12.6 Experimental Evidence 

Key Takeaways

welcome back to the last section of a 20 special relativity so as we discussed the special relativity itself um also general relativity is a theory which requires experimental evidence to be confirmed and there's plenty of experimental evidence for general relativity we talked about a few examples already but let's go through this one by one also a little bit within historic context so one of the first experimental pieces of evidence pointed out by einstein was the procession of mercury and also of other planets this was always this this this problem that the procession of mercury deviates from newton's prediction was well known and first recognized already in 1859 and it turned out that attempts to correct this are failed uh you can think about maybe there's other objects in the solar system which modify the trajectory of mercury around the sun but nothing really added up correctly and then when einstein calculated the effect of the possession he found that it's in good agreement with the observation this was already very strong evidence for general relativity effect and then there is gravitational lensing and here this was first measured by dyson and 1919 of light passing the sun in a total eclipse their observation was in brazil but also at the west coast of africa this wasn't the first attempt to measure this it was an eclipse a total eclipse in argentina in 1912 but unfortunately this expedition didn't lead to a result because it rained out there was it eclipsed shortly after in 1914 but that happened during the second world war and there's long stories and and accounts of how this failed but basically one of the expeditions wanted to travel to crimea in in russia and because russia was in war with germany um you know material was confiscated and people were imprisoned so this was cancelled if you want due to the second the first world war but then in 1919 this leads to the observation this the data was not as clear i think there was a little bit more hopes than science in the interpretation so there was um i was not a strong evident a strong significance of the results but the evidence nevertheless was there and as i was explaining earlier that led to you know the fame of the triumph of einstein where really uh his fame resulted out of the reporting of those events there's more uh experimental evidence live light travel time but round or close to masses object is modified we talked about gravitational time dilation which can be measured or has been measured um other tests of the equivalent principle but also the observation of gravitational waves gravitational waves where predicted by einstein by the theory of general relativity and only very recently we were able to observe those and then in addition there is plenty cosmological tests which require a precise understanding of general relativity in order to get to agreement between the observations and and the theoretical predictions but let's talk about let's talk about gravitational waves so those were predicted as i was say was saying but they're very very difficult to measure first indirect measurement was performed by hulse and taylor they were able to study a binary neutron star system and because the orbital the orbits of those two decayed required loss of energy and that loss of energy needs to happen somehow and it was theorized or predicted by general relativity that that loss of energy is due to the fact that gravitational waves are emitted and they received for their findings in nobel prize in physics in 1993. so how are gravitational waves generated you can ask you know i have a spinning sphere like our sun would that generate a gravitational wave and the answer is no it's a symmetrical situation there's no change um of the of the meta distribution and therefore space-time is not modified but if you have a sphere with a little bump that would create gravitational waves if you have a mass which is moving by maybe you know you know two passing galaxies that would not directly create gravitational waves but if you have those galaxies rotating or through stars rotating or neutron stars rotating or black holes rotating around each other those generate gravitational waves and the closer the objects the higher the masses of the objects the stronger the gravitational waves are so how can you measure gravitational waves very similar to the michael snowley experiment what you want to do is measure differences in arms of your of your parameter and you do this with very powerful lasers and with very precise mirrors so it's basically the very same uh experiment as michael's molly just much much bigger so we're talking about multiple miles of arms and very powerful lasers in order to conduct those experiments ligo which is one of those measurements of those devices experiments measures the change in the length of one arm with a precision smaller than the diameter of a proton so that's just really it's mind-blowing the level of precision the level of understanding needed in order to measure gravitational waves but nevertheless they succeeded so here you see two experiments like was actually two experiments two of those devices in the united states and there's other experiments similar uh worldwide now you see also highlighted here carltec and mit those are the leading communities of the leading universities in this endeavor and then the first observation of gravitational waves happened in september 14th 2015 and this first observation was rather spectacular because it was not just any observation but it was the observation of two collapsing black holes so we have two black holes they get close to each other then they circle each other and create a new heavier black hole so the collisions are of those two black holes this mass is around 30 times the mass of the sun it actually took place 1.3 billion years ago so the gravitational wave was traveling towards us for 1.3 billion years the energy of about three times the mass of the sun was emitted as gravitational waves in fractions of seconds so the huge amount of energy released in form of gravitational waves the collision happens with both black holes moving with half the speed of light so this is just a catastrophic kind of event in our universe and researchers or faculty at mit and caltech received the nobel prize in physics in 2017 for this discovery only two years after the discovery actually happened and very deserved very deserved let me close this lecture by just reminding you of a quote of einstein which i use in order to start this very same lecture um it is true that we are living through some difficult times some turbulent times but if you think about the bigger picture where i think we're making a lot of progress scientifically but also as humanity and i like this quote from albert einstein a lot it is not the result of scientific research that ennobles humans and enriches their nature but it's a struggle to understand by performing created and creative and open-minded intellectual work i think if there's one thing i want you to take away from this lecture it is this i want you to be encouraged to be creative to be open-minded to question and to perform high-level intellectual work thank you you