What happens when a ray of light hits the boundary of the universe?

[havonasun]

Okay, I have reread the thread. The balloon analogy is a theory. One person said the balloon was a 2d object with no edges. That seems to support the boundless theory. One person says that light traverses between objects along the surface of the balloon. Why not away? Expansion implies boundaries. Tessaracts have expanding and contracting cells which support the boundless theory. Is the balloon theory based on hyperdimensional math and stuffed into a 3d model? According to the theory, one could travel (at whatever speed) and never exit the universe...just go around and around and around... A closed system.

 

[cristo]

Okay, I have reread the thread. The balloon analogy is a theory. One person said the balloon was a 2d object with no edges.

The surface of the balloon is a 2d object with no boundary. That's the point of the model.

One person says that light traverses between objects along the surface of the balloon. Why not away?

Because, to beings living on the surface of the balloon, the third dimension does not exist.

Expansion implies boundaries.

No it doesn't: blowing up the balloon causes it to expand: but it has no boundary.

I really don't see where the problem is here!

 

[russ_watters]

The balloon analogy is a theory.

Yes, it is. And there is very, very good evidence to support it. For example, we can look a very, very long distance and detect none of this "thinning" that you suggest could exist. The universe looks exactly the same in every direction to a very large degree. This implies that there is no boundary. There are other pieces of evidence as well, but until you can understand what the model says, we're getting ahead of ourselves to talk about the evidence for it.

Anyway, you're still not getting it, though cristo did a good job repeating the exact same thing already said a dozen times.

Please read more carefully!

 

[havonasun]

Ok, I'm really trying to grasp what is being said here and I appreciate your comments.

Because, to beings living on the surface of the balloon, the third dimension does not exist.

But, we're not 2d beings. How does this model apply in reality?

 

[russ_watters]

But, we're not 2d beings. How does this model apply in reality?

That's why it is an analogy. The same type of phenomena (finite, but without boundary) that you see on the 2-d surface of a balloon appears to occur in the 3d space of our universe. As the 2d balloon is curved in a 3rd dimenson that isn't necessarily visible from the surface of the balloon (to an amoeba, it may look flat), the universe may also be curved in higher dimensions we cannot see.

 

[havonasun]

Ok, I agree with that.
I found this quote...

Is the universe finite or infinite?

"The observable universe is finite," Sweitzer said, which is to say that it had boundaries -- physical limits. Sort of. "It's a boundary to the events we can see directly, but not a boundary in the sense that New York State has a boundary."

And in an expanding universe, this boundary is constantly moving, as is everything within it. Cosmologists typically invoke a balloon with spots on its surface, representing galaxies, to explain the expanding universe. As the balloon is inflated, the spots grow farther apart. If you stood within one of these spots, you'd see all the others moving away from you, and the most distant spots would move appear to move the fastest.

Sweitzer goes on to say that the observable universe is probably part of a much larger universe, "which could be finite or infinite. Any global statements about the universe, such as overall extent, are speculative because they require extrapolating local mathematical theories and measurements beyond the observable universe."

What I'm understanding is that the balloon analogy was a way to explain the physical movement of the galaxies relative to one another. Ok, so the universe is expanding. Thank you.

 

[russ_watters]

Well, you're missing something there. When they talk about the "observable universe", they are talking about an expanding sphere of what we can see due to the distance the speed of light can travel in a time limited by the age of the universe.

That is different from the entire universe, which need not have any boundary.

 

[marcus]

Havona, you are quoting something over 6 years old that was said for a general public audience by a guy who is not a working cosmologist
but is a "director of education" at something like a natural history museum

http://www.space.com/scienceastronomy/astronomy/universe_overview_010605-1.html

thats OK for starters Havona, but I'm confident you can do better than that (you sound reasonable and serious)

I see your quote comes from page 2 of the article. James Sweitzer, the guy you quote, is being imprecise in that article, or the SPACE.COM reporter was being sloppy. I guess you know not to take everything you read in Space.com for gospel Actually I'd believe either Cristo or Russ Watters over James Sweitzer and popular space media any day! Plus the extra thing about them is you can ask questions back and get clarification.

I'm trying to figure out what you want to understand. What, in this thread, do you find confusing or unsatisfactory? What do you NEED to understand that you might not realize yet that you need to understand it?

 

[marcus]

Well, you're missing something there. When they talk about the "observable universe", they are talking about an expanding sphere of what we can see due to the distance the speed of light can travel in a time limited by the age of the universe.

That is different from the entire universe, which need not have any boundary.

I agree strongly. The observable really is a different kettle of fish. It is merely the part of the whole which we have got light from SO FAR.
The observable universe constantly increases as we get signals coming in from farther and farther

I also think you put your finger on what Havona and a lot of other newcomers are MISSING. they can't picture the 3D analog of the expanding boundaryless 2D surface of the balloon.

Maybe we need a tutorial that everybody gets put thru if they want to discuss cosmology.

(BTW I don't understand why this thread was started in Astrophysics since the topic is cosmology---the shape and dynamics of the universe---not individual stars and star systems.)

As a math student you get told about the 3-sphere early on and a way it may get introduced is as the ONE POINT COMPACTIFICATION OF EUCLIDEAN THREESPACE.

Euclidean R³ is just your familiar x,y,z space, as usual as graph paper. And then you picture adjoining just ONE EXTRA POINT the point at infinity in all directions. And that is the topological space S³,
It is easy to imagine walking around in because it is just like an infinitely big classroom with the extra feature that a straight line heading off in one given direction eventually comes back.

Another way to picture the 3-sphere S³ is as the "skin" on a 4D BALL. And then you have a radius r and you can write down the finite 3D volume of S³ in terms of r.

It is 2 pi² r³
================

Maybe it is too much to expect of newcomers like Havona that they teach themselves to picture living in S³
and pick up a little understanding of it. There is not much, it's so basic.

My hunch is that in Havona case it is not too much to expect at all, but it might be as a general rule.

The good thing is that S³ is FINITE AND BOUNDARYLESS THREE DIMENSIONAL SPACE WHICH CAN EXPAND in an easy-to-picture way: just like the balloon expands which is simply the lower dimensional version of it.

So if a newcomer can picture S³ then there might not be so much reluctance to accept basic cosmology ideas. Just a thought.

 

[M.g.f.s.n.]

Could the universe's boundary simply be defined as the point at which light becomes visible?

A spot 500 Trillion Million light years away obviously hasn't any light for which to define anything, no energy for which to create anything, and no medium for which to house matter. In essence, wouldn't that basically be non-existant?

 

[marcus]

Could the universe's boundary simply be defined as the point at which light becomes visible?

A spot 500 Trillion Million light years away obviously hasn't any light for which to define anything, no energy for which to create anything, and no medium for which to house matter. In essence, wouldn't that basically be non-existant?

just to try to be extra clear, M.g., we are talking about the extent of SPACE.

Professional cosmologists have a small set of models, basically mostly all versions of the same model with different numbers plugged in.

Some models have space be INFINITE in extent. Some have space be FINITE.

In no case do they ever have space be BOUNDED That is a no-no.

the models with infinite space fit the data roughly as well as those with finite space. In the future, to decide which, we need to get more data.
=============

Now your question is about some point that is currently something like a billion trillion lightyears away from us.
that is perfectly OK, it could be a place in space that looks rather much like the Milkyway galaxy or the Andromeda, except it wouldn't be exactly the same. It would differ in details.
The infinite-space model would accommodate that just fine. A billion trillion is no problem.
In that model, expansion began over a broad front----she was already infinite at bigbang time.

But you have a different idea which I never heard a cosmologist say:

A spot 500 Trillion Million light years away obviously hasn't any light for which to define anything, no energy for which to create anything,...

I don't see any reason to suppose this, so I guess you are mistaken.

 

[blighty]

How you see many things has little to do with what "IS" happening.
Eyes are ment to help you find food and evade preditors, not for
scientific purposes, that is what the brain is for.

we can create our own optical illusions and so can the universe.

 

[Gmanme]

To OP:

Mybe the the universe isn't growing at all, and has always been the same size. Mybe the only thing that has changed is TIME in the universe, Making the illusion of a changing distance. You never know.



Naw, just messin with you... :P


Theres no way too, but you have to imagine a finite 3d object with no boundries. The baloon is really only 2d object with no boundry, by simply making a circle. You have to assume anything outside the surface of the baloon is outside the universe, Which is imaginary, like thoughts, outside the universe doesn't exis't physically in any way, Its just a concept.

 

[zankaon]

Within GRT formalism, one can speak of a 4-sphere with 3-faces; or more easily, a 4-cube with 3-faces. So a light ray (null geodesic) would be described in such 4-D. Even more easily, what about a time-like geodesic, like us, in free fall, as for our star. Then such time-like geodesic would never bounce off of anything; rather one could consider it as if it 'rolls over' even if near some conceived near to maximum extent of the manifold (i.e. continuum).

 

[cbd1]

Yes, it is. And there is very, very good evidence to support it. For example, we can look a very, very long distance and detect none of this "thinning" that you suggest could exist. The universe looks exactly the same in every direction to a very large degree. This implies that there is no boundary. There are other pieces of evidence as well, but until you can understand what the model says, we're getting ahead of ourselves to talk about the evidence for it.

Anyway, you're still not getting it, though cristo did a good job repeating the exact same thing already said a dozen times.

Please read more carefully!

There are thin areas in the cosmic microwave background. It is not uniform in all directions. This means space cannot be infinite in all directions, for there would be infinite stars in all directions and light would be shining equally from all directions. right?

 

[marcus]

There are thin areas in the cosmic microwave background. It is not uniform in all directions. This means space cannot be infinite in all directions, for there would be infinite stars in all directions and light would be shining equally from all directions. right?

Your reasoning doesn't make sense to me.

The CMB is uniform to with one part in 100,000. There are what you call "thin" places, that is places where the temperature is about one thousandth of one percent less than average. The false color pictures exaggerate, of course.
In any case, that doesn't say anything about space being finite.

http://en.wikipedia.org/wiki/Cosmic_microwave_background

You haven't given us any reason that space couldn't be infinite in all directions, and approximately uniformly filled with matter (stars, galaxies, etc.)

Space might be either infinite volume or finite volume, we can't tell yet with the data we have so far.

Since the oldest stars are only about 13 billion years old, even if space were infinite volume uniformly full of an infinite number of stars we still would not SEE an infinite number of stars in the sky because finite age limits how far starlight can have traveled.
Even if space is infinite we are only in touch with a finite piece of it.

You might try reading up on "Olber's Paradox" It's not really a paradox, they just call it that for historical reasons. Try Wikipedia on it, might be OK.

 

[russ_watters]

In addition to the "thin" places not being very thin, they are not arranged in such a way as to suggest a direction for a center or edge. They are spread randomly and are very localized.

 

[SideReal]

Hmmm... the balloon you are speaking of is just an approximation of the Klein-bottle that is the real universe.

A Klein bottle is, effectively, a 4 dimensional spherical 'balloon.'

It's expanding. Or at least it looks like it is because astronomers have noticed that everything appears to be red-shifted away from us.

More accurately, it appears that all of the objects are accelerating towards something they have termed 'The great attractor' located somewhere in the Centaurus Supercluster. The Great Attractor is an anomalous mass weighing in at an approximated tens of thousands of Milky Ways.

Another theory, postulated by Senovilla is that the universe isn't really expanding so much, per se, as that time is literally slowing down. This argument has merit since the latest en vogue creation theory: Inflationism, indicates a period where the universe actually expanded superluminally. If you postulate that time is variable, however, you open the doors to allowing time to be passing at a much accelerated rate at the early stage of the universe and thus the expansion is still at or near light speed.

Everyone's question as to 'what would the edge of the universe look like' is non-sensical. What does the edge of the world look like? It doesn't, because there isn't one.

Therefore, to the OP's question: What would happen if a EM wave hit the edge of the universe? Nothing, there isn't an edge of the universe.

Just my 2 cents.

 

[SideReal]

There are thin areas in the cosmic microwave background. It is not uniform in all directions. This means space cannot be infinite in all directions, for there would be infinite stars in all directions and light would be shining equally from all directions. right?

No one on the forum has suggested that space is infinite. In fact, they have repeatedly said it is boundless and finite.

 

[cragar]

light would then circle the universe.

 

[cbd1]

Wow. That is true genius. Now I understand it.. The light circles the sphere, that way, no energy is lost from the system.

Thank you cragar

 

[cragar]

your welcome.

 

[stalon34]

light can never escape the universe simply because there is no space outside of our universe. so when light reaches the boundaries of universe it bends to one side.

 

[havonasun]

Finite, yet boundless. I think the confusion comes from trying to wrap a 3d brain around a tesseract. A point is flat. A line is flat. A plane is flat. By extension, we are flat. The 3d being rejects this because experience dictates otherwise.
There is a certain quantity of universe; that makes it finite. Yet, to be boundless, the 3d "plane" forms a sphere. The void (not space, space is 3d) inside and outside the sphere are one and the same; unseparated. The 3d mind interprets "sphere" as a ball, but this is a 4d thing - we wouldn't understand. That is why, a plane/line seems to curve as it moves away from the point of observation. It is still perfectly straight. The curvature is a pseudo-illusion. It's not really there, but it is still significant. As 3d beings, without our math instruments, the curvature is a sensory perception of 4d reality.
There is no boundary, just as a circle has no end.

 

[stalon34]

dear havonasun,

thank you for your reply, however i think you got the whole thing wrong; points,lines and planes are not flat. they are not 2D. in fact there is no such thing as 2D as apposed to our " 3D mind". 2D world is mathematical abstract. points, lines and planes are only mathematical concepts. every thing in this universe is 3D. and there is actually a boundary, but it depends on where you draw this boundary. if the universe that we see and perceive in any possible way through our instruments finishes at 13 or 15 billion light years around, then that is the boundary and light can never escape that. but one might ask what is beyond the boundary of this universe if we assume that there is a boundary. the answer is that because we and every thing else in our present universe is made out of the fabric of space and there is no space beyond our universe then it is impossible for us to imagine. we just simply say beyond is nothingness.

 

[cragar]

isn't everything in our universe 4d

 

[havonasun]

Stalon, that's where I got tripped up. It's a real paradox, its a boundary that is there but doesn't exist. The void or nothingness, by definition, can't be pointed to; you just end up pointing at the back of your head.

if the universe that we see and perceive in any possible way through our instruments finishes at 13 or 15 billion light years around, then that is the boundary and light can never escape that.

Until the following year when the Pro version comes out and then we can see 18 billion ly! That is the strangeness of this concept; light can't escape because there's nothing to escape. A beam of light will travel in a straight line through this universe for eons of eons and never reach the end. Its like Asteroids on Atari when your ship goes off the edge and appears on the other side of the screen.

I might still be wrong, everyone feel free to correct this.

 

[cragar]

A beam of light will travel in a straight line through this universe for eons of eons and never reach the end.

light would eventually come back around to where is was emitted .
There is a book called Mr. Tompkins in wonderland written by the physicist George gamow
and he talks about throwing a book off a planet and seeing it go away from the planet
it gets smaller and smaller as it goes away , then it starts to get bigger again then it appears as if it is right next to the person that threw it , he reaches out to touch it and he can't , because the lights rays are converging just like the meridian lines on our globe
when we see the imaginary image next to us the book is halfway around the universe
and then it eventually comes back to use , This would happen in our universe
it would just take a long time .

 

[havonasun]

Yep, that's what I meant. I love the mental visuals from paraphrase you gave. I think I'm going to look that book up.

 

[stalon34]

I just don't understand why havonasun keeps insisting on light traveling on a straight line. straight line is a very old and wrong concept. it has been proven that there is no such thing as straight line any where in universe due to the presence of gravity. if we accept straight line then we have to believe that our universe is a flat plane. while flat plane, straight line and point are only abstract mathematical concepts and not physical reality. every thing in universe is 3D; mathematical concepts are 2D. True way of knowing our universe is to collect the actual and factual observations and then allow these observations lead us towards imagining the unobserved aspects or features of our universe. In plain words, observe and then imagine the unobserved parts. What you guys are doing is quite the opposite. You guys plunge yourselves right at the start into a type of universe that is supported by nothing but your pure imagination.

 

[peterdorey]

It's all just movement in Stillness & stillness in Movement (a bit of Manhamudra there).
With e-m radiation "sloshing" around (this sentence loosley quoting ProfBECox seem to recall).

 

[Skaffen]

The only constant is change.

The Universe is 'Infinitely Bounded'. - Just as explorers (hundreds of years ago) thought there was an edge to the world, contemporary scientific observations lead us to think we have found the edge of the Universe...Dark Energy anyone?

From recent images (Keck, I think), the observable Universe appears to be an oblate sphere, which to me suggests it is spinning (Why not? Everything else does). This spin would imply a greater shearing force the further from the origin (Increased radius), hence the observed increased acceleration in spatial expansion over Time.

The balloon analogy invokes an inflationary process which does not address the principle of Conservation and consequently is misleading - a more useful description is to term it 'unfolding'...like an umbrella being spun.

Imagine a planet composed of a perfect liquid that is disturbed by a pebble (breaking symmetry) - the wave would reach the pole and make it's return journey 'indefinitely', however all is connected so the energy is transferred partially to another 'object' and the result/remainder is returned to the origin (feedback)..and evolves, becoming increasingly complex and diverse until it stabilizes through synchronisation.

Gravity is the tension of our liquid, light is the breaking of the wave.