[Geezer (OP)]

Is there any limit to how far photons travel/propagate through space? In other words, would they eventually come to a place where they are no longer able to travel/propagate at the edge of "space".

My intuition says there is no limit, but my intuition is usually wrong when it comes to this stuff.

[yor_on]

Well, as intuition is nothing and that you in all other matters are correct, even though you once thought yourself wrong, I think your intuition will become a certainty, any moment now

A photon is immortal, at least if you asked it, which you can't I haste to add, as it then would have to die. So the Photon Boundary, which is a new one that I really can dig, me man, have to be, at the very least, as big as SpaceTime.

I think?

[sherya]

nothing travels faster than light........... so there should come a time when photon reaches the edge of he universe..... or else the universe should not have edges!!!!!! as we see in general relativity its curved. again the question pops up is there a high gravitational field at the edges or the space is indistinguishable at the curvatures

[Farsight]

Is there any limit to how far photons travel/propagate through space? In other words, would they eventually come to a place where they are no longer able to travel/propagate at the edge of "space". My intuition says there is no limit, but my intuition is usually wrong when it comes to this stuff.

Yes and no, IMHO. The universe is expanding much faster than light, so if you fire off photons in various directions they can't catch up with the edge. But let's use a bit of artistic licence and get rid of the expansion of the universe. It's thought to be "flat", which means it doesn't curve back round on itself like a hypersphere. Whilst some claim that its size is infinite, we're happy enough with the big bang 13.7 billion years ago, and with the observable universe being circa 93 billion light years across. So a universe with a diameter of 150 billion light years sounds reasonable.

Given that the universe is flat and finite, you can think of space as something like a vary large globule of water. In this analogy, light waves are ripples inside the water. They can't propagate beyond the water. There is no water beyond the water, so ripples in the water cannot go any further. Instead they bounce back.

Don't think of yourself as some kind of fish who can jump out of the water. Remember pair production, and how we can literally make electrons out of light. Or think quantum mechanics and wavefunction if you prefer. You're made out of waves too. And once you've reached the edge of space, there's nowhere else to go.

[yor_on]

It's a weird thing this flatness

It's true that WMAP have measured  our universe to be flat with only a two percent expectation of it being wrong, according to them that is But imagining it becomes slightly weird, think of a flat plane stretching in all directions. Do we fall of the edge? Does it have an edge? When you thought of it you probably did it the same way as you would draw it too, right? Well, then you imagined it in two dimensions. But SpaceTime have three dimensions plus time.

So how will a flat plane look in three dimensions?
Width, length= two, and now we add 'height' and gets three.

Such a universe might be flat in the sense that we can't observe any significant curvature to the lights paths in it, but that doesn't say that it can't have a curvature. There are several solutions mathematically where the universe can be both seen as flat but still have a closed aspect like a torus. And to me a closed one is the one making most sense. So the light-waves 'traveling' here might in fact be able to take a round-trip if that's the way light 'propagates'.

[SuperPrincipia]

yor_on  me thinks  yor_on!!!  []

[LeeE]

nothing travels faster than light........... so there should come a time when photon reaches the edge of he universe.....

The speed of light is the slowest that the universe has ever expanded.  At one point in the fairly early stages of the Big Bang it's widely accepted (but still open to debate) that the universe expanded faster than the speed of light.

It needs to be remembered too, that although we may tend to think of photons as being the most fundamental expression of energy, it is significant that the Standard Model predicts that the very early universe was opaque to light.

During this period then, the universe appears to have expanded faster than the speed of light, but at the same time, the universe was nearly uniform and once photons started to appear as discrete particles, and existed as such for a significant amount of time before interacting, the did so pretty much uniformally throughout the universe.

The 'boundary' of the universe then, that's expanding, will have emitted photons in the direction of expansion, as well as photons in every other direction as well, and those that reach us are what we call the Cosmic Microwave Background Radiation (CMBR).  So just as some of the light from the BB has reached us, some of it is still travelling outwards, right at the forefront of the expanding universe.

Arguably, it is the presence of those photons, moving outwards at the speed of light, that are creating the expanding universe.

[Geezer (OP)]

nothing travels faster than light........... so there should come a time when photon reaches the edge of he universe.....

The speed of light is the slowest that the universe has ever expanded.  At one point in the fairly early stages of the Big Bang it's widely accepted (but still open to debate) that the universe expanded faster than the speed of light.

It needs to be remembered too, that although we may tend to think of photons as being the most fundamental expression of energy, it is significant that the Standard Model predicts that the very early universe was opaque to light.

During this period then, the universe appears to have expanded faster than the speed of light, but at the same time, the universe was nearly uniform and once photons started to appear as discrete particles, and existed as such for a significant amount of time before interacting, the did so pretty much uniformally throughout the universe.

The 'boundary' of the universe then, that's expanding, will have emitted photons in the direction of expansion, as well as photons in every other direction as well, and those that reach us are what we call the Cosmic Microwave Background Radiation (CMBR).  So just as some of the light from the BB has reached us, some of it is still travelling outwards, right at the forefront of the expanding universe.

Arguably, it is the presence of those photons, moving outwards at the speed of light, that are creating the expanding universe.

Not that I know much about any of this stuff, but that seems about right. After all, if a photon sort of "ran out of" space, what would it do? Unless the energy went down some sort of ethereal plughole, it would have to reflect off the edge. But that seems highly unlikely (to me anyway).

[peppercorn]

Not that I know much about any of this stuff, but that seems about right. After all, if a photon sort of "ran out of" space, what would it do? Unless the energy went down some sort of ethereal plughole, it would have to reflect off the edge. But that seems highly unlikely (to me anyway).

I didn't think there was an edge with 4D space-time []
Is the analogy (at least for a non-expansionist universe) of being able to see the back of one's own head, not the correct view these days?
[Like the 2D 'dwellers' who sit on the surface of a sphere...]

[Farsight]

No, that's the "hypersphere" like the top illustration here:



People are pretty sure nowadays that the universe is "flat" like the bottom illustration. If you jet off in one direction, you don't end up coming back from the opposite direction. See the WMAP page for more.

[JP]

I'm a bit confused by the hypersphere picture.  Since its space-time, does that mean that if you jet off in one direction, you'll eventually come back to the same coordinates in space-time?  That would mean you'd traveled back in time to the point from which you left...

[Farsight]

I don't think so, JP. The WMAP article says:

"If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere. This implies that initially parallel photon paths converge slowly, eventually cross, and return back to their starting point (if the universe lasts long enough)..."

Besides, you don't actually travel "through" spacetime. You travel through space, and plot your motion over time as a worldline in Minkowski spacetime. This gives a block-universe view that depicts time and space together. It's like you've filmed a red ball arcing across a room, and then cut the film into individual frames and stacked them on top of each other. There's this red streak in there, but there's nothing actually moving. A photon in a hyperspere universe has a worldline that moves out at a diagonal from the vertical origin worldline, but gradually tilts and spirals around to come back towards the origin higher up.

[peppercorn]

People are pretty sure nowadays that the universe is "flat" like the bottom illustration. If you jet off in one direction, you don't end up coming back from the opposite direction.

I don't understand how a 'surface' that is universal can be open, but as I have sort of passed the time in my life where I care much about these sort of questions, I'll take your word for it.

I'm a bit confused by the hypersphere picture.  Since its space-time, does that mean that if you jet off in one direction, you'll eventually come back to the same coordinates in space-time?  That would mean you'd traveled back in time to the point from which you left...

JP - that seems a fair point to me
I should have said I only envisage setting off relative to one spacial dimension.  In the 'hypersphere' view I would think this sort of makes sense  []

[yor_on]

Hmm

Let's split this question into pieces.

Is the universe 'flat'? Yeah, maybe, seems so, sure, why not?
Does that mean that its 'intrinsic shape' have to be flat in a 'SpaceTime'?
Nope.. And I guess JP suspects that As well as some others.

So can we both have the cake as well as eat it?

I think 'Does space have a shape' explains the concept fairly nice. He* even I could understand that one, but if we follow its conjecture the results become weird. There is something called a 'Cauchy surfaces' in math. Which Definition comes out like this: "Intuitively, a Cauchy surface is a plane in space-time which is like an instant of time; its significance is that giving the initial conditions on this plane determines the future (and the past) uniquely." Image here.

Now, that's a mighty big bite to swallow right? But they seem to come as a proposal from our former discussion about if a 'flat space' still could be 'closed' in some way. So under what circumstances could such 'surfaces' exist? On the Determination of Cauchy Surfaces from Intrinsic Properties. 

Ahh? a Ok, so what would that mean? Well, I don't really know? That we in some weird way could have a 'deterministic universe'? Tipler seems a nice fellow, maybe we will get some better insight from him? Casually symmetric SpaceTimes. And somehow the idea seems to spring from the discussions of what a 'Black Hole' might be described as. Read this one for that, it's actually nice in its own right describing a lot of different ideas. Black Holes by P.K. Townsend.

And now I'm bicycling in the great beyond

[yor_on]

"One possible shape is the triple torus. At first glance, the triple torus appears to be an ordinary cube. But each face of the cube is glued to the face on the opposite side. Imagine that you’re in a spaceship that’s flying inside a large cube. You head toward the top of the cube. You wouldn’t smash yourself flat once you made contact. Instead, you’d appear in a corresponding spot at the base of the cube. In other words, you’ve gone up through the top and came back in through the bottom. If you traveled far enough in any direction, you’d eventually come back to where you started. This isn’t that foreign of a concept, since on Earth if you travel in a straight line, you’ll eventually come back to your starting point. You’ll just be very tired."


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Weird stuff