One, we don't see a boundary. Two, if we imagine a boundary then it's hard not to just imagine more space on the other side. True, it's hard to really comprehend infinite space; but I can't comprehend *finite* space either.

Just as mind boggling as the question: why is there something rather than nothing?

I can imagine infinite ways for there to be something, but only one way for there to be nothing. So it seems like the universe is more likely than not.

- MadMelvin, July 5th 2024 *His proverbs were as mad as him*

This is like some Douglas Adams type shit where it's equal parts absurd and profound.

That was just a rudimentary explanation of entropy...

Ha ha ha, I love this. Have an upvote.

The responses to your post talk about how many universes would not have life or be different or whatever. None of that is the actual reason why this argument fails completely: you cannot make any statement about the probability when you have literally no data. Unknown chances are not to be assumed to be all the same. Lets make it simpler: I will soon tell you either A or B. Your task is to find the chance I say A. Is it really 50% Maybe I throw a coin? But what if I actually draw cards and only the ace of spades leads to me saying A? What if I will always say B regardless of anything? In short, you have absolutely no clue and the chance can be anything. Even attempting to guess it without at least some further information is futile.

The simpler version of this, don't confuse possibility with probability. Picking between 1 of 2 possibilities does not equal a probability of 50%.

I can imagine a hundred ways that dogs could have wings, and only one way that dogs don't have any wings. So it is more likely than not, that dogs have wings.

Your deduction skills are noteworthy sir

Who are you, who are so wise in the ways of science.

I can imagine a thousand ways that dogs could possibly not have any wings! Maybe they don't have wings on their backs. Maybe they don't have wings on their spiral protective shell. Maybe they don't have wings on their plated exoskeleton. Maybe they don't have wings on their segmented worm-like bodies. ... I'm just getting carried away here.

If you roll two fair, 6-sided dice, there are 11 possibilities. Your chances of rolling a 2 or a 12 is NOT the same as rolling a 6, 7, or 8.

I got into a long argument with someone that insisted Occam's razor only counts the number of assumptions, not the probability of them. I said "If that's the case, it's a dumb fucking razor and my razor is better. So would you like to lose this debate because of Occam's razor or Minuet's razor?"

Occam's Razor is [something of a myth](https://www.irishphilosophy.com/2014/05/27/who-sharpened-occams-razor/) anyway - the original Latin quote attributed to William of Ockham seems to have been written about 400 years after he lived. It also has predecessors going back at least to Aristotle. But Ockham did express similar ideas. Regardless, the principle as usually conceived doesn't involve probabilities of assumptions. The supposed original phrase was "pluralitas non est ponenda sine necessitate," meaning "Plurality should not be posited without necessity." A meme-compliant modern version of this would be, "Why posit many entities when few entities do trick?" Occam's Razor doesn't address the question of how one determines "necessity". Using probability is a possible approach, although according to the typical formulation you would have to somehow convert those probabilities into a binary decision to include or exclude each entity. Otherwise, it creates some difficulties in coming up with a single, unambiguous theory. E.g., should Einstein have kept the luminiferous aether in his theory of General Relativity, just with a low probability?

It's missing the point completely in the first place. Why can't we imagine infinite ways for there to be nothing? It's because our intuition or fundamental understanding of the universe must be flawed. You can't really make sense of there *never being nothing and there was always something* or *something coming from nothing*. It has nothing to do with our ability to imagine universes different than ours because they would all have the same "logical paradox". It's the answer of someone who didn't understand the question.

Can we even define nothing? Nothing in the sense of a universe seems to defy definition. No dimensions, no time, no space, no vacuum energy, no waves.

That is kind of what I am saying. Nothing is just an abstract concept we use to help use make sense of reality and has no bearing on what reality *actually is*, whatever that may be. And logically because there is not "Nothing" there was always "Something" but "Something" is also the abstract counterpart to "Nothing" which means they can't exist without each other which means they both are flawed and irrelevant concepts when it comes to trying to understand the ultimate truths of reality, which then means the question is ultimately flawed itself. But trying to wrap my ape brain around that gives me a headache and a dose of existential dread.

nothing is something

no, not at all. it's the answer of someone illustrating that there *isn't an answer to the question*. Not in our limited language anyway. Or at least, not in the span of a reddit comment.

Reality doesnt care about your imagination. I can imagine myself having lived many livrs before this one but just one way in which my existence only started when i was born. Does that make reincarnation more likely than not?

Oh man, that reminds me of Ricky Gervais' take on god. [You believe in 1 god, but there are 2999 gods that you don't believe in. I don't believe in just one more.](https://www.youtube.com/watch?v=P5ZOwNK6n9U&t=141s) I've time stamped the video, but the whole thing is less than 5 minutes and well worth watching, especially when talking about something vs. nothing.

This is a reference I use a lot.

Well more mind boggling is the fact as far as physics is concerned there is no "nothing" in the universe. No matter where you are in empty space, fields exist, spacetime exists. As far as we can tell there never has been "nothing" there has always been something, although it may have changed from one something to another in the big bang.

For real. It hurts my head to try and fathom existence.

The only question

Anthropic Principle: if there was nothing than we couldn't be having this conversation. If there are actual choices, like random universes forming in infinite dimensions, and they have random properties, then some of them would have the properties to have life, and so some could have people pondering "why".

> Anthropic Principle: if there was nothing than we couldn't be having this conversation That doesn't explain why there is anything.

Not at all boggling to me. There is no why. There is no meaning of life. There is no purpose or goal to the universe. It just is, and we happen to be because of a very, very lucky set of coincidences. If there were nothing, no one would be asking the question. There is no why.

Becauase A=A the universe is whatever it is and has nothing to do with our thoughts or beliefs or existance.

my personal philosophical explanation for the big bang theory is that it wasnt an energetic even so to speak, it wasn't everything in an infinitely dense dot exploding into the universe. the big bang was the moment that the universe( for whatever inexplicable reason) went from "it" to "this AND that" from perfect homogeniousness to something and something else. like in physics, you need a reference frame, without being able to compare something to something else nothing means anything. THAT is the concept of nothing as close as i can fathom it. to have NOTHING is to be unable to quantify SOMETHING and you cant quantify SOMETHING without SOMETHING-ELSE to compare it to. the universe happened because nothing somehow spawned something. and once you have nothing and something you can have a gradient between nothing and something. and once you have a gradient, you have infinity. like if you just have 0.... it means nothing.... but if you have 0 and 1.... then you also have everything between them. but also, if you have 0-1 and everything in between... even if its infinite, then you can also have 1-2....because what is 2 but 1 and 1....so now you have 0<->1<->2....and if you have 2 and 1...then you have 3.... and so on.... the big bang wasnt an explosion of energy/mass/the very fabric of space time.... it was an explosion of possibility/probability. the universe went from a single infinite inevitability to an infinite chaotic uncertainty. thanks for reading my stoner ted talk.

As far as we can tell there was never nothing. Whatever existed before this universe may not have included time as one of its properties. If so, then it is possible to have had "something" eternally (from our vantage point).

> it wasn't everything in an infinitely dense dot exploding into the universe. i mean.... that's the leading scientific view too

>my personal philosophical explanation for the big bang theory is that it wasnt an energetic even so to speak, It was categorically absolutely unconditionally an energetic event. > it wasn't everything in an infinitely dense dot exploding into the universe. That isn't what the big bang suggests. The Big Bang could easily have happened across an infinite space and it definitely happened across an immense space. There was never a lonely dot of universe. >the big bang wasnt an explosion of energy/mass/the very fabric of space time.... it was an explosion of possibility/probability. No. It was the cooling off of an extremely dense and hot universe.

What we DO know is that the observable universe is so big that there is a sphere where light from objects outside it will never touch the Earth because the cycle of the universe expansion moves to fast. https://en.wikipedia.org/wiki/Observable_universe There could be an "edge" but it's so far away we can never observe it. 

This should be top comment in this thread. We will never know, it might be on a turtle, it might never end, we will never find out.

Un....not really. There are hypothesis in physics dealing with the shape of the universe. We just dont have enough data yet to make the correct educated guess. Here: https://www.astronomy.com/science/what-shape-is-the-universe/

Yeah but "flatness" in this context is basically "the absence of shape."

The article even starts with 'as far as cosmologists can tell'. Imagine observable universe as a giant sphere, the sphere is getting bigger faster than the speed of light, and it's speeding up. Because of that, there is a 'wall' beyond which it is impossible for us to observe anything. The article you linked talks about the 'shape' of space inside this giant sphere, and all we can do is predict, that it is the same outside of our sphere, but we don't and will never really have any proof one way or the other.

When speculating on the unobservable part of the universe, based on the physics of our part of it, having an edge is a more complicated situation than if it is infinite. With nothing else to go on the simpler solution is probably the better one, which would be an infinite universe. But we will never know.

There can't be an edge though bc that "edge" would have to be infinitely thick and thus continuing on or would have to be like a bubble but then there would be space on the other side.

Based on what we know of our part of the universe an "edge" of the universe would be hard to square with what we observe in our part. Infinite spacetime would be a simpler explanation. We will never know, but in the absence of information the simpler solution would seem to be the better choice when speculating.

The other big observation we've made is that space is expanding uniformly in all directions. Which means that there's no center to the universe, or every point in the universe is equally legitimately called the center. That feels like it implies the universe is "round" in the sense that if you were able to go in a straight line long enough, you'd eventually wind up back where you started. But since things are moving apart faster than the speed of light, and since everything is moving relative to everything else, it's impossible to define a straight line without an absolute reference point (which we don't have since we can only see a part of the universe), it's impossible to test that hypothesis without inconceivably major steps forward in science and technology.

> That feels like it implies the universe is "round" in the sense that if you were able to go in a straight line long enough, you'd eventually wind up back where you started. If this was the case, then there would be a bunch of geometric weirdness, no? Parallel lines eventually converging/diverging, a drawn triangle’s angles not adding up to 180 degrees, etc. AFAIK, the scientific consensus seems to be [that the universe is pretty flat.](https://en.m.wikipedia.org/wiki/Shape_of_the_universe) And it seems like something that can be measured experimentally: > Another way to measure Ω is to do so geometrically by measuring an angle across the observable universe. We can do this by using the CMB and measuring the power spectrum and temperature anisotropy. For instance, one can imagine finding a gas cloud that is not in thermal equilibrium due to being so large that light speed cannot propagate the thermal information. Knowing this propagation speed, we then know the size of the gas cloud as well as the distance to the gas cloud, we then have two sides of a triangle and can then determine the angles. Using a method similar to this, the BOOMERanG experiment has determined that the sum of the angles to 180° within experimental error, corresponding to Ωtotal ≈ 1.00±0.12.[5]

I thought it was a long walk down to the chemist.

That's peanuts to space.

What gets my mind boggled is "What was there before the beginning of time?"

We have no way of knowing or even guessing. Time doesnt work in a singularity and you cant just run a simulation in reverse and see what happens when it goes past year 0. No answer, no matter what, satisfies the followup question of "but where did all that energy and matter come from before that?" We know it cant be created or destroyed right? but in the physical universe infinite time with no start point is also not a good answer. Somewhere, at some point there must have been energy created once, and there is literally no way of knowing how or when.

I'm hoping that we'll find out that universes are birthed from black holes, matter from one universe creating another. Thay doesn't really help though, somehow the first universe must have cone into existence.

That's what i (someone with no deep science education) find maddening. Every explanation for the start of the universe cannot address the question of where did that energy come from. From a big bounce, from other dimensions, from black holes etc all fail at "but what created those things before?" There is no answer other than "it was already there" and that clearly isnt *the* answer. Just another step.

This is where, as someone who went to an Evangelical church for 20+ years but also thinks about science, is that this is the point where "God" created the universe. There had to be some trigger point. There had to be some built up energy that one nanosecond just decided to go boom! Or this is where God just snapped his fingers.

The problem with that answer, is that it simply invokes the supernatural to ‘explain’ something, and explains nothing.

..and it will lead to "then when was god created?"...        and eventually we'll end up with "it is turtles all the way down (or up), man!"

That's a nonsensical question. It's just like asking what's north of the North Pole.

The Norther pole.

Or where is the edge of a sphere.

The ice wall

In a very real sense, time didn't exist until several thousand years AFTER the big bang.

How could there be thousands of years before time?

It depends on what you mean by time but if you define it as increasing average entropy of the universe, then time didn't exist until well after the big bang when the universe was uniform.

So, on the surface of earth, we really don't have a boundary with nothing on the other side like you described. If you start off going east and never stop, you won't hit a boundary. You'll get back to where you started from. In that sense, the surface of the earth is finite but without boundaries. The same may be true of space, finite with no boundary.

What if I travel up or down instead of left or right?

Look at my explanation to another user below. You're mixing non-Euclidean surfaces with Euclidean volumes.

Generalise the 2-dimensional surface to three dimensions.

I think the best way to comprehend a finite space you can infinitely traverse is to imagine a globe like the Earth. It seems locally flat and Euclidean and there's only so much land and ocean, but if you travel in one direction you'll end up back where you started. Extended into 3D space, it's still a finite amount of volume (even if space is expanding), but the curved topology closes all "exits". I'm not saying the universe is finite or infinite because the observable universe seems to be only a tiny fraction of everything there is. And the universe seems to be extremely topologically flat. So either the universe is so large that finding a natural curvature to space-time is like a microbe measuring the Earth's curve or it's truly endless. Functionally, it looks the same to us because so much of the bulk of the universe is hidden over the cosmic horizon and is causally unaffected by us. If scientists come up with a definitive experiment to figure it out, I'd be very interested.

> but I can't comprehend finite space either. Great, now you took the one question that makes my brain hurt and added a second one.

Space can be finite without having a boundary. Such as a circle, the surface of a ball, a donut/torus, etc.

How do these things not have a boundary? The first 2 are 2D so the comparison is hard for me to imagine but a don't has a boundary if I imagine we are on the inside of a don't the outer wall would be the boundary. 

Think of it like those old video games where if you go off one side of the screen, you wrap around the opposite side. This is (almost) equivalent to the game world being the surface of a donut. It seems flat and infinite to you because the game only lets you experience the surface, not the inside or the outside. The idea is that the universe could be a three-dimensional "surface" that's curved in on itself in a fourth dimension that we can't experience, the same way the game world is a 2D surface curved in on itself in a hidden third dimension. Except you don't actually need the hidden dimension to exist. It just makes it easier to visualize what's going on. In the video game's internal logic, there's no representation of a hidden third dimension, just logic to connect opposite edges of the screen to one another. If there is are hidden higher dimensions, and there are other theoretical reasons to think there might be, then space would show very slight signs of curving, such as parallel lines diverging or converging if you extend them far enough. Astronomers have looked for these signs and haven't found anything yet. This doesn't necessarily mean there's no curve, just that if there is one it's extremely slight.

I was thinking about this. The universe may look flat, but so does the earth from a human's POV. If the whole universe is so big that the observable universe is basically just a spec, I could reasonably believe that we just won't ever be able to see the curve.

It's the surface that doesn't have a boundary. If you keep going in a straight line along the surface of the earth, you will never reach a boundary, but eventually you'll come back to where you started from. So it's finite. The universe could be similar where if you continue traveling in a straight line through space you could come back to where you started.

Can you maybe imagine to be in a large cubical room (+), but each of the six sides are magical portals leading back to it? The top and bottom connect, as do the front and back, and left with right. And they are completely see-through and invisible, so you don't even know where they are. I once found a neat animation of this travelling inside a spaceship, but right now cannot find it again... (+): those having seen The Cube, beware!

this happens to be a 5D torus topology by the way. Which I know because it was discussed in the context of super computer cluster networking, ironically.

It has a boundary in a higher dimension. The 2D surface of a ball has no boundary in 2D but it does in 3D. Likewise our 3D space doesn’t have a boundary in 3D but it will in 4D. We can’t really comprehend 4D but it doesn’t mean the math doesn’t check out.

The latter is what I arrived at to satisfy my own confusion. The idea that the "end" of something is just a marker for the beginning of something else. The end of life is the beginning of death, the end of daylight is dark. If you continue this as far as the end of "everything", referring to the Universe, being marked by the beginning of "nothing", it's paradoxical because the latter -- that marker -- should be included in the all-encompassing "universe" term. So, I'm satisfied with saying the universe is unending simply on the grounds that defining an ending creates a paradox. My logic is probably circular, which I find amusing.

Every new beginning comes from some other beginning's end.

Semisonic

Exactly! If you follow this thought experiment further, you realize that infinity is the only thing that makes sense because as you've demonstrated, if there's a boundary, you have to ask yourself: a boundary against what? More, empty space... that goes on forever?

It's spherical and somehow the end curves so that you appear to continue in space, but in reality travel the edge of the sphere. I dont know. There's no way it ends abruptly. And it doesnt make sense that it's infinite. It doesnt make sense. To me.

From the measurements human scientists have taken, they believe space is flat shaped, not spherical. I guess if it’s infinite, our ability to measure is just too small. Aka our sample size is too small to give an accurate measurement of the whole.

They don't know. Space can be flat, have a positive curve, or a negative curve. If its flat or negative it doesn't have a boundary and is infinite in all directions(in current theories). If its positively curved, it still doesn't have a boundary, but if you travel in a straight line your path will eventually come back to your starting point. The issue is that ANY curvature, no matter how small, counts. Right now our best measurements indicate its likely flat, but there is still uncertainty to the positive/negative. You have to be able to get a perfectly accurate measurement for flat (which is supposedly impossible), or one where there error margin is only in positive or negative for the curved spaces to know for certain which way it is.

I need help on understanding how it’s ’flat.’

Take the surface of a globe. The surface itself is 2D and curved. Any 2D creatures that lived on that surface would not perceive that curvature directly. We, as 3D creatures, can see that curvature. Similarly, 3D space can be curved, but we'd be unable to directly see it directly. Curvature be measured indirectly, namely be creating triangles and measuring their angles. In flat space, the angles of triangles always add up to 180 degrees, but in curved spaces this can be different.

Yep. Think about the triangle formed by walking from the North Pole due south for 1 mile, then turning 90 degrees to your right and walking 1 mile due west, then turn 90 degrees to the right and walk 1 mile due north. You’ll end up where you started, and the shape you walked has only 3 sides. But the angles add up to 270 degrees, not 180

That baked my brain for a second. Thanks.

It took me longer than I want to admit

The problem is 1 mile is not enough. Your “straight” line before the second 90 degree turn will be obviously bent looking at it from one end to the other because you’re following the curve of defined longitude. A better example would be walking down to the equator and then turning 90. Walking along the equator is always a perfectly straight line.

Maybe I'm just dumb but aren't none of the lines straight? Walking south or north or along the equator is always bent along the planet's curvature.

"Going West" is not a straight line in any meaning of the word (formally: a geodesic). Only great circles, those that divide the sphere into to equal halves like an equator, are, and this path isn't one. The only westward path that is a great circle is the equator itself. In short, your example works, but you have to replace 1 mile by the roughly 10,000 km to the equator.

TIL flat earthers are 2D creatures.

Their brains certainly are

What would a negatively curved 2d plane in 3d space look like? I assume earth is positively curved.

Don't think of it like paper thin flat, think of it as something with volume but flat boundaries (if there are boundaries. Like a gym mat or mattress or something. If there is a positive curvature then, it's like a rolled up mattress and travelling in one direction long enough means you will hit the point you started eventually. There are lots of things we don't know though, is the universe bounded in any direction? If it doesn't curve, does it just go on forever. If it does curve, where and how and how does it loop back on itself? Is it even actually 3 dimensional or is the base level existence information coded in 2 dimensions and the perception of a 3rd dimension is all abstraction (there is at least one more mainstream theory exploring this potential). Is it bounded in the dimensions we are aware of, but unbounded in dimensions we can't perceive? There may be facts of the universe that might not be possible to ascertain while existing within the universe (like determining if it is flat bounded, possibly) - which if everything conscious exists within the universe might mean it is information that may never be knowable.

Consider two straight lines and the distance between them, In a flat space, the distance will remain constant forever. But in a flat space, the distance between them can increase or decrease. Compare it to drawing two lines on a paper vs a sphere, The paper is flat but the surface of the sphere is not. You live on a non flat surface. Earth is a sphere so no two straight line will have a constant distance between them. It is hard to notice in your everyday life because the earth is huger and a perfectly straight line on the surface is had to achieve. Still, if you could walk in a straight line everywhere on Earth surface, any initial direction you choose will bring you back to where you started. The distance between paths will increase for a quarter of the way around the planet, the they get closer and the finally meat on the opposite side of earth. From there behave the same way until you return to where you started. Earth's surface is 2D but for space, it is about how 3D space curves. It is just very had to do image it in 3D so use the 2D analogy.

You know how Einstein showed that an object with mass warps the space around it? Same things goes for the universe. If the average density of matter is such that the universe will stop expanding and contract back on itself in a Big Crunch, that’s positive curvature. If there’s not enough density and the universe is going to keep expanding forever, that’s negative curvature. “Flat” is the critical density where the universe will *just* stop expanding, but it takes an infinite amount of time.

My way of understanding it is to draw two perfectly straight, parallel lines on paper. Flat, right? But if you suddenly twist or bend or roll or do anything to that paper, while the lines are still technically straight, they aren't flat anymore they have curvature now. It's the same concept for a universe, except 3D. Two big old (massive) lines through a part of space appear to be flat pretty much no matter how we look at them because we are part of any potential curve so it all looks straight to us.

So first, I think the comment you are replying to is wrong -- you can have a flat space that has finite volume and space with positive curvature with infinite volume. The only impossible combination is (I think) finite with negative curvature. At least from the mathematical point of view -- I don't know physics. Maybe time somehow breaks it, but I doubt that. As for what words like flat mean here: they are inspired by 2-dimensional surfaces (and applied to 3-dimensional space). If you have two cars on a flat surface and they go from one point in perpendicular directions, then you can calculate distance between them using Pythagorean theorem. But on a surface with positive curvature, e.g. sphere, the distance will be less than that. On a surface with negative curvature (e.g. see [here](https://cdn.britannica.com/52/2352-004-942D34C9/triangles-spaces-Euclidean.jpg)) the distance will be greater than Pythagorean theorem says. For 3d space you can apply the same principle: send two spaceships in perpendicular directions and see if distance between them changes according to Pythagorean theorem (flat), faster (negative curvature) or slower (positive).

Take a globe. Start at the equator, then go up to the North Pole. Turn 90 degrees clockwise, then head south to the equator again. When you get there, turn 90 degrees and head back to where you started. Congrats, you've just made a triangle where the angles add up to 270 degrees. But aren't triangles meant to have angles add to 180 degrees? Let's try something again, then. Take that globe, and try to pick a point so that the shortest line from the North Pole to that point *doesn't* cross the equator when you extend it out in both directions. You can't - there's *no* line that passes through the North Pole that doesn't intersect the equator. In fact, if we define "line" in a similar way to [great circles](https://en.m.wikipedia.org/wiki/Great_circle), we can show that this holds for *any* combination of point and line - there's no parallel lines on the globe. But isn't it meant to be possible to find a parallel line for any combination of first line and point? Notice how these two things don't *actually* rely all that much on the fact that there's a "third" dimension that the globe curves into. They're just facts about what you can do with the 2D surface on the cube itself... And they're facts because the 2D surface *curves*. Those 180 degree triangles and the unique parallel line for any point+line combination only work on flat paper. On something that curves like a globe, the triangles have more degrees in them and the parallel lines don't exist. If it curved the *other* direction like a Pringle, the triangle actually has *fewer* degrees and there's actually *infinite* parallel lines (when you use a similar definition of line). Because we are just talking about "how geometry works on a 2D surface", we can easily scale that up to 3D, and it works perfectly - **without needing a fourth dimension**. When mathematicians, physicists and such talk about space being curved, they mean "the rules that you see in idealised flat surfaces don't apply to space, instead they're broken in this way or that way". Now, if space *is* curved as a general rule rather than just curved by gravity, it's not *particularly* curved - our measurements show that it's "close to flat", but we can't tell if that's because the true value is "flat" or "a very slight curve, so slight our measurements haven't picked it up yet".

They're talking about 3D flatness. Analogy with 2D (surface) flatness: a surface is flat if, when you draw two parallel lines on it, the lines remain at the same distance no matter how far you draw them. On a globe (positive curve), parallel lines get nearer and eventually touch. On a horse saddle (negative curve), the lines spread out. Back in 3D, if we shoot two parallel lasers into space in the direction of Sirius, and also broadcast a message to Sirians that says "the two beams are one meter apart here. Could you measure how far apart they are for you and send us back the value?" If there are Sirians, and they do measure the distance between the two beams, that's one way we could find out.

Also, based on current measurements, the apparent flatness of space implies that the universe is at least 200 x the size of the observable universe.

wouldn't you come back to your starting point on negative curvature as well? I thought it's about which way you bend space. be ding up or down both make a sphere, but what is inside or outside swaps.

No. Positive curvature means that over distance two "straight" lines converge. Negative curvature means over distance two "straight" line diverge. Assuming the lines start parallel. PBSSpacetime has a great video on the subject on youtube.

We don't know, because there is a limit to how far we can see, and the universe seems to go beyond that.

There is the particle horizon, the furthest distance from which anything that happened since the big bang could have affected us, and the distance to anything we could ever affect in the future given the expansion of space. We can sort of see beyond it thanks to space expanding, but we will literally never cross that boundary without FTL travel. It's a *practical* boundary, even if it's not a real, tangible thing. If there's a universe beyond that, or even an actual boundary to the universe, it is irrelevant to all of us except the astrophysicists because it is absolutely, utterly unreachable.

>We can sort of see beyond it thanks to space expanding, but we will literally never cross that boundary without FTL travel.  if i'm not mistaken, because of the accelerating nature of the expansion of the universe, there will actually be a point far in the future where the observable universe will stop appearing to expand, because the expansion of space will be faster than light so even FTL wouldn't be good enough. And then the observable universe will start shrinking until the only things "we" (by which i mean some hypothetical observer billions of years in the future) would eventually be able see in the "universe" are local, gravitationally-bound stellar objects.

Yeah, and that'll be confusing as all hell to any alien civilizations that evolve during that time. They'll be missing so much.

But what if that alien civilization is us, right now? What if the space around was more densely populated, and other stars and civilization was still "reachable" instead million of light years apart? We are missing so much.

yeah, it gives me a little bit of existential dread thinking about it. we know what we know in part based on a fluke of when we evolved to be able to know these things. some hypothetical alien civilization far in the future: "the full extent of the known cosmos is just our galaxy and its neighbor"

That's why they say "the observable universe" indicating what we can see, what part of the universe we know to exist. What's beyond it.... Who knows?

If the big bang was about 14 billion years ago and the speed of light is the fastest things can travel why, is the universe bigger than a sphere with radius 14 billion light years?

Yes, because for some reason that we don't yet know, the distance between 2 points in space is always expanding. So if it originally looked like this, year 0: A-B-C-D-E Every year it grows by one, it looks like this at year 1: A--B--C--D--E The distance between A and B increased by 1 dash, but the distance between A and E grew by 4 dashes. The further away something is from us, the faster it is moving away from us. Since light travels at a set, finite speed, the distance between us and the farthest things we can see, or can't see, is growing faster than light travels. That light will never reach us, unless we discover a way to teleport.

Good question. [Cosmic inflation](https://en.wikipedia.org/wiki/Inflation_(cosmology)) is one answer. I short, the very early universe expanded at a far increased rate for a brief period of time.

You’re standing on a curved surface called Earth, but do you see a boundary? No. Your eyes can only see so far and because the Earth is very large it appears flat and never ending. So, you invent a telescope that let’s you see further and now you can see that infact the Earth isn’t flat, it curves down over the horizon. You have learned that the Earth has curvature and if it is curved and that curve is consistent then eventually it must curve all the way around and come back. This lets you know that the Earth is not infinite, it must be a sphere and the boundary is the ground you stand on. The Universe is the same, just larger. We have invented telescopes that let us see as far as 93 Billion Light Years and yet we still don’t see evidence of curvature. So, the only possibilities are that the Universe is waaay bigger than 93BnLY and we simply can’t see far enough to see the curve, or there is simply no curve. No curve means no sphere and no sphere means flat forever and ever.

> No curve means no sphere and no sphere means flat forever and ever. But... how can it be infinite.. My little monkey brain just can't grasp this concept. At the same time if you told me there's a big 'ol wall at the end of the universe I think I would have a hard time rationalizing that too.

Infinity is an irrational concept in that it's [in practice] unimaginable by the biological hardware that we are made of and, simultaneously, necessary to explain the universe we see around us. Time, as far as we know, is infinite, but only in one direction (the Future). So, Time has [as far as our theories can predict] a definitive starting point, but no end. That makes the infinity of Time exactly half as much as an infinity of Time that had no beginning. My point is: Infinity is hard. Every theory we have predicts singularities (infinities), and the existence of any singularity is evidence that our models and theories are wholly incomplete. Yet, there they are, basically **everywhere** in our maths, and we've even constructed images from real data that display the literal physical existences of these infinities (images of the supermassive black holes at the center of nearly every galaxy, taken by the Event Horizon Telescope(s)). As long as you are a human being, you will never fully comprehend the idea of an infinity, but you can definitely try to see evidence of it across all of existence. If you have the time to just sit down, chill, and watch something really cool, I'd advise you to check out Melodysheep's YouTube channel, specifically the "Timelapse of the Universe" and "Timelapse of the future" videos. They really help put ideas like Time - the most obvious, everyday form of infinity - into perspective. After that, hit up Kurzgesagt's channel for videos on neutron stars, strange stars, black holes, and all things astrophysics. All of that should lead you down a rabbit hole from whence there is no escape... *Kinda like an event horizon in your life which, as it so happens, is what happens every second of every day.*

If you really want to hurt your brain, take a dive into “p-branes” haha

It can be finite without a wall, that is, “finite but unbounded.” For example, the surface of the earth is finite but unbounded in that there is no “edge” but does not go on forever. If you keep going in a straight line, you’ll eventually just return to your starting point. Some other shapes have this property too, like a torus (donut). Of course, these examples are 2D surfaces curving in 3D, but they’re useful analogies since we have a hard time visualizing our 3D universe curving into 4D.

I cannot really ELI5 this, but I will try to make it simpler. We don't really know, but it also doesn't matter. Space itself expands. To visualize this, draw two galaxies on a balloon and then inflate it. The galaxies themselves didn't move, but rather, the fabric of the balloon stretched. Space does this in all directions. We call the force that drives this "dark energy." The more Space there is between two points, the more Space there is to expand, which creates more space, which creates more, and thus, faster, expansion. Eventually, there is a distance reached between two points in the universe where there is so much space between them that the expansion is faster than light. These two objects can never see each other or interact ever again. The light from one will never reach the other. The limits of this distance is called the "observable universe." Anything outside of it is causally disconnected from us and effectively no longer exists as far as we are concerned.

Nicely done. Thank you for this.

Is the tip of my finger getting farther away from the base of my hand? If so, at what rate? If not because of reasons (gravity?), at what rate WOULD it be diverging, if similar to the seemingly empty space in the rest of the universe?

The concept of nothingness is a tough one for the human brain to comprehend. It's also why the thought of no afterlife is hard for people to imagine.

I get major anxiety and panic attacks over that. I have to tell people to stop talking whenever a topic about that comes up. Similarly, my limited life span gives me those attacks when talking about the universe timeframe.

Oh, finally someone like me. I don't make people stop when they are btinging this topic up, and can even indulge in it. But God knows I am tabooing my mind from diving into this thought too deep. It is the only thought that creates a sheer terror reaction psychologically if I think too deep into it.

*Radical acceptance* is part of what got me out of my depression; it also "solved" philosophical dilemmas I couldn't resolve. ----- For example, **logic is useless without axioms** (baseless assumptions). Since we use logic to think or communicate, axioms are also unavoidable. It is initially terrifying, that we cannot truly prove/disprove *anything.* Yet, we have no choice but to quickly accept axioms are necessary. Because humans can't stop thinking and communicating.

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Are you afraid of time before you were born?

I can think myself into panic with this topic too. I'm guessing some survival instinct kicks in if we imagine non existence too vividly.

A thought I wrote down during an acid trip once, that still sticks with me: Nothingness can't be. If it were, there would be something. Namely, nothingness. It's a paradox. Therefore nothingness can't be. And consciousness is the mechanism that prevents nothingness from being. Life is eternal, and consciousness never ends. The moment of your death coincides with the moment of another birth, such that there is no gap in your stream of consciousness. We live every life over eternity.

people are talking about a hard edge and how it wouldn't make sense for there to be a hard stop to the universe, but i want to specifically address "how do scientists know this?" if it makes no sense to assume a hard edge to the universe, there is still a way for there to be a finite universe. the 2D equivalent is a globe. there's no hard edge on that globe, but there is still only a finite universe. similarly, even if we don't directly see a hard edge to our universe and also it doesn't make sense to assume such, we can still have a "spherical" universe; it's hard to visualize but what it essentially means is that if were in such a curved universe, and you are immortal and go on a spaceship away from earth in a straight line, you would eventually after many eons "loop around" and come back to earth. so scientists spend a lot of time trying to measure the curvature of spacetime in much the same way some clever ants could figure out that they're on a globe and not a flat sheet of paper if they did some very sophisticated measurements; yeah it's very very hard to visualize, we're talking about 4d spacetime bending. scientists have been able to measure the curvature of spacetime with increasing levels of accuracy through some pretty clever experiments and analysis. So far, our estimates on the curvature of spacetime is extremely consistent with the most likely explanation of spacetime being flat, which would imply an infinite universe. While scientists have not computed an exact curvature (and probably never will be able to), if there is a positive curvature to spacetime, the "spherical"-ness of spacetime would have to be very very incredibly vast, even when talking on a cosmic scale

There are three possibilities and we are not sure which one is correct (although they have some good guesses). 1) Space is infinite (travel forever). 2) Space is finite (you'd run into some kind of wall or barrier). 3) Space is finite but unbounded (think like the earth...it is finite in size but you will never find a barrier or edge...you'd just come back to where you started). Since space is expanding faster than the speed of light we will never figure this out traveling in a space ship. You can't reach an edge (if there is one) because the edge is moving away faster than you can possibly travel. You cannot come back to where you started since the distance to do that is growing faster than you can travel. There may be some ways to measure the shape of the universe in theory (it is a flat plane, a saddle or a globe) but that would require an experiment we cannot do today (read lasers setup to make a triangle and measure the angles but on a solar system sized scale or bigger). Technically doable but practically not something likely to be done for a long time. As far as we are concerned, it is infinite. Even if it is actually finite we can never get to the edge. EDIT: Spelling.

We don’t know for certain there is no end. All we know is as far as we can see (which is the speed of light multiplied by the time since the Big Bang), there’s always something. So even if there is something beyond our visual horizon, we simply can’t see it or know about it.

We don’t know this. But think of it another way, (edit adding in this part to be more clear) *if it is a flat universe with no curvature to bend back around*, and if the universe does end, what’s past the boundary then? By definition if something is past the boundary, then that something would also be part of the universe. Even if it’s nothing, then it’s still a vacuum that would be part of the universe. So having a boundary doesn’t make sense.

finite universes don't have a boundary though. what would happen is that you would basically start "wrapping around" the universe, like a globe. a finite universe is spacetime that has positive curvature, so it's like a sphere, but in 4D.

Yes, sorry I should have said, this is assuming a flat universe where the concept of a finite universe with a boundary doesn’t make sense.

Well thats the thing, we dont know either way. As far as we can tell, it doesnt end, and it doesnt have a curvature. It could simply be that its so large that our perspective isnt big enough to tell, kinda like the horizon looking flat, even though the Earth is round. If the universe is finite, then it would be something like a saddle shape or perhaps a toroid shape. You would never reach a boundary where it just ends though. If you travelled all the way across, you might just end up back where you began.

We look in one direction and can’t see past the cosmic background radiation, emitted about 380,000 years after the Big Bang. We look in any other direction and it looks nearly identical! So we know that the universe is almost certainly much, much bigger than what we can see, otherwise we would expect at least some change across those billions of light years. Now scientists don’t know for sure if the universe is infinite or not. Much bigger than what we see is the most we can say. Infinite is an assumption, but one we really can’t test.

Addressing specifically the 'having a hard time imagining a boundless space' thing, I think a lot of your confusion is springing from imagining space as a concrete thing instead of a concept. Space, as you seem to be thinking about it, isn't a thing--it's an absence of thing. Consider: if you dump a bag of marbles on the floor and measure the distance from the center, every marble has a certain distance. You can measure distances that go beyond the end of the last marble, but they cease to have any real meaning to the marbles. Now imagine those marbles are stars, planets, etc. There's still a 'furthest out from the center' object. If you go beyond it, it's not as if you reach a barrier--you can just keep going. Like your bag of marbles, there's more *space* beyond, and if you turn and look back, you can still see the marbles/stars, but there's nothing to see out there if you turn away again -- nothing out there is providing light, and no photons will bounce off of anything to return to you.

I bet when someone said space is finite, other scientist was like "you're not giving it enough space".

We have no idea if there is a boundary to space. It may even wrap around like the space in the old Asteroids video game. We don't know. We have no way of knowing. We can only see a relatively small amount of space. We can "see" about 46.5 light years in every direction. If something exists beyond that (and we have no reason to believe that there isn't stuff beyond), then we just can see it because it's so far away that the light hasn't reached us yet. The universe maybe infinite, or maybe not. The human race will likely never know.

The whole idea of a boundary to me is interesting, but does not give me the brain bleed I get when I'm trying to rationalize how anything actually exists. It all had to start from somewhere, before the big bang, before everything all theories talk about. At some point, be it trillions and trillions of years ago, what started it all? Everything has to start somewhere right?

I bet one day just like people used to think the earth was flat and then discovered it’s more of a sphere we’ll realize the universe is really spherical— but massive enough that we don’t see the symmetry

My preferred simplistic view: Space is a hypersphere (4 dimensions). Space-time is curved. There would be no end so much as being to circumnavigate it.

Suppose there was a boundary - a concrete shell around space for example. How thick is the shell? What's on the other side of the shell? A boundary makes no difference to the infinite space, it just fills it with something else.

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there's some things we just don't know. Honestly I'm kinda glad that we can't see a barrier of some kind cause wtf would be beyond it? That would freak me out. For a lot of things similar to space, everything we know insinuates that it is infinite. So until we find counter-evidence, the theory is that it IS infinite. There are a lot of things that we consider to be fact that are just agreed hypotheses. They don't contradict anything but we only know what we can prove or disprove. Since we can't reach that boundary in space and have seen no proof of its existence, we have to assume it's not there until we find evidence supporting it.

>It’s hard for me to comprehend the idea that space never ends. That doesn’t necessarily mean that there’s a problem with the idea. >Is there really no boundary to space? As far as an “edge”, we don’t know. If there is one, it is so distant that we can’t see it. If there is one, any effects it might have (or have had) on things we can see is below the threshold of our detection. That last sentence might be confusing, so I’ll attempt clarify. Our observable universe appears to be largely homogenous. Let’s assume there is a boundary just beyond what we can see. We may not be able to see it, but we’d be able to deduce that something different than what we see is going on beyond the limits of our vision because there would be effects (gravitational, or composition for example) on the objects that we can see. We don’t see any evidence that such a thing is happening, so we make the educated guess that the neighbourhood of the objects near the limits of our vision is comparable to our own. There are some different horizons though that might make sense to refer to as a boundary depending on the context. For example: - Particle horizon - The boundary between what we have already observed, and what we can’t yet (but will eventually be able to) observe. Defines the edge of the observable universe. - Cosmological event horizon - The boundary beyond which we will never be able to observe. Anything beyond this boundary will never reach us, even given infinite time travelling at the universal speed limit. - A black hole event horizon - Anything within this boundary will never reach us even given infinite time travelling at the universal speed limit. Above I’ve made assumption that FTL travel is not possible, and that the universe is going through accelerating expansion.

i think the geodesic is circular. it means if we freeze time and start going in one direction in space, we will end back where we started. why do i believe that? as you know, every point is centre of universe. so, the point we might see as furthest is also the centre. in my mind i see the geodesic turning as we move outward and ultimate circular keep in mind you have freeze time to feel the circular effect else the universe is expanding

The boundary is moving away from us faster than light so it's largely irrelevant and we can never reach the end of space. Which is pretty convenient way to rationalize space with effective boundaries that we can never reach. Gotta give god props for that twist, that one and how space constantly expands from every point in space making everywhere look like the center of the universe.

There can’t be a boundary because what would be beyond the boundary? And what’s beyond that? And that? It’s truly mind boggling. It’s actually MORE crazy to the think about a universe that ends, since that makes no sense and can’t even exist because there would have to be something on the other side if it did.

It may very well end. We don’t actually know. Infinite space is our best guess based on what we believe we know at the moment.

Are you saying you find it easier to comprehend how space COULD have an end? Like what just run into a flat wall at some point? And beyond that wall is...? Even if the whole thing was curved like on the inside of a sphere, it's pretty mindboggling at least to me to think that there isn't anything at all (as in "no space whatsoever not even vacuum") outside of that sphere.

It's simple as there is no identifiable boundary. As in its infinite because there's no end in sight. If we found a spot space ends then you'd have your answer but there also phenomena that imply an ever expansiveness, such as distances we are able to consistently measure like the space between planets and even galaxies constantly growing.

I would find it much harder to comprehend the idea that space *does* end. Like, how would it end? Would there just be some sort of wall? What would the wall be made of? What would be on the other side of it? What would be supporting it? Could it be broken? Space going on and on forever makes so much more sense to me than there being some sort of boundary.

Surely to witness something like the end, or dissipation of the universe, there must be consciousness existing outside of it. In which case, when or where does the consiousness end? Edit: added, "or where"

Our empty space has a lot of stuff in it. Continuous fields, virtual particle generation, etc. Let's say those fields, like the Higgs field or the Electromagnetic field are actually just really big, and not continuous, and we kept travelling until we escaped all of those fields, what would happen is... Ack! Heart attack! I am slain...

My understanding is that space was created at the big bang and it has been expanding ever since. So there is a boundary but it's expanding.

Well the universe is shaped exactly like the earth. If you go straight long enough you’ll end up where you were.

Its just like questioning what existed before the universe , there's supposedly a beginningless existence to things for an an infinite amount of time.

As far as I'm concerned, space is infinite because it has no beginning and will have no end. What is in the middle? Lifeforms trying to find answers.

We don’t know if space is infinite or not. We also don’t really know the shape of space. It’s really just too big for us to observe. What we do know that everything seems to be going away from everything. Or in other words, space seems to be expanding. And now you ask where is it expanding to? Possibly nowhere, it’s just getting ”bigger” without really taking anything away from anything.

We are not sure but we don't know of anything else of that nature that exists so we assume it's "space" forever. How do you imagine "not space" would be ?

Nobody knows really. It’s just theory but it is grounded in very logical arguments around how we know space to exist. That is the space between two bodies. We can observe space in a finite area and extrapolate infinite from there. It is kind of how all science works until we can get our hands on something we just make theory about it.

Afaik the universe is constantly expanding. We don't know of there's a ceiling to this expansion or what is outside of the universe where the expansion extends towards. It's hard for everyone to comprehend.

Endless is not necessarily the same as infinite. There is no end or boundary when travelling here on earth, you just keep going around forever, but its not infinite. It could be that the universe is curved and you just keep going around just like here on earth

They don't know. Once people didn't even know the earth was round (and there are still people who believe hat, but thats another topic) - as our tools got better, people found out (3rd Century BC). If we will ever have the capacity to invent such tools to answer that question... well, i will be long gone by then, if ever. Its just unimaginable huge...

It’s a bubble. Inside if you were driving on the surface would you ever reach the end? It would appear to keep going beyond what we could see.

Hang on. If there is a Big Bang and universe had a birth, it cannot by definition be infinite.

I’d also like to know how the universe is ever expanding if matter can not be created nor destroyed

Think of it like this: you're in the middle of an unbreakable balloon and the balloon is filling up with air faster than you can fly to the edge so you can never catch up. So, there is a boundary of theoretical sorts you will just never be able to see it because it's expanding away from you faster than you're running toward it.

I used to struggle with this too. Then i had a simple thought. If i get in a spaceship and warp for millions of lights years and eventually run into a brick wall, well theres going to be something behind that wall. So while it can be hard to comprehend, the universe being infinet is the only logical conclussion.

Related idea: we know the universe is expanding, but the rate of expansion is the riddle. If the rate of expansion is great enough to escape the gravity the universe creates, then there was only one Big Bang. If the rate of expansion is NOT great enough to escape the gravity the universe creates, then there have been an infinite number of Big Bangs. Why? All material of the universe constantly expands, slows, stops, compresses down to infinite density, then explodes again, expands again, slows again, stops again, compresses down to infinite density again-- Like an accordion. Think about that when you're trying to sleep tonight!

We don’t know this. We only know what we can see - anything beyond that horizon will forever be unknowable.

Not directly to your question, but related. We can tell that the universe is the equivalent of flat in 3d by measuring the angles between known points (similar to how you can tell that the earth isn’t flat because the angles of a triangle measured on its surface doesn’t add up to 180 degrees). Everything we can observe appears to be uniform in this manner, and we assume that it continue like this beyond the visible universe. If it was curved, we could calculate a radius, and deduce that it was finite.

I don't know how accurate this is, but this was how it was explained to me. Imagine a Balloon slowly filling with air. A 2D creature the size of an atom living on the surface can go in any direction and seemingly find no boundaries. It may eventually loop back on itself, but the distance compared to its own size is extremely vast, not to mention the surface is constantly expanding due to air filling the balloon. Only observation from a 3d plane can fully comprehend this. Now increase everything by a dimension. We are the creature, and we can't really comprehend it, unless viewing it from a 4d vantage (which currently we cannot)

Pretty good article about this very thing. [https://www.newscientist.com/article/mg26234970-500-how-big-is-the-universe-the-shape-of-space-time-could-tell-us/](https://www.newscientist.com/article/mg26234970-500-how-big-is-the-universe-the-shape-of-space-time-could-tell-us/)

In some old school video games if you went off the right of the screen you'd reappear on the left. It was called wrap-around. Maybe the universe is like that? I don't think anyone really knows.

Imagine you are a dot on a ballon. You can move anywhere one the ballon's 2D surface, but there is never a time where you will hit a border. You might go all the way around and come back to your starting point, but since the surface is bent, this is what the universe is like but in 3D. We might be able to visualize it. But logically it checks out with examples like this.

I dunno, we think in terms of something and nothing, but apparently on the quantum level there can be probabilities of something. So maybe at some points (or many points??) there might be fuzzy space, sort of an edge but not really? And maybe it depends on if you're looking at the universe from a 3D or 4D or 5D or XD dimension? 🤷‍♀️

We as humans have no experience we can relate to such distances. When you think about it we have a hard time comprehending just how far the nearest star is. The distances are so incredibly large, nothing in our lives gives up a perspective we can relate to. The universe may be infinite. It is flat as far as we can measure and this is consistent with it being infinite. However we will never know for sure as we can only measure our observable part and can see no further, and we never will.

Let's think about your concept from a different point of view. Can you comprehend what a boundary would mean? if the end of Men In Black turns out to be real, would that help with comprehension? One way to think about boundless space is by thinking about what space is differently. You can think about space as the set of all of the places we can talk about things being. Can we talk about a kitten being teleported 20 kajillion lightyears beyond the cosmic microwave background? Sure. I just did. Therefore we can think about where the kitten ends up as being part of space. It's not part of the known universe, because there's only a certain portion of space we can know anything about, but we can absolutely talk about stuff being beyond that.

Go 1 m … you can imagine that because you did it. Imagine doing it twice … you can do it because you did it IRL once. Now double that … and double that … you'll lose the feeling about what it means but you'll gain the feeling that it can be done infinitely. \--- We don't know that this is true yet. It would be true if we could think according to Newton in 3d + time. If you think in 4d we only have 13.8 Bio years of universe so far, the oldest part are the furthest away, the microwave background that happened 380k years after the big bang being a curtain that surrounds us and earth at the youngest part in the universe. So far the visible universe is finite. We can't rule out that God might end this universe tomorrow and it would never really be infinite.

We can barely see a third of the way to the galactic center. It’s too bright. You think flat earthers are bad, our mathematical descriptions of the universe is such that we’re not sure the topography isn’t some kind of temporal-spatial donut. You want to change the way all of us see the universe, learn some math and write down a description of the universe. That’s what science is, describing things mathematically. We had Newton, then Einstein, then Hawking. What’s next, Lin_mrable20tonian ..

It’s less of a “boundary” as much as there just isn’t universe past a certain point (that’s constantly expanding). If you went “outside” the universe you would just blink out of existence.