial infinity
Who is holding the atom at a fixed radial distance?
circular geodesic motion
bound circular orbits for massive particles
stable orbits
How does the circular Unruh effect contributes to the Lamb shift?
Numerical estimation
Summary
Spacetime curvature affects the atomic Lamb shift.
It weakens the Lamb shift!
The curvature induced Lamb shift can be remarkably significant
outside a compact massive astrophysical body, e.g., the
correction is ~25% at r~4M, ~16% at r~10M, ~1.6% at r~100M.
The results suggest a possible way of detecting fundamental
quantum effects in astronomical observations.