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Beards
2006-Aug-14, 06:51 PM
So my first thread - oooh how exciting!!

Right guys I've got a bit of a poser for you. As I understand it anything that travels at the speed of light travels (from it's reference point) instantaneously from one place to another. So in effect all the light we see is of the same "age" and never gets any "older". Is this correct? If so how can that be? Could it be that what we see as light may be a glimpse into some other dimension which appears frozen in time from our perspective?

I'm having real trouble trying to understand exactly how light "works" and its properties at relatavistic speeds. I'm rather new at this and have no grounding in physics or maths so if you could try and explain in laymans terms as best as possible I'd be eternally grateful.:)

Beards.

grant hutchison
2006-Aug-14, 07:01 PM
Think of it as a letter.
Your friend in the next town sends you a letter, and it arrives a couple of days later. Your friend on the other side of the world sends you a letter, and it arrives a week later. The text in both letters has stayed exactly the same during transit, and yet one friend is two days older when you read her letter, while the other is a week older.
Likewise, there's no paradox in a photon that experiences no passage of time bringing you news from places that are ageing while the photon travels.

Grant Hutchison

Ken G
2006-Aug-14, 11:55 PM
And to extend Grant's argument to cosmological redshifts, it is as though light coming from billions of years in the past is a letter being sent to a Lilliputian-- the letters appear much larger when opened by our tiny selves, just like the wavelengths from the distant past.

grant hutchison
2006-Aug-15, 12:08 AM
And to extend Grant's argument to cosmological redshifts, it is as though light coming from billions of years in the past is a letter being sent to a Lilliputian-- the letters appear much larger when opened by our tiny selves, just like the wavelengths from the distant past.Yikes.
I think you just redshifted my analogy to invisibility, Ken ... :)

Grant Hutchison

Ken G
2006-Aug-15, 12:11 AM
That's because I've always preferred the "matter is shrinking" view to the "space is expanding" view. Physics is about matter, space is meant to be a spectator.

Beards
2006-Aug-15, 08:15 AM
Thank you guys for the replies (Oh I'm a poet and don't know it!)

I think I understand the idea of time dilation but I still don't understand if photons age. A letter is a fine analogy but does the letter change in any way during it's journey? As I understand it if something moves at c then from our perspective it doesn't age. So from it's own perspective does it age? Photons are created and travel for their whole "life" at c until they are absorbed, which could be called the end of their life. So in my understanding they only "age" when they are absorbed.

Am I being a total philistine or am I failing to grab some significant morsel of information?:wall:

WaxRubiks
2006-Aug-15, 08:39 AM
so, if, from its own perspective, the photon doesn't age then light leaving the Sun heading for deap space has already redshifted, in the future as it were?

cjbirch
2006-Aug-15, 09:55 AM
For a photon 'age' is somewhat of a meaningless concept, however you can surmise roughly how long the detected photon has been travelling. Yet photons are not subject to an ageing process they would look the same for all points in time that they exist.

Beards
2006-Aug-15, 10:58 AM
So am I then to understand that photons don't have a "life span" they just exist? Or do they have a miniscule life span yet we see them as existing for millions of years (in the case of light hitting my eye from very distant galaxies) from our frame of reference?

grant hutchison
2006-Aug-15, 11:02 AM
That's because I've always preferred the "matter is shrinking" view to the "space is expanding" view. Physics is about matter, space is meant to be a spectator.It wasn't that I was fretting about: it was this introduction of people who get progressively smaller as time goes by, while swapping letters that retain the size of the person who wrote them. You weirded out my analogy. :)


A letter is a fine analogy but does the letter change in any way during it's journey?The text content of the letter doesn't change, just faithfully preserves the ideas in the letter; it's timeless. Likewise, the photon doesn't change. When it's emitted, it comes into existence; when it's absorbed, it ceases to exist. In between, nothing happens.


so, if, from its own perspective, the photon doesn't age then light leaving the Sun heading for deap space has already redshifted, in the future as it were?We think of it as redshifting progressively, while time proceeds as usual for us. But we can only see that redshift as a one-off experiment by absorbing the photon; we can't come back later and check again on the photon's redshift progress. So the photon just moves some energy from one place to another, but it takes more energy to create than it subsequently delivers, if it is absorbed at some location cosmologically distant from its creation.

Grant Hutchison

Beards
2006-Aug-15, 11:22 AM
the photon doesn't change. When it's emitted, it comes into existence; when it's absorbed, it ceases to exist. In between, nothing happens.


Thanks Grant. So you are saying light is timeless then? It exists outside our time reference?

Sorry - may seem like a dumb question to you but this has got me very confused.

Ken G
2006-Aug-15, 02:19 PM
It wasn't that I was fretting about: it was this introduction of people who get progressively smaller as time goes by, while swapping letters that retain the size of the person who wrote them. You weirded out my analogy.
I see, well I hate to weird out an excellent analogy, but I was anticipating the redshift question. If the light doesn't age, how does it "redshift"? It is easier to imagine the redshfit as being a result of what is happening to the stuff that does age-- the universe that is considering that timeless light.

Ken G
2006-Aug-15, 02:21 PM
So you are saying light is timeless then? It exists outside our time reference?


Yes, in a manner much like the way the North pole exists "outside" the reference of longitude-- in a sense it exists at all longitudes at once, yet it is no different to stand on than any other point.

grant hutchison
2006-Aug-15, 05:17 PM
Thanks Grant. So you are saying light is timeless then? It exists outside our time reference?To Ken's comment, I'd add the remark that any object moving relative to us is "outside our time reference" with regard to its own internal clock: we see time pass differently for it.
The equations that predict how such things work for moving objects develop a crop of zeros and infinities at lightspeed, which makes the idea of trying to measure time "aboard" a photon a little problematic: hence, I think, cjbirch's use of the phrase "somewhat meaningless", and Ken's invocation of the singularity in longitude at the poles.

Grant Hutchison

grant hutchison
2006-Aug-15, 05:22 PM
... well I hate to weird out an excellent analogy, but I was anticipating the redshift question.I had an annexe to the analogy, which I was prepared to unlimber if that cropped up.
The longer a letter is in transit (preserving its original content in a timeless state during that period), the less of its meaning will be transmitted to its eventual recipient, simply because language and circumstances change over time.
Likewise, the Universe in which a photon is absorbed after travelling for a cosmological time period is a different place from the one in which it was emitted.

Grant Hutchison

WaxRubiks
2006-Aug-15, 05:32 PM
what if this letter contained betting tips for a long list of races over many months? Then the longer the letter was in transit the more tips are for races that have been run making the tip useless. Or is does that make the analogy more messy?

Ken G
2006-Aug-16, 01:37 AM
Likewise, the Universe in which a photon is absorbed after travelling for a cosmological time period is a different place from the one in which it was emitted.

Yeah-- the people are smaller! (But it's your analogy, and a good one, so I'll leave it be.)

ZaphodBeeblebrox
2006-Aug-16, 02:20 AM
Yeah-- the people are smaller! (But it's your analogy, and a good one, so I'll leave it be.)
This Actually Reminds me of a Short Story I Read a While Back ...

It Was About a Drunk Physicist Who Invents a Cabinet, Through Which Objects May Be Placed Into the Future ...

One of The Characters Actually Winds Up Crushing himself from a Week Before, Because The Size Difference Is SO Great!

:eek:

grant hutchison
2006-Aug-16, 07:24 AM
what if this letter contained betting tips for a long list of races over many months? Then the longer the letter was in transit the more tips are for races that have been run making the tip useless. Or is does that make the analogy more messy?I'm not sure it makes things any clearer.
But now I seem to have hijacked the thread into a discussion of the effectiveness or otherwise of my analogy instead of about Beards' question ... :o

Grant Hutchison

Beards
2006-Aug-16, 08:09 AM
So talking about photons ageing is a bit like talking about netrinos smelling - they've not got a great deal to do with each other?

I always thought that light was similar to a wave and a particle and was a small quantified lump of "stuff" which could be affected by everything in the universe, such as gravity, matter etc so trying to think of it as something which isn't affected by time feels a little odd.
I think I need to do some reading! :eh:

grant hutchison
2006-Aug-16, 08:44 AM
Yeah-- the people are smaller!On that topic:
The expansion of space seems to give an intuitive (read, "handwaving") reason for photons to lose energy as they travel; the contraction of matter doesn't.
How does that work, in your view of things?

Grant Hutchison

Ken G
2006-Aug-16, 01:12 PM
On that topic:
The expansion of space seems to give an intuitive (read, "handwaving") reason for photons to lose energy as they travel; the contraction of matter doesn't.
How does that work, in your view of things?


The key is the ignorable transformation that makes everything smaller and time faster, so as to keep c the same. Clearly, any such transformation is perceived by all measurements as an identity transformation, except for observers that did not undergo the transformation. If it is indistinguishable from an identity transformation, it is an identity transformation, for those within it. This is somewhat similar to what we already do in special relativity for a moving frame (length contraction and time dilation), except that time slows down and the identity transformation is recovered by desynchronization effects due to the motion. We talk about contraction of physical lengths, not expansion of space, in SR. In the form I'm talking about, time speeds up so there's no desynchronization, and there's also no relative motion. Now take all your normal pictures of cosmology and apply this transformation more and more as the universe ages, such that the distances between galaxy clusters, and the wavelengths of light, no longer change with age. Voila. It is merely a new pedagogy, not a new physics, as it is indistinguishable. The subtle difference is that now the "comoving coordinates" are truly a stationary coordinate system, which seems the most natural choice to me in the first place. And in terms of the loss of energy by photons, that's part of the beauty of it-- nothing at all happens to the photons, just like nothing happens to your letters. But they are perceived as having longer wavelengths and smaller energies, relative to us.

Edit: added special relativity analogy to show we already do something similar.

grant hutchison
2006-Aug-16, 05:14 PM
Ah. "Time faster" is what I was missing out on. Thanks. :)

Grant Hutchison

Ken G
2006-Aug-16, 07:03 PM
Yeah, in this view, supernovae took longer to happen in the distant past simply because time was proceeding more slowly back then, relative to now. You don't have to stretch the light train, something I was never very clear on how "space" accomplishes anyway.

Peter Wilson
2006-Aug-16, 11:53 PM
...the photon just moves some energy from one place to another, but it takes more energy to create than it subsequently delivers, if it is absorbed at some location cosmologically distant from its creation.

Grant Hutchison
But the total energy in the universe before and after is the same. Your head will start spinning, however, if you try to figure out where the missing energy went. Don't try it at home ;)

Ken G
2006-Aug-17, 04:58 AM
Actually, cosmological models use comoving coordinates and thus are not constrained to conserve total energy. But I've also heard the idea used that the radiation expands adiabatically with the space, and adiabatic expansion does work against the hypothetical "boundaries". I prefer the first point of view though. Some have also tried to put the energy into the gravitational potential, I'm not sure what the status of that effort is, but note that conservation of energy was always an artificial law-- potential energy was invented to make it work, it's not a fundamental rule unless potential energy is afforded a physical significance it is not clear that it deserves (for example, if gravity is not a force, why should we associate potential energy with it?).

grant hutchison
2006-Aug-17, 12:31 PM
But I've also heard the idea used that the radiation expands adiabatically with the space, and adiabatic expansion does work against the hypothetical "boundaries".Yes, the adiabatic idea was the one I was thinking about when I referred to an intuitive (handwaving) mechanism that could be invoked to dispose of the photon's energy if space is considered to be expanding.

Grant Hutchison

Ken G
2006-Aug-17, 12:53 PM
And actually, since there are no boundaries to do work on, what adiabatic expansion does is trade randomly directed (thermal) kinetic energy for ordered (bulk flow) kinetic energy. Only we don't see the latter, in the comoving coordinate system. That's what I mean about the coordinate system not conserving energy. But in the better spirit of general relativity, we don't even say the coordinates "rob" us of the bulk-flow kinetic energy, we say there is no flow at all and we just don't expect to have conservation of energy in the first place. It's a tricky business, to be sure.