Why Is Gravity So Weak?

humanity is obsessed with the superlative the biggest the fastest the strongest the most destructive even after the world witnessed the devastation wrought by atomic weaponry during world war ii we inexplicably built them bigger bolder and more deadly on october the 30th 1961 a tupolov tu95v bomber opens its doors and drops its 27 ton load over nevaya zamla island in the soviet arctic ocean the immense bomb descends almost gracefully beneath parachutes allowing its plane to speed away as fast as its propellers will take it which is just as well for when this tsar bomba or king of bombs detonates it produces the largest man-made blast the world has ever seen the thermonuclear weapon explodes with a force of 50 megatons of tnt more than 3 800 times stronger than the bomb dropped on hiroshima its mushroom cloud reaches 60 kilometers in height and the flash of the detonation could be seen a thousand kilometers away the heat from the explosion is enough to cause third degree burns at the distance of a hundred kilometers the colossal destructive force of the tsar bomber explosion was hard to fathom but earlier that year astronomers had witnessed an explosion that was some 10 billion billion times stronger in a spiral galaxy 45 million light years away from our own a giant star reached the end of its life in spectacular fashion as fusion came to a halt in its core the repulsion of its electrons could no longer resist the crushing force of gravity and it collapsed in on itself at nearly a quarter of the speed of light the sudden implosion sends temperatures soaring and results in a supernova explosion that spews stellar material over many light years but if supernova explosions are more powerful than anything we can conceive of on earth hypernovas put even them to shame these rare super powerful stellar explosions result from the death of stars more than 30 times more massive than our sun a black hole is created in the center and scorching winds of newly forged nickel and jets of radiation plough through and ignite what was left of the star hypernovae can be 10 to 100 times brighter and more powerful than a supernova they are believed to be the most powerful single event in our observable universe and all of this destructive force results ultimately from the pull of gravity on the atoms that make up these dying stars so you could be forgiven for thinking of gravity as vastly powerful indeed every moment of our lives is shaped and defined by gravity from the convenience of remaining stuck to the surface of the earth to the mental calculations we make when playing sports even to the immense challenge of engineering rockets to escape our planet's inexorable pull but gravity is just one of four fundamental forces that control our universe the strong nuclear force holds our atoms together the electromagnetic force permeates all of space and delivers the radiation which lights and warms the cosmos even the weak nuclear force has a vital purpose driving the nuclear fusion in the hearts of stars indeed when compared to these other heavy hitters the force of gravity pales in comparison the strong force is fittingly the strongest force known to nature followed by the electromagnetic force and then unsurprisingly the weak force and at the bottom of the scale gravity by some distance it is weaker even than the so-called weak force to be exact by 10 to the 25th power or 10 trillion trillion times in order for us to feel the effects of gravity we must work on cosmological scales with entire stars and planets worth of accumulated matter in simple terms the stored chemical energy in the legs of an ant can overcome the gravity of an entire world [Music] gravity's relative weakness has baffled scientists for years and has come to be known as the hierarchy problem and this mystery may be linked to another cosmological puzzle where does this omnipresent but practically powerless force originate to answer these century-old questions we will have to take a trip billions of years back in time billions of light years away in space and possibly even to another dimension [Music] this video is sponsored by curiositystream the documentary subscription service question how big was the largest black hole ever discovered a a billion times the size of our sun b roughly the size of the milky way c very big or d not very big ton 618 is 66 billion solar masses similar in size to the entire milky way and so my recommendation from curiositystream this month is the fascinating zenith advances in space exploration a new 13-part series exploring just how humanity will be able to journey to the stars and for this and more curiosity stream is a bargain at less than 20 dollars a year with a wide range of documentaries on topics ranging from science to music nature to sports with new shows popping up every week so go to curiositystream.com forward slash history of the universe to sign up and using the promo code history of the universe will save you 25 percent [Music] gravity was the first force to be recognized by scientists and yet to this day remains the most shrouded in mystery isaac newton may have been a brilliant mathematician but he was by all the counts a dower and serious man with no time or interest for the opposite sex or indeed any sex at all he remained a virgin all his life and what joy he experienced was scarcely visible on his face legend has it he only laughed in public twice but of course it was under this ultimate focus distracted by neither jolity or intimacy that newton was able to formulate his famous theory of universal gravitation [Music] in 1666 the deadly outbreak of plague forced cambridge university to close and newton returned to his family home wolves thought manor in lincolnshire here in an isolation much like that experienced by many of us in the last two years newton found himself uniquely inspired he continued to experiment with light and optics and took long walks around the grounds where it is said he witnessed something that would come to define physics whether or not this is myth it was a myth perpetuated by newton himself after dinner the weather being warm we went into the garden and drank tea under the shade of some apple trees he told me he was just in the same situation as when formally the notion of gravitation came into his mind it was occasioned by the fall of an apple as he sat in contemplative mood why should that apple always descend perpendicularly to the ground thought he to himself letting his mind wander above the branches of the apple tree he further considered this attractive force extending beyond the atmosphere of the earth in that moment he didn't understand gravity in its entirety but as the apple struck the ground it sowed the seeds of his pivotal theory that theory simply is that all masses experience an attractive force pulling them together the strength of that force is proportional to their respective masses and inversely proportional to the distance between them early 18th century scholars applying newton's theory of universal gravitation found that it worked remarkably well to explain the motions of the planet's moons and stars but newton in characteristically dour and pessimistic fashion was still not satisfied that gravity should be innate inherent and essential to matter or that one body may act upon another at a distance is to me so great in absurdity that i believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it gravity must be caused by an agent acting constantly according to certain laws but whether this agent be material or immaterial i have left to the consideration of my readers in other words the maths is correct but he has no idea why but while this mysterious agent remained undiscovered newton's theory persisted for some 200 years over that time more precise astronomical observations gradually revealed phenomena that the so-called universal gravitation was unable to explain the orbit of mercury for instance didn't conform to the mathematical dance predicted by newton's theory and the equations also came short of explaining how a light with no measurable mass of its own could be bent and deflected by other objects newton's self-skepticism was justified and ahead of its time the gravity of the situation became increasingly apparent to early 20th century astronomers as more and more advanced telescopes and instruments began to probe the cosmos on scales never before imagined time and time again newton's theory fell short and it became clear that a more nuanced mathematical explanation for gravity would be necessary and so in 1913 an alternative theory was proposed but not by the man you may imagine gunnar nordstrom was a finnish physicist hugely inspired by certain germans recent theories of special relativity in which space and time themselves are redefined as phenomena intrinsically tied to frames of reference nordstrom seized upon and solved the equations set by the german and went on to expand his own theories to consider how this might apply to gravitational attractions in them he posited a notion that must have seemed outrageous as it was difficult to grasp what if nordstrom suggested space and time were not flat but instead curved [Music] such a bizarre dimensional geometry would be enough to explain gravitation just as well as newton's theory even better in some circumstances unfortunately although nordstrom's mathematics were internally consistent they couldn't be confirmed by experiment or observation a solar eclipse in 1919 revealed bending of light that didn't fit with his theory's predictions nordstrom's ideas were dead in the water but the seed of curved space-time had already been sown the very german physicist whose equations first guided the fin now seized upon his revolutionary ideas and made them his own two years after nordstrom the german albert einstein published his own theory of general relativity with curved space-time central to all observable features of gravity but far from being resentful nordstrom nominated his friend and colleague for the nobel prize twice einstein himself found it helpful to think about the naughty questions surrounding the nature of gravity with his gadankan experiments he created an entire laboratory of thought experiments in one notionally inspired by seeing a painter fall from a roof opposite the painted building where he worked he imagines a man falling from an impossibly tall building as he falls untroubled by fear of any imminent impact he may consider whether he is in fact falling at all if he releases any object he was holding it would remain in the same position beside him he would be forgiven for thinking that there was no gravitational field at all that he was floating weightless and at rest in space such a comparison serves to demonstrate that our perception of gravity is strongly dependent on our frame of reference only by looking at the windows of the building scrolling past would our luckless man be aware of his imminent fate another of einstein's thought experiments places our unfortunate falling man inside a large closed box that is this time floating freely in space he would be truly weightless within it that box then has a rope attached to a surface that was to become its roof and that rope is pulled on by some divine and omnipotent being with a constant force the box would be accelerated uniformly upwards and the man inside would drop to the floor all his senses would tell him that he now has weight he's indeed sitting on the floor of the box as he would if the box was sitting on the surface of a planet like earth he could be forgiven for thinking that he was in a uniform gravitational field when in reality no such field exists this example serves to demonstrate that the effects of gravitational acceleration are indistinguishable from those of a more traditional force-induced acceleration the likes of which we experience in a speeding car or a cornering roller coaster this so-called equivalence may seem obvious to us but it became an important facet of einstein's theory of general relativity although physicists are still looking for ways to experimentally verify this equivalence and other assumptions that underpin general relativity the advanced theory first proposed in 1915 still holds up to detailed examination general relativity relates mata's energy and momentum to the curvature of space-time which in turn dictates how everything embedded within it both material and intangible behaves importantly the theory is able to explain the strange wobble in mercury's orbit since the tiny planet passes so close to the massive sun that it receives an extra relativistic push and general relativity also satisfactorily explains the curvature of massless light all of our perceived reality resides within the four dimensions of space-time and is a slave to the geometry of that fabric if space-time is curved by massive objects then light has no choice but to curve as well today we have tested general relativity time and time again and it does appear to hold true but while einstein's theories broke ground in describing how gravity works on cosmological scales they do little to reveal the true nature and origin of this mysterious force in other words echoing newton's statement 250 years earlier the maths checks out but we still don't know why physicists have spent the last hundred years searching for gravity's beginnings in the hope that it will shed light on its true nature why it is ultimately so weak and even how the universe around it came into existence [Music] one billion years ago on earth not a lot was happening [Music] for roughly 800 million years the planet has been stuck in a stagnant stinking stasis no major continental reshuffles no waxing and waning ice ages no fluctuations in oceanic or atmospheric chemistry life which has made promising advances since its appearance diversifying throughout the oceans inventing photosynthesis and even learning how to cooperate now just seems to stop in its tracks evolution stalled and for this unthinkably vast span of time amounting to nearly a quarter of all of earth's history nothing very interesting happened at all this period has been fittingly named the boring billion by geologists and yet in the earth's distant southern skies far beyond the magellanic clouds an event is taking place that is by no means boring [Music] two black holes each more than thirty times the mass of our own sun are locked in a fatal dance spinning faster and faster around one another until eventually catastrophically they collide merging to become one's super-sized body unfortunately for the proterozoic earth this spectacular event is unable to punctuate the tedium as not only are the black holes themselves non-luminous and therefore invisible but they are also more than a billion light years away there's nothing for anyone or anything to see hear or feel for a billion years until at 9 50 a.m on the 14th of september 2015 a remarkable instrument comes online two advanced laser interferometer gravity wave observatories known as ligo had just been completed in hanford washington and in livingston louisiana and had been switched on just two days before they were still in their test mode with no formal research having begun when researchers picked up an incredibly loud and clear signal it was so perfect that nobody believed at first that it was real they thought it was just another calibration signal designed to test the instrument's sensitivity and the researcher's analytical skills but when no one owned up to devising such a drill they were forced to conclude that their brand new detector had made its first detection this was in itself remarkable throughout its proposal funding and construction ligo received more than its fair share of criticism indeed the instrument had been conceived as a way to detect a phenomenon that had only ever existed as a theory it was einstein himself who posited the idea of gravitational waves radiating out through space-time from a sudden and massive gravitational disturbance but space-time is our reality so how could we possibly detect ripples within it without anything to compare it against ligo intended to search for these minute distortions using two lasers reflected back and forth between a sequence of mirrors which then converge at a single point to cancel each other out if a gravitational wave passed through they would change the lengths of these laser beams causing a mismatch that could be detected at the point of convergence critics doubted that the instrument would be sensitive enough to detect these faint ripples or indeed that they existed at all so it's little wonder that scientists were surprised by their discovery in 2015. the signal they detected was short lasting just a fifth of a second but corresponded to a vibration that was in the range of human hearing [Music] and the source of this chirp the cataclysmic collision of two black holes more than a billion years ago the merger had released nearly a quindicilian joules of energy that's 10 to the 48th power as gravitational waves radiated out in all directions traveling at light speed for a billion years until they finally reached ligo just two days after it came online and since that first remarkable detection scientists have heard countless more chirps and beeps from gravitational waves rippling through the fabric of the cosmos their success was confirmed beyond a doubt in 2017 when the detection of gravitational waves coincided with a burst of gamma rays and then observational evidence of a bright new star the source this time was not merging black holes but merging neutron stars ligo's critics were firmly silenced as the researchers were awarded the 2017 nobel prize in physics for their definitive detection of gravitational waves and further confirmation of einstein's general relativity but what does this cutting-edge science tell us about the nature of gravity in the cosmos the existence of gravitational waves implies that gravity is not simply a force between individual objects but a field that extends throughout all space and time ripples move through that field at the speed of light not instantaneously as newton's original formulations implied that means that if the sun were to suddenly blink out of existence the earth would continue orbiting the space it had inhabited for a full eight minutes unaware that anything had changed before the change in gravitational distortion caught up disconcerting though this may seem for physicists it is a comforting conclusion the other fundamental forces are also manifest as fields with energy being transmitted through them as waves light and heat are just ripples in the pervasive electromagnetic field of the cosmos and are two limited to a maximum speed so we would be ignorant to the disappearance of the sun's light too for a full eight minutes gravity therefore behaves similarly to the other forces and it's not unreasonable to think that its origin can be tied to them as well and it is in the pursuit of this mystery that lycos scientists hope gravitational waves can provide another clue today we detect space-time ripples all the time from things like black holes neutron stars and supernovae but theoretically there should be another wave lurking in the background behind all the noise of the lives and deaths of stars astronomers believe that there should be faint gravitational ripples pervading the universe which have been travelling the vast cosmic ocean since its birth [Music] currently accepted models for the universe's earliest moments see the newborn cosmos suddenly and almost instantaneously expand inflating to many uncountable trillions times its original size in less than a billionth of a billionth of a billionth of a second whatever heat and energy that had existed in the universe before inflation was now spread unimaginably thinly leaving space cold and empty yet of course that is not how the cosmos is today so cosmologists imagine a period of reheating that immediately followed they hypothesized that whatever force field or particle that was responsible for inflation in the first place decayed away and transformed into energy and particles to fill the new universe this is considered by some to be the real big bang since this is the event that filled the cosmos with everything needed to make our universe as we know it and theory suggests that this reheating event would have released energy in the form of gravitational waves which would have spent the last 13.8 billion years traveling through the fabric of space-time they would be faint but these primordial echoes if our theories are correct are washing over us right now unfortunately the groundbreaking ligo instruments in washington and louisiana may be able to pick up a black hole merger a billion light years away but they are not sensitive enough to probe the beginning of the universe new instruments are being developed right now including the european space agency's laser interferometer space antenna or lisa which it is hoped will have the resolution to filter out the clamor from the stars to see the swell of space-time itself the types of waves these new instruments detect also offer a chance for us to better understand the process and end of inflation it is a period of time we will never be able to probe with traditional astronomy since the universe was no more than an opaque plasma until it was around 380 000 years old and inflation occurred less than a second after the true big bang we don't know if reheating was slow or rapid but the pattern of gravitational waves from each scenario would be sufficiently different to allow us to reconstruct this remarkable event the detection of primordial gravitational waves from the end of inflation would also confirm another assumption that the gravitational field itself is intrinsic to the universe as we know it while it may seem obvious and reinforced by gravity's similarity to the other four fundamental forces it is an important experimental result that provides boundaries on what is and isn't likely during this improbable period in the history of the universe today the shape of space-time is warped by the presence of massive objects does the gravitational field require mass in order to manifest results from the large hadron collider in 2012 verified that the mass of normal matter is derived from the higgs field and its corresponding messenger particle the higgs boson but modelling suggests that the higgs field didn't come into existence until long after the end of inflation so is this the origin of the gravitational field as well while the link between mass and gravity in the modern universe is indisputable they are in fact separate and mutually exclusive phenomena the einsteinian notion of gravity is in fact created by a combination of energy and momentum not explicitly mass this much is apparent by gravity's effect on the massless photons that make up light and so gravity doesn't need the higgs field to work its origin can long predate the origin of mass in the cosmos although its effects become unrecognizable in a universe where matter and mass are not yet related and so the general consensus among early universe cosmologists is that gravity emerged during the bizarre and incomprehensible planck epoch when the universe was just a planck length in size around a hundred billion billionths the size of a proton but the nature of that gravity would be very different to that in the modern cosmos efforts to probe the behavior of gravitational space-time on such small scales have yielded troubling and unsatisfactory answers but new ideas are gaining ground which offered to transform our understanding again just as einstein's revelations and ligo's discoveries have done before these theories are as mind-boggling as any in the pre-inflationary universe but they offer to resolve the long-standing and as yet unresolved question of why gravity is so mysterious and so weak lisa randle is filled with confidence and adrenaline as she chalks her hands and flexes her fingers ready for the climb ahead of her stepping back to check her planned route for the final time she sees chalk on handholds favored by previous climbers and pins and hooks ready for securing ropes at stages along the vertical rock face she isn't nervous just the day before she had scaled yosemite's famous half dome and is feeling comfortable in her climbing prime happy and confident she steps up to the rock eases her toes into a crack and begins her ascent randle is no stranger to gravity and to its weakness in comparison with the other fundamental forces by 2003 she had made a name for herself as an instinctive and especially talented theoretical physicist with some seemingly outlandish but potentially inspired theories on the nature of reality but right now her thoughts are not of her theories or of the upcoming conference in which hundreds of like-minded physicists will discuss her work but rather the rock that is mere inches from her nose and the rough surface of the stone beneath her fingers gravity is so weak that here in yosemite as with everywhere on earth the tension in a single rope is enough to overcome it and hold a human body when fingers slip unfortunately for lisa randle she is near the top of the first leg of her climb and is yet to secure a rope to the first anchor point just a few feet above and it is here her hold fails and gravity takes over for all its weakness when there is nothing to stand in its way its speeds randall back to her starting point accelerating her by 22 miles an hour for every second perform one of the major barriers to understanding gravity in the way that we do the other forces is the fundamental incompatibility of the two leading theories in physics general relativity as defined by einstein describes the large-scale behavior and interaction of objects within a curved spacetime but at the other end of the physics spectrum quantum mechanics has been adopted to explain behaviors on the smallest possible scales almost all large-scale phenomena have been broken down into their smallest constituent parts the fundamental particles or virtual particles that represent the quantized universe thus the electromagnetic field that permeates all of space has been quantized into individual packets or particles known as photons the other forces likewise have virtual quantum particles in the shape of gluons and w and z bosons that communicate their action across various scales even the fundamental particles of mata have been recognized as quantum manifestations of a pervasive energy field but gravity is the exception to the rule a quantum messenger particle for the gravitational field has been theorized and even given a name the graviton but it has never been detected some doubt that it ever could be detected if the gravitational field is woven into space-time then isolating the quantum packet of that field would be nion impossible tantamount to reducing our reality to the 4d pixels that make it up such pixels would of course only appear when viewed from outside the fabric itself something which we are simply unable to do and if it were possible the energies required to resolve a graviton in a particle accelerator would require an instrument with a diameter larger than our solar system and on top of this traditionally mathematically supported theories of gravity begin to break down at the smallest scales of quarks and leptons general relativity suggests that any source of energy or momentum should distort space-time this means when you attempt to quantize space-time to the smallest degree then gravity itself becomes an excitation a source of energy that energy is enough to distort space-time to create more gravity which creates more excitations more energy more gravity more excitation and so on unresolvable infinities are bad news for any theory this self-excitation like the uncontrollable fishtailing of a trailer at high speed is actually not uncommon in quantum theory but physicists have found a way to mathematically constrain and correct for the so-called self-energy but when it comes to quantizing gravity and space-time there are no simple measurements that could be made to rein in the self-excitation physicists describe gravity as non-re-normalizable which essentially means that space time and gravity cannot behave in the way we expect at scales smaller than the planck scale gravity and the quantum world remain irrevocably at odds scientists have suggested alternate theories for the earliest gravity that existed during the planck epoch loop quantum gravity is a scheme that conveniently avoids the need to renormalize quantum gravitational measurements by considering gravity in a purely geometric sense rather than as a force in it space and time are constructed from closed loops that are woven into an extremely fine network transforming the fabric of space-time from mere metaphor to reality string theory provides an alternative but no less mind-melting approach in it reality is composed of tiny vibrating strings that exist over more dimensions than we can possibly ever experience it suggests that the three dimensions of space and one of time are merely emergent properties of a bigger reality and so it is here we return to yosemite lisa randall wakes as she is airlifted from the site of her decisive defeat in the battle against gravity her head swimming she sees not stars but strings but as her consciousness trickles back the tantalizing glimpse of multiple dimensions granted to her by concussive imagination is replaced with disappointing lucidity she's back in the limited three-dimensional world of the living for the next few months under enforced bed rest she finally has time to finish her book and to try to explain her bizarre multi-dimensional theories in a way that everyone can understand scholars of string theory generally assume that the other seven dimensions necessary for the theory to work aren't visible to us because they are curled up to an imperceivably tiny scale but randall went so far as to suggest that these extra dimensions could be infinite in extent provided space had a warped geometry taking nordstrom and einstein's concepts of curved space-time to another level randle suggests that our entire universe inhabits a three-dimensional pocket of higher dimensional space these pockets are known as brains from the concept of membranes just like a pearl on a string can move side to side and experience only one linear dimension in our three-dimensional universe so our entire universe may be limited to three dimensions even though many more exist beyond our reach and randle invokes these brains as an elegant explanation for gravity's elusiveness and its weakness she suggests that gravity is not a fundamental property of the brain we inhabit but that it is concentrated in another brain altogether this helps to resolve the unanswered question of why gravity is so weak and is indeed so much weaker than it should be indeed mathematical calculations from first principles suggest that gravity should in fact be 10 million billion times as strong as it is the fact that it is not has been explained by another theoretical scheme known as supersymmetry in which every particle has a supersymmetric sparticle that helps to shield it from some of the effects of the ambient universe but following this theory through to its natural conclusion that many spartacles throughout the cosmos would produce reactions that are simply not observed so lisa randle and her colleague raman sundrum found a way to keep the maths and explain what we actually see by effectively quarantining some parts of the theory in another universe and so placing gravity on another four-dimensional brain which is separated from our own by a short distance in a five-dimensional bulk helps to resolve the contradictions inherent in supersymmetry in this scheme the effects of gravity decrease exponentially the further we are from the so-called gravibrane thus in our reality we experience only those gravitational effects that leak through from the dimensions where gravity is king our lack of success in detecting gravitons thus far may be a symptom not of their diminutiveness but rather the very small quantum probability that they will stray from their own brain into our universe lisa randall's ideas are tantalizing ones and super symmetry string theory and the multiverse are concepts that are rapidly gaining ground within the physics community [Music] but it is hard to set such store by mathematics alone just as curved space-time seemed unfathomably complex to nordstrom and einstein's predecessors so these new theories skirt the worryingly incomprehensible and just as gravitational waves were criticized until the moment of their discovery many scientists won't take these concepts seriously until there is definitive proof if definitive proof is even possible with hundreds of physicists astronomers and mathematicians now working on the problem all over the world we will just have to wait to finally unravel the true nature of gravity and reality itself [Music] 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

Channel: History of the Universe

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Id: 4IxuXuLbMoQ

Length: 41min 24sec (2484 seconds)

Published: Fri Jan 07 2022