# How Many Dimensions Are There? **URL:** \ **Category:** Factual Questions\ **Created:** [January 22, 2018, 4:00pm UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215 "2018-01-22T16:00:33Z")\ **Posts on this page:** 9\ **Page:** 7
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [February 1, 2018, 10:50am UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/121 "2018-02-01T10:50:14Z")
Continuing… > [@Scylla](#): > > On the cosmological scale there is about 5 times as much gravity being generated as we would expect based on the amount of matter we can account for. This is a telling summary statement. The situation is not that there is “five times as much gravity being generated”. The measurements can not be converted into a “gravity meter” wherein each probe gives a reading of “amount of gravity” and they all say there is too much gravity. It is quite different. Consider just the Bullet Cluster example above. That observation provides clear evidence that galaxy clusters have two components of material – one that interacts and one that doesn’t. This is determined by comparing where the gravity comes from in non-colliding clusters with where the gravity comes from in colliding clusters, and comparing those maps with where the luminous matter lies in the two cases. The measurement is not that “there is too much gravity”. It’s a much richer observation about the flow and interaction – the kinematics and dynamics – of whatever is causing the gravity. Again, it is not that there is just too much gravity. It is that there is too much gravity that has a very specific set of features in colliding and non-colliding clusters. And that’s just this one probe. Consider the CMB example above. If you take another look at the [linked graph](https://en.wikipedia.org/wiki/Cosmic_microwave_background#/media/File:PowerSpectrumExt.svg), there is so much structure there that depends on much more than just a single “amount of gravity” knob. All that data with its precise patterns of up and down matches our understanding perfectly. (Note that the red line with a varying width [due to known statistical uncertainties] is what the model with dark matter says. Beautiful agreement with the data!) But the data does not required some vague "extra gravity. It requires the presence of non-interacting particles with a certain range of possible masses and densities, and it further requires that these particles co-exist in the broiling early universe with baryons and photons that themselves behave in precisely the way we expect them to. All of that can be inferred from such an intricate and involved phenomenon. In no sense does this data just say that gravity seems off. The real, physical kinetic energy of these particles enters the story. The real, physical differences in interaction strengths between the baryonic and non-baryonic particles enters the story. And that’s just this one probe. Consider the vast differences between the above two independent probes. If you gloss over the details of these and revert to an assumption that they are just saying “We need more gravity,” you will be stuck. It’s much more rich than that. There is no false leap here taking us from “We need more gravity” to “That gravity must come from particles”. On the contrary, the actual data already requires that there is some new kind of matter present. It’s part of what is being directly measured. Hopefully this helps make it clearer. If not, it may be helpful if you could quote specific statements that you have issue with so that I may address those, as I have done with your statements. I _do_ understand what you are saying, so restating it without addressing any of my points directly will not be helpful in reaching a mutual understanding. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [February 1, 2018, 11:11am UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/122 "2018-02-01T11:11:18Z")
> [@septimus](#): > > **Pasta** , will you indulge me and let me re-ask one of my favorite questions? 🙂 > > _ **How do “we” know that dark matter does not interact with photons?** _ **Chronos** has answered this question for me at least twice before — and his answers seem better than the answers at Physics.Stackexchange, but I didn’t fully understand them. I guess I’m not sure what you are looking to clarify. A surface interpretation would be that you’re asking “How do we know that matter that doesn’t interact with photons doesn’t interact with photons?” but I know you are trying to ask something more meaningful. As a first stab that stays away from 2nd Law arguments: CMB data, large-scale structure data, and cluster collision data all require that dark matter particles do not interact with photons. It’s not just that we can’t see the dark matter visibly in halos that makes it “dark” – though that is certainly true and is the origin of the name. The “dark” matter also has to be approximately collisionless to do everything it is seen to do. And if a particle interacts with photons, it is definitely not going to be collisionless. ---
**Author:** ![Scylla](https://avatars.discourse-cdn.com/v4/letter/s/a5b964/32.png) [@Scylla](https://boards.straightdope.com/u/Scylla)\ **Post date:** [February 1, 2018, 2:19pm UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/123 "2018-02-01T14:19:14Z")
Pasta: When you kept going on about the bullet cluster I thought “really, isn’t this where all the non-localized and modified gravity comes from since the collision speed of the bullet cluster is so fast as to make it statistically impossible? I’ll gather up a cite or two and show this dark matter optimist just how lost in the woods we still are.” Apparently, there have been some developments. So, let’s say you’re still skeptical of my dragon sitting on my bag of rice. I say “I’ll prove it!” And cackle appropriately. I propose a rice bag launcher that will slam my bag of rice into an anvil. It will have enough velocity that it will dislodge my dragon. We will examine the pattern of rice splatter to determine whether it has splattered like 100 pounds of rice that has just dislodged a dragon, or whether it has splattered like 500 pounds of wet rice. Rice splatter physics is highly precise so this is no big deal. Further, I propose to place my cat on the other side of the room. A split second after the collision, I will take a picture of my cat who will be just barely to the left and behind the rice. If it’s 500 pounds of soggy rice than the gravitational lensing will look one way. If the dragon flies off at the time of the collision as we predict, his center of gravity will be different from the rice and the picture will look another way. You agree to this lunatic scheme. Slam goes the rice. Click goes the camera, splatter go the rice grains. What we have on the ground is 100 pounds of dry rice in pretty much the exact pattern we would expect it to be if it weighed 100 pounds. We look at the picture we took and it is distorted in precisely the way we would expect it to be if 400 pounds of dragon was flying away from a rice explosion. The cat appears startled. Here there be dragons. This is what you have been pointing me towards, and I appreciate your patience. The evidence for dark matter is much stronger than I had thought. ::muttering:: I can’t beleive that bullshit turned out to be real. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [February 1, 2018, 2:27pm UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/124 "2018-02-01T14:27:27Z")
> [@Scylla](#): > > I appreciate your patience. And I yours. Glad to have the discussion. ---
**Author:** ![Scylla](https://avatars.discourse-cdn.com/v4/letter/s/a5b964/32.png) [@Scylla](https://boards.straightdope.com/u/Scylla)\ **Post date:** [February 1, 2018, 2:32pm UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/125 "2018-02-01T14:32:18Z")
> [@Pasta](#): > > And I yours. Glad to have the discussion. Yes. It was fun. Thanks to you I cleared up a misconception. So, thanks again. Odd that the rice analogy ended up fitting as well as it did. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [February 1, 2018, 2:55pm UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/126 "2018-02-01T14:55:14Z")
Now I’m hungry for some rice. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [February 3, 2018, 3:32am UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/127 "2018-02-03T03:32:35Z")
I’m sure I seem obstinate but I still feel that some of the argumentation about dark matter is based on circular reasoning. Let’s start with “collisionlessness” in The Bullet Cluster collision. > [@Wikipedia](#): > > The major components of the cluster pair—stars, gas and the putative dark matter—behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-rays, represents most of the baryonic, i.e. ordinary, matter in the cluster pair. The gases interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the gravitational lensing of background objects. In theories without dark matter, such as Modified Newtonian Dynamics (MOND), the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. IOW there are three matter components: (1) The hot baryonic gas which is slowed, (2) stars (also baryonic, no?) which are NOT slowed, and (3) the dark matter which, like the stars, is not slowed. Is this correct so far? Obviously a main difference between dark matters and stars is that dark matter is … dark! Some stars are also somewhat dark. Let’s see what StackExchange responded when someone asked why dark matter isn’t made up of brown dwarf stars: > [@https://astronomy.stackexchange.com/questions/10384/why-arent-brown-dwarfs-the-dark-matter](#): > > We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of “dark matter” is some 30% of the density of the universe. But on the other hand, estimates of the abundances of deuterium, helium, tritium and lithium produced in the big bang indicate that only 5% of the density of the universe can be in the form of normal matter (e.g atoms, protons, neutrons etc). As brown dwarfs are made of these things, they can only contribute to this small percentage and cannot be responsible for most of the dark matter (the same is true for anything made of normal, “baryonic matter”). > > Astronomers have counted how many brown dwarfs there are, both in the local neighbourhood and in star clusters (see for example Andersen et al. 2008; Kirkpatrick et al. 2011; Burningham et al. 2013; . In other words we can see brown dwarfs - they emit most of their light in the infrared part of the spectrum, and surveys like SDSS, 2MASS and WISE have uncovered them in their thousands. It turns out that there is about 1 brown dwarf for every 4 more massive stars. So, although numerous, they contribute a very tiny fraction of the normal matter in our Galaxy. In addition, microlensing experiments towards the Galactic bulge and Magellanic clouds suggest that although there are lots of brown dwarfs elsewhere in our Galaxy (e.g. Alcock et al. 2000; Novati et al. 2008), there are not enough to make much of a contribution to dark matter, and so the results from our solar neighbourhood appear representative of the Galaxy as a whole. There seem to be two arguments made here: (1) Deduced abundances of baryons from the Big Bang; just as Chronos answered in an earlier thread; (2) Brown dwarfs, while somewhat dark, are not completely dark. So … suppose there are stars which are _much_ darker than brown dwarfs. Why can’t these be the dark matter? I see two, _and only two_, answers to that question: (1) Contradictions with Big Bang nucleosynthesis theory. (2) Completely dark “stars” would violate laws of thermodynamics. What am I missing? ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [February 3, 2018, 4:26am UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/128 "2018-02-03T04:26:01Z")
> [@septimus](#): > > I’m sure I seem obstinate but I still feel that some of the argumentation about dark matter is based on circular reasoning. Let’s start with “collisionlessness” in The Bullet Cluster collision. > > IOW there are three matter components: (1) The hot baryonic gas which is slowed, (2) stars (also baryonic, no?) which are NOT slowed, and (3) the dark matter which, like the stars, is not slowed. Is this correct so far? > > Obviously a main difference between dark matters and stars is that dark matter is … dark! Some stars are also somewhat dark. Let’s see what StackExchange responded when someone asked why dark matter isn’t made up of brown dwarf stars: > > There seem to be two arguments made here: (1) Deduced abundances of baryons from the Big Bang; just as Chronos answered in an earlier thread; (2) Brown dwarfs, while somewhat dark, are not completely dark. > > So … suppose there are stars which are _much_ darker than brown dwarfs. Why can’t these be the dark matter? I see two, _and only two_, answers to that question: (1) Contradictions with Big Bang nucleosynthesis theory. (2) Completely dark “stars” would violate laws of thermodynamics. > > What am I missing? Because you are focusing on just strong gravitational lensing. Which is not really your fault except you are experiencing the same crisis of faith that everyone does when learning about this. It is hard to realize that we have to move past our faulty perceptions, and physicists like to show pretty pictures, thus the bullet cluster and the micro bullet being common, sexy and a pivotal part of the discovery. Strong lensing, weak leansing and CMB and other observations need to be considered as a whole as hypothesis need to explain multiple cases and not just the extreme cases. Realizing that I am simplifying, if the majority of the dark matter strongly interacted with the electromagnetic field it would “block light” during these other cases and it does not. The evidence related to shifting, distorting and magnifying effects would be much different under your above assumptions. There is no possible way for the observed effects to explained by some form of baryonic matter. Do not assume because you can “see” because your eyes detect electromagnetic radiation that it is the only way to “see”. This is a very common type of confirmation bias, and it is hard to not view evidence as confirmation of one’s known experiences. Remember to not conflate the naming of something with an assumption that we know what it is. Non-luminous matter in the outskirts of the galaxy exists, and it is called dark matter. Multiple experiments demonstrate this. Several experiments also demonstrate that it is is most likely not primarily composed of baryonic matter. It only seems circular because you are only focusing on a small portion of the available data while attempting to fit that information into your known experiences. This is normal, but don’t let it limit your ability to understand. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [February 3, 2018, 8:30am UTC](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215/129 "2018-02-03T08:30:12Z")
> [@septimus](#): > > So … suppose there are stars which are _much_ darker than brown dwarfs. Why can’t these be the dark matter? I see two, _and only two_, answers to that question: (1) Contradictions with Big Bang nucleosynthesis theory. (2) Completely dark “stars” would violate laws of thermodynamics. > What am I missing? Well IASNA astrophysicist but I feel emboldened by the fact my last response didn’t get pulled up for errors, so here goes: Firstly the two objections that you mentioned are pretty big ones. Anything that big made of conventional matter should undergo fusion. But secondly there is no model by which you can have that much conventional matter around and still not detect it. For example, one of the main reasons we first suspected the existence of dark matter was because we saw that galaxies were rotating too fast to hold onto all of their matter. Furthermore, the amount of matter required is about 3 times as much as we can see – and that includes all the supergiant stars, supermassive black holes etc. If the explanation for this is many brown dwarfs then it needs to be at least 3.62 Fuckloads (FLs) of brown dwarfs, and that is something we should be able to see; they should regularly obscure visible matter for one thing, if not have more collisions and black holes. [Previous page](https://boards.straightdope.com/t/how-many-dimensions-are-there/807215.md?page=6)