First quantum measurement of gravity: What does it mean?

the great thing about physics is that it makes the simplest things mind-bogglingly complicated take space what is it where space is whatever's between here and there not that complicated but but physicist say space should have Quantum properties space itself might be both here and there and what the heck does this mean well no one knows so much about the theory but we also have experiments in physics and this new experiment which I just read about the other day brought us a step closer to figuri

ng out what space is or did it let's have a look this video comes with a quiz that lets you check how much you remember time is what prevents everything from happening at once as John weeder put it and according to John Barrow space is what prevents everything from happening in Cambridge I'm just back from a trip to Cambridge and for all I can tell the only thing that happens there is rain but that let not make things more complicated than they need to be both Johns were referring to space and t

ime in Einstein's good oldfashioned theories Einstein developed his theories before quantum mechanics ruined everything and this is why they don't contain stuff like the uncertainty principle or dead and alive cats in Einstein's theory space and time are coordinate grits basically they tell you when and where things happen like rain in Cambridge or on Monday Tuesday Wednesday you get the idea Einstein also told us however that space and time react to matter Inside by curving trouble is that this

responsiveness of SpaceTime doesn't sit well with quantum physics not at all the issue is that according to quantum physics particles can be in two places at the same time and particles have masses that curve SpaceTime so where do they curve SpaceTime if they're in two places quantum mechanics doesn't tell us because it knows nothing about the curvature of SpaceTime and Einstein's theory can't tell us because it knows nothing about quantum physics what we do know though is that the way that spa

ce Works in Einstein's theory can't be right because somehow we need to account for those particles which can be in two places at once to sort this out we need a theory in which space can be in two places at the same time time which makes no sense and that basically is the problem since gravity is just the curvature of space and time a theory that gives them Quantum properties is also a theory of quantum gravity now I've worked on quantum gravity after I finished my PhD 20 years ago back then ev

eryone kept telling me that quantum gravity is basically philosophy because it doesn't have any measurable consequences that's because gravity is an incredibly weak force compared to the other forces think about it if you pin a magnet to your fridge then the magnetic force of that tiny thing is stronger than the gravitational pull of the entire planet and the forces inside Atomic nuclei are even stronger that's why nuclei stick together and good thing that they do and so the story went we can't

test the quantum properties of space that had require enormously High energies a particle Collide at the size of the Milky Way that's a common estimate but I've tried to tell people that makes no sense gravity is different from all those forces because it can add up it doesn't neutralize like all the other forces do that's why we experience it so prominently in daily life to measure Quantum gravitational effects you just have to measure the quantum properties of objects that are heavy enough I n

o longer work in the field that's a long story but I'm super excited to see that there are now several experimental groups trying to test quantum gravity and not with Milky Way size cidus but in the laboratory this then brings me to the new experiment because they found a new way to measure very small gravitational forces you see the issue is that if you take Elementary particles like those in the standard model electrons quarks muons and all our best friends their gravitational pull is so tiny

we can't measure it so you can't test quantum gravity with them if you take something heavy like a planet then you can measure the gravitational field all right but you can't measure its Quantum properties that's because normally Quantum properties go away the larger the object unless you treat them very very carefully and that's what they did in this experiment they put a tiny magnet about the weight of a milligram into a superconducting container which is called to near absolute zero that supe

rconducting container generates a magnetic field and that keeps the magnet trapped it levitates on the container and because it's so carefully isolated it can do Quantum things like being in two places at once have a look at their experiment it looks a little like the quantum computers at Google and IBM doesn't it these different levels you see here are all noise buffers with these tubes belonging to the different stages of cooling then they take a fairly heavy weight of about 2.4 kg put it on a

wheel rotate the wheel and move it from one side to the other of this container with the levitating magnet the thing is now that the gravitational pull from the moving weight should affect the levitating magnet this will make the tiny magnet swing relative to the container with the frequency that depends on the position of the wheel and that they can measure because it creates a current for which they have a super sensitive detector in the container the the reason they put this weight on a whee

l is that this way they know the effect must come with a particular frequency and that makes it easier to identify it stands out against the noise basically and what you can see here is that the motion of the magnet indeed responds to the wheel being moved the force that they measured was a tiny 30 Aton neon that's not the smallest Force ever measured and that also brings me to the question what do you want to do with it remember that what we want to know is what the quantum properties of space

and time are for this you need to measure the gravitational field of an object in a superposition of two places but this is not what this experiment measures it measures the gravitational field of the heavy weight with a Quantum sensor that's the levitating magnet this isn't what you want to measure to test quantum gravity yes it's something with Quantum and something with gravity but that doesn't mean it's quantum gravity however one thing that you can do instead of directly measuring the gravi

tational pull of a Quantum object is to measure the effect of the gravitational pull of two Quantum objects on each other in a setup like this that basically becomes a question of how the thing responds to vibrations but I'm not sure how you'd extract Quantum gravitational effects from that so I still think that the best approach is that pursued by the group of Marcus aspor in Vienna which tries to directly measure the gravitational pull of small Quantum objects by bringing microscopic sensors a

s close to these objects as possible then again it's good to have a variety of experiments and whatever they'll do next I'll let you know so stay tuned oh yes and subscribe I'm not doing this YouTube thing right am I did you know I have a quantum mechanics course on brilliant.org it's a beginner's course that you can take without any background knowledge it'll introduce you to topics such as interference superpositions and entanglement the uncertainty principle and Bs theorem and afterwards you

can continue learning more about your favorite topics in science computer science or maths all courses on brilliant come with interactive visualizations and follow-up questions it's really an easy and fun way to learn something new if you want to try it out for free use our link brilliant.org Saina first 30 days are free and the first 200 of you to use this link will get 20% off the annual premium subscription thanks for watching see you tomorrow