Does anti-gravity explain dark matter?

one of the lesser known facts about me is that i'm one of the few world experts on anti-gravity that's because 20 years ago i was convinced that repulsive gravity could explain some of the puzzling observations astrophysicists have made which they normally attribute to dark matter and dark energy in today's video i'll tell you why that didn't work what i learned from that and also why antimatter doesn't fall up newton's law of gravity says that the gravitational force between two masses is the product of the masses divided by the square of the distance between them and then there's a constant that tells you how strong the forces for the electric force between two charges we have coulomb's law that says the force is the product of the charges divided by the square of the distance between them and again there's a constant that tells you how strong the force is these two force laws look pretty much the same but the electric force can be both repulsive and attractive depending on whether you have two negative or two positive charges or a positive and a negative one the gravitational force on the other hand is always attractive because we don't have any negative muscles but why not well we've never seen anything fall upright then again if there was any anti-gravitating matter it would be repelled by our planet so maybe it's not so surprising that we don't see any anti-gravitating matter here but it could be out there somewhere why aren't physicists looking for it one argument that you may have heard physicists bring up is that negative masses can't exist because that would make the vacuum decay that's because if negative masses exist then so do negative energies because e equals m c squared and so on yes that guy again and if we had negative energies then you could create pairs of particles with negative and positive energy from nothing and particle pairs would spontaneously pop up all around us a theory with negative masses would therefore predict that the universe doesn't exist which is in conflict with evidence i've heard that argument many times unfortunately it doesn't work this argument doesn't work because it confuses two different types of mass if you remember einstein's theory of general relativity is based on the equivalence principle that's the idea that gravitational mass equals inertial mass the gravitational mass is the mass that appears in the law of gravity the inertial mass is the mass that resists acceleration but if we had anti-gravitating matter only its gravitational mass would be negative the inertial mass always remains positive and since the energy equivalent of inertial mass is as usual conserved you can't make gravitating and anti-gravitating particles out of nothing some physicists may argue that you can't make anti-gravity compatible with general relativity because particles in einstein's theory will always obey the equivalence principle but this is wrong of course you can't do it in general relativity as it is but i wrote a paper many years ago in which i show how general relativity can be extended to include anti-gravitating matter so that the equivalence principle only holds up to a sine that means gravitational mass is either plus or minus inertial mass so in theory that's possible the real problem is well we don't see any anti-gravitating matter is it maybe that antimatter antigravitates antimatter is made of anti-particles anti-particles are particles which have the opposite electric charge to normal particles the anti-particle of an electron for example is the same as the electron just with a positive electric charge it's called the positron we don't normally see anti-particles around us because they annihilate when they come in contact with normal matter then they disappear and leave behind a flash of light or in other words a bunch of photons and it's difficult to avoid contact with normal matter on a planet made of normal matter this is why we observe antimatter only in cosmic radiation or if it's created in particle colliders but if there is so little antimatter around us and it lasts only for such short amounts of time how do we know it falls down and not up we know this because both mata and antimatter particles hold together the quarks that make up neutrons and protons inside a neutron and proton there aren't just three quarks there's really a soup of particles that holds the quarks together and some of the particles in the soup are anti-particles why don't those anti-particles annihilate they do they are created and annihilated all the time we therefore call them virtual particles but they still make a substantial contribution to the gravitational mass of neutrons and protons that means crazy as it sounds the masses of anti-particles make a contribution to the total mass of everything around us so if antimatter had a negative gravitational mass the equivalence principle would be violated it isn't this is why we know antimatter doesn't anti-gravitate but that's just theory you may say maybe it's possible to find another theory in which antiparticles only antigravitate sometimes so that the masses of neutrons and protons aren't affected i don't know any way to do this consistently but even so three experiments at cern are measuring the gravitational behavior of antimatter those experiments have been running for several years but so far the results are not very restrictive the alpha experiment has ruled out that anti-particles have anti-gravitating masses but only if the absolute value of the mass is much larger than the mass of the corresponding normal particle this means so far they ruled out something one wouldn't expect in the first place however give it a few more years and they'll get there i don't expect surprises from this experiment that's not to say that i think it shouldn't be done just that i think the theoretical arguments for why antimatter can't antigravitate are solid okay so antimatter almost certainly doesn't antigravitate but maybe there's another type of matter out there something new entirely and that anti-gravitates if that was the case how would it behave for example if anti-gravitating matter repels normal matter then does it also repel among itself like electrons repel among themselves or does it attract its own type this question interestingly enough is pretty easy to answer with a little maths forces are mediated by fields and those fields have a spin which is a positive integer so 0 1 2 etc for gravity the gravitational mass plays the role of a charge and the force between two charges is always proportional to the product of those charges times -1 to the power of the spin for spin zero field the force is attractive between like charges but electromagnetism is mediated by a spin-one field that's electromagnetic radiation or photons if you quantize it and this is why for electromagnetism the force between like charges is repulsive but unlike charges attract gravity is mediated by a spin to field that's gravitational radiation or gravitons if you quantize it and so for gravity it's just the other way around again like charges attract and unlike charges repel keep in mind that for gravity the charge is the gravitational mass this means if there is anti-gravitating matter it would be repelled by the stuff we are made of but clump among itself indeed it could form planets and galaxies just like ours the only way we would know about it is its gravitational effect that sounds kind of like dark matter and dark energy right indeed that's why i thought it would be interesting because i had this idea that anti-gravitating matter could surround normal galaxies and push in on them which would create an additional force that looks much like dark matter normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies but aren't those situations very similar more positive mass and side are negative mass outside pushing in and if you remember the important thing about dark energy is that it has negative pressure certainly if you have negative energy you can also get negative pressure somehow so using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight but at second side neither of those ideas work the idea that galaxies would be surrounded by antigravitating matter doesn't work because such an arrangement would be dramatically unstable remember the anti-gravitating stuff wants to clump just like normal matter it wouldn't enclose galaxies of normal matter it would just form its own galaxies so getting anti-gravity to explain dark matter doesn't work even for galaxies and that's leaving aside or the other evidence for dark matter and dark energy well the reason that dark energy makes the expansion of the universe speed up it's actually not that it has negative pressure it's that the ratio of the energy density over the pressure is negative and for antigravitating matter they both turn negative so that the ratio is the same contrary to what you expect that does not speed up the expansion of the universe another way to see this is by noting that antigravitating matter is still matter and behaves like matter dark energy on the contrary does not behave like matter regardless of what type of matter this is why i get a little annoyed when people claim that dark energy is kind of like anti-gravity it isn't so in the end i developed this beautiful theory with the new symmetry between gravity and anti-gravity and it turned out to be entirely useless what did i learn from this well that i wasted a considerable amount of my time on this was one of the reasons i began thinking about more promising ways to develop new theories clearly just guessing something because it's pretty is not a good strategy in the end i wrote an entire book about this today i try to listen to my own advice at least some of the time i don't always listen to myself but sometimes it's worth the effort this video was sponsored by brilliant whether or not you believe that anti-gravitating matter exists it's fun playing around with the idea what you could do with it if it exists and it's really basic physics if you want to freshen up your knowledge about physics or learn something new brilliant is a great place to start brilliant is a website and app that offers interactive courses on a large variety of topics in science and mathematics i love their courses because they are so to the point and they allow you to explore the depth of each topic on your own for more background on the physics i talked about today check out brilliant courses on classical mechanics and gravitational physics all their courses will challenge you with questions so you can check your understanding along the way to support this channel and learn more about brilliant go to brilliant.org sabine and sign up for free the first 200 subscribers using this link will get 20 of the annual premium subscription thanks for watching see you next week you

Channel: Sabine Hossenfelder

Views: 270,107

Keywords: dark matter, dark energy, what is dark matter, is dark energy anti-gravity, anti gravity, anti-gravity, anti-gravitation, is anti gravity real, does anti-matter anti-gravitate, antimatter, antimatter antigravity, antigravity, does antigravity work, is antigravity real, does antimatter antigravitate, is dark matter negative mass, does negative mass exist, is dark energy antigravity, hossenfelder, science without the gobbledygook, universe

Id: EeAguTk6_2E

Length: 12min 56sec (776 seconds)

Published: Sat Nov 27 2021