What If Gravity is NOT Quantum?

the Holy Grail of theoretical physics is to come up with a quantum theory of gravity but after a century of try we really have no idea how close we are or if it's even possible but we shouldn't feel bad because it turns out that the universe is doing everything in its power to make this as difficult as possible well perhaps it's just telling us that quantum gravity isn't possible should we take the hint our modern theory of gravity was discovered a little over a century ago with Albert Einstein's general theory of relativity and then just a little under a century ago we discovered quantum mechanics which would become our modern theory of everything except gravity it was an exciting decade or so for physics but then things slowed down we've spent the following 100 years trying to reconcile these two theories and bring them together into a single Master Theory of Everything the most common approach to this reconciliation has been to try to make gravity quantum after all we got a theory of quantum electromagnetism by quantizing the electromagnetic field the result was quantum electrodynamics in which the force of electromagnetism can be described by the exchange of a single Quantum of this field which turns out to be the photon the same basic procedure led to the Discovery and quantization of the strong and weak nuclear forces with their Associated particles gluons carrying the strong force and w and z bosons carrying the weak so if three the four forces of nature are quantum surely quantizing gravity is an essential step on the path to a final Theory and if the gravitational field is quantized then it should be made up of Quant gravity should be mediated by its own Force carrying particle we call this hypothetical particle the graviton detection of the graviton would allow us to confirm gravity's Quantum nature and even test out theories of quantum gravity such as string theory and loop quantum gravity we've talked about these theories in the past they are mathematically very dense and involve quite a bit of speculation and some have argued that we are getting way way ahead of ourselves with these theories So today we're going to get right back to the basics to do that we'll follow some of the thinking of Freeman Dyson who helped shape quantum theory from near the beginning and thought about the most fundamental questions for all his long life we'll see what he had to say about whether it's even imposs to detect a graviton something we need to do in order to verify that gravity really is quantum but first we're going to follow another musing of Dyson's in which he asks whether the same trick that told us that electromagnetism must be a Quantum force can also be applied to gravity the quantum nature of electromagnetism was the very first clue that led to the quantum Revolution it first showed up in the mathematical trick that Max plank used to explain thermal radiation and this inspired Einstein to take the quantization of electromagnetism seriously in order to explain the photoelectric effect now we understand that the electromagnetic field and electromagnetic waves AKA light can be described as being composed of countless tiny and individual packets of energy called photons Plank and Einstein's discoveries were clues that led to the full development of quantum mechanics in the mid 192 which was followed quickly by our full quantum theory of electromagnetism Quantum electrodynamics but even before electromagnetism was properly quantized Neils B and Leon Rosenfeld came up with a strong argument that this Force must be fundamentally Quantum I'm going to go through this thought experiment because maybe if it works for electromagnetism we can also use it to argue for gravity being Quantum let's start with a simple particle in motion at any point in time particle has a position and a momentum if this is a Quantum particle then it's impossible to simultaneously measure both of these properties with perfect Precision if we try to measure the position very precisely then the uncertainty in the momentum increases if we try to measure the momentum as perfectly as possible then the position becomes undefined and it's not just that we lost the certainty in one property by bumping it or whatever when we tried to or the other the Heisenberg uncertainty principle is a fundamental limit to the knowability of the quantum world and we talk about this fundamental in this video this trade-off between the knowledge we can possess about a Quantum system applies to many pairs of properties position versus momentum energy versus time one AIS of polarization or spin versus a perpendicular axis and many more so if the electromagnetic field is quantum in nature then concy principle should apply to our attempts to measure this field okay back to our particle in fact let's have two particles and give them both a negative electric charge we start them off moving towards each other we know that light charges repel so these particles will interact by the electromagnetic field when they get close and be deflected back we know that there's a Quantum restriction on how precisely we can measure the position and momentum of these particles but we also know that the particles motions are entirely defined by their interactions via the electromagnetic field so bour and rosenell argued that the same restrictions on measurement of particle motion have to apply to the field that governs that motion after all measurements of the electromagnetic field can only happen by observing its interactions if those interactions are subject to fundamental Quantum uncertainty then the field must be also and if that's true then it's reasonable to think that the electromagnetic field is truly a Quantum entity as indeed it turns out to be okay so if this argument applies to electromagnetism why can't it also apply to the gravitational field if we can only measure the gravitational field through the interaction of massive particles and those particles are subject to Quantum uncertainty then surely our measurement of gravity is subject to the same here it's important to pay attention to the details of the bore Rosenfeld argument they realized that in order to confidently state that the Heisenberg uncertainty principle applies to electromagnetism we need to consider only a pristine electromagnetic interaction between the two particles the interaction needs to be mediated by the most Quantum possible influence of the EM field a socalled Quantum of action that's the part of the EM field that we're trying to measure if there are any extra bits of electromagnetic field then they'll add to our uncertainty in measuring the field responsible for the interaction but electromagnetism is pretty messy for example we know that moving charges create magnetic fields those extraneous components of the EM field prevent us from concluding that our knowledge of the EM field is limited to the same degree as our knowledge of particle motion only with a pristine interaction can we show that electromagnetism also obeys the Heisenberg uncertainty principle but war and Rosenfeld came up with a clever trick to clean up the EM field in their thought experiment instead of individual particles moving towards each other they imagined in of particles one positive and one negative that cancels out any electromagnetic field emerging from the particle motion allowing us to describe the most fundamentally Quantum interaction via the EM field and it allows us to show that the EM field really is subject to True Quantum uncertainty but this is where we get stuck with gravity particles with electric charge are subject to the electromagnetic force the analogous charge for gravity is mass we can imagine a pair of massive particles moving towards each other and interacting via a Quantum of gravity our ability to measure that gravitational interaction should be limited by our ability to measure the motion of those particles but in order to show that the limit is truly the Heisenberg limit we need to rule out complex interactions for the gravitational field just as we did for the EM field so why not apply the same trick as B and Rosenfeld just add an opposite gravitational charge to each particle but that means adding negative masses and as far as we know negative Mass doesn't exist and it's not just that we haven't discovered it yet there are very very good reasons to believe that negative mass is fundamentally impossible its existence would lead to Major paradoxes so it seems that the very nature of gravity forbids us from using B and rosenfeld's argument that might on the surface sound like a bit of bad luck but follow me through the next thought experiment and it starts to feel like the universe is really conspiring to prevent us from finding evidence of quantum gravity perhaps the most direct evidence of quantum gravity would be the observation of a graviton or at least of its effect after all the observation of the influence of individual photons in the photoelectric effect was a pretty clear demonstration of the quantization of electromagnetism so on to the next thought experiment from Freeman Dyson he figured out what it would take to detect an individual graviton with a gravitational wave detector gravitational waves are rieles in the fabric of SpaceTime caused by Massive objects undergoing certain types of motion when a gravitational wave passes by it causes distances to change as space is ultimately stretched and compacted by a very tiny amount at least that's how gravitational waves look in general relativity Einstein's very unquity In classical electromagnetism electromagnetic waves are caused by accelerated charges but we now know that those waves are really made up of individual photons so if gravity is quantum then a gravitational wave should be made up of many gravitons in 2015 we detected our first gravitational waves caused by the merger of black holes with the laser interferometer gravitational wave Observatory the two Lio facilities sense the extremely tiny relative changes in the lengths between their 4 km Arms by bouncing lasers many times along each arm and watching for subtle changes in how those beams recombined so what would it take to measure a single graviton probably is quite a bit more difficult than measuring a single Photon but there must be some Far Future gravitational W of detector that could do it that's what Dyson wanted to find out so we'll start by estimating how many gravitons are in a typical gravitational wave like the ones detected by ligo if we want to do that for an electromagnetic wave we take the total energy of the wave and divide it by the energy of a single Photon which is just the plank constant time its frequency that tells us that a ping 5 m 630 NM red laser pointer blasts out 10 ^ of 16 photons per second the typical gravitational wave detectable by ligo has an energy density of approximately 10 ^ of -1 Jew per cubic M with an angular frequency of 1 khz that's 1,000 Herz according to Dyson the energy density of a single graviton in this frequency is at most 3x 10^ -48 Jew per Cub M and that gives us around 3x 10^ 37 gravitons per cubic meter in these waves so what would it take to detect just one of these well if the rotational wave that I described has around 10 37 gravitons and that's just at the edge of ligo sensitivity then we need to improve that sensitivity by a factor of 10 ^ of 37 that sounds challenging but surely not impossible even if it would take some science fiction level device to do it to see just how science fictiony let's simplify our gravitational wave detector we're going to detect incoming waves by measuring the change in the distance between two masses we'll assume the masses are free floating in space but the argument also works for masses that are fixed to a device Dyson argues that in order to say that we detected a signal graviton we need to be able to measure a change in distance on the order of the plank length and that this requirement is actually independent of the frequency of the graviton you might recall from previous videos that the plank length is essentially the smallest distance that we can consider before the meaning of distance and space breaks down it's a pretty small distance so what sort of device could measure a change on that scale for our simplified gravitational wave detector the question becomes what combination of masses and distances between them would we need as our lonely graviton passes our detector the masses move in and out by a tiny amount in order to be sensitive to that tiny change in distance we need to measure the positions of each of those masses with a Precision equal to half of that change but the Precision with which we can measure those Mass positions is limited by the Heisenberg uncertainty principle which by now you're very familiar with while a mass is being moved by a graviton its speed changes it changes roughly by the distance it travels divided by the time it takes for a single graviton to pass by that time is just the separation of the masses divided by the speed of light that gives us the variation in the speed during our measurement multiply that variation by the mass itself and we get the change in momentum due to the passage of the graviton so now we have an estimate for the uncertainty in the position needed to detect the graviton and that's just around half the plank length as well as the uncertainty in the momentum generated by the motion of the masses caused by the graviton if we plug these into the Heisenberg uncertainty principle we get a relationship between the masses and their separation in order to be able to detect a single grow graviton it's a simple enough equation the mass separation has to be less than or equal to the gravitational constant time is the mass of the masses divided by the speed of light squared but that expression is familiar to all physicists and he's very bad in this context it's the expression for the SWA Shield radius any Mass compacted to a size smaller than this radius becomes permanently trapped by its own gravitational field it becomes a black coal this is really strange we found that a gravitational wave detector capable of detecting a single graviton inevitably forms a black hole that means even if it detects the graviton it swallows any information about that measurement and so prohibits us from confirming the graviton really any attempt to measure distances small than the plank length threatens black holes as we've discussed before so it seems that nature isn't just conspiring to th our theoretical Arguments for quantum gravity but also to stop us from building the detector we need to test these theories none of this means that gravity isn't really Quantum or that the existence of the graviton can't be ever proved there are several proposals for how to do this like searching for the extremely rare interactions with particles of matter and gravitons but these events are going to be so rare that it may be practically impossible to see enough of them to confidently confirm their nature unless we could come up with a clean source of gravitons immensely more powerful than is currently known like a laser of gravitational waves but that's really in the realm of extreme far future technology there are also indirect measures of quantum gravity in the same vein as the B Rosenfeld argument for electromagnetism for example if we could cause two particles to become Quantum entangled through a gravitational interaction then that interaction itself would have to be Quantum this is more promising than direct graviton detection but has not yet been achieved and who knows perhaps nature will continue to conspire to make new tests of quantum gravity impossible and maybe that's because gravity isn't Quantum in the same way that the other forces are not that this is going to stop us from continuing down the rabbit holes of speculative theories in the hopes that one day we'll find a way to test and maybe verify the quantum nature of SpaceTime is that time of year again where we ask you to take the annual PBS digital Studios audience survey the space time audience has always been amazing at filling out the survey and we'd love to continue that Trend by doing so you can help us pick out what new shows should be made and what types of topics you'd like to see new shows cover it'll only take a few minutes but it's extremely valuable to us the whole network really Dives deep into the data and it helps give us tremendous insights into what you're thinking there is a link in the description thanks in advance

Channel: PBS Space Time

Views: 1,569,063

Keywords: general relativity, quantum gravity, quantum mechanics, black holes, quantum gravity explained, gravity explained, space time, what is gravity, quantum gravity theory, albert einstein, dark matter, general relativity vs quantum mechanics, outer space, dark energy, the universe, general relativity explained, pbs space time, matt odowd

Id: 8aR77s9RLck

Length: 18min 30sec (1110 seconds)

Published: Thu Nov 09 2023