Article reviewed and commented on superposition definition
by G.Haven d’Amaury III [GhA]
[Comments in green]
Spooky action and beyond
Viennese physicist Anton Zeilinger talks about teleportation, the information stored in a human being and freedom in physics.
Die Weltwoche: Professor Zeilinger, the media
calls you "Mister Beam". You personally were absolutely against the
association with beaming. Why?
Anton Zeilinger. Photo: Jacqueline Godany
Anton Zeilinger: Because it gives the wrong
impression of my work. "Beaming" exists only in science
fiction films, where it was invented as a money-saving device. Actually having
to land on all those planets runs up huge production costs. Beaming is cheaper:
1,2,3 and you're somewhere else. But that's a long way from anything we're
doing here.
What are you doing?
Transferring the properties of light particles over certain distances onto
other light particles, with no time delay. The procedure is based on phenomena
which exist only in the quantum world, and is known as "quantum
teleportation."
It sounds almost as exciting as "beaming".
Yes, but there are two major differences. Firstly, we transfer properties,
not matter. And secondly, until now we have had more success with light
particles and occasionally with atoms, not with larger objects.
In 1997 your team successfully performed the first quantum teleportation.
What distances can be crossed with this technique today?
illustration of the teleportation of light
particles under the Danube
Last year we teleported light particles across a distance of 600 metres under
the Danube – that's the current world record. In theory the range is limitless.
I always say that when the Americans really start their Mars mission, the
280-day journey will be deadly boring for the astronauts. They might be
interested in taking part in a few teleportation experiments on the way, and
increase the record by a hundred million kilometres or so.
You said that you only transfer properties, not particles. Would
"copying" not be a more accurate expression than
"teleportation"?
No. Firstly it differs from simple copying in that the original loses all
its properties. That is something so crazy that it could only exist in the
quantum world. You can actually remove all the properties of a particle and
give them to another particle.
But both particles remain where they are.
Yes, but the question is: how do I recognise an original? I maintain: solely
through its properties. Matter itself is completely irrelevant. If swap all my
carbon atoms for other carbon atoms, I am still Anton Zeilinger.
This happens over the course of our lives. We are continually changing our
cells.
Exactly. The only important thing are my properties, and they are based on the
order of the atoms – that what makes me who I am. The atoms are unimportant in
themselves. So when we transfer characteristics during teleportation, in this
sense we are actually transferring the originals.
Some teams of physicists are already teleporting single atoms. So what
really stands in the way of beaming humans?
We are talking about quantum phenomena here – we have no idea how we could
produce these with larger objects. [As of 2012, we
can do electrons, atoms and molecules. GhA] And even if it was possible,
the problems involved would be huge. Firstly: for physical reasons, the
original has to be completely isolated from its environment for the
transfer to work. There has to be a total vacuum for it to work. And it is a
well-known fact that this is not particularly healthy for human beings.
Secondly, you would take all the properties from a person and transfer them
onto another. This means producing a being who no longer has any hair colour,
no eye colour, nix. A man without qualities! This is not only unethical – it's
so crazy that it's impossible to imagine.
Well, in Vienna perhaps... But you said that another problem was the mass of
information. You once calculated that if you burnt all the information in one
human being onto CDs there would be enough of them to make a tower that reached
from here to the centre of the milky way.
That was a few years ago – with today's technology the tower would not be quite
so tall. But we can do a rough calculation. The atoms in a human being are the
equivalent to the information mass of about a thousand billion billion
billion bits. Even with today's top technology, this means it would take
about 30 billion years to transfer this mass of data. That's twice the age of
the universe. So we'll need a number of major breakthroughs in technology
first.
What do think are the limits of teleportation?
Who knows, perhaps in a thousand years we really will be able to teleport a
coffee cup. But beware: even the tiniest interference can mean that the cup
arrives without its handle. This method of transport would be far too dangerous
for humans.
Why is the procedure so sensitive to disturbance?
Because any disturbance – and this goes for a measurement or observation –
alters the state of the particles which are involved in the teleportation. The
rules of quantum physics are completely different from those of the world we
live in. For quantum teleportation we use methods of entanglement. This is a particular state
which can connect two or more particles, but which disappears as soon as it is
observed from outside.
Entanglement – we should imagine this as...
...there is no way of imagining it. The Austrian physicist Erwin
Schrödinger coined
the term in 1935 and also said that the twist in the phenomenon of quantum
physics is that it forces us to bid farewell to all our dearly held
ideas about the world.
Help us do this!
When two particles collide like billiard balls at a quantum level, then they
are immediately linked or "entangled". Neither of the two particles
has a clearly defined position or a clearly-defined momentum: location and
speed are uncertain, as we say.
Heisenberg's famous uncertainty principle.
Exactly. But then I can go and measure, say, the momentum of one of the
entangled particles. By way of this measurement, the momentum which was
previously uncertain can now be determined. The peculiar thing is that in the
same instant, the second particle also gains a clearly-defined momentum. No
matter how far away it is.
Albert Einstein called this effect "spooky
action at a distance"
Right. But the truly strange is yet to come.
I can't wait.
The result of my measurement of the first particle is completely random.
There is no way of predicting it, on principle. But as soon as I have the
result, I can deduct the momentum of the second participle.
NB:So I can accurately measure the
momentum of the second particle, even if it
is hundreds of billions of kilometres away.GhA
illustration of a non-local four-proton entangled
state
Theoretically yes. The effect has so far been proved across a distance of a
hundred kilometres. The amazing thing is that there can be no exchange of information between the two
particles. They react absolutely in synch, although they could could
never know anything of each other's existence. You can think of it as two dice
far away from each other that always land on the same number, without there
being any kind of mechanism which connects them. Absurd! [Sounds like that: But: If I superimposed some molecules, after
I had first encoded with a book by Shirl, onto the A half, then transport the B half to anywhere
in the Universe. Then anyone there can read the encoded book, it will be exactly the same book. Ipso facto: Before
B is moved too far away, I will simply see if encoding any other book on A and
trying to read it to see how long it took to be able read it on B, instantaneously. If so, I will claim a
Noble Prize for Physics. GhA]
Uncertainty, coincidence, spooky effects – doesn't it make you dizzy
sometimes?
It's all pretty crazy. The spooky effect at a distance is a process outside
time and space that even I can't really imagine. But I believe that quantum
physics tells us something very profound about the world. And that is that the
world is not the way it is independently of us. That the characteristics of the
world are to a certain extent dependent on us.
That almost has a New Age ring.
You have to be careful not to be cubby-holed. I mean it's like this: an
experimenter can determine through his choice of measuring equipment which
physical size becomes reality. Take a particle with an uncertain location and
an uncertain velocity. When you look at it through a microscope and locate it,
the particle gives you an answer: "Here I am." That means, the
location becomes reality at that moment. Beforehand, the particle had no
location at all. With the choice of the measuring equipment we've had a major
impact on reality. But the answer that nature gives is completely random.
I choose the measuring equipment, and nature chooses the result?
That's right. I call that the two freedoms: first the freedom of the
experimenter in choosing the measuring equipment - that depends on my freedom
of will; and then the freedom of nature in giving me the answer it pleases. The
one freedom conditions the other, so to speak. This is a very fine property.
It's too bad the philosophers don't spend more time thinking about it.
I'd like to come back to these freedoms. First, if you assumed there were no
freedom of the will – and there are said to be people who take this position –
then you could do away with all the craziness of quantum mechanics in one go.
True – but only if you assume a completely determined world where everything
that happened, absolutely everything, were fixed in a vast network of cause and
effect. Then sometime in the past there would be an event that determined both
my choice of the measuring instrument and the particle's behaviour. Then my
choice would no longer be a choice, the random accident would be no accident
and the action at a distance would not be action at a distance.
Could you get used to such an idea?
I can't rule out that the world is in fact like that. But for me the freedom to
ask questions to nature is one of the most essential achievements of natural
science. It's a discovery of the Renaissance. For the philosophers and
theologians of the time, it must have seemed incredibly presumptuousness
that people suddenly started carrying out experiments and asking questions of
nature and deducing laws of nature, which are in fact the business of God. For
me every experiment stands or falls with the fact that I'm free to ask the
questions and carry out the measurements I want. If that were all determined,
then the laws of nature would only appear to be laws, and the entire natural
sciences would collapse.
Are there physicists who advocate complete determinism?
I've met one. At the time I was a lot younger and cheekier than I am today, and
I intentionally insulted him publicly at a conference. He was incensed. I said
to him: "Why are you getting so upset? Neither you nor I are free in what
we do."
I'd like to come to the second freedom: the freedom of nature. You said that
for example the velocity or the location of a particle are only determined at
the moment of the measurement, and entirely at random.
I maintain: it is so random that not even God knows the answer.
Ultimately that implies something monstrous: namely that the particle had
absolutely no characteristics before it was measured. The great Danish
physicist Niels
Bohr once
said: no one has ever seen a chair. There is no objective reality. Only that
which is measured exists. We construct reality, and only in the moment of
measurement or observation.
I think you have to make a distinction: in my view there is something that
exists independently of us – in physics we call that the singular event.
For example the activity of a particle detector. Or the activity of a certain
cell in my eye, which registers a certain number of light particles and then
provokes a chemical reaction that is then registered in the brain. The images
that we form on the basis of this are our constructs. Bohr's chair or on a much
more abstract level, the quantum mechanics equation of states, are our concepts
of an object. Of course they are very purpose-oriented, because they've
been corroborated with repeated use.
So there is in fact something that exists independently of us. And the moon
is also there when I'm not looking at it.
Something exists, but it is not directly accessible to us. Only indirectly. And
whether this thing must really be called the "moon" is another
question. That is also a construct.
But there is something up there...
... the word "there" is yet another construct. Space and time are
concepts aimed at giving meaning to our world of appearances. So they are
entirely reasonable constructs. By no means do I want to give the impression
that I believe everything is just our imagination.
The world as a huge theatre that only plays in our heads.
That is certainly not my view of things.
Then what would you call it, this something that you can't call moon or
space or time – this something that exists independently of us?
Wouldn't I be making another qualification if I tried to give it a name?
Isn't it enough if I just say it exists? As soon as you use words like
"world" or "universe", you start lugging about all that conceptual
ballast again.
But you defend the thesis that there is an "original matter of the
universe": information.
Yes. For me the concept of "information" is at the basis of
everything we call "nature". The moon, the chair, the equation of
states, anything and everything, because we can't talk about anything without
de facto speaking about the information we have of these things. In this sense
the information is the basic building block of our world.
But just now you spoke of a world that exists independently of us.
That's right. But this world is not directly ascertainable or describable.
Because every description must be done in terms of the information, and so you
inevitably get into circular reasoning. There's a limit we can't cross.
And even a civilisation on Alpha Centauri can't cross it. For me that's
something almost mystical.
In your last book you wrote: "Laws of nature should make no distinction
between reality and information." Why?
We've learnt in the natural sciences that the key to understanding can often be
found if we lift certain dividing lines in our minds. Newton showed that
the apple falls to the ground according to the same laws that govern the Moon's
orbit of the Earth. And with this he made the old differentiation between
earthly and heavenly phenomena obsolete. Darwin showed that there is no
dividing line between man and animal. And Einstein lifted the line dividing
space and time. But in our heads, we still draw a dividing line between
"reality" and "knowledge about reality", in other words
between reality and information. And you cannot draw this line. There is
no recipe, no process for distinguishing between reality and information. All this
thinking and talking about reality is about information, which is why one
should not make a distinction in the formulation of laws of nature. Quantum
theory, correctly interpreted, is information theory.
And can you explain all these strange quantum phenomena conclusively with
your information concept?
Not all of them yet, but we're working on it. With limitation it works
excellently.
How?
I imagine that a quantum system can carry only a limited amount of information,
which is sufficient only for a single measurement. Let's come back to the
situation of two particles colliding like billiard balls, and in so doing
entering a state of limitation. In terms of information theory that
means that after the collision the entire information is smeared over both
particles, rather than the individual particles carrying the information. And
that means the entire information we have pertains to the relationship between
both particles. For that reason, by measuring the first particle I can
anticipate the speed of the second. But the speed of the first particle is
entirely random.
Because the information isn't sufficient.
Exactly. Its randomness is ultimately a consequence of the finiteness of the
information.
Dr. Zeilinger, you belong to the rare species of philosophising physicists.
Earlier there were more, especially in Austria: Wolfgang
Pauli,
Schrödinger, Ludwig
Boltzmann, Ernst Mach...
Not only in Austria. It may be that Vienna is a special city, but there was and
still is a tradition in Europe of philosophical thinking among physicists. I
saw that in 1977 when I went to America for the first time. Already after a
couple of weeks I started to miss philosophical discussion. Here we're more
ready to ask really fundamental questions. In Europe it's important to question
things. In America it's important to be able to build something. I don't mean
that at all negatively.
That's probably what accounts for American superiority, especially in
technology.
Certainly. And it also has to do with the American pioneer spirit and
the "success" of natural sciences in World War II. But I think the
European approach is more successful in the long run. Precisely in terms of the
major problems facing physics. We've now been working on the unification of
gravitation and quantum physics for almost eighty years – there must be
something wrong with our concepts. I'm convinced we can only succeed
with an entirely new philosophical approach.
But in recent times you've also proceeded with a very American-like idea: an
elite university for Austria.
Yes, we've been looking for a good name for some time. It's now called
"Austrian Institute of Advanced Science and Technology". The idea is
to create a scientific institution with an absolutely world-class status, one
that attracts the very best people. Something like the ETH in Zurich but...
... a little better?
Well, in some areas the ETH is very, very good, but not in all. That's the
major disadvantage of European universities, this mix of excellence and
mediocrity. You find it in almost every university in Europe. In America,
by contrast, differences of quality exist more between the various
institutions. There are some absolutely first rate ones, but there are also
many mediocre, and even poor ones. My idea is a university solely for doctoral
and post-doctoral work. 500 people at the most, a campus where people are constantly
discussing and exchanging ideas. In my experience the best results are achieved
where there's a high degree of interdisciplinary cooperation.
How far along is the Austrian Institute?
The project is ready, the government has expressed its approval, all that's
missing is the money.
It's probably not cheap.
The cost is no higher than a few kilometres of motorway: fifty to eighty
million euros in initial capital and roughly the same amount again every year
in operating costs. I'm convinced Austria needs it. And I'm convinced that in
ten or twenty years our region will have such a top-class university. The
question is only where. Here, in Bratislava, or in Warsaw?
*
Here a list of books by Anton Zeilinger.
His most recent work, "Einsteins Spuk. Die neue
Welt der Quantenphysik." (Einstein's Veil. The New World of Quantum
Physics) is published by Bertelsmann.
*
The interview originally
appeared in
German in Die Weltwoche on January 3, 2006.
Anton Zeilinger is a professor of physics at Vienna
University.
Translation: lp, jab.
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