Was Einstein "wrong"? | Testing new theories of gravity

one of the things that was physicists find ourselves saying all the time is Einstein was right his theory of general relativity is the best description we have for how gravity works everything from launching satellites into space to how planets orbit around the Sun and how the Sun orbits on the Milky Way and also how stuff moves around black holes every test we've put it through it's passed with flying colors so does that mean that we can all just sort of pat ourselves on the back and brush our hands of it and all go home because jobs done no we are still testing gravity and crucially we're coming up with different theories of gravity which we could also test our understanding of in the hope of better understanding how our universe works so let's kick things off with a little bit of a reminder of some science history Isaac Newton is credited with the first explanation of what gravity is that every single particle is attracted to every other particle in the universe by a force and that force depends on how massive those particles are and how far apart they are Newton wrote that down as a few equations and those equations allowed us to predict where cannonballs would land and how high we could throw a ball and even the orbits of the planets now when those predictions were accurate it became clear that this was the way that gravity was working at least in our local backyard of the universe and in the physics labs on earth ie the places that we were able to test it in at the time however the planet Mercury through a bit of a spanner in the works Newton's equations didn't give accurate predictions for Mercury's orbit so that suggested that his theory and his equations were flawed somehow so a long came in Stein in 1915 with his theory of general relativity and I've taught before on this channel about how Mercury's orbit was actually one of the very first tests of general activity linked the video up here so you can go check out if you want now the way that Einstein's thought about gravity was very different to how Newton thought about it instead of just sort of this force acting at a distance he thought of gravity as mass curving space-time and here it's sort of representative in 2d but really you have to try and picture it in 3d that's just incredibly incredibly difficult to do now Einstein's equations that describe how gravity works look very different to Newton's and that's because Einstein worked with something called tensors that actually appreciate the full four dimensions of space-time so the three dimensions of space so forward back left right up down and then also time what the tensors do is mathematically describe how space is curved by an object with mass you can take Einstein's equation and the full tensor form that it starts in work it all the way through for Mercury's orbit for example and you'll get out something that actually looks very similar to Newton's equations it just has this extra term and that extra term is divided by C squared the speed of light squared and the speed of light is a huge number and if you take a huge number and times it by a huge number you can again a bigger number and so if you divide a term by such a big number you essentially get incredibly close to zero so in most cases that term is just not going to matter at all for example here on earth throwing a ball up into the air that term vanishes to zero and except when either the mass is very large or the radius is small so for example in Mercury's case mercury is very close to the Sun which is very massive so it makes a difference it also makes a difference around massive objects like black holes and we were able to test that properly for the first time when we had that first ever image of a black hole back in 2019 thanks to the event horizon telescope everywhere and every way we've been able to test general relativity its predictions Ben right so everything from planet orbits to pulsars to black holes to gravitational waves it's all evidence in favor of Einstein's theory of general relativity those equations that he came up with a thought to govern everything from the expansion of the universe to the behavior of black holes to how gravitational waves move through space to the formation of every single structure in the universe from the smallest planet to the biggest galaxy to the biggest clusters of galaxies but what about the areas that we've not been able to test general relativity for example around bigger and more energetic objects the entire universe perhaps or on the opposite scale the tiniest objects in the universe such as particles the quantum level do you guys just put the word quantum in front of everything just general relativity hold there well we assume it does at the minute because of all the scales we've been able to test it has held but what we're doing is extrapolating there we're saying because it works on the smaller scales that we've been able to test it must work on even smaller and because it works on the biggest and the most energetic scales we've been able to test it must work on even bigger ones than those but just like how Newton's equations work perfectly on everyday scales here on earth but then don't four other scales such as large masses and small radii then you might also start to think well what if that's also applicable for general relativity what if there's also some scales where you need that extra term maybe and you need to also general relativity in some way in order to make accurate predictions now you might be thinking this is very interesting at all Becky but like this is just speculation so why should you care well understanding gravity has huge implications for how we understand how the universe was formed how it's evolving and what it's made of as well it's a branch of astrophysics that we call cosmology with general relativity proven on all the scales that we can test it on it's actually the problems in cosmology that I've talked about before on this channel about the current crisis in cosmology that's going on that is actually driving forward the need for a slightly altered theory of gravity if general relativity is correct then it predicts that 96% of the universe is dark and doesn't interact with light it predicts that dark energy makes up 70% of the universe and we don't know what that is it's this mysterious force or energy that's acting against gravity to cause the expansion of the universe to speed up then the other chunk of the dark universe is dark matter there's so much evidence for dark matter it's just all gravitational in nature ie we don't see it with our eyes necessarily but we see its influence due to gravity so all of those things dark energy dark matter really do raise the question of whether general activity that has served us so well so far and all the scales that we've been able to test it on is maybe not the full picture it's not the Einstein was wrong just like Newton wasn't wrong it's just that in the scales that we're looking at it just maybe needs modifying slightly so there are currently astrophysicists in institutions across the world that are trying to come up with new theories of gravity and crucially test whether they are right another thing like this is a new thing either like physicists have been trying to you know make general activity better with some edits you know ever since Einstein first proposed it in 1915 you know everyone from Eddington to Dirac to Sakharov so like I said the majority of these new theories are modifications or extensions to Einstein's theory of general relativity because 80 new theory of gravity you come up with still has to work as well as general activity has on all the scales that we've tested it on but sometimes people have been known to basically just crumple up everything we've currently gotten through out the window and start from scratch which I'm sure that sounds overwhelming as does trying to adapt general activity as well so how do people actually go about doing this well here's three different ways number one is starting from an observation and working backwards from there so for example theories like munde modified Newtonian dynamics have started with this idea that Dark Matter might not necessarily need to exist to explain something that we've observed so one of the strongest pieces of evidence for dark matter is the idea of galaxy rotation curves so the speed that stars are moving around the center of a galaxy when we look at a galaxy all the light is concentrated in the center pretty much that's where the brightest spot is that's the same as what we see in the solar system right 99% of all the stuff in the solar system is basically the sun's mass so we look at what speed the planets are going around the Sun we see that speed drop-off as we get further out so we'd expect to see the same thing in galaxies but we don't speed actually increases as you go out to the outskirts and that suggests that actually instead of the mass being concentrated in the center there's actually a load of mass on the outskirts but that's not where we see stars so it's assumed there's lots of dark matter out on the outskirts of a galaxy but that's all based on the assumption that general relativity is right what if we had a different theory of gravity would that be able to explain these galaxies rotation curves so that's what Mon tries to do actually starts with Newton's equations not Einsteins those out einstein's completely and it actually says well what if we just added this extra little factor into Newton's famous equation F equals MA this factor though only matters for very Oh acceleration the kind of numbers we only ever see for stars orbiting around galaxies then the speed that Monde predicts that stars should be going round the center of galaxies is exactly what we observe but crucially it also predicts what we observe for planets going around the Sun as well so it kind of seems to solve the problem of dark matter the problem however is that it doesn't get rid of the need for dark matter entirely you still need it to explain the masses that we see in for example galaxy clusters so it started out as a theory to explain away dark matter but it can't get rid of it entirely it could only sort of reduce its majority percentage in the universe but the biggest blow to mond came in 2017 because what modern predicts is that gravitational waves should travel a different speed than the speed of light but in 2017 we detected from the exact same object at the exact same time a burst of light and gravitational wave suggesting that they travel at the same speed and that was a huge blow for that theory number two adding an extra field to general activity I you have a gravitational field that gives you the force of gravity but what about if you had an extra field the coupled to the gravitational field to give you a fifth force as people know it because you've got four fundamental forces in the universe and this could be one extra one that we haven't thought of before for example scalar vector tensor gravity which ridiculous I'll admit they had an extra field into general relativity that at very very small distances produces a force that acts against gravity it sort of like a repulsive force to weaken it but then at very very large distances it couples to gravity to actually make gravity stronger so sort of in the middle in ground from everywhere from you know the solar system to black holes to the Milky Way galaxy clusters it reproduces everything that general relativity has been able to do but it can also explain things on these big cosmological scales and for example the galaxy rotation curves without the need for dark matter but the problem with this theory is once again with the speed that it predicts gravitational waves should travel at if you have this extra field that's coupled to gravity it will slow down the speed that gravitational waves travel at and as I said before in 2017 we found that they traveled at the speed of light ie the fastest speed that is in the universe so that result back in 2017 really sent a lot of people that are working on all these different theories not just this scalar vector tensor gravity theory but all the ones that try and add an extra field to general relativity back to the physicists drawing board for blackboard I guess number three adding extra dimensions now people might have come across this idea before in the context of string theory string theory was a bit of a buzzword in physics for the past decade or two it's actually a particle physics theory one that attempts to sort of explain everything by getting rid of the idea that everything around as matter is made of particles and saying actually it's made of one-dimensional vibrating strings yeah the vibrational properties of the string ie how the strings vibrate is supposed to give us this of properties of particles that were familiar with like mass and charge but crucially what string theory also gives you is a theory of quantum gravity ie how gravity behaves on the very small scales it says that gravity is governed by the exchange of a particle actually a string called the graviton just like how electromagnetism is governed by the exchange of a particle a photon a photon of light particle of light to work though string theory needs ten dimensions not the four of space and time that we're familiar with now could indeed be that there are ten dimensions but on our scales we're just not able to see them but adding those extra dimensions has these huge ripple effects for how gravity behaves on small and large scales now there are many many problems with string theory which is why you don't really hear about it that much anymore because it's falling out of favour with the majority of physicists in the context of this video though about leading a new theory of gravity and explaining crisis in cosmology the issue with string theory is that it doesn't give you any explanation for dark energy can't reduce any sort of repulsive force that could explain the accelerated expansion of the universe and okay general relativity doesn't give you that but generality does a much better job of explaining everything else then string theory does so our best theory for how the universe works is still one where general activity is the best description we have of gravity because it fits our observations the best the consequence of that is the prediction of the dark side of the universe so if you accept Einstein's theory of general relativity then you have to accept the dark matter and dark energy are things but it's by no means are done in dust in topic like science never is but that's the beauty of it that our understanding can change and evolve over time as new evidence comes to light and then new scientific theories that are rooted in that new evidence and also the mathematical understanding and then born I for one am incredibly excited to see where the next centuries worth of research will take us first of all thank you so much to you for watching this video if you liked it subscribe for more content like this and also hit that like button so more people who like watching space videos will see it second of all a big thank you to my old office mate dr. Ollie Tattersall who is an expert in modified gravity theories and he pointed me in there right direction to a couple of really good review papers on this topic which I have linked in the description below if you want to do like a really big deep dive into this and then last but not least a huge thank you to this week's sponsor which is brilliant brilliant is a problem-solving website that gets you to learn by doing now those of you who have been following this channel for a while will know that if people ask me you know I want to be an astrophysicist when I get older what's the best thing that I can do to prepare for that I won't tell them it's to learn to code my language of choice is Python and you can find courses on Python on brilliant as well as on sort of the fundamentals of computer science and algorithms as well brilliant teaches you to think like a scientist breaking down problems into easy-to-understand chunks but crucially having interactive sessions that give them like a fun application so you know how to apply the knowledge that you're learning if you like the sound of that then go to brilliant org forward slash dr. Becky that's the rpe CK why and sign up for free also just for you luck the first 200 people that go to that link will get 20% off an annual premium subscription sponsorships like this from brilliant allow me to keep tracking science with you guys and for you guys to afford any skills you need so head over that you're brilliant and say thanks for me made of it's a branch of astrophysics that we call cosmology cosmology I feel like it did cosmology didn't change generality but see ever since Einstein first proposed it in 1915 including people like edited and edited editor editing Becky yeah famous physicist Arthur editing it's by no means a done and often dusted are done and dusted topic though science never is and that's the beauty of of the villa

Channel: Dr. Becky

Views: 234,114

Rating: 4.9340692 out of 5

Keywords: astronomy, space, physics, astrophysics, university, college, collegelife, universitylife, academiclife, phdlife, scientist, astrophysicist, cosmology, drbecky, Becky Smethurst, Rebecca Smethurst, gravity, general relativity, einstein, newton, modified gravity, scientific process, testing theories, dark matter, dark energy, mercury, planets, orbits, solar system, galaxies, galaxy clusters, stars, early universe, bullet cluster, womeninSTEM, womeninscience, female scientist, age of the universe

Id: dtfEzDAlL5k

Length: 19min 54sec (1194 seconds)

Published: Thu Jun 11 2020