Thread: Wandering Photons

What happens to photons that are traveling away from the universe -never to return?

Do they have no interreaction with it ?

Do they cease to exist on the basis that all existence implies a relationship? (you know the old question about the tree in the forest that nobody sees fall or maybe even the sound of one hand clapping )

Or are they still in a relationship with their "mother" universe even though they will never interreact directly again?

Could they bump into a "virtual" particle ?(I "understand" that these are particles that spontaneously pop out of the vacuum)



Apologies in advance for any elementary misunderstandings.

I'm having trouble with the definition of the word "universe" here.

Originally Posted by geordief

What happens to photons that are traveling away from the universe

I agree with adelady; what does this mean? Photons are in the universe. (There is no escape! Bwhahaha!)

As a definition ,yes.

But if they cannot interrreact with it (or can they?) then are they some kind of second class universe citizens ?

I am not suggesting they could have "escaped" but "died".

Or is that just the fate that awaits everything in the universe eventually (heat death) and are these photons just the first to make the transition? (The Little Escape)

A photon will just carry on for ever in a straight line (strictly speaking, a geodesic) until it interacts with something and is absorbed. Photons interact with things in the universe all the time. That is how we can see things!

Surely some photons will never (or can never) encounter anything. Nothing travels as fast and so they are "on their own" for eternity. They will never be absorbed or allow us to see anything.

Are you saying that all photons have at least a slight probability of encountering another object?

Am I labouring under a misapprehension that there is some kind of an "outer limit" to the universe (even if only poorly demarcated) ?

if spacetime is curved then photons can never leave our universe and will eventually inter act. as far as we know there are no other universes. this is the only universe. to believe otherwise is to belive in pure speculations.

Originally Posted by geordief

Surely some photons will never (or can never) encounter anything.

We see photons that have been travelling for 13.7 billion years without bumping into anything. Until they hit our instruments! There may well be photons that never hit anything. I suppose that as the universe expands, their chances of remaining free improves.

Are you saying that all photons have at least a slight probability of encountering another object?

They all have a probability of hitting something. The longer they are around, the greater that probability gets.

Am I labouring under a misapprehension that there is some kind of an "outer limit" to the universe (even if only poorly demarcated) ?

Yes. There is, as far as we know, no "edge" of the universe. It is assumed to be either infinite, or finite but unbounded.

Not even a fuzzy edge?

By the way what about Hawkin Radiation?That applies to Black Holes.Could there be anything like that that would apply to the universe in its entirety ? So that the universe would radiate away in time.

Or is that just fuzzy speculation? (I realise i have changed the subject and I won't persist as my initial question has been answered)

Originally Posted by geordief

Not even a fuzzy edge?

By the way what about Hawkin Radiation?That applies to Black Holes.Could there be anything like that that would apply to the universe in its entirety ? So that the universe would radiate away in time.

Or is that just fuzzy speculation? (I realise i have changed the subject and I won't persist as my initial question has been answered)

good question. i think hawking radiation implies partical creation in spacetime. so this would be have to happen within our universe.

Originally Posted by geordief

By the way what about Hawkin Radiation?That applies to Black Holes.Could there be anything like that that would apply to the universe in its entirety ? So that the universe would radiate away in time.

But radiate to where? There is no edge because there is no outside ...

i have a question about hawking radiation and wandering photons. hawking radiation implies that black holes will eventaully evaporate. but wandering photons are constantly falling into black holes. would this addition of mass-energy to black hole counteract any loss due to hawking radiation ?

Originally Posted by Strange

Originally Posted by geordief

By the way what about Hawkin Radiation?That applies to Black Holes.Could there be anything like that that would apply to the universe in its entirety ? So that the universe would radiate away in time.

But radiate to where? There is no edge because there is no outside ...

Well just to try and answer the question as very,very pure speculation.Could it radiate inwards? Back where it came from . Space Time expands from the vacuum (stop me when the gibberish gets too bad) and so the radiation could be directed back into those "spaces"

Originally Posted by Chucknorium

i have a question about hawking radiation and wandering photons. hawking radiation implies that black holes will eventaully evaporate. but wandering photons are constantly falling into black holes. would this addition of mass-energy to black hole counteract any loss due to hawking radiation ?

Yes, any realistically sized black hole will gain more mass than it loses from Hawking radiation. The amount of Hawking radiation is inversely proportional to mass: smaller black holes radiate more. A black hole would have to have less mass than the moon in order to radiate more than it receives from cosmic microwave background.

Originally Posted by geordief

Well just to try and answer the question as very,very pure speculation.Could it radiate inwards? Back where it came from . Space Time expands from the vacuum (stop me when the gibberish gets too bad) and so the radiation could be directed back into those "spaces"

But Hawking radiation occurs at an event horizon. The universe doesn't have an event horizon, or any sort of boundary. So ... no.

Originally Posted by geordief

Space Time expands from the vacuum

what we call the vacuum of space is actually space-time itself. spacetime does not come from a vacuum. there is spacetime (our universe) and that is all.

Originally Posted by Chucknorium

i have a question about hawking radiation and wandering photons. hawking radiation implies that black holes will eventaully evaporate. but wandering photons are constantly falling into black holes. would this addition of mass-energy to black hole counteract any loss due to hawking radiation ?

Wouldn't the effect of ordinary matter falling in be much greater? At a guess I would say that both would counteract the Hawkin effect but that is just my layman's (if I can flatter myself thus) opinion.

Originally Posted by geordief

Wouldn't the effect of ordinary matter falling in be much greater? At a guess I would say that both would counteract the Hawkin effect but that is just my layman's (if I can flatter myself thus) opinion.

Normally, yes. However, for a small black hole like this (a black hole with the mass of the moon will have a radius of about 0.1mm, I think) in intergalactic space (where there is about 1 hydrogen atom very cubic metre) the chances of any mass falling into the black hole is really small. But it will still be bathed in photons from the CMB.

Could I just ask a bit about the expansion of the universe (I think it is called inflation -is it the same thing , just different terminology ?).

I am curious as to whether this is a theory or an observation. Well I think I know it was an observation (didn't they suddenly one day observe that all the galaxies were all moving apart from each other?) but was there any theory prior to that that predicted that there would be this expansion? Or was this an observation that led to the theory?

Here is a rough timeline:

1915: Einstein introduces General Relativity. Initial solutions to the field equations contained within showed that the universe would contract due to gravity.
1917: Einstein publishes his cosmological considerations of General Relativity, where he added a cosmological constant to the field equations, to keep the universe static.
1924: Friedmann publishes solutions to the original 1915 field equations that show the universe must either expand or contract, depending on the initial conditions.
1929: Hubble observes the redshift - distance relationship, which implies the universe is expanding.
Einstein thereafter abandons the cosmological constant.

So theory came before observation.

Originally Posted by geordief

Could I just ask a bit about the expansion of the universe (I think it is called inflation -is it the same thing , just different terminology ?).

And just to clarify this one point: inflation refers to an initial very rapid expansion that is thought to be necessary to explain some specific features of the universe.

thanks.

What about the fact that the expansion is everywhere and not from any centre? Was that the original theory ? I mean I always thought that there must be a centre (like somewhere in the middle of the hole in a spherical doughnut - or the centre of 2 concentric spheres) but I have heard it said often enough now that "the centre is everywhere" and ,I think "the big bang was everywhere".

Was that accepted from the start?

Absolutely. Perhaps the easiest way to get your head round that is to run things backwards. We are here at the unfashionable end of our galxy, there are other galaxies scattered all over the universe. At one point they were all slightly closer together. And then closer ... and closer. Eventually everything was in a very small spot. The is the centre and it is everywhere that is now widely separated. (It is important to bear in mind, that this was not an explosion in space, it was an expansion or unfolding or creation of space.)

Originally Posted by geordief

Was that accepted from the start?

Yes indeed it was, although even back then people misinterpreted what the theory actually said. Here is an article from Popular Science magazine, published in December 1932. Take no heed of the sensationalist headline, but read the content:

Blast of Giant Atom Created Our Universe | Modern Mechanix

"In every direction, there would be an apparent withdrawal, which would be greater for more distant objects. Everybody would be similarly affected and each would believe himself to be the center away from which the other objects had moved.

The case of the universe is analogous, except that the expansion, being of a three-dimensional volume, cannot be visualized. The phenomena are, however, comparable. The nebulae are not running away from us. Their recession is due to expansion of space. This may, perhaps, seem to be quibbling over terms, since it amounts to the same thing in the end. Nevertheless, the distinction is worth keeping. According to the relativity theory, there is a difference between the running away of the nebulae and expansion of the medium in which they are imbedded."

Originally Posted by SpeedFreek

Originally Posted by geordief

Was that accepted from the start?

Yes indeed it was, although even back then people misinterpreted what the theory actually said. Here is an article from Popular Science magazine, published in December 1932. Take no heed of the sensationalist headline, but read the content:

Blast of Giant Atom Created Our Universe | Modern Mechanix

"In every direction, there would be an apparent withdrawal, which would be greater for more distant objects. Everybody would be similarly affected and each would believe himself to be the center away from which the other objects had moved.

The case of the universe is analogous, except that the expansion, being of a three-dimensional volume, cannot be visualized. The phenomena are, however, comparable. The nebulae are not running away from us. Their recession is due to expansion of space. This may, perhaps, seem to be quibbling over terms, since it amounts to the same thing in the end. Nevertheless, the distinction is worth keeping. According to the relativity theory, there is a difference between the running away of the nebulae and expansion of the medium in which they are imbedded."

They mention in this article " a medium in which they are embedded".I assume this refers to SpaceTime but I thought that terms like the "fabric of SpaceTime " were erroneous and that it shouldn't be considered as a "thing" at all.

Is that not what this quote is saying? Is it also erroneous in that respect?

Yes, that is what the quote is saying - the "fabric of spacetime", but that terminology has indeed fallen out of fashion and nowadays we just call it the metric. No, it shouldn't be considered as a physical thing - the metric is purely the result of what goes on within it, it is how distance is defined due to the geometry of spacetime. This has always been the case, and the mathematics hasn't changed in that respect.

Metric expansion of space - Wikipedia, the free encyclopedia