What happens when I reach the edge of the universe?

Factual Questions
21

There is no edge to the universe, any more than there’s an edge to the Earth.

If we imagine a small universe, what happens is you sail farther and farther away from Earth, and eventually you see a sun in front of you and orbiting that sun is Earth.

Except in our universe, even if our universe turns out to be finite and curved, you’d never be able to actually do this because even travelling at 99.99% of the speed of light you’d never reach the antipode–the antipode is receding away from you faster than the speed of light.

Imagine trying to sail around the earth, but the earth is expanding faster than your ship can sail. You’ll never get back to your home harbor because the ocean keeps getting bigger.

It could have been that we lived in a universe that was small enough that it would be possible to travel back to your starting point by going away far enough. Or to put it another way, we could train our telescopes out into distant space, and see a very faint galaxy and recognize it as our own milky way, except in the distant past. But we don’t live in such a small universe, light from the early milky way can never travel all the way around the universe and back to our telescopes because the universe is too large, and is expanding fast enough that the light from that early milky way isn’t closer to us every day, it’s farther away from us every day and therefore will never reach us.

This is assuming we’re talking about a spaceship that can travel Nearly As Fast As Light. If you’re asking about a spaceship that can travel Faster Than Light, who knows what would happen, because if FTL travel is possible in our universe then our current understanding of the universe is seriously flawed.

22

Well, here’s your first mistake. You’re thinking in terms of traveling in a straight line away from the earth. But while you’re doing this, for billions of years, think of the entire universe rotating for those same billions of years, so when you look back, you don’t see a straight line between you and the earth. You see the entirety of the universe, including you and the earth, curving and expanding. So while you think you’re going in a straight line toward some kind of “edge,” you’re just curving along with everything else in the universe.

Another way to look at it: You’re an ant, and you spend your life following the ant in front of you, while he’s following the ant in front of him. There’s another ant behind you that is following you, and another ant behind him, etc. Each ant thinks it’s part of a straight line of ants, and assumes that someday they will reach the end of the line. What they don’t realize is that they’re all traveling around a large tree.

23

No, you can’t imagine it, and you won’t be able to. It is not your fault, though. The human imagination is capable of imagining things in no more than three spatial dimensions, since our brains and perceptual systems have evolved in and for a 3D world. However, just as a 2D surface can be curved back on itself through the third dimension, so a 3D space could be, and may actually be, curved back on itself through a 4th dimension. We can represent this mathematically, but we can’t visualize it.

24

And you don’t even actually need the 4th dimension to do it.

It’s also not impossible to visualize, just really, really difficult. Like, you might be able to if you spend a decade practicing it.

25

I had always heard/assumed that with a closed universe you would reach a point on your travels where you would begin to head back towards your point of origin. You wouldn’t change direction any more than you would if circumnavigating the Earth - keep travelling east and you would eventually end up back home. Is that no longer correct?

26

:dubious: I know you know lots more about this sort of stuff than I do, but I feel like I am being whooshed here. Would you care to explain?

Actually, I do know rather a lot about visual imagination, and I have never heard of any visualization that, if it can be done at all, takes years to learn to do. (It might take a while to learn to accurately and completely visually imagine something showing huge amounts of intricate detail, but that does not seem likely to be to the point here.) However, as I have no idea what you are getting at in your first sentence, I cannot really assess what you might be on about in the subsequent two.

27

Conceptually it’s like travelling on the surface of a near infinite sphere. There is no point at which you will reach an edge.

28

Well I know less than both of you, but here’s a model anyway. Consider the 2D game of Asteroids. Asteroids drift towards one end of the screen and come out the other end. That’s a simulation of an endless plane (a rather small endless plane, actually). Asteroids can keep moving in the same direction indefinitely.

It’s not hard to imagine that in a 3D context. I don’t know whether any physicist thinks of the universe that way though.

29

It does depend on the nature of the curved space. There are some curved spaces (toroids are one) where there are some straight-line paths that never curve back to their origin (or at least, take multiple traverses of the entire space before they do). So the universe could be somewhat smaller, but all visible lightpaths are long …

I don’t actually think that this is the case, and I am sure that there are proofs of that, but it is interesting to speculate. :wink:

30

Ahem…maybe this visualization will help? From like, the top of the thread? I was hoping someone else might weigh in on this and explain it better, but the conversation has drifted completely in other directions.

In addition to above, if you are travelling through space in this manner, then when you finally got far enough away form earth that it began to “curve around the ball”, so much time would have passed that you would never be able to get back to it, because so much time would have passed (relativistically) that you’d be going back to the area where earth used to be, and probably long since disappeared, swallowed by the sun. Just because you can travel faster than c in this thought experiment, doesn’t mean the rest of the universe isn’t plugging along as normal. Something about time travelling at diverse speeds in diverse places - or some such rubbish :slight_smile:

I hope someone with some smarts can address the idea I just spewed out and either explain why it’s totally wrong, or reinforce it with better words so the **OP **can understand.

31

Maybe where you live. :stuck_out_tongue:

32

As mentioned, we do. But remember, if the light is just now reaching us, these stars are very, very far away, so they’re very very dim. We only see them when we’re looking very very hard. It’s not like a new Arcturus is going to suddenly appear and change our constellations.

33

Because the math for the data cosmological physicists have works either way.

Nobody has disproven the 3d-space in a 4d-shape that bends around on itself (at least, not as of the writing of “The Fabric of the Cosmos.” And I’d think it’d be a big news story).

Nobody has disproven the model of space where it’s 3-d and flat and infinite (the ‘obvious’ model).

So nobody knows, but I think the flat model is more popular these days.

34

A nitpick. It’s not certain that the world *is *3-D. Three dimensions just happens to be a convenient calculation space for solving the sorts of problems that faced our ancestors on the African veldt.

35

We do. I would imagine that a lot of it is lost because the “new” star happens to be in a fairly straight line with another and gets occluded. But keep in mind theCrab Nebula, , which exploded a long time ago, but the light from the explosion didn’t get here until somewhere around 1054 CE.

36

Huh, didn’t know that. Interesting.

37

The game area of Asteroids can be projected onto a torus, a three dimensional shape that looks like a donut or ring. Go off the top, end up on the bottom, go off the right, end up on the left. It doesn’t work on a sphere. When you hear talk about the curvature of space, it’s kind of like the pilot of the Asteroids ship trying to detirming what 3D solid his game is being played on.

38

The only dimensions physicists concern themselves with are those that can have an effect on our observations. One can describe, say, a sphere in terms of two coordinates (say, latitude and longitude), and so we say that a sphere is two-dimensional. And based on measurements confined entirely to those two dimensions, we can determine that a sphere is curved (for instance, the sum of the angles of a large triangle will be found to be greater than 180 degrees). Now, one could also measure the curvature of a sphere (such as the surface of the Earth) by experiments involving drilling down into the bulk of the Earth, or towers that extend above the surface of the Earth. In that case, the third dimension is relevant. But drilling or towers aren’t necessary for measuring the curvature.

In the Universe we live in, we know of no way to “drill” or “tower” at right angles to our familiar three spatial dimensions, so we say that, so far as our experiments can determine, we have three spatial dimensions (some models say more, but those always come with experiments that could be done, at least in principle, to “drill” or “tower”). At the same time, though, we can also do experiments, confined entirely to our three-dimensional space, to measure the curvature, and those experiments tell us that we do have some curvature to space. When we say that, though, it doesn’t necessarily imply anything about any other dimensions: It just says that we live in a world where those sorts of measurements give those sorts of results.
One analogy that’s due to Einstein: Suppose we have a very large flat tabletop, and suppose that we measure distances on that tabletop by means of a bunch of uniform brass rods of some length. If something is as long as three brass rods laid end to end, then we can say that that thing is three rods long. Now, if the table is flat, we can set up a nice uniform grid on the table: Put four rods on the table at equal angles from each other, then at the end of each of those rods put three more at the same angles, and so on. You’ll find that the ends of rods all exactly meet up with the ends of other rods, and you get something like graph paper.

Try the same thing with a bulging table, and it won’t work: You might have something that approximately resembles a grid at first, but as you get further from your starting point, you’ll find that you need to distort the angles, or stretch or compress some of the rods, or have the rods not all meet exactly end-to-end. Based on this distortion of the grid, you can say that the table is curved.

But now suppose that instead of a bulging table, you have a table that’s heated unevenly. There are some hot spots on the table and some cold spots. Wherever the table is hot, the brass rods expand slightly, and wherever it’s cold, they contract slightly. If you try to arrange your rods on this heated table into a grid, you’ll find that it doesn’t work: You’ll get distortions in angles, or lengths, or meeting points, just like if the table was bulged. And so, you could again say (based on those measurements of the grid) that the table is curved. Except there’s no bulge in it, no extra dimension involved: The curvature is just there, in the dimensions you have.

39

Sorry to nitpick, but this one of my pet peeves: just because something lies outside of our Hubble sphere (i.e. is receding from us faster than light), doesn’t mean that it is outside of our event horizon (i.e. it doesn’t mean that we can never reach it). The Hubble sphere isn’t a horizon except for when it is degenerate with horizon.

In the current standard cosmological model the event horizon is larger than the Hubble sphere and has been since the big bang and will be at any point in the future. The Hubble sphere is asymptotically approaching the event horizon as our current model is asymptotically de Sitter and in de Sitter space the event horizon and the Hubble sphere are degenerate.

The question of whether you can reach the antipodal point or circumnavigate the Universe (as it doesn’t follow from the fact that you can reach the antipodal point that you can circumnavigate the Universe) is very interesting as I believe it simply boils down to whether you can travel the comoving distance to the antipodal point (and get back, if you want to circumnavigate the Universe) before conformal time runs out. I think though you’re on pretty safe ground when you say that if our Universe was spatially a 3-sphere we would never be able to reach the antipodal point under a reasonable set of assumptions which tally with observation.

40

the Bungee Cord of Descartes snaps you back.