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Transcript
0:00 · I got a lot of questions last week about an article in Quanta Magazine about Dark
0:05 · Dimensions. And for a change this is a case where I think I'm actually the right person to
0:10 · ask. What's this all about? Let's have a look. This recent work is based on string theory,
0:16 · an approach to a theory of everything that was invented in the 1970s. It was quite popular in
0:22 · the 80s and 90s. One of its main features is that it requires 9 dimensions of space. Now,
0:28 · as you've probably noticed, we don't live in 9 dimensions. So
0:32 · string theorists assumed that 6 of these dimensions are not infinitely large,
0:38 · they are rolled up to such small sizes that we wouldn't notice them.
0:42 · The new paper says that the extra dimensions could explain dark matter, if that exists,
0:48 · which it may not, hence the combination dark dimensions. Dark matter is what astrophysicists
0:55 · think makes up 80 percent of all the matter in the universe, but we can't see it and we have
1:00 · never managed to directly detect it. We only indirectly infer the presence of dark matter
1:06 · from its gravitational pull. This is why there's an alternative idea, that there's no dark matter,
1:12 · it's that we've got the law of gravity wrong. Though Albert doesn't like that at all.
1:19 · But back to the extra dimensions. In the original idea of string theory, these additional dimensions
1:25 · were so small that we can't measure them at all, about 10 to the minus 35 meters,
1:31 · a size know as the Planck length. It's named after Max Planck who also made the quip that
1:36 · science progresses one funeral at a time. Though I think that was very optimistic.
1:42 · Then, in the late 1990s some people had the ingenious idea to just conjecture that one or
1:48 · several of these hypothetical dimensions are much larger than the Planck length,
1:54 · so that they could become measurable with the next generations of experiments. These
1:59 · were called the "Large Extra Dimensions". You see, the way this kind of research
2:04 · work is that it's always the next experiment that will test these ideas. And if that next
2:10 · experiment doesn't find the stuff, then it's the next after that, and so on. Physicists
2:15 · usually justify this by a pseudoscientific argument called "naturalness" according to
2:21 · which some otherwise arbitrary values of model parameters are preferred by nature.
2:27 · These "naturalness" predictions can be shifted because they're not scientific to begin with.
2:31 · 20 years ago, there were a number of experiments that looked for
2:36 · these large extra dimensions, particle colliders, astrophysics,
2:40 · tabletop and so on. And would you believe it they didn't find them.
2:44 · The new paper is now about a revival of this old idea of large extra dimensions. Indeed,
2:50 · these "Dark Dimensions" are a new research program that seems to have begun in 2022 around
2:56 · Cumrun Vafa, a string theorist at Harvard who has been doing this stuff for decades.
3:01 · In their scenario, there is one extra dimension that is particularly large, and they ignore the
3:08 · other 5 as being too small to be measurable. This is not new, it was a rather common setup 20 years
3:14 · ago. The new thing is the justification for why this one dimension has a size of
3:20 · about one micrometer. This is supposedly "natural" because it's related to the cosmological constant
3:26 · and something to do with the swampland. Doesn't really matter exactly what this means because the
3:32 · swampland isn't real and naturalness arguments have failed over and over again in the past.
3:39 · In any case, this supposedly "natural" size of the extra dimension is great because,
3:46 · guess what, it could be tested with one of the next experiments. The current
3:51 · experimental constraints on the size of this dimension is currently about 52 micrometres.
3:58 · Okay, but what does this have to do with dark matter?
4:01 · In such large extra dimensions, you must assume that forces which we have measured on very short
4:08 · distances, that's the nuclear forces and also electromagnetism, don't notice the
4:14 · additional dimensions. And actually, all the matter that we are made of can't travel into
4:20 · these directions either. In string theory speech, this normal matter is "confined to the brane".
4:26 · That's b r a n e not b r a i n, is derived from "membrane" and is our normal three
4:33 · dimensional space. So all the normal stuff needs to stay on that 3 dimensional brane.
4:40 · The reason is that atomic nuclei are much smaller than this micrometer which is the
4:45 · supposed size of the extra dimension. And if the constituents of nuclei
4:50 · could spread into more than 3 dimensions nuclear physics wouldn't properly work.
4:55 · But here's the thing. There's no such problem for gravity,
5:00 · so they can assume that gravity does experience the large extra dimensions.
5:05 · And these extra dimensions then explain dark matter as follows. Whenever you have rolled
5:11 · something up you get standing waves in this rolled up direction. For gravity, these standing waves
5:18 · are quanta of the gravitational field, called gravitons. And if they're standing waves,
5:24 · they have masses that depend on how many wavelengths fit into the extra dimension.
5:30 · These massive gravitons can make up dark matter. Again this isn't a new idea, this is the same
5:36 · thing people did 25 years ago. They were even looking for this stuff at the LHC and didn't
5:42 · find it. For this new theory they just use some parameter ranges that have not yet been excluded.
5:49 · The novelty of the Dark Dimensions scenario is that the extra dimension doesn't have
5:54 · the same radius everywhere. This has the effect that the mass of these gravitons
5:59 · isn't the same everywhere and the heavier ones can decay. This basically heats up
6:04 · the gravitons with lower masses and since they're hotter that changes the behaviour
6:09 · of the dark stuff. They say that this heating effect would affect the large-scale structure
6:14 · of galactic filaments and guess what, that future experiments could soon rule that out.
6:21 · You know what I'll go out on a limb and say they
6:24 · won't find any evidence for the decay of these massive gravitons.
6:28 · Last year, I coincidentally noticed that if you asked Google for my cell
6:33 · phone number it gave you an answer. Luckily I changed my number three years ago and the
6:39 · one that Google spat out was my old one. But that was somewhat of an eye opening.
6:46 · I have since signed up to Incogni, who have been sponsoring this video.
6:50 · You see, each time you open a website, it'll try to collect data about who you are and
6:56 · where you are and what other websites you've visited. If you then sign up for a website
7:01 · and fill in your personal details, they can, and often do, make money by selling
7:06 · your private information to data brokers. Most countries have laws against that,
7:11 · and you can ask for your data to be removed. But doing this takes up a lot of time.
7:17 · Incogni automates the process of getting you out of those databases. You sign up and
7:22 · they'll contact the big sinners, request that your personal details be removed. They'll keep on doing
7:29 · that and, if you want, send you updates about the progress they're making. I am glad there's
7:35 · now a simple solution to stop unfriendly people doing nasty things with my personal details.
7:41 · Incogni is super easy to use. You sign up, give them the information they should look for,
7:41 · and they go to work, like, within a minute basically. I now sleep better at night,
7:42 · and maybe I can help you sleep better too. If you use our code SABINE or the custom link in the info
7:42 · below, you will get sixty percent off of incogni. That's an amazing deal, so go and check this out.
7:42 · Incogni is super easy to use. You sign up, give them the information they should look for, and
7:46 · they go to work, like, within a minute basically. I now sleep better at night, and maybe I can help
7:52 · you sleep better too. The first 100 people who use our code SABINE or the custom link in the
7:59 · info below will get sixty percent off of incogni. That's an amazing deal, so go and check this out.
8:06 · Thanks for watching, see you next week.

Transcript
0:00	·	I got a lot of questions last week about an article in Quanta Magazine about Dark
0:05	·	Dimensions. And for a change this is a case where I think I'm actually the right person to
0:10	·	ask. What's this all about? Let's have a look. This recent work is based on string theory,
0:16	·	an approach to a theory of everything that was invented in the 1970s. It was quite popular in
0:22	·	the 80s and 90s. One of its main features is that it requires 9 dimensions of space. Now,
0:28	·	as you've probably noticed, we don't live in 9 dimensions. So
0:32	·	string theorists assumed that 6 of these dimensions are not infinitely large,
0:38	·	they are rolled up to such small sizes that we wouldn't notice them.
0:42	·	The new paper says that the extra dimensions could explain dark matter, if that exists,
0:48	·	which it may not, hence the combination dark dimensions. Dark matter is what astrophysicists
0:55	·	think makes up 80 percent of all the matter in the universe, but we can't see it and we have
1:00	·	never managed to directly detect it. We only indirectly infer the presence of dark matter
1:06	·	from its gravitational pull. This is why there's an alternative idea, that there's no dark matter,
1:12	·	it's that we've got the law of gravity wrong. Though Albert doesn't like that at all.
1:19	·	But back to the extra dimensions. In the original idea of string theory, these additional dimensions
1:25	·	were so small that we can't measure them at all, about 10 to the minus 35 meters,
1:31	·	a size know as the Planck length. It's named after Max Planck who also made the quip that
1:36	·	science progresses one funeral at a time. Though I think that was very optimistic.
1:42	·	Then, in the late 1990s some people had the ingenious idea to just conjecture that one or
1:48	·	several of these hypothetical dimensions are much larger than the Planck length,
1:54	·	so that they could become measurable with the next generations of experiments. These
1:59	·	were called the "Large Extra Dimensions". You see, the way this kind of research
2:04	·	work is that it's always the next experiment that will test these ideas. And if that next
2:10	·	experiment doesn't find the stuff, then it's the next after that, and so on. Physicists
2:15	·	usually justify this by a pseudoscientific argument called "naturalness" according to
2:21	·	which some otherwise arbitrary values of model parameters are preferred by nature.
2:27	·	These "naturalness" predictions can be shifted because they're not scientific to begin with.
2:31	·	20 years ago, there were a number of experiments that looked for
2:36	·	these large extra dimensions, particle colliders, astrophysics,
2:40	·	tabletop and so on. And would you believe it they didn't find them.
2:44	·	The new paper is now about a revival of this old idea of large extra dimensions. Indeed,
2:50	·	these "Dark Dimensions" are a new research program that seems to have begun in 2022 around
2:56	·	Cumrun Vafa, a string theorist at Harvard who has been doing this stuff for decades.
3:01	·	In their scenario, there is one extra dimension that is particularly large, and they ignore the
3:08	·	other 5 as being too small to be measurable. This is not new, it was a rather common setup 20 years
3:14	·	ago. The new thing is the justification for why this one dimension has a size of
3:20	·	about one micrometer. This is supposedly "natural" because it's related to the cosmological constant
3:26	·	and something to do with the swampland. Doesn't really matter exactly what this means because the
3:32	·	swampland isn't real and naturalness arguments have failed over and over again in the past.
3:39	·	In any case, this supposedly "natural" size of the extra dimension is great because,
3:46	·	guess what, it could be tested with one of the next experiments. The current
3:51	·	experimental constraints on the size of this dimension is currently about 52 micrometres.
3:58	·	Okay, but what does this have to do with dark matter?
4:01	·	In such large extra dimensions, you must assume that forces which we have measured on very short
4:08	·	distances, that's the nuclear forces and also electromagnetism, don't notice the
4:14	·	additional dimensions. And actually, all the matter that we are made of can't travel into
4:20	·	these directions either. In string theory speech, this normal matter is "confined to the brane".
4:26	·	That's b r a n e not b r a i n, is derived from "membrane" and is our normal three
4:33	·	dimensional space. So all the normal stuff needs to stay on that 3 dimensional brane.
4:40	·	The reason is that atomic nuclei are much smaller than this micrometer which is the
4:45	·	supposed size of the extra dimension. And if the constituents of nuclei
4:50	·	could spread into more than 3 dimensions nuclear physics wouldn't properly work.
4:55	·	But here's the thing. There's no such problem for gravity,
5:00	·	so they can assume that gravity does experience the large extra dimensions.
5:05	·	And these extra dimensions then explain dark matter as follows. Whenever you have rolled
5:11	·	something up you get standing waves in this rolled up direction. For gravity, these standing waves
5:18	·	are quanta of the gravitational field, called gravitons. And if they're standing waves,
5:24	·	they have masses that depend on how many wavelengths fit into the extra dimension.
5:30	·	These massive gravitons can make up dark matter. Again this isn't a new idea, this is the same
5:36	·	thing people did 25 years ago. They were even looking for this stuff at the LHC and didn't
5:42	·	find it. For this new theory they just use some parameter ranges that have not yet been excluded.
5:49	·	The novelty of the Dark Dimensions scenario is that the extra dimension doesn't have
5:54	·	the same radius everywhere. This has the effect that the mass of these gravitons
5:59	·	isn't the same everywhere and the heavier ones can decay. This basically heats up
6:04	·	the gravitons with lower masses and since they're hotter that changes the behaviour
6:09	·	of the dark stuff. They say that this heating effect would affect the large-scale structure
6:14	·	of galactic filaments and guess what, that future experiments could soon rule that out.
6:21	·	You know what I'll go out on a limb and say they
6:24	·	won't find any evidence for the decay of these massive gravitons.
6:28	·	Last year, I coincidentally noticed that if you asked Google for my cell
6:33	·	phone number it gave you an answer. Luckily I changed my number three years ago and the
6:39	·	one that Google spat out was my old one. But that was somewhat of an eye opening.
6:46	·	I have since signed up to Incogni, who have been sponsoring this video.
6:50	·	You see, each time you open a website, it'll try to collect data about who you are and
6:56	·	where you are and what other websites you've visited. If you then sign up for a website
7:01	·	and fill in your personal details, they can, and often do, make money by selling
7:06	·	your private information to data brokers. Most countries have laws against that,
7:11	·	and you can ask for your data to be removed. But doing this takes up a lot of time.
7:17	·	Incogni automates the process of getting you out of those databases. You sign up and
7:22	·	they'll contact the big sinners, request that your personal details be removed. They'll keep on doing
7:29	·	that and, if you want, send you updates about the progress they're making. I am glad there's
7:35	·	now a simple solution to stop unfriendly people doing nasty things with my personal details.
7:41	·	Incogni is super easy to use. You sign up, give them the information they should look for,
7:41	·	and they go to work, like, within a minute basically. I now sleep better at night,
7:42	·	and maybe I can help you sleep better too. If you use our code SABINE or the custom link in the info
7:42	·	below, you will get sixty percent off of incogni. That's an amazing deal, so go and check this out.
7:42	·	Incogni is super easy to use. You sign up, give them the information they should look for, and
7:46	·	they go to work, like, within a minute basically. I now sleep better at night, and maybe I can help
7:52	·	you sleep better too. The first 100 people who use our code SABINE or the custom link in the
7:59	·	info below will get sixty percent off of incogni. That's an amazing deal, so go and check this out.
8:06	·	Thanks for watching, see you next week.