Anirbit Pound and Rebka

7/31/2019 Anirbit Pound and Rebka

1/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The Pound and Rebka experiment.

Anirbit

October 21, 2008
http://find/


2/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Miles to go before you sleep...

Dreams and Reality

Removing sources of extranneous frequency shifts

The mechanics and the electronics of the experiment

It begins where it ends
http://find/http://goback/


3/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The aim of the experiment

For various reasons (NOT necessarily coming from GeneralRelativity) theoretical physicists had started believing thatgravitational force affects energy of the photon and hencethere was a pressing need to experimentally test thisapparently non-intuitive prediction.
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4/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends



Thats what Pound and Rebka tested in their famous 1960experiment published on 1st April 1960 in Physical ReviewLetters.


5/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends



Thats what Pound and Rebka tested in their famous 1960experiment published on 1st April 1960 in Physical ReviewLetters.

Rebka was a graduate student under Pound at the HarvardUniversity.
http://find/


6/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

References

Apparent Weight of Photons by Pound and Rebka on1stApril 1960PRL

Lectures by Mester Experimental Tests of GeneralRelativity

The Mossbauer Effect review paper by A.F.Boyle andH.E.Hoyle

Weighing Photons, I by R. V. Pound, Physics in

Perspective 2, 224 (2000) Weighing Photons, IIR. V. Pound, Physics in

Perspective 3, 4 (2001)
http://find/


7/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Thanks

I would also thank Prof.Arnab for the references and

Arnab Sen (senior DTP student) and Prof.Vikram forhelpful disucssions


8/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

A rough idea of the magnitude of the effect

The Jefferson Physical Laboratory at Harvard has anenclosed and isolated tower that is 22.5m high. Thisgravitational force over this height difference can produce a

fractional frequency shift of 2.5 10

15 (gh

c2 ).
http://find/


9/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

A rough idea of the magnitude of the effect

The Jefferson Physical Laboratory at Harvard has anenclosed and isolated tower that is 22.5m high. Thisgravitational force over this height difference can produce a

fractional frequency shift of 2.5 10

15 (gh

c2 ).

So that kind of number that they were trying tomeasure is of the order of 1015!


10/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

A rough idea of the magnitude of the probe

Any source of photons has a Natural Linewidth which is a

completely quantum phenomemon coming from theuncertainity principle.
http://find/


11/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends




The gamma photon source they had was 14Kev transition of

Fe57 and that has a Natural Lifetime of the order of = 107sec.
http://find/


12/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends






Hence a Natural Linewidth of = h

= 108eV.
http://find/


13/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends






Hence a Natural Linewidth of = h

= 108eV.

So the probe has a natural spread that is a fractionalFWHM of

E= 1012
http://find/


14/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The constraints

So far the probe sounds precise enough for the purpose but avariety of phenomenomenon can ruin this precision. Twomost important of these are:
http://find/


15/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The constraints


Doppler broadening due to thermal motion of theemitter.

Th P d d
http://find/


16/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The constraints



A difference of 0.6C between the emitter and theabsorber can swamp out the gravitational effect.

Th P d d
http://find/


17/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The constraints




The recoil of the emitting nuleus broadens the emissionspectra.

The Pound and

h


18/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The constraints




The recoil of the emitting nuleus broadens the emissionspectra.

A free recoil will cause a spread of about 105 timesthe natural width

The Pound and


19/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

endsFrom these numbers it is obvious that Pound andRebka have to do everything they can to reduce theeffect of the recoil and thermal motion.

The Pound and

F i i i


20/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

From constraints to creativity...

The Pound and

O i /R i d f th b i t


21/74

The Pound and

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Overview/Reminder of the basic concepts

As explained earlier we need a precise source of gammaphotons (of high energy (10 100Kev) so that thenatural line width

is very small of the order of 1012)

The Pound and

O i /R i d f th b i t


22/74

u

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


As explained earlier we need a precise source of gammaphotons (of high energy (10 100Kev) so that thenatural line width


Thats where the then recent, 1958 discovery ofMossbauer effect (the so calledrecoilless photons)

comes in to achive this.

The Pound and

O e ie /Re i de of the basic co ce ts
http://find/


23/74

Rebka experiment.

Anirbit

Dreams and

Reality

Removing sourcesof extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


As explained earlier we need a precise source of gamma

photons (of high energy (10 100Kev) so that thenatural line width



comes in to achive this. Once we have a precise source of photons the idea is to

measure the frequency shift of it due to gravitation

The Pound and

http://find/


24/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends








NOT directly

The Pound and



25/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends








NOT directly

but by determining the velocity of the source whichcancels this shift by the doppler shift.

The Pound and

R bkOverview/Reminder of the basic concepts
http://find/


26/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends








NOT directly

but by determining the velocity of the source whichcancels this shift by the doppler shift.

The Pound and

R bk iWhat is Mossbauser Effect?
http://find/


27/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

What is Mossbauser Effect?

The crude picture:

The Pound and

Rebk e e i e tWhat is Mossbauser Effect?
http://find/


28/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


The crude picture:

A nucleus inside a crystal lattice undergoes a quantumtransition and emits a gamma photon.

The Pound and

Rebka experimentWhat is Mossbauser Effect?
http://find/


29/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


The crude picture:


But classically because of the various possible latticevibration modes the emitted photons will have a considerablespread in energy.

The Pound and

http://find/


30/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


The crude picture:



Fortunately the world is quantum theoretic!

The Pound and

http://find/


31/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


The crude picture:



Fortunately the world is quantum theoretic!

Energy is trivially conserved in the process but momentumconservation is the subtlety.

The Pound and

Rebka experiment.
http://find/


32/74

Rebka experiment.

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

If the crystal lattice is very close packed the sound wavesin the lattice, Phonons, can propagate very fast at timescales comparable to nuclear transition times (or

life-time of the excited nucleus) and hence take away themomentum without needing the nucleus to recoil.

The Pound and

Rebka experiment.
http://find/


33/74

p

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends



Hence giving a sharp emission frequency limitedpredominantly by the energy uncertainity and the thermalvibration.

The Pound and

Rebka experiment.
http://find/


34/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends



Hence giving a sharp emission frequency limitedpredominantly by the energy uncertainity and the thermalvibration.

Further the stiffness of the crystal causes the energylevels of the emitter to be far spaced and that alsoreduces the broadening by suppressing the transitionprobabilities.

The Pound and

Rebka experiment.
http://find/


35/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

There are more precise arguments using uncertainityprinciple and these can also be justified by elaborateperturbation theory.

The Pound and

Rebka experiment.
http://find/


36/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

There are more precise arguments using uncertainityprinciple and these can also be justified by elaborateperturbation theory.

These arguments shows that we have an upper bound oforder of 100Kev for the gamma photons to show MossbauerEffect.

The Pound and

Rebka experiment.Miles to go before you sleep...
http://find/


37/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Dreams and Reality



http://find/


38/74

The Pound and

Rebka experiment.


39/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Note the following important things:

Mossbauer Effect is a purely quantum phenomenon.

If the emitter de-excites to the same atoms as thelattice then phonon propagation is facilitated and theemission line sharpens.

The Pound and

Rebka experiment.
http://find/


40/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Note the following important things:

Mossbauer Effect is a purely quantum phenomenon.

If the emitter de-excites to the same atoms as thelattice then phonon propagation is facilitated and theemission line sharpens.

The Pound and

Rebka experiment.
http://find/


41/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

endsGetting a great Mossbauer Effect was an important

aspect where Pound and Rebka won over theircompetitors by carefully annealing the source so thatthe emitter (0.4 curie of 270 day Co57) diffuses to adistance of 3 105 cm into a 0.005 inch of iron disk.

The Pound and

Rebka experiment.The two important aspects of the experiment
http://find/


42/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

What is required to get the Mossbauser effect?

The Pound and

http://find/


43/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

What is required to get the Mossbauser effect?which was discussed in the previous slide

The Pound and

http://find/


44/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


What are the other sources of frequency shifts thathave to be eliminated?

The Pound and

http://find/


45/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


What are the other sources of frequency shifts thathave to be eliminated?

The Pound and

Rebka experiment.

A i bi

Other sources of frequency shiftsd h i l


46/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

and their removal

The Pound and

Rebka experiment.

A i bit

Other sources of frequency shiftsd th i l
http://find/


47/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

and their removal

The energy of the emitted gamma photons is sensitiveto temperature. A difference of 0.6C can swamp outthe gravitational effect.

The Pound and

Rebka experiment.

Anirbit

Other sources of frequency shiftsd th i l
http://find/


48/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

and their removal


Hence the source and the absorber were kept at thesame temperature maintained by a thermocouple.

The Pound and

Rebka experiment.

Anirbit

Other sources of frequency shiftsa d thei e o al
http://find/


49/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

and their removal



There were very high intrinsic frequncy shifts between

the source and the emitter.

The Pound and

Rebka experiment.

Anirbit

Other sources of frequency shiftsand their removal
http://find/


50/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

and their removal



There were very high intrinsic frequncy shifts between

the source and the emitter.

This effect was elimated by exchanging the sourceand the absorber many times during the experiment.

The Pound and

Rebka experiment.

Anirbit
http://find/


51/74

Anirbit

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The Pound and

Rebka experiment.

Anirbit
http://find/


52/74

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

endsThese two effects were not considered by the previous

experimenters like Cranshaw and hence through theydid similar experiments their data was not convincingthat the shift was coming completely due togravitational effect.

The Pound and

Rebka experiment.

Anirbit

Reduction of the absorption
http://find/


53/74

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Pound and Rebka used the 14Kev transition and these giveabout 70% recoilless gamma photons at room temperature.

The Pound and

Rebka experiment.

Anirbit

http://find/


54/74

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


These gamma photons can be absorbed by air.

The Pound and

Rebka experiment.

Anirbit

http://find/


55/74

Dreams and

Reality


frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


These gamma photons can be absorbed by air.

So they kept a constant flow ofHe through the Mylar Bagat the rate of 30liters/hr.

The Pound and

Rebka experiment.

Anirbit

Miles to go before you sleep...
http://find/


56/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Dreams and Reality




The Pound and

Rebka experiment.

Anirbit

Some idea of the instrumentation Mechanics
http://find/


57/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Mechanics

The Pound and

Rebka experiment.

Anirbit



58/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Mechanics

The source of gamma photons was sinosoidally moved usinga loudspeaker kind of apparatus oscillating at a frequency of10 15 Htz.(intentionally kept much higher than the shiftthat gravity would cause).

The Pound and

Rebka experiment.

Anirbit

http://find/


59/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


The frequency shift between the emission point and theabsorption point was measured during both the cycles andthe difference between then gave information about the

frequency shift due to gravitation.

The Pound and

Rebka experiment.

Anirbit

http://find/


60/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends


The frequency shift between the emission point and theabsorption point was measured during both the cycles andthe difference between then gave information about the

frequency shift due to gravitation.

This method of measurement increased the sensitivity and

removed errors due to background shifts of the modulation. The Pound and

Rebka experiment.

Anirbit

Some idea of the instrumentation Electronics
http://find/


61/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

The Pound andRebka experiment.

Anirbit

http://find/


62/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

There is an elaborate electronic circuit which essentiallyamplifies the signals from the photomultiplier connected to

the absorber which is a scintillation crystal.


Anirbit

http://find/


63/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

There is an elaborate electronic circuit which essentiallyamplifies the signals from the photomultiplier connected to

the absorber which is a scintillation crystal.

There were 2 such circuits and alternately in each quatercyle of the source the signal was fed into it. The difference

in counts was a measure of the assymmetry and hence of thefrequency shift due to gravitation.
http://find/


64/74


Anirbit

Their Results


65/74

Dreams and

Reality

Removing sources

of extranneous

frequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Pound and Rebka found the frequency shift as12

(5.13 0.51) 1015 good to 10%.


Anirbit

Their Results
http://find/


66/74

Dreams and

Reality

Removing sources

of extranneousfrequency shifts

The mechanics

and the electronics

of the experiment

It begins where it

ends

Pound and Rebka found the frequency shift as12

(5.13 0.51) 1015 good to 10%.

Pound and Snider later improved this to an uncertainity of1% (PRL 13 539(1964))
http://find/


67/74


68/74


Anirbit

Final reflections


69/74

Dreams and

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Removing sources


The mechanics

and the electronics

of the experiment

It begins where it

ends

We must keep in mind that this experiment is NOT atest of General Relativity.

This was a truly a path-breaking experiment which forthe first time showed that gravity-photon interaction

can be measured on laboratory length scales.


Anirbit

Final reflections
http://find/


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Removing sources


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can be measured on laboratory length scales.I would like to draw a parallel of this experiment withthe Michelson-Morley Experiment.


Anirbit

D d

Final reflections
http://find/


71/74

Dreams and

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Removing sources


The mechanics

and the electronics

of the experiment

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ends



can be measured on laboratory length scales.I would like to draw a parallel of this experiment withthe Michelson-Morley Experiment.Both of these experiments pushed the limits of precisionand resolution of measurement of length and time.


Anirbit

D d

Final reflections
http://find/


72/74

Dreams and

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Removing sources


The mechanics

and the electronics

of the experiment

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can be measured on laboratory length scales.I would like to draw a parallel of this experiment withthe Michelson-Morley Experiment.Both of these experiments pushed the limits of precisionand resolution of measurement of length and time.

We should note that the time difference between thediscovery of Mossbauer Effect and its miraculous use totest gravity-photon interaction is only 2-years!


Anirbit

Dreams and
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73/74

Dreams and

Reality

Removing sources


The mechanics

and the electronics

of the experiment

It begins where it

ends

I see here a lesson to learn in terms of research that we mustalways be aware of the current frontiers and...


Anirbit

Dreams and
http://find/


74/74

Dreams and

Reality

Removing sources


The mechanics

and the electronics

of the experiment

It begins where it

endsThe chief enemy of creativity is good sense. Picasso
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Anirbit Pound and Rebka

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Transcript of Anirbit Pound and Rebka