What Are The Paradoxes In Quantum Mechanics?
What are the paradoxes in quantum mechanics? originally appeared on Quora: the knowledge sharing network where compelling questions are answered by people with unique insights.
Answer by Allan Steinhardt, PhD, Author "Radar in the Quantum Limit", Formerly DARPA's Chief Scientist, on Quora:
(Photo: DAVID MCNEW/AFP/Getty Images)
Answer by Allan Steinhardt, PhD, Author "Radar in the Quantum Limit", Formerly DARPA's Chief Scientist, on Quora:
What are the paradoxes in quantum mechanics? I can
give you five, all of which have been experimentally confirmed. Some
are already commercial products, other may be soon. As quantum is
rapidly entering commercial markets, it is insightful to approach the
question as in "what are the capabilities that quantum mechanics
can provide in the real world that most laymen would interpret as
startling, or impossible, based on intuition?" I then close with some
comments about paradoxes writ large and why they arise in Quantum. In
each case I provide links to science literature, usually to original
sources.
1) Teleportation: Quantum
technology allows us to "beam" an exact replica (down to quantum numbers
i.e. superposition state) of one system at point A to another system at
point B, arbitrarily far away. This topic is discussed in this paper Experimental quantum teleportation, and also this answer on Quora.
Caveat: We must first send clumps of
matter, i.e., ship raw matter to "clone" from point A to point B. We
also must learn, and communicate, the desired state of this matter (i.e.
the superposition state) through a "standard" (though suitably
conditioned) comms channel, at less than or equal the speed of light.
This probably will destroy the original system, being, human, whatever.
So there is no violation of light speed constraint on information
transfer. Read more here. The "paradox" here is that teleportation even
of living humans is theoretically feasible, though (likely) technically
impossible or impractical.
2) Remote detection of eavesdropping: This
is the basis of secure quantum encryption. Using entanglement we can
detect whether a (quantum) message between Sender and Receiver has been
intercepted in transit. It makes no difference how far away the
eavesdropper was, the mere impact of his/her measurement is detected.
Caveat: Sender and Receiver must compare notes, no speed of light violation. See Quantum cryptography without Bell's theorem.
The "paradox" here is we can learn if someone somewhere read a private
message without knowing who or how, even though we cannot access the
reader!
3) "Spooky" calculation: Here
we use superposition to obtain more equivalent computations than
hardware and clock cycle enumeration would suggest is feasible. This
"hack" exploits the fact that quantum states can be in superposition and
in so doing can "remotely" interfere with each other. Thus we can
store, in a quantum computer, ones and zeros, and do calculations simultaneously on both without duplicating hardware. See Quantum Computing since Democritus.
Caveat: Only some problems can be sped up
this way. As of February 2016, we have no quantum computers that are
faster than regular computers for any compute problem, but that is
expected to change. Protein folding has been shown to be among the class
of problems where quantum can help. It would be great if we could
actually get there commercially. The "paradox" here is that we can have,
in one computer, more calculations in one nanosecond than there are
atoms in the universe! See What is a quantum computer?.
4) Interaction free measurement: The
best way to explain this is that we can measure something in quantum
without measuring it. Of all the animals in the Quantum Zoo this is the
most exotic! See: PHYSICS ILLINOIS, and How does the quantum zeno effect work?.
Caveat: We need to get the object "near" the sensor, we just don't actually allow the sensor to disturb the object in any way. This does allow
us to measure things that normally would be harmed through measurement,
such as cold atoms. The "paradox" here is that we can measure things
remotely in ways that, while limited, defy our sense of time and space.
5) Life extension of particles: Because
quantum causes particles to interact with each other in weird ways we
actually can slow down the decay of particles. Note: this is not special relativity here, the particle remains at rest, it just "feels" time differently, unlike its surroundings. See nist.gov.
Caveat: Unlike time dilation in relativity,
this effect is limited to particles that evolve through state
superposition. The "paradox" is we can selectively slow down time
"simply" by interacting with a (certain type of) dynamic system.
Note for Nerds: Paradox
is an ambiguous word. Unresolved ambiguity in language usage makes
quantum impossible to explain satisfactorily without defining terms.
(The quantum particles are in a state of superposition, don't let our
language follow suit!) Paradox can mean one of three things: (i) we get
different contradictory answers using logic, (ii) we appear to get different contradictory answers using logic, and (iii) we observe something that defies common sense. We always use paradox in quantum in this third sense. There is never a logic paradox in quantum. If
(i) occurred in any branch of mathematics this would be an epic event.
If it were possible to get two different answers in arithmetic using two
different solution paths we would need to dismantle and rebuild math
from scratch. Item (ii) occurs all the time in "pure" math and math as
applied to physics. But as the word "appear" suggests this is a
subjective event, unlike (i), which is about math not us.
As a child we are puzzled by things we perceive later as "obvious" as an
adult. Like (ii), (iiii) is subjective. But unlike (ii), item (iii) is
about our sense of the physical realm, not our sense of theory. When laymen say there is a paradox in Quantum they never mean "hey
the Hermitian operator in Quantum appears to give contradictory results
when I select different basis functions in Hilbert space". If they
did we would be at paradox definition (ii) [if resolvable], or (i) [if
unresolvable]. In Quantum (sans strong gravity) the paradox is always
physical, i.e. case (iii). The fact that we call Quantum paradoxical is a
testament to (A) its awesome predictive power and explanatory success,
and (B) its deep mathematical basis. No one ever accuses, say, cell
biology, or social science, of being paradoxical!
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