Quantum physics just keeps getting weirder, even as it gets more fascinating.
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Light is well known to exhibit both wave-like and
particle-like properties, as imaged here in this 2015 photograph. What’s less
well appreciated is that matter particles also exhibit those wave-like
properties. Even something as massive as a human being should have wave
properties as well, although measuring them will be difficult. (FABRIZIO
CARBONE/EPFL (2015)) |
“Is it a wave or is it a particle?” Never has such a simple
question had such a complicated answer as in the quantum realm. The answer,
perhaps frighteningly, depends on how you ask the question. Pass a beam of
light through two slits, and it acts like a wave. Fire that same beam of light
into a conducting plate of metal, and it acts like a particle. Under
appropriate conditions, we can measure either wave-like or particle-like
behavior for photons — the fundamental quantum of light — confirming the dual,
and very weird, nature of reality.
This dual nature of reality isn’t just restricted to light,
either, but has been observed to apply to all quantum particles: electrons,
protons, neutrons, even significantly large collections of atoms. In fact, if
we can define it, we can quantify just how “wave-like” a particle or set of
particles is. Even an entire human being, under the right conditions, can act
like a quantum wave. (Although, good luck with measuring that.) Here’s the
science behind what that all means.
This illustration, of light passing through a dispersive
prism and separating into clearly defined colors, is what happens when many
medium-to-high energy photons strike a crystal. If we struck this prism with a
single photon and space were discrete, the crystal could only possibly move a
discrete, finite number of spatial steps, but only a single photon would either
reflect or transmit. (WIKIMEDIA COMMONS USER SPIGGET) |
The debate over whether light behaves as a wave or a
particle goes all the way back to the 17th century, when two titanic figures in
physics history took opposite sides on the issue. On the one hand, Isaac Newton
put forth a “corpuscular” theory of light, where it behaved the same way that
particles did: moving in straight lines (rays) and refracting, reflecting, and
carrying momentum just as any other kind of material would. Newton was able to
predict many phenomena this way, and could explain how white light was composed
of many other colors.
On the other hand, Christiaan Huygens favored the wave
theory of light, noting features like interference and diffraction, which are
inherently wave-like. Huygens’ work on waves couldn’t explain some of the
phenomena that Newton’s corpuscular theory could, and vice versa. Things
started to get more interesting in the early 1800s, however, as novel
experiments began to truly reveal the ways in which light was intrinsically
wave-like.
The wave-like properties of light, originally hypothesized
by Christiaan Huygens, became even better understood thanks to Thomas Young’s
two-slit experiments, where constructive and destructive interference effects
showed themselves dramatically. (THOMAS YOUNG, 1801) |
If you take a tank filled with water and create waves in it,
and then set up a barrier with two “slits” that allow the waves on one side to
pass through to the other, you’ll notice that the ripples interfere with one
another. At some locations, the ripples will add up, creating larger magnitude
ripples than a single wave alone would permit. At other locations, the ripples
cancel one another out, leaving the water perfectly flat even as the ripples go
by. This combination of an interference pattern — with alternating regions of constructive
(additive) and destructive (subtractive) interference — is a hallmark of wave
behavior.
That same wave-like pattern shows up for light, as first
noted by Thomas Young in a series of experiments performed over 200 years ago.
In subsequent years, scientists began to uncover some of the more
counterintuitive wave properties of light, such as an experiment where
monochromatic light shines around a sphere, creating not only a wave-like
pattern on the outside of the sphere, but a central peak in the middle of the
shadow as well.
The results of an experiment, showcased using laser light
around a spherical object, with the actual optical data. Note the extraordinary
validation of Fresnel’s wave theory of light prediction: that a bright, central
spot would appear in the shadow cast by the sphere, verifying the “absurd”
prediction of the wave theory of light. The original experiment was performed
by Francois Arago. (THOMAS BAUER AT WELLESLEY) |
Later in the 1800s, Maxwell’s theory of electromagnetism
allowed us to derive a form of charge-free radiation: an electromagnetic wave
that travels at the speed of light. At last, the light wave had a mathematical
footing where it was simply a consequence of electricity and magnetism, an
inevitable result of a self-consistent theory. It was by thinking about these
very light waves that Einstein was able to devise and establish the special
theory of relativity. The wave nature of light was a fundamental reality of the
Universe.
But it wasn’t a universal one. Light also behaves as a quantum particle in a number of important ways.
- Its energy is quantized into individual packets called photons, where each photon contains a specific amount of energy.
- Photons above a certain energy can ionize electrons off of atoms; photons below that energy, no matter what the intensity of that light is, cannot.
- And that it’s possible to create and send individual photons, one-at-a-time, through any experimental apparatus we can devise.
Those developments and realizations, when synthesized together, led to arguably the most mind-bending demonstration of quantum “weirdness” of all.
Double slit experiments performed with light produce
interference patterns, as they do for any wave you can imagine. The properties
of different light colors is understood to be due to the differing wavelengths
of monochromatic light of various colors. Redder colors have longer
wavelengths, lower energies, and more spread-out interference patterns; bluer
colors have shorter wavelengths, higher energies, and more closely bunched
maxima and minima in the interference pattern. (TECHNICAL SERVICES GROUP (TSG)
AT MIT’S DEPARTMENT OF PHYSICS) |
If you take a photon and fire it at a barrier that has two
slits in it, you can measure where that photon strikes a screen a significant
distance away on the other side. If you start adding up these photons,
one-at-a-time, you’ll start to see a pattern emerge: an interference pattern.
The same pattern that emerged when we had a continuous beam of light — where we
assumed that many different photons were all interfering with one
another — emerges when we shoot photons one-at-a-time through this apparatus. Somehow,
the individual photons are interfering with themselves.
Normally, conversations proceed around this experiment by
talking about the various experimental setups you can make to attempt to
measure (or not measure) which slit the photon goes through, destroying or
maintaining the interference pattern in the process. That discussion is a vital
part of exploring the nature of the dual nature of quanta, as they behave as
both waves and particles depending on how you interact with them. But we can do
something else that’s equally fascinating: replace the photons in the
experiment with massive particles of matter.
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