How Does Light Actually Work?

the newly born Universe buzzed and frothed with Boundless Energy even after the Raging furnace of the first few minutes had died away temperatures Universe wide were more than 100 million de for thousands of years this Primal heat burned a cosmos of plasma a super hot mix of particles and radiation until one day it changed forever that day arrived when the universe was almost 400,000 years old and had cooled to about 3,000 Kelvin in this now comparatively tepid soup lone electrons met lone protons and finally could stick together forming the first atoms but this was not

all for as each electron and proton bound together a small amount of energy was released a packet of energy that raced away the cosmic speed limit the speed of light a particle of energy born in the formation of a hydrogen atom a particle we call a photon a particle of light itself this Photon was far from the first but as the universe began transitioning from plasma into neutral gas light could then for the first time stream freely through its reaches and so our photons long long journey began it headed out first into the universe's Dark

Ages a time before the first Stars burned a time before the first galaxies formed in the Eerie Darkness gravity pulled on mass to mold the first seeds of cosmic structure but the photon sped on and noticed nothing eventually the first Stars burst into life around it massive and Bloated these ancient Suns burned themselves out in a cosmic blink of an eye as the first super giant black holes grew rapidly between them as they e devoured Mass but the photon sped on and noticed nothing the first galaxies began to assemble the sky lit up with the

fires of uncountable young Stars across the cosmos as they began to fuse the initial hydrogen and helium atoms into heavier elements but the photon sped on and noticed nothing Millions steadily turned into billions of years and as galaxies grew and matured eventually the intense light of young Stars began to settle the photon's journey could have potentially lasted forever into eternity but after almost 14 billion light years of travel a large spiral galaxy steadily came into view its Destiny was set near a small blue dot orbiting a small white star after crossing the last few thousand

light years the photon collided with a piece of metal part of a telescope built by humans and orbiting near the planet Earth the photons energy was completely absorbed energizing electrons and registering on detectors but as the photon vanished from existence its billions year-long Journey complete it still simply did not notice because to the photon itself the journey never took place 13.8 billion years of cosmic time disappeared in an instant yet how can this be light has existed in the universe from its earliest moments and will continue to exist long after humanity and the stars are

shred to dust but just how does it work and how can it seemingly last forever and perhaps most importantly what even is it light is fast it only takes 0.13 seconds for it to circulate the entire Globe through fiber optic cables that slows down by a third but that still means that with surf shark changing your virtual location is almost instantaneous surf shark have been kind enough to sponsor to this video and they are the best simplest VPN service out there you can change your country quickly and easily to stream whatever you want from wherever

you want in the world with more than 100 countries to choose from light seems to escape definition as it exhibits properties of both a particle and a wave and now you too can be similarly inscrutable to Big tech companies and cyber criminals by using surf shark to mask your IP address and encrypting your data on public Wi-Fi it's the best way to secure your personal information when using the internet and even warns you of potential data breaches so check out surf shark by clicking on the link in the description and get 83% off and 3

months for free as well as a 30-day money back guarantee thanks to Surf shark for supporting educational content on [Music] YouTube light had played a pivotal role since the cosmos's very beginning in these earlier times it had only existed for the very briefest of moments slamming into speeding particles before it had a chance to travel anywhere one piece of light dying as another was born however our parcel of light was born into a very different very transparent Universe with the cosmic Maelstrom of the Big Bang finally abated our Photon could begin its immense Journey unhindered

as it traveled many generations of stars led to our sun and billions of years after humans began to walk on the surface of our small rocky planet and So eventually as our Photon was a couple of thousand light years distant from Earth those humans began to wander all men by Nature desire to know an indication of this is the Delight we take in our senses and above all others the sense of sight this most of all the senses makes us know and brings to light many different between things the ancient Greeks wondered if light emanated

from the eyes touching and feeling the world around us but clearly there are times when it is dark when we can't see anything so they concluded light must be something external something captured by our eyes Islamic scientists went on to unravel the properties of light finding rules of reflection and the magnification properties of glass lenses light was clearly a natural part of the universe around us but it took the coming of the Scientific Revolution and a fight between two giants of science for light's deepest secrets to be finally uncovered the year was 1652 and Dutch

physicist astronomer mathematician and allround genius Christian haggens was exploring Optical phenomena he had noted how light traveled through lenses and bounced off mirror surface surfes and was particularly interested in the phenomenon of refraction where the path of light is bent when it passes from one medium to another haggens noticed that light was often split into the colors of the rainbow by his instruments and sometimes strange patterns of dark and light would be produced clear evidence to him that light was some sort of wave some sort of traveling oscillating phenomenon oscillations can be found throughout nature

from planetary orbits to vibrating electrons but let's start with a simple picture think of a child on a swing as they swing their position oscillates from one position to the next and then back again just like a pendulum that drives the regular ticking and talking of a grandfather clock when oscillations act in unison but slightly out of Step waves are formed a stone thrown in a flat Pond pulls the water down with it but the water bounces back this splash of water pulls on its neighbors inducing them to oscillate which in turn pulls on their

own neighbors these oscillations fan out across the pond as a steadily Rippling pattern of waves waves are everywhere from sound waves csing in the air to ocean water waves driven by the wind and moon seismic shifts can generate violent and destructive earthquakes in our planetary crust whilst similar waves Ripple in the atmosphere of the Sun and other stars and so light seemed also to be a wave but this left an obvious question if light is a wave just what was doing the waving in Britain Robert Hook also reached a similar conclusion about the nature of

light and he realized that this picture of wavy light could explain a lot of the phenomenon he had seen this was cutting edge science at the time but hook had a problem and that problem was a man there are no surviving portraits of Robert Hook and over the years a rumor passed down the generations that this powerful man was to blame having conveniently lost the painting when taking over his head of the Royal Society in London for Robert Hook had a powerful enemy and that enemy's name was Isaac Newton since cleared of any wrongdoing in

the absence of contemporary images of hook there is still little question that the two men were not friends for as well as his far-reaching discoveries in mathematics and gravity Newton also had an interest in Optics and the nature of light itself and he did not like what hook or haggens had to say indeed it was Newton who discovered that white light could be split into a rainbow by passing it through a prism and like hook he had kept musing on this but unlike hook Newton did not conclude that light was some sort of wave to

Newton light consisted of cor pusles to Newton light was made of tiny individual particles Newton's Focus was the phenomenon of defraction the fact that waves bend around a sharp edge he knew that sound a wave in the air bent as it traveled past sharp edges it was clear that you could Eaves drop on a ation from around a corner without being able to see the gossipers you could hear from behind an object but you could not see so he reasoned light simply could not be a wave and he didn't stop there Newton went further much

much further he reasoned that light as a stream of particles would even feel the pull of gravity in his book Optics published in 1704 he wrote do not bodies act upon light at a distance and by their action Bend its rays and though on this he was correct he was not proved correct for centuries so it was Newton's corpuscular theory of light that reigned Supreme due more to his weight of personality and scientific standing as opposed to its ability to explain the complex observations of light over the years however steadily the tide began to turn

away from Newton in 1800 polymath Thomas Young sha light through a pair of narrow slits and observed a pattern of interference on a background screen this wasn't the first demonstration of interference but it was the clearest how could Newton's picture of light as particles explain Young's observation of interference how could light as Tiny bullets passing through either one slit or the other produce The observed pattern simply throwing a couple of Pebbles into a Still Pond reveals that interference is naturally produced by waves either in water or in light other observations of light supported its wav

likee nature including light's polarization through a material called calite but for centuries the big secondary question remained unanswered if light was a wave just what was doing the waving [Music] our Cosmic parcel of light was born when a proton captured an electron It sped out into the universe powerful and energetic but as it traveled and the universe expanded it started to lose some of that energy the light originally blue to our eyes steadily morphed through the colors of the rainbow and into the red soon it was joined by other energetic light shining from countless billions

of newly formed stars there were different types of light as well light of exceptionally high energy and light with barely any energy at all the universe was a wash of course our light did not know that these would be invisible to human eyes for it would be many billions of years until eyes existed and indeed these X-rays and radio waves as we call them were unknown to us until the very end of the 19th century in this new era thought itself will be transmitted by radio gulel Mo Marone was at his father's estate near bologna

in Italy he was still a young man aged only 20 but his education had opened his eyes to an invisible world in the decades before the nature of light had steadily been unraveled and Marone was ready to use this newfound knowledge to change everything staring at his equipment Marcone was waiting to see a faint spark in the darkness with the bundle of wires and coils of his Workshop such a spark would not be surprising but the impetus for this spark was not in the equipment before him it was in similar equipment located several miles away

of course the 19th century had seen the arrival of the telegraph where electronic pulses were sent along wires that crossed entire countries and continents but this needed wires to be strung through the air air and under the oceans Marone had no need for such wires he would be sending messages not a long bits of copper his messages would simply fly completely unseen through the air but how the answer lies with one of science's greatest Geniuses the answer lies with James Clark Maxwell when a young James Clark Maxwell arrived at the University of Cambridge in 1850

he was told that attendance at the 6 a.m. church service was compulsory for all students the Scottish born Prodigy had long been a night owl and simply responded hi I suppose I could stay up that late his name is RIT large across the modern world but his crowning achievement was uniting two seemingly disperate phenomena and creating something remarkable electricity and magnetism had been known about since ancient times seen in the strange attraction of rubbed materials and mysterious stones that knew how to find North but by the 19th century it was becoming clear that these two

were not truly distinct a flow of an electric current could generate a magnetic field and a changing magnetic field could generate a current in a wire but as Maxwell stared at these equations he began to see a deeper picture instead of separate relationships he saw that electricity and magnetism M could be united into a single whole a United set of mathematics that encompassed all electric and all magnetic phenomena but Maxwell's great Insight was not only concerned with electromagnetic complexity for he wandered about the simplest situation of all electromagnetism is a nothingness of a vacuum how

does light travel through the emptiness of space Maxwell knew that electromagnetic fields filled all of space even in vacuums but it was imagined that in empty space these fields would be null effectively not there but what if you plucked one of these fields either electric or magnetic so that these fields were not at zero at some location Maxwell pondered this question using his equations to explore how the situation would evolve and the answer was astounding think about pinching the skin on the back of your hand what happens when you let go of your pinch your

skin sinks back to its unpinch self quickly if you are young and somewhat slower if you are older Maxwell's equations told him that the electromagnetic pinch would evolve away back to zero but that this was not the end of the story pinching the electric field would generate a similar pinch in the magnetic field and the pinch in the magnetic field would generate a pinch in the electric field and these pinches did not simply fade back to zero instead they oscillated regenerating each other from one moment to the next and just like ripples on a pond

these oscillations traveled away as waves and so Maxwell realized these oscillations had the property of light light he realized is a self-propagating electromagnetic wave but what was causing the ripples what was the electromagnetic equivalent of the stone thrown in the pond he saw that it was electric charges something we now know as electrons as these charges jiggle and oscillate they disturb nearby electric and magnetic fields and these disturbances Ripple away as electromagnetic radiation what we call light he also realized that the inverse must be true as light entered the eye and fell on the retina

the oscillations of the light must cause electrons in atoms in the eye to jiggle and it is this jiggling of electrons in the eye sent us signals to the brain that we perceive as Vision finally Maxwell understood what it was that was waving and what caused the waves and one more thing he knew that light had waves with a length of about a millionth of a meter but his equations showed no limitation on the wavelength of of his electromagnetic waves and so he concluded that there must be light with both long and short wavelengths that

is invisible to the eye it would take two more decades for the answer to this puzzle to present itself decades in which Maxwell died of cancer at the age of only 48 in 1886 Heinrich Herz working at the University of CaRu was the first to find these invisible waves named herian waves after their discover a new revolution had been born we now refer to these herzan waves as radio Herz was understandably very pleased with his Discovery but when asked about the Practical use these radio waves have he apparently responded nothing I guess and yet it

was these radio waves that only a few years later Marone was using to send messages across miles and then across counties oceans and all across the immensity of the [Music] globe in 1909 Marone received the Nobel Prize for his work on wireless telegraphy Herz however died in 1894 at the youthful age of 36 never seeing the true promise of his Discovery the world was set to become full of invisible light as the 20th century began and the Mystery of light's properties seemed settled that was until 1905 and a remarkable year for one German patent [Music]

clock the universe continued to change and evolve as our particle of light traveled the mixture of light joining it on its Journey reflected that change bursts of radio waves and high energy gamma rays becoming more and more frequent this energy surged through space much of it flowing between the stars and into the darkness but some encountered lone atoms in The Emptiness of the Void the low energy radio waves very gently Shook and energized these atoms like a calm ocean wave lapping at a Sandy Shore just as we would expect from Maxwell's picture of electromagnetic waves

but the behavior of the high energy gamma rays was different they delivered their energy to the atoms with a violent punch that ripped electrons clean away not a lapping Ripple but an isolated smash the gamma rays hit the atoms not like waves but like hard little energetic particles but how could Maxwell be wrong could on some occasions light be more like Newton's vision and act like a particle and if so just what would those occasions [Music] be for the present we have to work on both theories on Mondays Wednesdays and Fridays we use the wave

theory on Tuesdays Thursdays and Saturdays we think in streams of flying energy Quant or cor [Music] pusles Alfred Nobel had made his Fortune through his inventions and his businesses especially in the field of explosives and weapons and so perhaps not unfairly in an 1888 obituary in a French newspaper he was called The Merchant of death this surprised to no Bell firstly as he was still very much alive but secondly and more distressingly because he realized what his historical Legacy was to be so in his will he decided to leave most of his fortune to a

series of prizes prizes that would honor those that have conferred the greatest benefit to humankind across science the Nobel prizes are perhaps the most prestigious the list of winners repak with the Giants of science over more than a century and in 1901 the inaugural Nobel Prize in physics was awarded to the German vilhelm runan for his discoveries about the nature of light for it was he who discovered the X-ray ronon's experiments with various materials found that only the denist could halt x-rays he even managed to convince his wife Bera to place her hand in the

beam after realizing his x-rays should stream through her flesh but be partly blocked by her denser bones thus producing the first x-ray photograph whilst it was suspected that X-rays were electromagnetic radiation with a wavelength much smaller than visible light as opposed to radar which was much longer it took several decades to conclusively show that this was the case although in the meantime the medical application of x-rays to fix bones and save lives grew without bounds and so this meant that by the early 1900s Maxwell's vision of electromagnetic waves beyond the visible had been undoubtedly confirmed

all of light Secrets uncovered even the electron having been discovered all that remained was to find the rest of the light we could not see gamma rays microwaves and more to fill out the last gaps on the electromagnetic spectrum and wrap up the story but of course if physics feels that the job is done a rude shock is usually just around the corner in Maxwell's picture of light it could be thought of as a continuous wave scientists had found that when light crashed into most materials it continuously the dumped energy that energized electrons causing them

to be emitted this was called the photoelectric effect by lowering the intensity of light it took longer for energy to be deposited and it usually took longer for the electrons to begin to be spat out usually for that was not what was observed when light was shown on certain Metals electrons would seemingly be ejected instantaneously from the metal surface and the really confusing observation came from adjusting the color of the light being Shone blue light would result in very energetic electrons being emitted green light resulted in less energetic electrons and red light produced no electrons

at all this made no sense if all colors of light carry energy why did Red Light fail to energize the electrons this mystery was solved and new Mysteries were born in a very special year this was no ordinary year it was a miraculous year for it was the year that Albert Einstein changed physics forever most people are familiar with Einstein's Anis marabis 1905 the year he wrote down the special theory of relativity but that was just the beginning Einstein was awarded the Nobel Prize in physics in 1921 the citation noted the the award was for

his services to theoretical physics yet one Topic in particular was singled out for recognition and it was not his work on relativity it was especially for his discovery of the law of the photoelectric effect when Einstein explored the photoelectric effect he had to radically revise Maxwell's vision of light he realized that when light interacted with electrons it could not do so as a continuous wave of energy instead the energy must be concentrated and dumped into an electron as an instantaneous packet light Einstein declared must be quantized it must be chunks of energy it must interact

like a particle Einstein went on to explain that each packet of energy was proportional to the frequency of the light a packet of red light carries less energy than a packet of green light a packet of green light carries less energy than a packet of blue and so in experiments the red light simply didn't deliver enough energy for an electron to [Music] escape this enigmatic packet of energy didn't get its current name until 1926 when in an article in the journal Nature Gilbert Lewis coined the name Photon evidence for the particle nature of light swiftly

grew and it was in 1923 Arthur Compton put together an important experiment but one that relied on a bizarre fact light can push this might seem like a strange thing to say how can light which has no Mass push but Maxwell's equations showed that in carrying energy light also carries momentum you can easily buy a Crooks radiometer today is an executive toy for your desk consisting of four veins and an evacuated glass tube one side black and the other side white when placed in bright sunlight the veins begin to spin pushed supposedly by the momentum

of nothing more than sunlight as however the physics of the crooks radiometer is more complex than this simple explanation but the force of sunlight pushing on the veins is real with some Visionaries imagining future Humanity coursing amongst the planets solar sailing on sunlight in 1923 Arthur Compton's experiment however was a little different in his experiment Compton aimed a beam of high energy X-rays at an atomic Target thus ripping electrons from the outer part of the atoms but when examining the rebounding X-rays and recoiling electrons Compton found that Maxwell's picture of a wave of energy and

momentum simply did not work instead Compton had to treat the x-rays and electrons like colliding billiard balls for when an x-ray hit an electron it delivered its energy and its momentum as a discrete packet when an x-ray hit an electron they definitively interacted like hard particles Newton's vision of particles of light was reborn but was this definitive proof that light was a particle not quite there was of course still a mountain of evidence for its wav like nature if anything scientists were more confused than ever before despite Maxwell's picture of electromagnetic waves proving extremely powerful

and successful these experiments in the early 1900s demanded that light must be a particle and so people began to wonder was there even an answer to be found we began this story following a photon of light as it traveled across the universe Maxwell tells us that this Photon was formed by the changing energy of an electron and that it vanishes when finally absorbed by electrons at its Journey's End but what happens in between is this Epic Journey simply governed by Fate does the photon really fly off in a random Direction coring into an electron at

some arbitrary point in its future this question is the next part of our story for in the early 20th century it was realized that this simply could not be the case in the language of quantum mechanics the photon's journey has nothing to do with chance not only is the universe Stranger than we think it is stranger than we can think we begin on a chilly morning in France in January 17 1993 with the swish of a guillotine Blade the King Louis the 16th was no more throughout this upheaval chaos reigned across France Victor franois the

second Duke de Bry battled for his King but eventually like many others of the aristocracy he fled France for safety abroad the dees eventually returned to their native France to shape the country after the upheaval of Revolution and after a series of Statesmen diplomats and writers in 1892 who into the de Bry family was born the man who would change our understanding of everything his name was Louie Victor Pierre Ron 7eventh Duke de BR but in the annals of physics history he is simply known as de BR in the early 20th century he bore witness

to the birth of quantum mechanics and the growing confusion about whether light was a particle or a wave to De Bry however there was one obvious solution though a counterintuitive one light was neither and both at the same time it was clear that light when it traveled traveled as a wave producing the effects of interference and defraction but when it interacted it interacted like a particle it seemed to exhibit properties of being both a particle and a wave but was never really either de br's remarkable Insight was to realize that this was true not only

for light but for the entire Quantum World here he claimed there were no true particles and no true waves everything de BR told us was some sort of quantum thing and so in his PhD in 1924 he claimed that electrons which were clearly particles should also exhibit wavelike properties and in 1929 he received the Nobel Prize when experiments bore out his predictions there has been significant philosophical discussion about this wave particle duality in quantum mechanics but its observational consequences are incontrovertible a series of single photons or electrons sent through multiple slits still result in interference

patterns and even large complex molecules have also been shown to have properties of both waves and particles the largest yet tested being 2,000 atoms in size and so with quantum mechanics in hand the quest was on to understand just how light and electrons interacted with classical physics the physics of Maxwell electrons jiggled as electromagnetic waves passed by just like seagulls bobbing on a choppy ocean and by their jiggling the electrons emitted their own electromagnetic waves adding to the mix but the quantum picture had to be different for the quantum world was one of Quant and

particle reactions despite these opposing situations it didn't take long for a solution to be found and it was another scientific Titan Paul Adrian Maurice durak that began to crack the code in the late 1920s he was working to unite two of the greatest breakthroughs in modern physics The Strange World of quantum mechanics and Einstein's special theory of relativity D's story has been told many times about his famous absent-mindedness and lack of communication skills Quantum Pioneer Neil bore went as far as to call him the strangest man but there was absolutely no doubt that durak was

a revolutionary genius and it was through his work on quantum mechanics that dur made his mark on scientific history to understand the modern view he helped to bring about we have to accept that everything is actually Fields these fields are different to things like classical electric and magnetic fields in Quantum field Theory there are electron Fields Photon Fields fields for the various quarks and more Ripple for example in the electron field is an electron and a ripple in the photon field a photon think of an atom what do you see in our minds we often

have the picture given to us by Neil's bore of electrons orbiting a nucleus like a planet orbiting a star and when an electron jumps from a higher orbit to a lower orbit a photon of light is emitted but when considering the quantum world this is not quite correct in Quantum field Theory we think of an orbiting electron as a vibrational pattern in the electron field the higher energy orbited is one particular pattern and the lower energy orbit is another in the language of physics the electron field and the photon field are coupled together and jumping

between the higher and lower orbits the electron field generates a vibration in the photon field Quantum field theory has grown to become arguably the most successful theory of our world to date describing almost everything in our un Universe across 24 Quantum Fields corresponding to the various possible interactions of the standard model and so simple everything is fields and the fields interact but of course as they often do in the quantum World things are about to get a lot stranger many of the greatest minds of quantum mechanics were involved in this move towards strangeness but perhaps

the most well-known is a man from Far Rockway with a broad Brooklyn accent a man named Richard feineman born in 1918 he started his career as part of the Manhattan Project and was recommended by Oppenheimer himself for Berkeley in a now famous letter sent in 1942 he is by All Odds the most brilliant young physicist here and everyone knows this I may give you two quotations from men with whom he has worked beta has said that he would rather lose any two other men than Fineman from this present job and vigner said he is a

second duraq only this time human though his 1985 autobiography surely you're joking Mr finan was an eye opener for many not only regarding his numerous contributions to science but also his extroverted personality and complex private life including a ponchon for strip clubs these aspects did not fit the stereo typical vision of a professor indeed Mari galman another giant of quantum mechanics once even equipped of Fineman fean was a great scientist but he spent a great deal of his effort generating anecdotes about himself and yet when it came to thinking about the quantum world for many

physicists Fineman changed [Music] everything whilst Fan's famous quip that no one truly understands quantum mechanics may have been true Fineman himself certainly understood the depth of the mathematics that underlies it and this gave him the insights to think about the true nature of light and how it interacts it all starts with a solitary electron in the electromagnetics of Maxwell the charge of the electron results in an electric field surrounding it and a charge in an electric field feels the presence of the electric field in this situation there must be energy in the interaction but the

question was how much the problem was every time Fineman tried to calculate the amount of energy the answer came out to be the same Infinity so Fineman did something quite radical he threw away the classical notion of the electric field as defined by Maxwell in the acceptance speech for the award of his Nobel Prize in 1965 Fineman said I suggested to myself that electrons canot not act on themselves they can only act on other electrons and a new bizar picture of the interaction of light and electrons emerged the best visual representation of this interaction is

the diagram named after feineman himself the feineman diagram they are often a complicated mixture of lines Wiggles and Loops but at their heart find diagrams describe all of the possible interactions in quantum mechanics to pick apart a Fineman diagram it is best to start with the simplest of interactions the interaction between electrons and light findan diagrams represent an interaction over space and time lone electrons Trace out straight line paths through SpaceTime a path known as its World line the electron is really just a vibration in the quantum electron fi field and with no interactions it

happily traces a simple straight line path we also know however that the electron field can couple with the photon field and when this happens the vibrations in the electron field change in an atom the electron jumps from a high energy orbit to a low energy orbit but for a free electron conservation of momentum means that the electron changes direction if we imagine this over SpaceTime the world line line of the electron possesses a distinct Kink and this occurs where and when the photon usually depicted as a wavy line is emitted this structure this Junction is

known as a vertex and it is the basic Lego piece for building all Fineman diagrams of course full Fineman diagrams are more complex than a single vertex they usually combine several distinct pieces the emitted Photon from one electron is eventually received by another electron two vert C are joined together to give the complete interaction two kinked electron paths joined with the wiggly line representing the photon but what governs the coupling between the electron field and the photon field this is related to the charge on the electron and one of Nature's constants the fine structure constant

this is electromagnetism and the exchange of the photon between two electrons is the electromagnetic force in action and so in fineman's view we wave goodbye to the electromagnetic field in its place we simply have two electrons interacting through the exchange of a photon and when huge numbers of these photons are exchanged it approximates the classical Force even though at its heart this electromagnetic force is a Quantum phenomenon and it's not just electromagnetism for it is also true for the fundamental weak and strong nuclear forces for the strong force it is gluons instead of photons that

are exchanged between quarks and for the weak force it is via the exchange of massive particles known as the w and z and again all of these forces can be presented by a combination of Fineman vertices but this was not the end Fineman had one even stranger card yet to play when it came to light he had said that one electron acts upon another and this happens through the exchange of a photon producing the complete Fineman diagram of the interaction but did this mean that an electron fired out a photon at random did this Photon

stream out into the universe with only a remote chance of being absorbed by another electron counterintuitively the answer fan said was no he told us that the photon is only past between two electrons that have agreed on the exchange but there is something odd happening here if we are in the middle of the photon's journey its emission from one electron occurred in the past while the absorption of the photon by the other electron is going to occur in the future so when did the electrons communicate and agree to exchange the photon how did they even

know of each other's presence it clearly cannot be via the electromagnetic force as this is precisely what the exchange of the photon actually is so what is the solution as with a lot of quantum mechanics was the mathematics just works the interpretation the question of what is really happening is the biggest challenge and so Fineman with his supervisor John Wheeler put a mindbending possibility on the [Music] table the suggestion is something we now call the transactional interpretation they said that the two electrons handshake their acceptance of exchanging the photon but that this handshake is taken

through time one electron messages from the past and the other from the future this might sound ridiculous but it completely fits with the mathematics of quantum mechanics and so on a dark night when you gaze at a distant star an electron in your eye and an electron in the atmosphere of that star spoke to each other through time and agreed to exchange the photon you see and going even further for the lonely Photon we met at the beginning of our story two distant electrons shook hands over billions of years of time billions of light years

of space and agreed that the photon should undertake its Cosmic Journey the bizarre world of quantum mechanics never never disappoints and yet there is one final even stranger mystery to unfold about light and our lonely Photon in the Blackness of space as it travels over many billions of light years just what does the photon experience we've followed our Photon over many billions of years eventually at Journey's End the universe it inhabits is very different to the one of its birth yet there is a disconnect for whilst this Photon was almost as old as the universe

itself it remained eternally youthful galaxies formed in the void stars were born lived and died whole superclusters splintered and collapsed and the photon missed it all because to the photon time itself meant nothing this might seem like a strange thing to say the photon clearly had an existence in time but with the coming of Einstein and his special theory of relativity it was realized that time was actually flexible it was relative dependent upon who or what was actually measuring it and light light takes this idea to the extreme what would the universe look like if

I were riding on the end of a light beam at the speed of light in the middle of the 17th century only Roma was baffled working at the Paris Observatory Roma was peering at IO one of the bright moons of Jupiter like clockwork the moon orbited the giant planet in just over 42 hours Vanishing from view as it ducked in and out of Jupiter's Shadow except there seemed to be something odd about this Cosmic clock Roma noticed that the timing of io's eclipses drifted he realized that the timing of the eclipse of IO was somehow

tied to the Earth's orbit changing from earlier to later and back again when the Earth was either closest to or further from Jupiter and it was then Roma realized the culprit was light and in particular its speed he reasoned that the drift in io's eclipses must be due to a finite speed of light as the Earth moved in its orbit the distance to Jupiter changed and the change in time was because light had to Traverse these differing distances his initial estimate for the speed was fast very fast 220,000 km every second and eventually more accurate

measurements tied the speed of light to almost 300,000 km/s but just what was this speed relative to it had been the belief for centuries that there existed a medium The Ether that carried light waves surely therefore light speed was relative to this medium from Plato to Newton this ether had long been suggested as a solution to various questions in physics but never firmly detected experiments in search of evidence having failed time and time again and so it was that in 19 5 during his miracle year Einstein rang the final death nail for this invisible medium

special relativity the truth was that it was the speed of light that was the universal absolute and invariant measured to be the same value for all observers across the cosmos A lot has been written about special relativity and although much of it seems confused and paradoxical there is a simple Central message at its heart particles with mass such as electrons chart out their own time as they travel through space Time Imagine two clocks sitting at the same location synced to show exactly the same time now take these clocks on two separate Journeys speeding them up

and slowing them down in neon's view of the universe of absolute time if you were to bring these clocks together again and compare their faces they would have remained synchronized but this is not the case in Einstein's the relative motion of the two clocks would have influenced their relative passage of time and as they trace out their different paths through SpaceTime when they reunite their times will now be out of sync this mind-bending aspect of Relativity seems too strange to be true but numerous experiments have shown this to be the way the universe works for

Globe trotting atomic clocks to high-speed particles and accelerators time is definitively relative but what does this mean for light light had taken a central place in Einstein's New Vision of the cosmos everyone in SpaceTime should measure the speed of light to be precisely the same value but in demanding this something else had to give and so space and time themselves VES had to bend become flexible and rubbery to accommodate the consistency of the speed of light indeed one immediate consequence of Einstein's insights was that light would feel the existence of gravity and as it traveled

through the universe light's path would be deflected by the presence of mass Newton's claim of two centuries prior reborn indeed experiments have worn this out again and again with the results becoming more and more accurate massive objects such as stars and galaxies can even behave as gravitational lenses magnifying distant galaxies from the very early universe and revealing the presence of dark matter the beauty of these natural telescopes is clearest in deep space images such as the first revealed by the James web Space Telescope and so the flexible nature of space and time had truly seen

the end of Newton's view of a rigid universe but again what about light what did this mean for light's experience of space and time traveling at the fastest speed possible in the universe the effects of Relativity become extreme very extreme all distances shrink to zero as does the time taken to cover these zero distances and so for photons no matter how far they travel across the universe not a single instant of time will tick by even though this light may have existed in time and space for many years or light years even though it would

have been clearly formed by one electron in one location and vanished when absorbed by another electron in another location the space-time distance between these two events would be exactly zero to the photon it is born and dies at precisely the same moment we began this story by following a lone Photon from its creation just after the beginning of time to its ultimate destruction in the detector of a telescope orbiting our planet today and yet the photon itself saw nothing of this not the intense light of Stellar birth the catastrophic explosions that come with Stellar death

or the formation of planets and events ual Rise Of Life on our own pale blue dot the photon notic [Music] nothing you've been watching the entire history of the universe don't forget to like And subscribe and leave a comment to tell us what you think thanks for watching and we'll see you next time [Music]