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 near 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 [Music] 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 michaelson 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 ether the invisible or 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 ether at least but in so doing it succeeded in cracking the supposedly unassailable foundations
of 19th century physics the death of the ether 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 ether 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 ether is the immutable and indestructible element that fills space and in crystalline state forms
celestial bodies that's right ether is the fifth element big badaboom medieval alchemists were all about the ether 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 ether therefore both filled and gave reality to the space between the celestial bodies he imagined the ether
as flowing moving in vortices these flows carried the celestial bodies for example in circles around the sun the dutch physicist christian huygens developed a detailed mathematical theory around this idea that gravity resulted from the fluid dynamics of the ether 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 ether 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 ether he called it the luminiferous or light-bearing ether this was in huygens 1690 treaties
on light published only five 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 etheric 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 ether and he was also an avid alchemist so must have studied the esoteric version of ether whether he also studied his own urine isn't clear but he eventually came to the position that huygens
luminiferous ether was bunk he reasoned that if any medium filled the space between the planets then surely it would cause a sort of drag that would impede their apparently perfect motion newton also favored his own corpuscular theory of light light as tiny particles rather than waves huygens vs newton light as a wave versus light as 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 point 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 ether seemed a done deal and so we get to the end of the 19th century with newton's mechanics in gravity huygens 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 ether 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 meters per second depending on conditions if you're standing still but hop in a jet plane and you can chase your own sound wave so they appear to stand still the apparent velocity of an object or of 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 if i were
a smug 19th century physicist i'd want to hope galilean relativity was right okay 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 ether there should be a sort of ether 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 michaelson 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 kilometers per second 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 the 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 you see your interference pattern to start with you wouldn't
know whether the arms were exactly equal lengths or whether the ether 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 ether 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 ether 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 set up in a tub of frictionless mercury so it would maintain a fixed orientation despite the earth rotating beneath it it was also beautifully designed and it should have detected the ether wind if it existed of course you know what's coming on rotating the device the interference pattern didn't budge okay so maybe the
ether happens to be moving exactly with the earth fine try it six months later when the earth is moving 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 ether 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 hendrick lorenz to derive his lorentz transformation and 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 eight 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 ether 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 ether is disproved but let's be clear the ether as a classical medium for light is dead however the general concept of the ether sort of has an afterlife einstein talked about the new ether as the medium of the gravitational field and which we now think of as the fabric of space-time paul durack suggested that a particular ether could explain the near-vacuum state of space-time 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 that he could never bring himself to accept his own result he appears to have believed in the luminiferous ether until the end when einstein visited michelson on his death bed in 1931 michaelson's daughter begged einstein not to get him started on the subject of the ether 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 reveal a much weirder but still totally luminiferous space-time last week we explored how it is that astronomers could possibly figure out the age of the universe given that we don't even have light from the very beginning there were some really smart questions let me see if i can answer them ravenlord 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 infinitesimally 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 one thousand and they're a thousand 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 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 to the power of negative 43 seconds the so-called planck time at that point inflation seems to give our best picture and i'm gonna have to direct you to our episodes on that for the deets
steve jeffries has another mind bender as we look further out into the universe we are looking at the inner surfaces of spheres which are progressively smaller well that's right because we're looking back in time at a younger and 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 it's so weird to think about the most distant horizon that 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 they were swimming against the expansion of space that whole time several of you pointed out how awesome a name vesto schleifer
is well yeah and frank henshi he does sound like a yu-gi-oh character the funny thing is slifer wasn't even some fancy european astronomer he was from indiana his brother's name was earl i'd say maybe giving your kid a weird name might help them achieve greatness but actually earl was a pretty successful astronomer also andermere the slifers were obnoxiously competitive but rumor has it that 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 hmm wait robert smith robert james smith
from the cure look dude you put on a few pounds who doesn't but you'll always be that scrawny goth scarecrow to me you
