PS700 Colloquia
Daniel Hayden 3rd Year Physics
The Bohr-Einstein Debate, an argument over the meaning of reality
Given by,
Professor Jim Al-Khalili
At Kent University
Colloquium on The Bohr-Einstein Debate, an argument over the meaning of reality”
given on 23rd January 2007 by Professor Jim Al-Khalili from the University of Surrey.
Professor Jim Al-Khalili’s colloquium addressed the following topics:
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The electron diffraction problem
The birth and history of quantum mechanics
The photoelectric effect
The on-going debate between Bohr and Einstein
Theories for the electron diffraction problems’ solution
Questions from audience
Professor Al-Khalili started by asking everyone “Does anyone not know what the 2 slit
experiment is?” (To which 4/50 people attending raised there hand). The two slit
experiment is where you have a source (usually light) being shone upon a surface with
two small slits in it. The light passes through the slits and is bent slightly causing
interference that can be detected on a far away screen by the appearance of bright and
dark fringes caused by the constructive and deconstructive interference of the waves. But
Professor Al-Khalili came up with a good metaphor that even though with wave-particle
duality particles can be thought of as waves in some circumstances and visa versa;
“Doing the experiment with sand isn’t the same as with light”, but why?
Quantum mechanics describes the way that subatomic and very small objects behave but
the same experiment with small particles yields a different result. He suggested you
could try to fool the experiment by letting one electron through at a time, and as you start
to do so the pattern does indeed appear to be random. But as you carry on you gradually
begin to see interference patterns building up again.
The question raised by this paradox was does the electron split in two and spread out like
a wave? Half detected through one slit and half through the other. Scientists set up an
experiment with a detector over one slit, fired a certain number of electrons and saw that
only half went through the slit they were observing and so obviously the other half were
going through the other slit. But when they looked at the diffraction pattern, no
interference pattern was seen, it was as if they were now acting as particles. Turn the
detector off and the interference pattern would appear again, no one knows why this is so
but Professor Al-Khalili stressed that “This isn’t theory, this actually happens”.
The birth of Quantum mechanics was around 1900-1905 with a man called Max Planck,
helped by the work of L.Boltzmann. Before them, professor Al-Khalili explained, people
used to think objects were infinitely divisible, but Boltzmann was the first to say that
there must be a small indivisible “something”. Planck said that heat radiation given off
by bodies comes in lumps or “quanta” and then Einstein came in and said actually all
electromagnetic radiation comes in lumps. Einstein actually won the Nobel Prize for this
using his photoelectric effect experiment.
The photoelectric effect is a quantum phenomenon where incoming electromagnetic
radiation of high enough energy can impart this energy to the electrons within a metal
giving them enough kinetic energy to overcome the binding energy and leave the metal.
It used to be thought that the higher the energy of the photons the more electrons would
be released but this experiment showed that in fact it depended on the frequency of the
photons, not the intensity, i.e. photons inbound with enough energy to electrons released
was 1-1 thus electromagnetic radiation must come in discrete packets.
Niels Bohr was working on quantum mechanics in an era where it was already a well
established theory (1924-26). Professor Al-Khalili actually compared Bohrs’ genius to
that of Einstein’s but said that “Bohr was however a very bad talker, not a very good
lecturer and although a good footballer, not very good at getting his point heard”. A
disciple of Bohr was Werner Heisenberg and although they fell out in the Second World
War Heisenberg was “one of the genius’s in the world” because he said that in quantum
mechanics properties don’t even exist until you measure them. W.Pauli, he said, also
deserved a mention for having a part in the success of quantum mechanics and is most
famous for his exclusion principle stating that no two electrons in an atom can be at in the
same state or configuration at any point in time, this laid the foundation for the whole of
modern chemistry.
Professor Jim Al-Khalili mentioned Einstein’s list of achievements but said that with time
he become almost old hat, “The longer you’re in a subject the harder it is to break free”.
The others were younger and now more revolutionary than Einstein so when all the best
physicists in the world descended upon Copenhagen as they did annually that year to
work on quantum mechanics, sparks were bound to fly. Professor Al-Khalili started then
to talk about the friendship that Einstein and Bose had, they would walk around for hours
at the conferences and debate on the meaning of quantum mechanics and associated
experiments like that of Stern Gerlach and Compton. Of all the Physicists that went to
the Solvay conference of Brussels in October 1927, two main groups emerged. One
consisting of people like Einstein, Pauli, and Schrodinger who believed that the world
was ultimately deterministic “God does not play dice” and the other with Bohr at the
helm saying that basically he did. At the time Einstein’s clan was not as good at
attacking the Copenhagen groups’ ideas and so Bohr won through (As an aside Al-Khalili
mentioned that apparently Dirac was above it all).
This is where the famous debate between Einstein and Bohr started, constantly going
back and forth with letters and meetings every so often where Einstein would think he
had something to prove Bohr wrong and Bohr disproving it so Einstein would then go off
again coming back next time with something else only to be disproved again, all the time
with the thought experiments becoming more and more sophisticated and complex trying
to prove that quantum mechanics was false (or at least incomplete). But at the 6th Solvay
conference in 1930, Einstein was convinced he had proof. Bohr was worried because
Einstein seemed supremely confident in his new idea to disprove quantum mechanics and
it did indeed cause Bohr some problems as shown below.
Einstein thought of a box that containing electromagnetic radiation and a clock that
controlled the opening of a shutter which covers a hole made in one of the walls of the
box. The shutter uncovers the hole for a time Δt which could be chosen arbitrarily.
During the opening, theoretically a photon would escape through the hole. In this way a
wave of limited spatial extension could be created. In order to challenge the
indeterminacy relation between time and energy that Bohr had always championed from
quantum mechanics, it was necessary to find a way to determine with an adequate
precision the energy that the photon has brought with it. At this point, Einstein turned to
his celebrated relation between mass and energy of special relativity:
. From
this it follows that knowing the mass of an object provides a precise knowledge about its
energy. The argument is therefore very simple, if one weighs the box before and after the
opening of the shutter and if a certain amount of energy has escaped from the box, the
box will be lighter. The change in mass multiplied by will provide precise knowledge
of the energy emitted. Also the clock would indicate the precise time the particle left the
box. Since in principle, the mass of the box can be determined to an arbitrary degree of
accuracy, the energy emitted can be determined with a precision ΔE as accurately as one
wanted. Therefore, the product ΔEΔt can be rendered less than what is implied by the
principle of indeterminacy.
But by the next morning Bohr had worked it out and worse still by again showing that
Einstein's subtle argument was not conclusive, using one of Einstein’s own great ideas:
The theory of relativity. Bohr showed that, in order for Einstein's experiment to work, the
box would have to be suspended on a spring in the middle of a gravitational field. In
order to obtain a measurement of weight, a pointer would have to be attached to the box
which corresponded with the index on a scale. After the release of a photon, weights
could be added to the box to restore it to its original position and this would allow us to
determine the weight. But in order to return the box to its original position, the box itself
would have to be measured. The inevitable uncertainty of the position of the box
translates into an uncertainty in the position of the pointer and of the determination of
weight and therefore of energy. On the other hand, since the system is immersed in a
gravitational field which varies with the position, according to the principle of
equivalence the uncertainty in the position of the clock implies an uncertainty with
respect to its measurement of time and therefore of the value of the interval Δt. A precise
evaluation of this effect leads to the conclusion that the relation
, cannot
be violated (Heisenberg Uncertainty principle). Bohr and the Copenhagen group had one
the debate.
The Copenhagen interpretation of the electron diffraction experiment is basically that the
act of looking affects the experiment, “We can never peak behind the curtain”. Quantum
mechanics shows you the answer and predicts what is seen but as soon as you look it
returns to what you would expect classically. Bohr said there was no point in looking
because to check is to change the outcome so you can’t win and quantum mechanics
underlies nearly everything we see around us. Professor Al-Khalili then went on to
explain other interpretations that have been hypothesized over the years like a rather good
one called the Bohm interpretation where all the electrons are considered point-like
particles that occupy precisely defined regions of space at all times. When one performs a
double slit experiment we want to know the positions on a screen at which the electrons
arrive individually, one at a time. Over time, the positions at which the electrons are
detected build up a pattern characteristic of wave interference. The usual Copenhagen
interpretation is puzzling in that a single entity, the electron, is said to exhibit
characteristics of both particle and wave. The Bohm interpretation accounts for the same
phenomena by saying that both a particle and a wave do exist. The particle aspect is
present because each electron traverses one slit or another, but never both. The wave
aspect is present because the electron's pilot wave traverses both slits. Also there is the
Transactional interpretation which is slightly more “out there” but basically says that a
signal is somehow sent back in time from when the electrons have already arrived at the
screen, telling the electrons where to go. This is however a rather ambitious theory as it
is almost a paradox fore the signal to be sent back there would have to theoretically be an
electron that has already traveled there at some time for the first time and therefore the
problem is still unexplained as to why the electron took that route in the first place.
However another good theory is the Many-world or Multi-verse theory in which there are
actually infinitely many universes stacked on top of each other and at every decision the
universe splits again, therefore we are just taking the specific route through the universes
that we are experiencing. Lastly there was the Sum over histories interpretation which is
likely one of the most well known of those mentioned. In this theory an atom explores
every possible path simultaneously, but all summed together cancel leaving just one,
which we see.
Professor Al-Khalili then went on to describe entanglement which is the apparent link
between two originally connected particles even when they are an arbitrary distance
apart. The EPR paradox of 1935 (Einstein, Podolsky, Rosen) said two particles leaving
back to back are quantum waves until measured. When measured greater than the speed
of light apart, measuring one defines the other instantaneously. But Bell’s theorem in its
simplest form says that “No physical theory of local hidden variables can ever reproduce
all of the predictions of quantum mechanics” and an experiment was thought of to see if
two particles were obviously one and the other because they were once together or as
quantum mechanics said, really were only defined when measured. This was Alain
Aspects’ experiment and it showed that Einstein was wrong, however illogical quantum
mechanics seemed it was correct and particles did indeed seem to be linked more than
could be accounted for. Many physicists have views on these conclusions; Mermin said
that despite this the explanation of the phenomenon is still incomplete (Which Professor
Jim Al-Khalili agreed with), Cushing claimed that the Copenhagen group just got their
idea out there first and that it is by far not the best explanation and lastly Gellmann
claimed Bohr had brainwashed the people around him into believing it was solved when
really it never was. Professor Al-Khalili finished by posing the audience a question; “Is
the difference just philosophical?” and that Bohr said it was wrong to think that physicists
were there to find out how nature is but merely too find out what we can about nature.
(The talk over ran by 15 minutes)
Questions:
Q: Does string theory come into this?
A: Yes, QM explains small things, Newtonian explains macroscopic things, Physicists
want to explain everything and string theory is a theory that joins QM with special
relativity.
Q: Is the next step to think of better experiments?
A: Yes, we are all the time thinking of experiments, some better than Einstein or Bohr
could have ever dreamed of, Quantum optics is a whole field doing just that.
Q: Can macroscopic things show quantum behaviour?
A: Yes, but still unknown where Macroscopic ends and the quantum world begins.
Talked about quantum de-coherence.
Q: Is Planck’s constant always the same value?
A: Not necessarily
Q: Does it matter?
A: Yes
Q: What would happen with the double slit experiment if you used marbles?
A: They are using bigger things all the time to do the experiment, Bucky balls (Carbon
60), and even possibly small viruses in the future and it still works, but eventually there
will come a limit. Marbles are too big.
Strengths
Professor Jim Al-Khalili was very clear and well spoken, getting his message across well.
The main thing that impressed me was the quality of diagrams to aid the audience as he
went through his talk, every slide had something to help you better understand what he
was talking about and I felt that lay and scientists alike could enjoy what they were
seeing. His talk was vibrant and passionate, one clearly being able to see his own interest
in the subject, but keeping it light and airy rather than serious (like a lecture). His
language flowed well and every subject he talked about was explained thoroughly so
again everyone was able to understand. I learnt a lot myself about the electron diffraction
problem and understood the many theories that try to answer it a lot better after attending
the talk. I was surprised and delighted to hear all about the annual Solvay conferences
and to see a picture of many of the best scientists that have ever lived sitting together in
one picture, it even sent a small shiver down my spine and a thought of jealousy that I
could not be there at that time to hear them all, it must have been awe inspiring. The
power point presentation was colourful and eye catching with the pace through the slides
being about right and able to read everything that was on each of the slides easily before
moving on to the next, basically a very well matched visual to oral presentation of the
ideas. Professor Al-Khalili seemed supremely confident throughout and answered all
questions at the end with accuracy and open mindedness even in the face of questions
that to a scientist may have at times seemed obvious.
Weaknesses
Professor Al-Khalili did over run the hour allotted by 15 minutes, yet still it felt like the
last 10 minutes of the talk were a little rushed (As If there was simply too much to fit in
the hour allowed). There were a couple of spelling mistakes on the slides but generally
not too bad. There was a tendency to do asides a little too often, it is cute but relatively
un-important to know that Niels Bohr was the sub goal keeper for his national side and
these facts a little too often felt a bit too off topic (especially as he over ran by
15minutes). I would personally have cut out so many of the unimportant asides and
concentrated on the topic more, still giving time to the relevant asides but keeping them
as asides rather than small sections of the talk. It surprised me in a bad way that
sometimes he seemed to skip back slides and then forward again to explain certain
aspects rather than make the presentation slides flow better, his speech and personal
presentation flowed very well with the slides and it was just a shame that although
visually and factually stunning his power point presentation was not as linear as it could
have been. Also there was a tendency to skip large periods of time from the birth of
quantum mechanics 1900-1905 straight to 1924-26 when it was already a well established
theory. This is fine for scientists who understand that things went on in between but to
lay it might have seemed that suddenly in 1924 it was accepted and started moving
forwards again when in fact work has never and probably will never stop on quantum
mechanics, and it is only the subsequent work in the many intervening years between
breakthroughs that lead to the next.