Dark Matter's Not Enough Q&A

hi I thought was very interesting nature and could you just tell me what the difference is if there is any difference between dark matter particles and neutrinos yes so neutrinos are another type of particle they're very closely related to the normal particles that the everyday stuff is made out of and neutrinos were in fact invented this is an example of great success actually in just making stuff up because neutrinos were invented by particle physicists they saw they were doing some particular collisions that's what particle visits do they just sort of smash stuff into other stuff and they did some of that and they saw some results they didn't understand and they said okay well these results would make sense if there was a tiny undetectable particle that was just flying away at the speed of light from our experiment taking away a bit of mass a bit of energy and it sounded kind of crazy but then later people actually managed to detect these things directly so it's actually an example of where things really went right just by taking an idea and running with it however it sounds like neutrinos would be great for dark matter right they're almost undetectable almost invisible and not quite unfortunately that just cannot be enough of them to make up this five times more than the stuff that we know and love so Dark Matters got to be something else if I if I knew the answer to that I would be delivering this lecture in Sweden collecting my nobel price nobody knows dark matter seems to be invented so that gravity can work right but why can't the laws of gravity themselves be like slightly changed instead of inventing this horny matter thing yeah it's very fair question why don't we just change the laws of gravity rather than invent new stuff and the answer is that people have tried so people have tried finding ways to change the laws of gravity so that you are not seeing evidence for extra stuff you're just doing your calculations wrong basically that that's that's this idea and it's perfectly fair enough because you know our idea of what gravity does it's largely based on experiments here on earth places like apple orchards and so on and and and if you just take that and you scale it up to the size of the universe it would be perfectly fair enough to go well maybe it just does something slightly different on those really large scales however however much people have tried they haven't been able to explain all the different things that we see where as dark matter pretty much has that's not totally fair to say that dark matter answers all the mysteries about the way stuff behaves in the universe but it does answer an awful lot of them just with that one idea that we've got this dark matter out there people have not been able to do that by changing the equations of gravity so it's totally practical it's just it's just we're going well yeah of course you could change the Lord's gravity it's just nobody's worked out how to do it and I suppose in a sense that's my point that it could be that this whole Dark Matter idea is wrong but we've got to stick with the thing that is actually giving us results at the moment otherwise we're just going to sit around going well could be Lego bricks or I mean it's just it's just too many possibilities if you if you allow yourself that that's why we stick with pretty much with with one idea but it's possible um how much space does Dark Matter take - no matter so like you said that the dark matter can come through the walls like as if the rules went there so like no matter how much stuff that of place um or space that the normal matter take the top dark matter just seems to be on its own bit of stuff even though that stuff is already occupied so how much stuff does it take up in continuing about the normal stuff as a really good way of looking at it so one way you can think about how is it the dark matter can come through walls it's by imagining that every little particle of dark matter is in fact itself incredibly tiny we call we the thing the thing is when we say how big how big is a particle of say I don't know how big is a hydrogen atom say then we have a particular way of answering that because when you get down to those very tiny scales even questions like that are quite hard to answer you have to be very very clear about exactly what you mean so we talked about something's cross-section which basically means if you throw it if you if you throw it it's a it's a a mesh so it's if I create a little mesh to see what I can fit through then I throw a hydrogen atom at that mesh then how closely space does the mesh need to be before the hydrogen atom will bounce off and that distance is what we base this idea of the cross section on now we can do the same for dark matter and we say dark matter has a cross section - so how finely spaced does your mesh need to be before Dark Matter bounces off it and it turns out we don't know yet because we haven't found it but we do know it's going to be incredibly incredibly fine much much finer than for any other particle we know about so you can take all of the other particles and throw them at something really really finely spaced and they just won't fit through dark matter still will so we don't know actually how much space a bit of dark matter takes up I will write up at the top hello um what would dark energy really need what properties would it need what properties would ask her more particle need dark energy well the the thing about dark energy is we don't think it can be a particle because it has this property that it's creating it seems to be creating energy to kind of push the universe apart faster and faster and we don't know of particles that can do that as far as we can tell particles don't just don't behave in that way so it you can't even really think of it like pockets but it's even more abstract and weird than that but we can say what properties it needs and the properties are that if you could take if you've got a box of dark energy right if I give you a box of dark energy that's got a certain amount of energy trapped within it associated with that dark energy just a certain amount a number and if I then sort of so make the box a bit larger so imagine I still have a have a have a handle I can pull out so that the Box gets larger then with normal particles they just get more spread out through the box so if I've got a certain amount of particles in here at certain amount of air and I I make the Box bigger they just spread out and they go through it so I've got the same total amount of stuff in there that I started off with the thing that we need for dark energy is that if I do the same thing if I have it in a box and then I make the Box twice as big then I end up with twice as much dark energy in the box as I started with which is just not something that you can do with particles so that's the property that it needs but it's very it's very abstract that's that answer your question it seems from your presentation the whole idea of dark matter is necessary if the measurement of matter in a galaxy from the light its meeting is accurate how sure we that that measurement is actually accurate and not just strong um yeah it's a great point so I think what you're saying is suppose because because you see a certain amount of light coming from a galaxy you need to convert that into how much stuff is actually there it's like a conversion ratio like swapping dollars for pounds or something you're swapping light for total amount of stuff and the question if I understand correctly is how certain is that conversion ratio and the answer is that it's not very certain actually people struggle with this but we do think we've got it under control and what I didn't have time to say so there are actually examples of galaxies where there are virtually no stars in there there's just a bit of gas and it turns out with gas we are very very good at going from what we can see and turning it into a total amount of gas and in those galaxies that's where we really really start to believe this evidence that there has to be extra stuff because we don't have that uncertainty there so yes it is worry but we think we're on top of it I can't I can't say yes one up there look at not Angie who were you like the first time and our energy it feels like there's like so similar the dark energy and the and the what sorry said similar when you first think about it it seems like that madam that I need yeah I was so similar oh I see yes well they they are very similar in a way no they they're both things that we can't see and that we've just can't we've just invented to to make things sort of seem to add up a bit better but they but we do know that they have to be different things because they do slightly different things to the universe so although they do have a lot in common we do have to be clear that they are differ things they do both say dark yeah so where's the next one there's a Deborah 20 over there hello yeah so if you've got these millions and millions and millions in part of dark particles dug dark matter particles streaming through this room and going out the other side of the earth and if dark matter particles are BAE gravity then have we got dark matter particles in orbit I mean are they with this and because if they obey gravity then you think they would be yeah and the the you're absolutely right I mean so dark matter particles can go into orbit however around say the earth they don't really and the reason is that this is all of course if we're doing the calculations correctly which I hope I've sort of shown you there is some room for doubt but assuming that we're doing this right we don't think they're in orbit around the Earth and the reason is that they are approaching the earth at about a hundred kilometers a second that's very fast indeed it's far far faster than the speed that you need to totally escape from the Earth's gravity so they certainly feel the Earth's gravity as they come through but they've got so much speed to start with but the earth is unable to really capture them however what people do think and and do calculations of is it's something much bigger like the Sun might be able to capture a significant amount of dark matter not not a huge amount compared to the Sun itself but the gravity of the Sun is that bit stronger so it may be that right at the center of the Sun there is a certain buildup of dark matter yeah well so the speed the speed comes from these computer simulations of what happens to dark matter where we where we where we build something that looks very much like our own galaxy and then we can go in and say all right in our virtual universe because we know everything we like God's brilliance we can go in and we can find out exactly what's happening to all the dark matter in there and we do see that there's this large speed at which the Dark Matter tends to be approaching stars so that's what that is based on and as I said that's a sort of general pattern rather than something very specific and that's why we kind of trust it more than we would if we were just trying to say what happens to this particular bit of dark matter over here why have you called it dark matter if you can't see it so you don't know why it's it if you don't know this dark yeah that's a real was it was pretty bad thing what would you call it invisible matter I I think we can all get behind that yeah I unfortunately people just make up names for things and once they've made up a name it sort of sticks quite hard so if I were to go into my into my work tomorrow and I were to write something in a scholarly magazine about invisible matter then all the people around the world would go what what's invisible matter so once somebody's called it dark matter unfortunates too late it's really hard to rename it after that but I agree with you it should be called invisible matter yeah by sorry thank you yeah hello sorry I've got two questions basically the first one is about a part and lot of flat metal talking about because what you said it's a one thing around toward the center of gauzy but well we know that the center of Goddess is a black hole and Frago suppose of a very strong gravitational falls there and follow-up to a question you just lady down there asked you about the water we can capture it I think black hole should be have a strong enough force to capture it but if it did get into the rack that if the dark matter to get into the map black hole whether they conned or did they just pass through it or they disappear or they not effect apply the black hole at all this is the I mean just how how the black hole and Dark Matter fit together and this is the first question second question is that it we see the Babri is if I if I now walk out of this building and point of a star in the sky without any device just using my eyes how do I know is a galaxy or is a star alright I might tell you a second question first because it's a bit easier if you go out tonight look at the sky and you see something it's definitely a star because you will not see galaxies from within london galaxies are really really faint not not because they're actually dim but just because they are really really long way away so you do need a telescope really I mean the Andromeda which is the closest galaxy to us you can just see with the unaided eye but it needs to be a really dark night and you need to be in a really dark place and your eyes need to adjust so yeah anything that you see with your naked eye pretty much is going to be a star now your first question was about black holes at the center of galaxies and it's absolutely right that we do now think we've got some really good evidence whole other topic that there are black holes at the center of galaxies and they are very good at capturing things if you throw something into a black hole it is not going to come out again and so you're absolutely right that if a piece of dark matter fired right into a black hole it will not come out it just disappears in gets eaten up the black hole gets a bit bigger it's the same thing that happens to normal matter we have no reason to think that matters any different the reason it doesn't matter too much for the stuff that I was talking about is that actually black holes are really very tiny compared to galaxies so even if you're talking about stuff going right towards the center of galaxies it needs to be incredibly well aligned needs to be going exactly the right angle towards the center of the galaxy to get to what is actually a very tiny black hole so most dark matter just kind of goes past the edge of the black hole and that's absolutely fine of course so we don't actually think black holes eat all that much dark matter ah we've got another one I'm trying to remember who we over there somewhere so what's the difference between dark matter and antimatter yeah dark so antimatter these is stuff that's very closely related Bhilai neutrinos very closely related to normal matter though it's familiar to us in fact it's it's almost like a mirror image of normal matter that for every normal particle that we are used to having around us there is a theoretical antiparticle which is if you like it's a sort of mirror image of the other of the normal particle and those things are absolutely known to exist there's no question about it you cannot make sense of the stuff that's going on say at the Large Hadron Collider for example unless you include antimatter in the calculations and what's more people have actually created antimatter they've even stored antimatter the only thing we haven't done with antimatter so far is blown up the Vatican but I don't think there are any plans to do that but you know who can tell so antimatter is absolutely definitely real dark matter is much more doubtful I mean personally I do actually think it's probably real but but it's much more doubtful we haven't been able to make it it's a completely different type of particle rather than just a sort of mirror image of one of the normal particles that we know about I drew with a Conniff with econo physics um how do they work out numbers from people throwing money at each other how do they get the graph it's bait so so really where the number comes from is you start off by assuming there is a certain amount of money to start with so if you assume there's a total amount of money and then you imagine people are just throwing it around at random and you apply a formula known as maximum entropy that's the technical name of what's going on here and it basically says what you're going to end up with if people really just slow this stuff around it's the most random thing that you could possibly end up with and believe it or not you can take something that sounds a bit vague like that and turn it into a mathematical equation this has been actually you know proved from axioms in a very mathematically rigorous way and and that's what maximum entropy is all about and it works incredibly well within physics about how energy gets divided between different particles and so on so this was just a sort of leap of faith and going well maybe it tells you how money gets divided between people and kind of does we know that we have matter and antimatter we have dark matter do we have anti dark matter the answer that is it depends for our best so particle physicists do play around with the equations of particle physics and try and work out okay how could we take the equations that we know love that that tell us what particles there are and extend them to try and to try and come up with some something that includes dark matter as well as all the stuff that we actually know exists and so the the best guess they have at how to do that then dark matter is actually its own antiparticle which sounds sounds a bit weird but that is actually possible within particle physics so for that for the best pasta best possible model they've got then there isn't really such a thing as anti dark matter but you know until we actually know what it is who knows yep if there's so many particles are dark matter bottles flying through everywhere why can't we see like the interactions with normal after particles and say oh there's a dark matter particle oh no it's gone but I mean there it was why can't we see that well that is exactly what people are trying to do so people have built experiments to try and do exactly that and they build enormous fats I think we're up to about a ton now a very very pure it turns out xenon is very good at this for reasons which I'm definitely not going to go into now but the the idea with those things is exactly what you say that Dark Matter particle comes through if it hits an atom of xenon you should be able to see that that's happened and that is precisely what people trying to do they build big I think it's a ton a ton of xenon now and they sit there and they watch it to see doesn't does it get stirred and they sit there and they sit there and they sit there and it's very boring because absolutely nothing happens as far as we can tell so far but that doesn't mean the darkman is not there that the trouble is as I said Dark Matter doesn't feel the normal forces that we do so for it to be able to hit something that does involve a force so it needs to need to feel some sort of force so what we know is that it only feels a very tiny amount of force and so very very hard to get it to do that or to go back to the question earlier on about how big is dark matter Adam another way you can think about the same thing it's just imagine that the dark matter particles are very very tiny so it's very easy for them to sit between all the xenon that's in your tank so that's the same physics explained in two different ways so yes so far absolutely nothing it doesn't mean that there's nothing to be found but we're trying ahh okay yeah gan you're going back to that messy thingamabob ii with them there's a theory called supersymmetry isn't there where it's that the particles are more than one TV or something which is significantly more than the existing particles that we know exists in the standard model so surely they'll be bigger so why do they pass through the mesh so they're smaller so well well first of all you're absolutely right that supersymmetry is exactly what the particle physicists think could explain dark matter it does predict all these extra particles and you're right that a lot of those particles are very massive indeed I think a TV might be pushing it a bit but 100 GeV would be fine and that the thing is that in particle physics mass doesn't have to go with size or to put it another way it doesn't mass and how much something feels forces are two separate things they just appear completely separately in the equations so you can kind of tune them separately from each other so you can actually have something that's really got loads of stuff there but yet it's very tiny so this is part of the weird world of quantum mechanics but yeah I hope that answers it I mean it's just that these two things are different in reference to dark matter like you see how it just goes really fast at really four speeds isn't it anywhere where it starts like anywhere where it stops ya know starts doesn't start because you know it has to build up the speed yeah um yeah I I have kind of edged rounds this whole question well where does everything start right I think I've kind of not said anything about that I showed you how galaxies formed I didn't say well where where do all these things start and where does the dark matter start it turns out actually we have a really great theory for what was going on in the very first few moments of the universe it's something called inflation and the best way I can describe it is that it's even more made-up than dark energy and it's totally made-up I mean it's crazy stuff but what's remarkable about that is it makes predictions again for what the early universe would have looked like and we can compare those predictions to a kind of record we've got of the early universe then something called the Cosmic Microwave Background I really don't want to start explaining that but you can go and look it up it's a record of the early universe and it agrees very well with this idea called inflation so that gives us a sort of a way of thinking about what was the early universe like and basically everything was very evenly spread out and so the reason dark matter ends up going very fast when it's in our galaxy is actually our galaxy started out as this enormous thing spread out over very very large regions and then because of gravity everything was pulled together so by the time it gets to what we now call the galaxy it's going very fast and it doesn't really have any way to get rid of all that speed that it's got so it just carries on you know looping around the galaxy and and that's our current theory of the way that galaxies form silence the question yeah I think that was what there was one up the top which I keep getting hello so I guess discussion has been slightly undermined by the other supersymmetry thing but um it's not to do a dark matter that easy he said it was like five times more than than weeks when we see from the light um so that that must be like a vast amount of maths that that we're talking about but then yet each so particle you say is really really small and then and I guess sort of like the neutrinos how can something so small how can it be enough that it adds up to such a vast amount yeah I mean I think it is it is partially the same question that says down here right that that um that you you know you're you're you're taking something yes it's very tiny but that when we say something is tiny we just need to be very clear about what do we mean by it being tiny and what we mean is it's packed into a very small space if you like it's very hard for us to actually catch hold of it because it tends to just go through things and it's just it's just true about the way that particle physics is put together that that doesn't mean that there can't be very much of it in there if the it can be very small but have an awful lot of stuff in there and then you take an awful lot of these tiny things and you can quite quite quickly add those up to be a large amount of mass when you've just got a lot of them okay go ahead I've got two questions the first one is if we made up dark matter almost and dark energy can we compare them to an imaginary number like the square root of -1 and also are there any other laws of physics that Dark Matter does obey yeah well you know what I've never thought about it like that but that's a really nice way of thinking about it you know in maths is there lucky in a way right because you can make up an idea in maths and as long as you are sort of consistent about it as long as you don't do anything that's against the rules then it's fine it becomes part of maths and so yes we are doing something very similar we're making something up the difference though is that in physics we have to go and test against the real world and eventually if we can't show that what we're saying is true is also happening in the real world then we're in a bit of trouble in a way that the mathematicians don't have to worry about so it is a very it's a very nice parallel there and it and the second one I've completely forgotten the second question now all right yes right are there other laws of physics yes I mean though there are really important laws of physics like conservation of energy and other things like conservation of momentum that you might have heard of dark matter certainly obeys those or at least in the way that we've made it up it does the trouble is if it didn't obey things like that then we couldn't even do this argument where we say well the patterns are going to come out about right because that's all based on things I mean remember when I was throwing out the pendulum I was talking about energy if you don't have things like conservation of energy anymore then even at that very basic level you can't make predictions so you run into real problems very fast so we do need it to be most of the laws of physics that we're familiar with so sort of main thing okay where are you oh you're there hello the sort main thing that I'm going to take away from tonight is that dark matter particles are very different from ordinary particles that sort of sort of struck a question in my mind and you know could we incorporate these dark matter particles into the stand model that we know of or are we going to have to scrap that theory entirely and make a new one like is there a dark model of particle physics or something yeah the outside is the particle physicists bless them they're trying very hard there they're her the trouble is almost there's too much freedom if you say all right we want something which has all the normal particles but it also has these weird dark particles as well then there are actually loads of different ways you can do that it's no longer the standard model because by definition when we say the standard model particle physics we just mean that the the model of the particles that we already definitely know about so by definition that's no longer the standard model but it sort of extends the standard model it's adding stuff on to it bolting it on the side and the real problem that they've got is they just have so many different ways of doing that and getting these extra types of particles though that it's too much choice in a way you know that there are literally dozens of different ways you can do it and until we actually find one of the particles and can say you know we've got it in the lab we know it has these properties we just we just won't know so it's the very last question I think yeah I actually I just want to know I was wondering is it possible to add extra dimensions to dark matter properties certainly people have have looked at adding extra dimensions to the universe if you've heard of something called string theory it's all about that it's about saying well you know the three dimensions of space are all very well but let's add a few more and see what happens and it turns out that that's a really interesting mathematical idea that can give you lots of kind of ideas about what might this it comes back to the same point in a sense it comes back to how might you extend what we understand as the standard laws of particle physics at the moment that's what it boils down to doing this it's just a very specific way of going beyond physics that we know about today the trouble with that idea is so far although it's been a lot of fun for a lot of mathematicians they haven't been able to show how the standard model that is the stuff that we know in love today that we can experiment on now they haven't shown exactly how that comes out of the maths of all these extra dimensions and strings and so on let alone used it to make really good predictions for what should the properties of dark matter be so it's a it's an idea that a lot of people are following at the moment we don't know whether it's going to lead us anyway and I think with that I've got to stop so I I know it is sad it is sad if you want please come and find me I'm very happy to talk within reason I'm sure we'll eventually get kicked out of the room but I just want to say thank you very much for listening tonight and thank you for coming

Channel: The Royal Institution

Views: 91,007

Rating: 4.8634 out of 5

Keywords: Science, Ri, Royal Institution, Science Communication, Education, physics, andrew pontzen, Dark Matter, dark energy, space, cosmos, universe, cosmology, chaos

Id: tNyLJ3lNp5U

Length: 34min 5sec (2045 seconds)

Published: Wed Nov 19 2014