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Author Topic: How can photons be localised if the universe is expanding?  (Read 47241 times)

Offline DoctorBeaver

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My little beaver brain is working overtime again. This is what its murky depths have churned out this time.

As has been explained elsewhere, for a photon travelling at c time has no meaning. In fact, time doesn't really exist for it. It therefore follows that a photon could be everywhere in the universe at the same time. But the universe is expanding, so there is now more "everywhere" for it to be in than there was at any time in the past. How can the photon be everywhere at once if everywhere is getting bigger?

 ???
« Last Edit: 21/12/2008 10:20:19 by chris »


 

Offline Soul Surfer

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Its not just photons beaver quanum mechanics implies that every particle and quantum is everywhere in the universe over all time.  its just that the probability of being in most places other than where it is, is rather small!!!
 

Offline DoctorBeaver

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Ian - I am aware of that, but I think you have missed my point; to whit, the infinite time dilation aspect of travelling at c.
 

lyner

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its just that the probability of being in most places other than where it is, is rather small!!!

Could we change that to "its just that the probability of appearing in most places other than where conventional Science would predict, is rather small!!!"

?


The existence of a quantum particle 'everywhere' is fine by me. Anything to avoid trying to specify where it 'is, exactly' rather than where it has an effect. At c, a photon is, effectively anywhere because it can 'choose to' reveal itself, instantly in any one of a number of places, depending on its probability function (the diffraction pattern with which it's associated), because it exists independently of time.
 

Offline Soul Surfer

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To answer your question the universe is not getting bigger from the edges its getting bigger from all over and that is why the cosmic microwave bacground radiation which was like starlight when it was generated is now srtetched right out into the microwave region.
 

Offline Soul Surfer

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Sophie I think you've got it a bit wrong there It depends a bit who's time you are talking about the photon doesn't "know anything about time between it being created in an interaction and it being destroyed in another interection.  from our point of view it can have been created as part of the cosmic microwave background flash a few hundred thousand years after the big bang travelling a long way from a specific direction and arriving at our detector and being absorbed at a clearly definable location.
 

Offline DoctorBeaver

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I know about the expansion not being from the edges.

Let me try saying this a different way. If nothing stops a photon from travelling then, theoretically, it could eventually actually have been at all points in the universe. Time has no meaning for a photon. Therefore it would have been at all those points simultaneously. But as the universe is expanding (and for this thought experiment it doesn't really matter how that expansion is happening) there are more points being created all the time that the photon could be at.
 

lyner

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SS
You don't appear to be disaggreeing with what I wrote. The production and destruction of a photon will be observed in 'our' space / time. The conditions which determine when and where are determined by the experimental setup (including, if you like, the BB). Because it is not 'experiencing time' the photon can know instanly (from our viewpoint) where it needs to have started and where it needs to finish its journey between source and detector. CBR would behave just the same as light from a light bulb in this respect.

In fact what I am saying is that many of the difficulties and 'paradoxes' can be resolved by looking at the problem this way and not thinking of a photon as being at a point, ever.
DrB
Your problem with points in between being generated by expansion is dealt with because the only thing that matters is where / when the ends of the experiment are. The photon doesn't have to 'be' anywhere in between.
« Last Edit: 20/12/2008 11:30:55 by sophiecentaur »
 

Offline DoctorBeaver

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SC - there is no experiment involved in what I'm saying; just a photon - any photon - travelling at c. It doesn't get measured in any way. Put yet another way, how does that photon "view" the expansion of the universe if time doesn't exist as such?
 

lyner

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If you're observing anything it can be an experiment. You can't have a photon unless it's produced by something and you have no idea where it is or when it is unless it is absorbed by something.  Of course it is 'measured' at both ends. The atom it interacts with has measured it, effectively, when it decides if it has the right energy for an interacton  and the one which chucked it out used a 'measured' amount of energy to make it with. We are considering a testable hypothesis in this discussion!


How does it 'view' the expansion? That's almost not a valid question because it would have to see a rate of change of displacement which implies dividing a distance by time. Dividing by, effectively, zero will give the impression of an 'infinite' or indeterminatly rapid step change between the emission and absorption events. But, as the photon doesn't really need to exist except at each end of the process, why should it (or we) care?

How's that, Dr B?
 

Offline DoctorBeaver

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Re: How can photons be localised if the universe is expanding?
« Reply #10 on: 20/12/2008 23:34:11 »
SC

What you said...
Quote
If you're observing anything it can be an experiment.

What I said...
Quote
It doesn't get measured in any way.

Right, forget that intelligent observers exist. Early enough in the universe's history there weren't any anyway.

A photon gets created somewhere, somehow - could be the CMBR or something else; it doesn't matter. Off the photon goes on its journey through the universe without interacting with anything else (unlikely, but theoretically possible). If spacetime is closed then there is the possibility that it will travel through every point in space - actually travel through every point. From the perspective of the photon, such a journey would take zero time. But, in fact, by the time the photon has indeed made its journey, the universe has expanded. So, is it true to say that from the photon's point of view, the universe has expanded in zero time?

Or, if you prefer to think of it this way, take a photon emitted from the Sun and heading towards the Earth. It takes roughly 8 minutes in our time, but zero time to the photon due to the infinite time dilation. In the 8 minutes of our time that elapse, the relative positions of the Sun and the Earth has changed. How does that change "look" to the photon in zero time? How can something happen if there is zero time for it to happen in?
« Last Edit: 20/12/2008 23:35:42 by DoctorBeaver »
 

lyner

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Re: How can photons be localised if the universe is expanding?
« Reply #11 on: 21/12/2008 00:50:25 »
I can see that you are still insisting that, somehow, the photon (like a bullet) is speeding on its way from a to b. If you look at it that way then I can see there may be a problem. But why does it have to exist between a and b? The only places and times that you can prove it's there are at each end of 'its' journey.
There is a great difference between a photon and all other so-called particles. An electron can make a journey and make its presence felt by many systems on the way - giving a little (in principle, measurable)  nudge to every charge it passes. But a photon only interacts at each end of its 'journey'. As far as I can see, then, it is not necessary to consider what it is getting up to on the way - it gets up to nothing, in fact. I also hold that the 'it', to which we refer , is just the energy, which will behave according to the wave with which it's associated. It can't  / needn't be considered as a particle except where it is interacting.

You seem to be over concerned about the changing distance between source and detector - why should that concern an object which doesn't really exist on the journey? What about the situation where source and detector happen to be approaching each other at the same speed that the space between them is expanding - wouldn't that situation be just as difficult to explain?

My way of looking at it is not to say "Off the photon goes on its journey through the universe without interacting with anything else" because, for a start, you can only say that it hasn't interacted yet. I say it is potentially everywhere at all times between its creation and its absorption. It's journey is only described once it has actually completed the two interactions.

There is, of course, a very finite time involved in the interactions at each end because they are, essentially, resonances which take time to build up. As far as the photon is concerned, the two ends could be one atom's width or the whole universe apart.

The bullet picture makes it extremely difficult to reconcile the effects of diffraction - requiring a message to travel instantly all around the Universe to tell all other atoms that the photon has arrived 'here' and can't be seen anywhere else. But, even in the simplest, two slits, experiment, something has to tell each photon that it is part of a number of others which must, somehow sort themselves out into an interference pattern - which will always occur when there are 'enough of them'. It's a nightmare this way round; stick to the wave interpretation.
 

Offline DoctorBeaver

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Re: How can photons be localised if the universe is expanding?
« Reply #12 on: 21/12/2008 08:47:46 »
For what I'm trying to get at, it doesn't matter whether it's a particle or a wave. So, let's say that "...the wave could propogate through every point in space..." and be throughout the whole of space at the same time. Space is still expanding and there is still zero time for the photon.
 

lyner

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How can photons be localised if the universe is expanding?
« Reply #13 on: 21/12/2008 12:48:25 »
OK but so what?
Are you after finding out what the photon 'experiences'? What do you mean by 'the photon'?
If ,as I suggest, it doesn't exist except where it interacts then it's not 'like anything' for the photon. There isn't a photon for it to be like for.

It's a bit like sending a TV picture over a link. The picture exists at each end but it doesn't travel across the link in the form of a picture. It would not make sense to talk in terms of the picture during the transit time of the signal - all you could say would be that the link characteristics could affect what the picture turned out like.

Or. I have this idea and you are receiving it (and understanding what I say - or gnashing your teeth about that idiot the other end). The text, the binary signal from the keyboard, the wi fi signal from computer to router, etc etc are not the IDEA, they only carry a representation of the idea.
For me, the photon is just what 'goes on' during the interactions each end. It would be meaningless to discuss its existence except where / when it exists.
 

Offline DoctorBeaver

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How can photons be localised if the universe is expanding?
« Reply #14 on: 21/12/2008 13:11:51 »
That sounds as if you're saying that in between the photon being emitted and being detected it doesn't exist anywhere. From everything I've read, that is simply not the case. Also, I've seen it proposed in physics text books that an electron can be everywhere at once.
 

lyner

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How can photons be localised if the universe is expanding?
« Reply #15 on: 21/12/2008 14:54:41 »
Yes - I'm saying just that. Or, rather, that it has no meaning or reason for considering it as existing. I suppose it's a bit a a Zen thing - like "when a tree falls in the forrest, does it make a sound if there's no one to hear it?". But that is a bit of Zen nonsense because there are plenty of things for a falling tree to interact with and there is no interaction for a photon to have on the way.

Yes- I've read the same thing but that doesn't mean it's the best way to look at things. There isn't a 'right answer' to any of this stuff. All you can do is get a theory together which predicts most events as accurately as possible.
The wave theory gives the right answer about where to expect light to turn up (it gives a probability distribution) and the particle theory gives a good answer about how the quanta of energy in the wave will interact (the quantum of energy and the momentum).
Until I thought out this way of looking at it I had a terrible problem reconciling the two approaches. I have yet to hear a coherent (no pun intended) way of explaining what happens, in terms of photons, during diffraction. And you have to remember that every 'photon' follows the rules of diffraction in every circumstance! Diffraction is always there.
 

Offline DoctorBeaver

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How can photons be localised if the universe is expanding?
« Reply #16 on: 21/12/2008 22:29:28 »
So the expansion of the universe has no effect on photons?
 

Offline Bikerman

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How can photons be localised if the universe is expanding?
« Reply #17 on: 21/12/2008 23:34:23 »
Yes expansion has an effect.
If you consider the point the photon was emitted and the point it is detected (presuming that the two are separated by large (intergalactic) distances) then the red-shift of the photon is a measure of the expansion between the two points (after we subtract the gravitational red-shift/blue-shift).
 

lyner

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How can photons be localised if the universe is expanding?
« Reply #18 on: 22/12/2008 00:44:29 »
Yes. That is an energy matter. Another problem for the 'traveling photon' model. The photon would need to have both energies at the same time because it could not be changing 'with time'.
« Last Edit: 22/12/2008 01:09:19 by sophiecentaur »
 

Offline Bikerman

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How can photons be localised if the universe is expanding?
« Reply #19 on: 22/12/2008 05:11:44 »
The problem with photons (I think) is there is no non-mathematical language which can
be applied. You cannot construct a spacetime diagram for a photon (well, you can, but you just end up with the y-axis, or you have to construct a Minkowski diagram with ct as the y-axis and the photon as a 45 degree plot). Therefore it is meaningless to talk about time from a photon's POV, unless you are doing a translation...
 

Offline 3lviis

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How can photons be localised if the universe is expanding?
« Reply #20 on: 22/12/2008 08:11:37 »
As has been explained elsewhere, for a photon travelling at c time has no meaning.
Your basic premise is flawed. Time has no meaning without the observable context of mass. Photons have mass(albeit infinitesimally small mass). This is the reason why black holes can capture light....if photons had no mass they would pass by black holes. Also effects like gravitational lensing would not be observed if photons had no mass.

"It therefore follows that a photon could be everywhere in the universe at the same time."

Time does exist for photons (because they have mass) which is why they cannot accelerate past the speed of light in a vacuum.

"But the universe is expanding, so there is now more "everywhere" for it to be in than there was at any time in the past."

Is it really? All of this conjecture is based on the Big Bang theory, the red shift and the premise that the Doppler effect of sound translates into the Doppler effect of light. In fact it doesn't and cannot be proven to. Why you ask? Because we have yet to discover a way to accelerate light past the speed of light.

When you decelerate sound the frequency becomes lower. When you decelerate light the frequency becomes lower (observable shift toward the red spectrum).
When you accelerate sound waves the frequency becomes higher. When you accelerate the speed of light what happens? The answer is no one know because you cannot accelerate the speed of light.

This explains why we see a red shift in the universe and no corresponding blue shift. And it explains why scientist incorrectly came to the conclusion that all matter originated from one point.

What science hasn't explained is why we have yet to discover the universe's edge. The point at which we can no longer see mass. Everywhere we have looked there has been mass.


I believe that the reason we haven't discovered the edge of the universe is because there is no edge and that the universe is infinite.

Other problems with the big bang theory is that the rate of expansion of the universe according to theory had to be improbably precise in order for the atoms to coalesce into the galaxies we have today. Any infinitesimally small increase would have resulted in  a universe of scattered atoms. Any infinitesimally small decrease would have resulted in all the matter coagulating into one massive lump. Many groups point to this improbability as evidence of a Prime mover. I point to it and say that it is evidence of a flawed theory.
 

Offline DoctorBeaver

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How can photons be localised if the universe is expanding?
« Reply #21 on: 22/12/2008 09:20:18 »
As has been explained elsewhere, for a photon travelling at c time has no meaning.
Your basic premise is flawed.

It's not my basic premise. Please take up the issue with Richard Feynman, Stephen Hawking & Lisa Randall as it was in books by them that I read it first.

Quote
Because we have yet to discover a way to accelerate light past the speed of light.

That statement is a total nonsense. If you accelerate light past the speed of light then the speed of light has increased and you would have to accelerate it again to get past it which will increase it... ad infinitum.
« Last Edit: 22/12/2008 09:22:54 by DoctorBeaver »
 

Offline DoctorBeaver

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How can photons be localised if the universe is expanding?
« Reply #22 on: 22/12/2008 09:24:07 »
I should also like to point out that the title of this thread has been changed and no longer reflects my initial question.
 

Offline DoctorBeaver

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How can photons be localised if the universe is expanding?
« Reply #23 on: 22/12/2008 09:28:37 »
Yes expansion has an effect.
If you consider the point the photon was emitted and the point it is detected (presuming that the two are separated by large (intergalactic) distances) then the red-shift of the photon is a measure of the expansion between the two points (after we subtract the gravitational red-shift/blue-shift).

But, surely, that is only from an observer's POV. The photon itself won't "notice" it.
 

lyner

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How can photons be localised if the universe is expanding?
« Reply #24 on: 22/12/2008 11:23:09 »
If, indeed, it's "there" to notice anything.
 

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How can photons be localised if the universe is expanding?
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