1. The quantum world is lumpy
The quanta here is the Planck constant, named after Max Planck, the godfather of
quantum physics. He was trying to solve a problem with our understanding of hot
objects like the sun. Our best theories couldn’t match the observations of the
energy they kick out. By proposing that energy is quantized, he was able to
bring theory neatly into line with the experiment.
2. Something can be both wave and particle
A telescope can focus light waves from distant stars,
and also acts like a giant light bucket for collecting photons. It also means
that light can exert pressure as photons slam into an object. This is something
we already use to propel spacecraft with solar sails, and it may be possible to
exploit it in order to maneuver a dangerous asteroid off a collision course with Earth, according to Rusty
Schweickart, chairman of the B612 Foundation.
3. Objects can be in two places at once
Wave-particle duality is an example of superposition. That is, a quantum object existing in
multiple states at once. An electron, for example, is both ‘here’ and ‘there’
simultaneously. It’s only once we do an experiment to find out where it is that
it settles down into one or the other.
This makes quantum physics all about probabilities. We can
only say which state an object is most likely to be in once we look. These odds
are encapsulated into a mathematical entity called the wave function. Making an
observation is said to ‘collapse’ the wave function, destroying the
superposition and forcing the object into just one of its many possible states.
This idea is behind the famous Schrödinger’s cat thought experiment. A cat in a
sealed box has its fate linked to a quantum device. As the device exists in
both states until a measurement is made, the cat is simultaneously alive and
dead until we look.
4. It may lead us towards a multiverse
The idea that observation collapses the wave function and
forces a quantum ‘choice’ is known as the Copenhagen interpretation of quantum
physics. However, it’s not the only option on the table. Advocates of the ‘many
worlds’ interpretation argue that there is no choice involved at all. Instead,
at the moment the measurement is made, reality fractures into two copies of
itself: one in which we experience outcome A, and another where we see outcome
B unfold. It gets around the thorny issue of needing an observer to make stuff
happen — does a dog count as an observer or a robot?
Instead, as far as a quantum particle is concerned, there’s
just one very weird reality consisting of many tangled-up layers. As we zoom
out towards the larger scales that we experience day to day, those layers
untangle into the worlds of the many
worlds theory. Physicists call this process decoherence.
5. It helps us characterize stars
Danish physicist Niels Bohr showed us that the orbits of
electrons inside atoms are also quantized. They come in predetermined sizes
called energy levels. When an electron drops from a higher energy level to a
lower energy level, it spits out a photon with an energy equal to the size of
the gap. Equally, an electron can absorb a particle of light and use its energy
to leap up to a higher energy level.
Astronomers use this effect all the time. We know what stars
are made of because when we break up their light into a rainbow-like spectrum,
we see colors that are missing. Different chemical elements have different
energy level spacings, so we can work out the constituents of the sun and other
stars from the precise colors that are absent.
6. Without it the sun wouldn’t shine
The sun makes its energy through a process called nuclear
fusion. It involves two protons — the positively charged particles in an atom —
sticking together. However, their identical charges make them repel each other,
just like the two north poles of a magnet. Physicists call this the Coulomb
barrier, and it’s like a wall between the two protons.
Think of protons as particles and they just collide with the
wall and move apart: No fusion, no sunlight. Yet think of them as waves, and
it’s a different story. When the wave’s crest reaches the wall, the leading
edge has already made it through. The wave’s height represents where the proton
is most likely to be. So although it is unlikely to be where the leading edge
is, it is there sometimes. It’s as if the proton has burrowed through the
barrier, and fusion occurs. Physicists call this effect "quantum
tunneling".
7. It stops dead stars from collapsing
Eventually, fusion in the sun will stop and our star will
die. Gravity will win and the sun will collapse, but not indefinitely. The
smaller it gets, the more material is crammed together. Eventually, a rule of
quantum physics called the Pauli exclusion principle comes into play. This says
that it is forbidden for certain kinds of particles — such as electrons — to
exist in the same quantum state. As gravity tries to do just that, it
encounters a resistance that astronomers call degeneracy pressure. The collapse
stops, and a new Earth-sized object called a white dwarf forms.
Degeneracy pressure can only put up so much resistance,
however. If a white dwarf grows and approaches a mass equal to 1.4 suns, it
triggers a wave of fusion that blasts it to bits. Astronomers call this
explosion a Type Ia supernova, and it’s bright enough to outshine an entire
galaxy.
8. It causes black holes to evaporate
A quantum rule called the Heisenberg uncertainty principle says that it’s
impossible to perfectly know two properties of a system simultaneously. The
more accurately you know one, the less precisely you know the other. This
applies to momentum and position, and separately to energy and time.
It’s a bit like taking out a loan. You can borrow a lot of
money for a short amount of time, or a little cash for longer. This leads us to
virtual particles. If enough energy is ‘borrowed’ from nature then a pair of
particles can fleetingly pop into existence, before rapidly disappearing so as
not to default on the loan.
Stephen Hawking imagined this process occurring at the
boundary of a black hole, where one particle escapes (as Hawking radiation),
but the other is swallowed. Over time the black hole slowly evaporates, as it’s
not paying back the full amount it has borrowed.
9. It explains the universe’s large-scale structure
Our best theory of the universe’s origin is the Big
Bang. Yet it was modified in the 1980s to include another theory
called inflation. In the first trillionth of a trillionth of a
trillionth of a second, the cosmos ballooned from smaller than an atom to about
the size of a grapefruit. That’s a whopping 10^78 times bigger. Inflating a red
blood cell by the same amount would make it larger than the entire observable
universe today.
As it was initially smaller than an atom, the infant
universe would have been dominated by quantum fluctuations linked to the
Heisenberg uncertainty principle. Inflation caused the universe to grow rapidly
before these fluctuations had a chance to fade away. This concentrated energy
into some areas rather than others — something astronomers believe acted as
seeds around which material could gather to form the clusters of galaxies we
observe now.
10. It is more than a little ‘spooky’
As well as helping to prove that light is quantum, Einstein
argued in favor of another effect that he dubbed ‘spooky action at distance’.
Today we know that this ‘quantum entanglement is real, but we still don’t
fully understand what’s going on. Let’s say that we bring two particles
together in such a way that their quantum states are inexorably bound, or
entangled. One is in state A, and the other in-state B.
The Pauli exclusion principle says that they can’t both be
in the same state. If we change one, the other instantly changes to compensate.
This happens even if we separate the two particles from each other on opposite
sides of the universe. It’s as if information about the change we’ve made has
traveled between them faster than the speed of light, something Einstein said
was impossible.