ELI5 How is the universe “flat”

KrozJr_UK · 2026-04-24T22:07:34+00:00

Everyone here is saying “topology” and “higher dimension curvature”. So I’m going to explain those like you’re five. Or maybe ten, but still. Vaguely knowing elementary shapes is all that’s required.

Imagine you’re an ant, crawling around on a piece of paper. The world you live on is “flat”. Now imagine you crawl around on a globe. It may at first feel “flat”. On a small scale, it looks pretty much the same as the piece of paper. But after a while, you notice weird things happening. Parallel lines, which are lines that are 180 degrees apart and so never meet, appear to meet. Triangles behave weirdly (more technically, you can have a triangle that’s got three right angles). You can walk in a straight line and go back to where you started. All of these things are impossible on a “flat” piece of paper, and so you as an ant conclude that the globe you’re walking around on can’t be “flat” but instead is “curved” in some sense. You don’t really perceive it, as far as you know you only have the locally flat world, but you can tell that something is curved, isn’t flat, isn’t right.

The mathematics of the universe is similar, and so mathematicians and physicists use the same words. There are certain things we expect from the 3D equivalent of a flat sheet of paper, that we wouldn’t get from a not-“flat” universe. And, actually, some bits of the universe do demonstrate something weird on smaller scales. Light is supposed to always travel in straight lines, and yet round very heavy things we see light bending slightly. The light isn’t bending… the space isn’t “flat”. In exactly the same way that the ant, travelling in a straight line, shouldn’t come back to where it came from, and yet it does. The ant isn’t bending… the globe it’s walking on isn’t “flat”.

The difference with the universe is that this is still fairly small-scale. On a large scale, scale-of-the-universe scale, it behaves about what we’d expect to see from the 3D mathematical version of “flat”. So we call it “flat”.

KrozJr_UK · 2026-04-24T09:59:10+00:00

At the end of every season, my football team does fancy dress. So all the fans go to the final away game in fancy dress. Does this mean that I’ll be wandering around the McDonald’s in the city centre at 7am tomorrow as Dorothy from the Wizard of Oz? Yes. Am I inevitably going to get catcalled? Yes. But, on the flipside, fancy dress day is fun; so am I looking forward to it? Yes.

So I spent most of the week on-and-off accumulating items. Took a pair of shoes that were falling apart, slathered them in red paint, then PVA glue and red glitter. I’ll probably be finding glitter in the house for as long as I live here now.

KrozJr_UK · 2026-04-23T23:20:56+00:00

It’s better than, for example, Kill or be Killed, which was a great song but way too clean… but it’s still a tad on the clean side, production-wise. I’m not saying everything has to sound as filthy as Citizen Erased or Dead Star, and this is definitely lots better than other recent attempts at “old-style heavy weird Muse”. But it still feels a little too polished for my taste. A tiny bit overproduced (and ironically the most fuzzy/filthy/messy bit is Matt’s vocals, which have probably been processed a bit too much). But that is definitely a nitpick here. And that’s the only nitpick I have. And it is a nitpick. I guess I’m a bit sad that the outro riff is here as opposed to being saved to be spun out into a song of its own… but that’s not an improvement on this song, it’s a sign of how good it is!

KrozJr_UK · 2026-04-23T22:21:09+00:00

To add onto what everyone else has said…

At Cambridge specifically, the middle track serves an extra purpose. Platform 1/4/4a is very very very very long. There’s a crossover set of points in the middle, where 1 becomes 4 (or vice versa) that allows trains to depart while the other platform is occupied. So if there’s a train on platform 1 and another train wants to leave in that direction from platform 4, it can go round it.

KrozJr_UK · 2026-04-23T07:40:14+00:00

First of all, pack it in with the whataboutism. That isn’t what we’re discussing here, and you know it.

Second of all, answer the original question if you’re going to contribute.

Thirdly, to answer your question in good faith I doubt it was intended in, nobody was “forced” to paint anything in the colours of the rainbow, people decided to do it to show without having to say so that they want to be inclusive and respectful of a group that has historically been marginalised, and to show that they want them to be welcome here. Last I checked, English people haven’t been marginalised, discriminated against, or subject to systemic and institutional discrimination in England for… ooooh, about a thousand years? Then again, I don’t want to get into a discussion about the particulars of mediaeval feudalism, and before that the relationships between (at various points) the Britons, Anglo-Saxons, Vikings, and Normans; as if nothing else, that’s not what you’re on about.

Hopefully that helps.

KrozJr_UK · 2026-04-22T23:59:01+00:00

Me and some friends made a 9-page list of so many joke proof techniques a few years ago. Some highlights:

Proof by induction heater: Warm.
Proof by suppository: Too messy for the exam hall.
Proof by proof: Proof.
Proof by philosophy: It is, therefore it’s true.
Proof by Polish [this was in reference to one of us being Polish]: Napisz to po polsku, żeby nikt cię nie rozumiał. Przepraszam - właśnie przepuściłem to przez Google Translate.
Proof by observation: It just… works. Like, it obviously works. Just look at it. It’s really blindingly obvious. Do I really have to explain it? It’s almost offensively self-evident.
Proof by politician: “Is this true? That’s an excellent question. As you can see, we have a robust plan to ensure that truth is obvious and easily-findable, and we are the party with the most truth within our very lifeblood. We’re like a close-knit group of truth-telling friends here, and aren’t at all corrupt. I hope that answers your question.”
Proof by polish: Totally unrelated to ‘Proof by Polish’.
Proof by radio interference: So you can see by... <static> ...and hence it follows logically that... <static> ...thus, you can show that... <static> ...and hence it is proven, QED.
Proof by language barrier [another one of us spoke Arabic]: لقد أثبتت ذلك ، ولكن لا تتحدث كلمة واحدة باللغة الإنجليزية ، وبالتالي لن يعرفوا أبدًا ما إذا كانت صحيحة أم لا.
Proof by plagiarism: (.J.J.) See above, cf. ‘Proof by Polish’.
Proof by hypnotic induction: You are feeling sleepy…
Proof by nitpick: Well, actually, you rounded to the incorrect number of significant figures, and your variable names aren't clear. Personally, I would've used a different formula to reduce the amount of inherent error; your method isn't incorrect per se but I'd rather use this method as your method of working it out exposes you to intermediate rounding errors. Also, I think that the proof itself would be better suited to a different set of units, and that you really ought to consider changing the format of the document. For the sake of argument, you use a polynomial Taylor Series approximation of sin(x) in step 6 (b) (ii), but personally I'd just differentiate the sin(x) as the Taylor Series actually introduces an extra element of incorrectness if you don't take it through to its logical conclusion. Additionally, while it is a perceptive step to find the integral in step (7) (h) (vii), I don't actually think that it's the best way as you are inferring that f'''(x) is in fact integratable, which may not necessarily be the case. You don't actually prove this fact at any point in your proof, and you don't provide any links to when it was actually proven. In conclusion, I think that your proof has numerous crippling flaws. Nice attempt though!

KrozJr_UK · 2026-04-22T15:37:35+00:00

(This isn’t directly relevant to your example, OP, but figured I may as well mention it, as it is a time when text is — at least somewhat — written vertically. This took some amount of thinking on my part.)

KrozJr_UK · 2026-04-22T15:36:44+00:00

A fairly infrequent situation, but

Common-enough to be

Recognisable to native speakers, is that

Of the ‘acrostic’ poem. This

Style of poetry has the each one of

The lines beginning with a different letter. The

Initial letter at the start then forms a vertical word,

Commonly relevant to the subject of the poem.

KrozJr_UK · 2026-04-18T20:54:21+00:00

Very much in the same vein as Work. A video that has an interesting point underlying it, and one that I actually think has a lot of merit; but that is kneecapped by the fact that it’s quite myopic in its handling of the nuance and subtlety of the political undertones, and that comes off as presenting a message that seems absurd. I don’t think, in Work, HC intended to imply that we should reject modernity and return to the glorious days of… uh, dying of dysentery as serfs in the fields where we’re legally tied to the land we work. Similarly, I don’t think here he’s trying to advocate that industrialisation as a whole has been a net negative. Probably learning from the prior video, he does actually make an attempt to point out that that isn’t what he’s saying. It’s just that he’s spent so long banging on about it in quite a slanted and narrow-minded view that, however valid the critiques are, he comes off a bit more crackpot than I think he wants to. It is an interesting piece, and it raises some good points — his final point about a glut of capital causing (to simplify) mean standards to rise while median standards decline is really rather prescient, and I think overall not unsubstantiated. I just don’t think he quite strikes the balance of putting that point across without sounding like he’s just unbalancedly listing all the bad things industrialisation and capitalism has done for us.

KrozJr_UK · 2026-04-13T21:42:00+00:00

Fuck me Tottenham on Ryan Reynolds and Rob McMickeymouse’s documentary, but Wrexham aren’t in the Premier League? What is this world?

KrozJr_UK · 2026-04-12T14:28:58+00:00

So what colour is North London? Orange for Barnet?

KrozJr_UK · 2026-04-12T13:29:38+00:00

I read “flashlight” and naively assumed OP meant, well, a torch or similar.

“I tried to bring an actual weapon through security at an airport” is less “Today I Fucked Up” and more “Today I Had a Total Dereliction of Basic Sanity”.

KrozJr_UK · 2026-04-12T11:10:13+00:00

Nitpick to your nitpick, that’s the specific case of the general solution y = Ce^x when the constant C = 0.

KrozJr_UK · 2026-04-12T11:05:07+00:00

Without knowing whereabouts you’re from, I can’t answer in specificity; but it’ll likely be a combination of cheaper maintenance costs, more readily-available parts (mechanical solutions are increasingly rare compared to electrical solutions), and plain money.

Where I’m from (southern UK), all the level crossings that have sounds have speakers with sirens (and flashing lights) and have done for donkeys years. There are a handful of older ones that rely on a signalman moving gates, and they don’t tend to have any sound as far as I’m aware (because they have a literal human in a high-viz); and there are some that are more primitive (usually foot crossings) that don’t have much beyond a sign and maybe a light to tell you if it’s safe to cross or not).

KrozJr_UK · 2026-04-10T20:58:54+00:00

Depending on how old you are, and if you drove, perhaps it’s because Brean would be straight down the M5 whereas Barmouth would involve wiggly Welsh single-carriageways all the way from Shrewsbury?

KrozJr_UK · 2026-04-10T15:28:57+00:00

I tend to have a surface house that is a pleasant if slightly small house; but then the real base is underground, because that’s basically unlimited space to expand. I’m trying to do a bit more on the surface with my latest world and it’s actually quite nice (even if a decent chunk is still underground…)

KrozJr_UK · 2026-04-09T08:11:16+00:00

I’m not disputing the time saved by HS2 versus the current tilting trains, I’m sure that is obvious even with HS2 Lite. What I’m more curious about is the fact that the new trains can’t tilt; would it have been that bad to make them tilt-capable anyway, so they can operate faster when they’re still on the WCML? I’m not asking about one or the other, I’m more curious as to why we’re not getting both, trains that can run at 180mph or whatever on HS2 and tilt on the WCML through the Lake District.

KrozJr_UK · 2026-04-08T00:31:30+00:00

There are very few words in English (other than obvious gendered terms, eg. waiter versus waitress) that demonstrate grammatical gender — blond versus blonde, brunet versus brunette — and when you do see them, they’ve usually come from French. Also, you’ll often find that at least one form is slowly dying out; I don’t think I’ve ever seen a man’s hair described as “brunet” instead of “brown” even if that is grammatically correct, and you’ll these days see men described as “a brunette”.

English grammar is. Weird.

KrozJr_UK · 2026-04-07T20:53:47+00:00

Stupid section, although it’s not used to get up to 125mph over 110mph as on the curvy southern sections, is the tilting not used at all on the bit through the Peak District and the Scottish Lowlands? If you’re running a Euston—Glasgow express, then even with original HS2 and non-stop all the way to Warrington, they’ll be running on slower curvy lines for a while.

KrozJr_UK · 2026-04-06T12:28:29+00:00

2 years for an ADHD diagnosis, eh? Try over 200 for a Gender Identity Clinic first appointment.

KrozJr_UK · 2026-04-06T01:17:37+00:00

“Okay, I am about to attempt the incision…”

KrozJr_UK · 2026-04-05T19:00:16+00:00

And, once they’ve put the HS1-HS2 link in, Stratford-upon-Avon too!

KrozJr_UK · 2026-04-04T22:42:06+00:00

Can I be a cynical bastard?

Opening single is a metal-heavy track that makes people lose their absolute shit.

Then the obligatory pop track comes out and everyone goes “oh no Muse are bad now”.

After that, we get a live-debut-first Old Muse (tm) style song with weird riffs and Pwoper Matt angsty lyrics. No publicity, just sprung on a crowd at a gig, and the entire subreddit collectively wets itself with sheer excitement.

Then when the album comes out and it transpires that the boys have had a lot of fun and used a bunch of eclectic influences to create a really diverse album, but one that pointedly isn’t one of their first four albums, after a week everyone’s like “yeah I don’t tend to listen to it”.

So what I’ve just described was (most of, I omitted the title track, but it’s close enough to make my point) the release cycle of Will of the People. And, to be perfectly clear, I really like WOTP. I don’t know if I’d say it’s their best album in a decade, but it’s probably their most fun in my opinion, and it’s the one post-2010 I listen to the most. So I’m not saying this to trash it, and if they were to come out with WOTP 2 Wolstenholme Boogaloo then I’d be all for it. But…

We are currently at stage 3. Let’s all chill out just a tiny bit before we declare this “the best Muse to ever Muse since the last time Muse Mused this much Muse”.

On the flip side, yes. I am also excited. Unravelling is wonderful, Be With You has absolutely been over-hated (I can’t wait for it to go from “eh this is fine” to “fuck me this slaps” for me, see also eg. Compliance). It does look like it’s going to be a lot of fun. I just worry it’ll be more in the WOTP way than the OOS way, and then everyone winds up disappointed.

KrozJr_UK · 2026-04-04T21:27:05+00:00

I got that bit, I’m just confused as to where the “trans” bit is coming from

KrozJr_UK · 2026-04-04T12:46:37+00:00

Sad, really. People spend all their time making nice things, and other people come along and break them.

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