As the 19th century came to a close, physicists were feeling pretty satisfied with the state
of their science. The great edifice of physical theory seemed complete. A few minor experiments
remained to verify everything. But little did those physicists know that one of those experiments
would bring the entire structure crashing down paving the way for the physics revolutions
of the 20th century.
In 1894, the eminent physicist Albert Abraham Michelson described the state of his science
like this “it seems probable that most of the grand underlying principles have been
firmly established and that further advances are to be sought chiefly in the rigorous application
of these principles to all the phenomena which come under our notice.” Michelson had lived
this principle. In 1887, he and fellow American Edward Morley attempted to verify the existence
of one of the oldest concepts in physics. They attempted to measure the luminiferous
aether - the invisible, all-pervasive medium through which light was thought to propagate.
Their experimental design was so ingenious it couldn’t possibly fail. And yet it did
fail - failed to detect the aether, at least. But in doing so it succeeded in cracking the
supposedly unassailable foundations of 19th century physics. The death of the aether helped
open the way for the acceptance of Einstein’s theory of relativity.
So today we’re going to celebrate the life and death of an idea. As with so many ideas,
the aether was born in the minds of the ancient Greeks. For them it was the air breathed by
the Gods, and was embodied as a primordial deity of light. In Aristotle’s cosmology,
earth, air, fire and water are the physical elements of the world, while aether is the
immutable and indestructible element that filled space and in crystalline state formed
celestial bodies. That’s right, aether is the fifth element. Big ba da boom.
Medieval alchemists were all about the aether, also called quintessence. They believed this
spiritually pure element had miraculous capabilities of transmutation. And in some cases that it
could be extracted by distilling huge quantities of human urine. It turned out that was phosphorus.
Still useful I guess.
Fast forward to the mid 17th century. French philosopher Rene Descartes asserted that there
could be no such thing as empty space, and Aristotle’s aether therefore both filled
and gave reality to the space between the celestial bodies. He imagined the aether as
flowing - moving in vortices. These flows carried the celestial bodies, for example
in circles around the Sun. The Dutch physicist Christiaan Huygens developed a detailed mathematical
theory around this idea that gravity results from the fluid dynamics of the aether. But
Huygens is most famous for his wave theory of light. By thinking of light as a wave,
he was able to build a theory of optics, explaining how light refracted - bent in its path - between
different substances. And in Huygens’ mind, that theory also required the aether in order
to make sense.
Consider a wave on a string: each string segment moves up and down only, tugging on neighboring
segments so that a sinusoidal pattern flows forwards, while the segments themselves just
oscillate in place. In sound waves, air molecules oscillate back and forth to propagate a pattern
in the density of the air. All familiar waves - what we’ll call classical waves - are
a chain reaction that propagates an energy pattern through some medium. So if light is
a wave, surely it also needs a medium. For Huygens, that medium was also the aether.
He called it the luminiferous - or light-bearing aether.
This was in Huygen’s 1690 Treatise on Light, published only 5 years before his death. Across
the channel, a much younger Isaac Newton was sitting in Cambridge making life difficult
for Huygens. First his theory on universal gravitation conflicted with the predictions
of Huygens’ aetheric gravity. And Newton also opposed this whole wave theory for light
business. Now Newton’s case is complicated - some of his early ideas on gravity incorporated
the aether, and he was also an avid alchemist, so must have studied the esoteric version
of aether. Whether he also studies his own urine isn’t clear. But he eventually came
to the position that Huygen’s luminiferous aether was bunk. He reasoned that if any medium
filled the space between the planets, surely it would cause a sort of drag that would impede
their apparently perfect motion. Newton also favoured his own corpuscular theory of light
- light as tiny particles rather than waves.
Huygens versus Newton. Light as a wave versus a particle. Most accepted Newton - as most
always did. This was until the beginning of the 19th century when Thomas Young performed
his famous double slit experiment. He showed that light produces an interference pattern
passing between a pair of slits, like water waves do. These bright bands are points
on the screen where the waveforms of light emerging from the two slits happen to line
up perfectly, reinforcing each other. In between the waveforms are out of phase and so cancel
out. We see exactly the same effect in a pair of expanding ripples on the surface of water.
So, light experiences refraction and interference like a wave. Add to that the fact that in
the 1860s, Maxwell’s equations predicted that electromagnetic waves should travel at
exactly the speed of … wait for it, light. It was becoming pretty clear that light was
indeed some type of wave. And waves need a medium, so the reality of the luminiferous
aether seemed a done deal.
And so we get to the end of the 19th Century. With Newton’s mechanics and gravity, Huygen’s
optics, Maxwell’s electromagnetism, physics seemed pretty much wrapped up. Albert Michelson
and Edward Morley thought they’d help tie a bow on it by demonstrating the existence
of the luminiferous aether as a classical medium for the propagation of light.
The key was to observe a change in the speed of light depending on the direction of motion.
All classical waves travel at a constant speed relative to their medium. For example, sound
waves travel at around 340 m/s, depending on conditions if you’re standing still.
But hop in a jet plane and you can chase your own sound waves so they appear to stand still.
The apparent velocity of an object - or a wave - depends in a simple way on the velocity
and direction of motion of the observer. This is Galilean relativity - after Galileo obviously.
You calculate what velocity everyone observes using the Galilean transformation, which is
part of the foundation of Newton’s mechanics. I were a smug 19th century physicist, I’d
wanna hope Galilean relativity is right
OK, so if light is a classical wave in some medium then we should see changes in the apparent
speed of light depending on our direction of motion. If you move relative to the aether
then there should be a sort of aether wind - like when you stick your hand out of a moving
car. Light should have a fixed velocity relative to that wind, but for you it should be carried
by that wind - faster moving behind you than ahead of you.
That is what Michelson and Morley sought to measure. All they needed was a measuring
device capable of traveling at a significant fraction of the speed of light, which in the
1890s I guess may have seemed challenging. But why build a fast-moving lab when you already
have a fast-moving planet? Earth hurtles around the Sun at 30 km/s - that’s only one one
hundredth of one percent of the speed of light - but it should produce a change in the speed
of light of the same degree. Michelson and Morley set out to measure that.
Thus was invented the ingenious Michelson-Morley interferometer. In it, a beam of light is
split in two by a semi-reflecting mirror and sent along two paths at right angles to each
other. The split beams bounce back and forth multiple times before being brought back together
again. And just like in Thomas Young’s double-slit experiment, these beams would then interfere
with each other to produce the bright and dark bands of an interference pattern.
Once set up, a couple of things could change that interference pattern. One would be changing
the length of either interferometer arm. That would cause the peaks and troughs to line
up in different places. Changes in length quite a bit smaller than a single wavelength
of light would produce observable shifts in the fringe pattern. And this is exactly the method
that LIGO uses to detect gravitational waves.
The other way to get a shift in the interference pattern is by changing the relative speed
of light along the two arms. That would cause the wave pattern in one arm to lag behind
the other, leading to a similar shift in the pattern. So let’s say you set up the interferometer
and see your interference pattern. To start with you wouldn’t know whether the arms were
exactly equal lengths or whether the aether wind was blowing one way or the other - but
actually none of that matters. You’d see the bright bands of the interference pattern
at the locations where the beams happened to give constructive interference.
That interference pattern should stay fixed as long as the path lengths stay fixed and as
long as the relative speed of light between the arms stays fixed. But if the speed depends on
the direction of the aether wind, then just rotate the interferometer by 90 degrees. If
the speed of light was faster in one arm than the other due to the aether wind, then the
speed difference gets flipped when the device is rotated. The interference pattern should
shift to reflect this. That is the Michelson-Morley experiment. For extra genius points, they
floated the whole setup in a tub of frictionless mercury so it would maintain a fixed orientation
despite the earth rotating beneath it.
It was all so beautifully designed,
and it should have detected the aether wind - if it existed. Of course you know what’s
coming - on rotating the device the interference pattern didn’t budge. OK, so maybe the aether
happens to be moving exactly with the earth - fine, try it 6 months later when the earth
is moving in the opposite direction - still no pattern movement.
This may be the most famous null result in the history of physics. There appeared to
be no luminiferous aether - at least, not one that resembled a classical medium for
wave propagation. The speed of light appeared to be independent of the motion of the observer.
That contradicts Galilean relativity, and so revealed a crack in the sacred Newtonian
mechanics. This directly inspired Hendrik Lorentz to derive his Lorentz transformation
- an update to the Galilean transformation that now allowed the speed of light to remain
constant, no matter your velocity.
And that brings us to Einstein. The constancy of the speed of light and the Lorentz transformation
are fundamental to his special theory of relativity, which was published 8 years after the Michelson-Morley
experiment. Now Einstein’s starting motivation seems to have been the fact that the Maxwell
equations are also a little broken under Galilean relativity. But the death of the aether gave
Einstein two things - it inspired the Lorentz transformation, critical to both special relativity
and then his general theory of relativity that ultimately overthrew Newton’s theory
of gravity, and, according to Einstein, it revealed the cracks in the foundations of
physics that allowed his ideas to be accepted.
It’s commonly stated that the aether is disproved - but let’s be clear. The aether
as a classical medium for light is dead. However the general concept of the aether
sort of has an afterlife. Einstein talked about the “new aether” as the medium of the
gravitational field, and which we now think of the fabric of spacetime. Paul Dirac suggested
that a “particulate aether” could explain the near vacuum state of spacetime in which
quantum physicists believe particle pairs are quickly born and destroyed. In retrospect,
Descartes seems prescient: empty space is not really empty. Its flowing fabric is the
source of the gravitational field, and its full of quantum fields.
And what about Albert Michelson? Well it seems he could never bring himself to accept his own result.
He appears to have believed in the luminiferous aether until the end. When Einstein visited
Michelson on his deathbed in 1931 Michelson's daughter begged Einstein “not get him started on the
subject of the aether.” Michelson may have mourned the death of the idea, and the passing of
the sensible, down-to-earth world of 19th century physics. But that death helped spark
the revolutions of relativity and then quantum theory that revealed a much weirder, but still
totally luminiferous space time.
Last week we explored how it is that astronomers could possibly calculate the age of the universe,
given that we don't even have light from its very beginning. There were some really smart
questions - let me see if I can answer them.
Raven Lord asks how we reconcile the idea that "the universe is infinite in size" and
"the universe has a finite age" if the universe started from a singularity. How do you go
from infinitessimally small to infinitely large?
Well let's pretend that the expansion rate is constant. And for an infinite universe the "size" is
just the average distance between galaxies. Half that age and the galaxies were half the
distance that they are today. Divide that age by 1000 and they're 1000 times closer together.
But infinity divided by any positive number is still infinity. So even when the universe
was a tiny fraction of a second old, it was still infinite, even though ALL of the galaxies
we can see were compacted into a tiny point. There were just infinitely many of those tiny points.
But the singularity happens when you go from a tiny positive age to zero age. Infinity
times zero is zero and that's your singularity.
But all that aside - we don't actually know A) if the universe truly is infinite, or B)
what happened before 10^-43 seconds - the so-called Planck time. At that point inflation
seems to give our best picture, and I'm going to have to direct you to our episodes on that
for the deets.
Steve Jeffryes has another mind-bender. As we look farther out into the universe, we
are looking at the inner surfaces of spheres which are progressively smaller. Right - because
we’re looking back in time at a younger, hence smaller universe. While we can’t see
the tiniest of those spheres right after the big bang because the universe was opaque back then,
Steve asks what was the diameter of the universe at the most distant point represented by the
cosmic microwave background radiation?
Well that’s easy - the universe has expanded by a factor of 1100 since the CMB was released.
It’s now 93 billion light years across - so the CMB sphere is around 85 million light
years across. That’s about half again the size of the local Virgo supercluster. That's
so weird to think about - the most distant horizon we can see is the same size as the
local bubble of galaxies. Those poor cosmic background photons should have reached us
billions of years ago - it's not that they had far to go, it's just that they were swimming
against the expansion of space that whole time.
Several of you pointed out how awesome a name Vesto Slipher is. Yes, Frank Henchy, he does
sound like a Yi gi oh character. The funny thing is, Slipher wasn't even some fancy European
astronomer. He was from Indiana. His brother's name was Earl. I'd say that maybe giving your
kid a weird name might help them achieve greatness, but actually Earl was a pretty successful astronomer
also. AND a mayor. The slipher's were obnoxiously competitive. But rumour has it Vesto solved
the Millennium Puzzle, so I guess he wins.
Robert Smith quips the universe was smaller and hotter when it was young, weren't we all?
Hm. Wait... Robert Smith? Robert JAMES Smith from the Cure. Look dude, so you put on a
few pounds. Who doesn't. But you'll always be that scrawny goth scarecrow to me.