Can gravitational waves INTERFERE with each other?

a few weeks ago one of my subscribers asked me a science question that has just been itching on my brain ever since Larry bber asked can gravitational waves interfere with each other do they cancel each other out when they Collide and my sort of like gut instinct answer to this question was like yeah sure they can they're waves right but do we actually know that because sure we call them waves but do they actually behave basically the same as a light wave or a sound wave or like water waves so I dived into the astrophysics literature on this and realize that not only is this a wonderful question but it also has some fascinating implications for our elusive theory of quantum gravity so in this video we're going to chat first about what gravitational waves are second what do we actually mean by interference three how we know that in theory yes they should interfere with each other for what this means for any theory of quantum gravity we might be able to come up with and five how we might be able to test this with observations of the universe in the future but before we get to all of that I first want to take a moment just to chat about our mental health because I know that a lot of people can feel overwhelmed when thinking about the universe and the vastness of space it can really compound with feelings of hopelessness and helplessness and isolation and if those feelings have ever gotten to the point where they start to interfere with your happiness or with you achieving your goals that's where therapy can help which brings me to better help this is a paid partnership with them better help makes connecting with a therapist so easy and convenient especially for those of us with busy lives everything is online and your therapy is done remotely so you can fit it around your lifestyle in any format that's comfortable for you whether by phone video call or even text by filling out a few questions better help can connect you with the credential therapist in under 48 hours in most cases just the simple Act of like speaking to a therapist like an expert in mental health who's just seen all of this before was so helpful to me like therapy gave me the tools to be able to manage and deal with any of my often like irrational feelings and thoughts that might pop up so if any of this is sounding familiar to you try giving better help a go and just see if therapy helps you look there's a link in the video description to betterhelp.com docky which if you click you know not only supports this channel but also gets you 10% off your first month's therapy with better help so thanks again to better help for partnering with me on this video and now let's chat first about what actually are gravitational waves well they're ripples through through space itself caused by huge disturbances to space our current best theory of gravity is Einstein's theory of general relativity which says that gravity is caused by heavy objects bending space the heavier the object the more that space is curved and anything traveling on that curved space has its path changed this is how like the mathematical equations in general relativity describe what gravity is and it means that we can picture say you know like planets in orbit around the sun as like pingpong balls traveling around the curved surface of a trampoline that has like a football placed in the middle now sticking with that analogy imagine you've now got two footballs on the trampoline and they're also circling each other so the areas of the trampoline will go through cycles of being pushed down by the football as it passes and then popping back up again so hold that picture in your mind and now imagine this for two very dense objects like neutron stars or black holes they're so dense that they cause such an extreme curvature of space that as they orbit around each other Cycles from one extreme to the other which sends out ripples into the universe around them and when they get closer and closer and finally merge there's an extreme Ripple that gets sent out it's these ripples through space that we call gravitational waves and we've actually been able to detect them here on Earth with detectors like ligo and Virgo which measure the squashing and stretching of space as a gravitation wave passes through so then what does it mean for waves to interfere with each other well interference is something that happens to waves all around us whether that's water waves on the surface of the sea or whether it's sound waves or light waves and waves can either add together that's something we call constructive interference or cancel each other out which is called deconstructive interference you know drop one Pebble into a pool or a pond and you'll see the water waves on the surface spread out undisturbed but if you drop two pebbles at the same time and the ripples will collide and add together in places to make a ripple twice as high or subtract and just get rid of the wave entirely if you have noise cancelling headphones they're using deconstructive interference so the headphone is recording the sound wve that's coming from the outside world inverting that sound wave and then adding them together so the two completely cancel out and you don't hear anything the same is true for light waves they can also interfere with each other which is actually the method we use to detect gravitational waves in the first place so in the ligo detector for example a laser is split and fired down two sides of this L-shaped detector and then bounced back off mirrors if the two distances are the same when they're recombined they cancel each other out but if the two distances are different because because of gravitational wave has passed through they don't perfectly cancel each other out and we detect some light from the lasers so if light waves and sound waves and water waves all do this then surely gravitational waves do as well but even though they're all called waves all of those things are very different physically so sound waves and water waves are what we call mechanical waves which means they need something to travel through like air or water light waves are are electromagnetic but light is also a particle as well photons of light that carry the information on a Quantum level the really small so gravitational waves are again something completely physically different from light and sound and water waves so we can't take it for granted that that is also how they will behave and they will interfere with each other so that brings us to how do we actually know in theory that they should interfere with each other well the only data or observations that we have to go on are the detections of gravitational waves that we've made with the ligo and Virgo observatories while from the merger of two black holes we only detect gravitational waves because we were never going to get any light from that the merger of two neutron stars tells us much more neutron stars are like the baby siblings of black holes that are not quite dense enough yet to actually trap light away so when they merge together we detect both gravitational waves and light from them as well the first one we ever detected was GW 170817 and it was a big deal when the detection was announced cuz we learned so much about gravitational waves and their behavior because a flash of gamma ray light was detected just 2 seconds after the gravitational waves that came from the same object 100 million light light years away which told us two very important things first of all that gravitational waves travel at the speed of light and they're affected by the expansion of the universe and the expansion of space itself in the same way that light is but we also have to remember that the path that the light took from the neutron star merger is not a straight path it will have been affected by everything in its path between us and the merger every bit of matter that it encountered will have curved space changing its path ever so slightly and causing a time delay compared to a straight path since the gravitational waves were detected at the same time as the light it means that they took the same path from the neutron star merger to us they were affected by the same curvature of space that the light was to put it another way gravity interacted with the gravitational waves gravity interacted with gravity curvature interacted with curvature gravitational waves can interfere with each other which brings me to the huge implications this has for any theory of quantum gravity you know two of the major pillars of modern physics are Einstein's theory of general relativity which describes the behavior of the very large and the very heavy in terms of gravity and then quantum mechanics which describes the behavior of the very tiny the fundamental particles that are the building blocks of our universe the problem is we've just never been able to combine them we can't describe gravity in terms of the behavior of these tiny Quantum particles this is quite honestly the dream for theoretical physicists is cracking how to do this but it's very difficult without any clues of where to even start but this idea that gravitational waves interfere with each other does give us a clue because like how light has photons the quantum particle that transfers the information a quantum theory of gravity would have to have a similar thing we often call this Quantum particle the graviton and so if gravitational waves interfere with each other that means gravity interacts with gravity which means in a quantum theory of gravity the graviton would interact with another graviton if two came together they could Collide they could scatter off each other that gives us a huge amount of information to go off when we're thinking about a quantum theory of gravity it helps us to rule out a lot of possibilities and narrow down our options now you could argue that all of this rests on very indirect evidence of gravitational waves interfering with each other because all we've done is inferred that from our two simultaneous detections of gamma ray light in gravitational waves so that brings me to how we might be able to test for this with observations in the future well there is somewhere that we expect this gravitational wave interference to happen and it's in the original merger of two black holes each black hole is a source of gravitational waves as it moves so those waves should meet in the middle and interfere with each other like when you drop two pebbles in a pond like that we talked about earlier but if gravity interacts with gravity then when these gravitational waves come together you wouldn't expect them to just add together in normal constructive interference like what we see with water waves for example on the surface of a pond what you would get is this interaction and this scattering some exchange of energy this kind of interference with the scattering is referred to as nonlinear Effects by people who model how this should happen these nonlinear effects are first of all incredibly complex to calculate and second they are like a secondary effect so they don't have a huge impact on the overall shape of the gravitational wave you get when two black holes merge add to the fact that our current gravitational wave detectors ligo and Virgo wouldn't be sensitive enough to tell the difference between a gravitational wave with or without these nonlinear effects people have just tended to use the models that just stick with the linear effects where the gravitational waves like interfere as you would necessarily expect them to if there was none of these sort of scattering and interactions going on but last year a paper published by mittman and collaborators worked through the mass and used a supercomputer to work out what the gravitational wave signals from two merging black holes looks like when you take into account the fact that they interfere with these nonlinear effects in the scattering all we have to do now is test those models against the detections of gravitational waves that we make ligo was offline for a while while getting a sensitivity upgrade but it is back online now as of mid 2023 so we'll have to see if that sensitivity upgrade was enough to be able to detect this or we might have to wait for future gravitational wave detectors like for example the Einstein telescope or Cosmic Explorer which are both still in the development stages but if we were able to detect these gravitational waves with enough sensitivity to be able to study these nonlinear effects that come from the fact the gravitons might be Interac with each other and scattering off them we have so much more information for our quantum theories of gravity which like I said is just the holy grail for so many theoretical physicists which is why they're hoping that we will be able to detect this and what I love is just that this this simple question that Larry asked has brought us here to one of the most fundamental problems in modern physics figuring out a quantum theory of gravity and we've been able to detect them with gravitational wav detected here on Earth like ligo and Virgo which time I say ligo I just hear that you know the meme that's like Lego my Lego ego legless Legolas I always want like ligo to be like the next step of that Meme like there's Legolas there's Lego there's EG ligo should be in there as well path to get to us they were a fact affected they were affected by the same curvature of course if you know we're really being honest with oursel it should be that mavity interacts with mavity which means that in a quantum theory of mavity that mavaton would interact with mavaton

Channel: Dr. Becky

Views: 168,950

Keywords: dr becky, becky smethurst, rebecca, dr becki, dr beccy, astronomy, astrophysics, cosmology, dark matter, black holes, modified gravity, einstein, gravity, general relativity, general relaitivty, GAIA, ESA, NASA, LIGO, interference, mechanical waves

Id: zFGnOqrq8j4

Length: 14min 37sec (877 seconds)

Published: Thu Mar 14 2024