I have heard that humans have a wavelength. Is this true?
Asked by: Brendan Playford
In 1932, a French scientist named Louis de Broglie suggested that the wave-particle duality
applied to not only light, but also to matter. That is to say, he proposed that all matter
possessed wave-like characteristics. To understand how he arrived to this conclusion, we
must explain how light can possess both wave and particle properties.
Until the eighteenth century, light was thought of purely as a wave, like sound. There
were several problems associated with this theory, however, one of the foremost being the
lack of medium in space. Waves require a medium through which to travel, and without such
substance, the wave cannot exist - - this is why sound cannot travel through a vacuum. In
space, however, there did not appear to be any medium that would allow light to travel, yet
light obviously traveled through space to reach the Earth. In order to explain this,
scientists visualized a material that existed everywhere and through which light could
propagate. This material came to be called the 'luminiferous ether'. The wave theory was
further promoted when, in 1803, a scientist named Thomas Young demonstrated the
interference of light in the famous 'double slit experiment'. This experiment could only
be explained by the wave-nature of light.
So, how does the particle theory enter the picture? The wave nature of light does not
explain everything, particularly the fact that light diffraction was not as readily
observed as was other wave diffraction, such as sound or water waves. Things were further
complicated with the photoelectric effect, a phenomenon where light striking metals
produced an emission of 'photoelectrons', that is, an electric current. The empirical data
of the photoelectric effect could only be explained by the corpuscular (particle) theory of
In the meanwhile, Clerk Maxwell synthesized everything that was then known about
electricity and magnetism in what are known as Maxwell's equations. These equations
described visible light as a portion of the electromagnetic spectrum, and said that the
luminiferous ether was not necessary for electromagnetic waves to propagate through space.
He proposed an experiment through which the absence of ether could be demonstrated, but
believed that the precise measurements required for the experiment were not possible.
Albert Michelson and Edward Morley proved him wrong in July of 1887. The famous
Michelson-Morley experiment sent light in two orthogonal (perpendicular) directions and
used an 'interferometer' (invented by Michelson) to detect the shift in the wavelengths of
the light beams. This shift was supposed to be caused by the speed of the Earth moving
through the luminiferous ether, but no such shift occurred. Thus the speed of light was
shown to be the same, regardless of the relative motion of the frames.
So, is light a wave or a particle? It is perhaps best to say that light is a complicated
phenomenon that is neither a wave nor a particle. The wave and particle theories are
simplified models of light, and in certain situations, one or the other of these 'models'
offers a more convenient explanation.
Now, enter Prince Louis de Broglie. Max Planck and Einstein had related energy to the
frequency of waves, and by Einstein's famous equation E=mc2, mass was related to energy.
Thus de Broglie supposed that matter might also be related to the frequency of waves. The
elementary particle of light, the photon, had been shown to exhibit wave-properties; de
Broglie wanted to extend this fact to all matter.
The momentum of a photon, p, was given by the ratio between Plank's constant and the
photon's wavelength. De Broglie applied this relationship to all matter. Since Plank's
constant is on the order of 10-34, the de Broglie wavelength is virtually undetectable for
large amounts of matter.
There is much evidence of matter possessing such a de Broglie wavelength. The double slit
experiment demonstrates interference effects in photons, electrons, and neutrons, the last
of which being very significant, considering that the neutron is perhaps the densest mass
What about humans? Well, theoretically, since all matter possess wave-like properties, so
do humans, and cats, and whatever you please. We could hypothetically demonstrate this
fact by performing the double slit experiment with these 'particles'. So here we go,
firing cats haphazardly at two slits, trying to get cats to interfere with each other.
Will it work? Well. . . kinda. There are a lot of little technicalities, so you'll have
to be careful not to aim at the slits (i.e., you must fire randomly to create a incoherent
cat-beam), and you'll have to space out the firings. You fire one cat, you wait for a
while, then you fire the other cat. Eventually, you'll form the familiar interference
pattern on the other side of the slits. Unfortunately, that waiting period between firings
is about the age of the universe when you're using cats.
Finally! What IS the wavelength of a human being? Assuming he/she weighs 70 kg, and is
being fired at 25 m/s, it's about 3.79 x 10-37 meters.
Answered by: Aman Ahuja, Physics student, WPI, Mass.