Hi, I'm John Skieswanne, and this post is part 3 of a series on physics. In this series I will explain a few pillars of modern physics. I won't be
using any complex maths. It is my hope that this series will introduce some of you brilliant, curious-minded laymen out there to the inner circles of
So, sit back and enjoy.
At the subatomic level, particles are represented with probability wave functions - their properties are blured into a cloud of probabilities. This is
a direct result of the Uncertainty Principle, in which one cannot measure a particle's property without affecting other of its properties. Wave
functions are rather good models for guessing the most likely properties of a particle. When two particles share the exact same properties, they may
be both represented with the exact same wave function. Only one wave function will be enough to predict the properties of both particles, since they
are exactly identical in nature. These two particles are "entangled".
Now, if you were to measure the property of one of these twin particles, you'd be knowing exaclty
one of its properties. You would have
collapsed its wave function (that is, you would have eliminated all other possibilities down to zero). The fascinating thing is, this means that you
also now know the property of the other twin particle (it's the same that the one you just observed) without having to measure it - no matter how
far this other particle is located
. It could be at the end of the Universe, it won't matter: as soon as you know one particle's property, you
also know the other's. Both particles are modelled with the same wave function; as soon as the latter collapses, both particles get to have their
property known, no matter how far apart they are.
An analogy to explain the phenomenon:
Imagine you have two coins, and a table covered with ink. You put both coins "heads"-up, and you glue them side by side. You flip the coins, but as
they land on the ink, you don't look. You just separate them, and seal them in two individual envelopes (envelope A and envelope B). You send
Envelope B to Bob, your friend on Mars.
Once Envelope B reaches Mars, you finally venture and open your own envelope. Until now you never checked on which side your coin (and your friend's)
landed after you have flipped them. What you do know, is that since you glued both of them together, they'll have both landed on the same side
(either heads for you and heads for Bob, or tails for you and tails for Bob). But until now you didn't know which side the coin chose, it could be
heads or tails, with 50/50 chances.
As you open your envelope, you discover that there's ink on, say, the "tails" side of your coin. Instantly, you know which side has ink on Envelope
B's coin, even though Envelope B is now in the hands of Bob some 60,000,000 kilometres away (or over three minutes at the speed of light). The
you saw your coin, and as you knew that Bob's was identical, then you knew that Bob's coin had ink on its "tails" side, just like
yours. Thus this effect enables information to apparently "travel" at speeds way, way higher than the speed of light.
Can an useful communication be transmitted using entanglement? After all, since upon opening his own envelope, Bob will also know which side of
coin has ink on it, then doesn't this mean one can send faster-than-light messages that way?
Well, the thing is, not only do you have to send an entangled particle (or coin) to your friend in the first place (and the delivery truck cannot go
faster than the speed of light). But also, even if you could send an entangled particle (or coin) in a matter of seconds, the outcome of the
entanglement is unpredictable. Remember that until you observe it, the coin has 50/50 chances of being on either sides. Which mean you won't know the
message you sent before sending it in the first place. If you and Bob agree that "ink on tails side" means "Hello mate", and "ink on heads side"
means "You are despicable", Then Bob has 50 percent chances of receiving "You are despicable" even though you really meant him to receive "Hello
mate" (and vice-versa).
The chance of him receiving the correct message is just as good as pure random chance.
You might be tempted to simply dip the tails side of Bob's coin in ink so to make sure he receives "Hello mate", but then you'd already know the
properties of the coin before sending it - defeating the whole point of entanglement in the first place, since you already collapsed the wave function
- might as well send him an actual call, with actual radio waves which already travel at light speed (and can carry way more meaningful messages).
I hope you enjoyed this read; Part 4 will be coming soon, and will deal about the Standard Model of Particle Physics - a very popular aspect of modern
Other parts of the series:
-Part 1: the Uncertainty Principle
-Part 2: the Special Theory of Relativity