[@/antic (OP)]

Hi

When we look out into space/time, we're looking back in time.

How is a photon of light able to convey an image (or a bit of it)?
Can it carry this image over an indefinite distance through space?
Why does the image stay intact over such great distances and not dissipate?
Why is the image not corrupted by other photons travelling through space/time, and the image remains consistent?

Thanks,

Atlantic

[Pmb]

How is a photon of light able to convey an image (or a bit of it)?

Look at a photo that was scanned into a digital image and then enlarge the image so that all you can see is one pixel. That should give you and idea of how it works. To see an actual image you need a lens. A lens redirects the light so that it forms an image. Each photon carries a bit of the image. Any more detail requires a course on optics to some  extent.

Can it carry this image over an indefinite distance through space?

No. There is a point where the imager degrades so much that no information can be collected from it. Eventually all you'll see are indiv idual photons.

Why does the image stay intact over such great distances and not dissipate?

Where did you get the idea that they did?

[@/antic (OP)]

Hi Pmb,

Got it from astronomers, who would say that they have "seen" a star or cosmic object, whose image has travelled billions of light years to get to the Earth......

[Pmb]

Hi Pmb,

Got it from astronomers, who would say that they have "seen" a star or cosmic object, whose image has travelled billions of light years to get to the Earth......

There is no way that images don't degrade over cosmic distances. I think you misunderstood him/her.

[JP]

The problem is that most astronomical objects emit photons in many directions (usually in all directions).  If you look at the angle our telescopes make out of the entire range of directions in which photons are emitted, it's tiny!  The number of photons collected tends to fall off as 1/distance².  There will always be a small number of photons making it to us from these distant objects, but the further away we are and the dimmer they are, the smaller number of photons there are to observe.  Eventually you can't form an image because there are so few photons, you are overwhelmed by stray photons from other sources or from noise in the camera electronics.

[imatfaal]

The image is also limited by angular resolution

Two point sources are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other.[1] If the distance is greater, the two points are well resolved and if it is smaller, they are regarded as not resolved. If one considers diffraction through a circular aperture, this translates into:

               
    where

        θ is the angular resolution in radians,
        λ is the wavelength of light,
        and D is the diameter of the lens' aperture. 

From here
http://en.wikipedia.org/wiki/Angular_resolution

[CliffordK]

There are a few issues with long distance astronomy.

The intensity of the light which has several factors including the intensity of the source, the distance, the area of the collector, and the time of observation.  What do you mean by a single photon?  A single photon striking all of Earth during one's lifetime?

Atmospheric Scattering, clouds, and light pollution, leading to the use of space based telescopes rather than terrestrial telescopes.  Likewise, there is "noise" in space.  A star's corona, while it gives light, and lights up surrounding planets, it can also cause a tremendous amount of interference for a clear image.

I can imagine building some very large lunar telescopes inside of large impact craters which could give unprecedented resolution.

The other issue is red-shifting.  As gamma rays get red-shifted into radio waves, they will eventually be at too long of a wavelength to be detectable.

[Soul Surfer]

A single photon carries the information of its existence and frequency together with the direction that it has come from but that depends on the nature of the instrument used to detect it.  A single photon does not carry any sort of image.  Many protons can contain the information required to make an image,  as long as their paths have not been significantly disturbed en route by gravity or matter on the path from the image source.  The image source however does not have to be where the photons originated.  For example a photon originating from a bright star can be reflected by a particle in a dust cloud to form an image of the dust cloud.  This is just the same as using a torch in the dark to light up a wall.

[MikeS]

For a photon image to degrade, somehow the photons that make up the image would have to interact with each other or something else and that implies an interaction over time.  Any interaction en-route obliterates those photons.  Only un-interacted photons survive to make up the image and they (the survivors) have no sensation of the passage of either time or distance.

The image can only degrade through
Photons being obliterated.
Red-shift.
Photon density decrease per unit space volume.
Non of the above degrades the image other than it looses intensity.

The warping of space-time by passing close to a large mass can distort the image by distorting the 'grid' the geodesics that the photons follow but this is a distortion of the 'fabric' of space-time not individual photons.  (This is similar to the distortion of the 'fabric' of a child's balloon as the balloon is inflated or distorted by poking it with a finger which is analogous to gravity.)

[imatfaal]

Mike - of course the number of photon once it slips below a threshold will lead to image loss.  As will red-shift - look a the equation I posted above - at longer wavelengths less angular resolution is possible for the same size light gatherer.  And please stop postulating about the sensitivities of inanimate massless particles (they hate it) and it is pure speculation - and please do not take this thread o/t by arguing that it is good physics

[evan_au]

• Photons spread out as they get further from the source, so to see really distant objects, you must build a larger telescope (collect photons from a larger area) and/or expose the image for much longer (collect photons for a longer time, like the Hubble deep-field images). Both of these cost money, so economic factors impact the quality of the images we see.
• Earth's atmosphere absorbs light and distorts images because of turbulence. Adaptive optics can help, but the best viewing is done by space-based telescopes.
• Photons can be absorbed by dust (blue is absorbed more than red), and certain frequencies in the light can be absorbed by gas atoms and molecules in space. So images do degrade as they pass through space. It's actually easier to see the shape of the Andromeda galaxy than our own, because we must look through so much gas and dust in the plane of our galactic disk. The James Webb space telescope works in the infra-red, so it is able to see better through this dust.
• Although light interacts with atoms, photons do not normally interact with other photons until you reach really high energies. For example, two gamma-ray photons can collide and produce an electron and a positron. But visible-light photons pass straight through each other, so they don't interfere.
• Light is red-shifted by the expansion of the universe (seeing events earlier in the life of the universe is another bonus for infra-red astronomy), and also as it escapes from an intense gravitational field such as a neutron star.
• Light can also be bent by the gravitational field of a galaxy, which distorts (and sometimes magnifies) light from more distant galaxies - but this information is now being used both to see galaxies further away than our telescopes could normally see, and also to infer information about the intermediate galaxies which we cannot see directly.

So seeing clearly into space requires some very large and expensive telescopes, preferably in space (the launch is expensive too...)

[evan_au]

Each of these image distortions records an interaction of light with something.

For example, light from distant quasars has chunks deleted from their spectrum. Astronomers now use these deletions to estimate the size and distance to gas clouds between us and the quasar - gas clouds we would not otherwise see.
 
By similar spectroscopic techniques, Astronomers have detected various atoms and molecules in space.

Light interacts so strongly with matter that it is actually not the best technique for seeing some astronomical phenomena.
 

• Estimates vary, but it is said that it can take of the order of a million years for light energy to percolate from the center of the sun to the visible surface, so it tells astronomers very little about what is happening in the sun now.
• However, sound waves propagate very quickly through the sun (similar to earthquake waves), so they are very informative about the internal structure of the sun.
• If you want to see images that are little distorted by matter, astronomers have used neutrinos to peer inside the sun's core, and have detected a supernova this way (SN1987A).
• Similarly, experiments are underway (like LIGO) to use gravitational waves to detect supernovae and merging black holes.
• The problem here is that neutrinos and gravitons interact so weakly with matter that it is extremely hard to detect them. The result: big, expensive detectors, that may detect only 1 astronomical event in many years.

[MikeS]

• Estimates vary, but it is said that it can take of the order of a million years for light energy to percolate from the center of the sun to the visible surface, so it tells astronomers very little about what is happening in the sun now.
  
  

Light energy probably.  Individual original photons probably not and it is individual photons that carry that information. 

[MikeS]

Light interacts so strongly with matter that it is actually not the best technique for seeing some astronomical phenomena.
 

I guess it depends upon what you mean by light and what you mean by interact.  Individual photons to the best of my knowledge can not interact in any way and survive. 

[Quantum Inquisitor]

Hi Everyone!

I just finished reading that entangled photons communicate in a way not currently understood; and transmit information to each other 100k times faster than the speed of light!

Ken

[evan_au]

Individual photons to the best of my knowledge can not interact in any way and survive.

There is a special case where photons can interact with matter and "survive": in a laser.

This requires special lasing materials with metastable states, which are "pumped" to have a population inversion. Then an incoming photon in the right wavelength band can trigger a cascade of many other photons with the same frequency and phase. (Quantum effects mean that it won't be an exact clone of the incoming photon, but it's pretty close..)

Laser-like effects have been reported at microwave frequencies in astronomical gas clouds (a "maser"), but I'm not aware of any observations of laser-like effects at optical frequencies in astronomical objects.

[yor_on]

It's a very interesting question. How does 'photons' transmit information? They have a momentum, a 'spin', and 'energy', as far as I know.

Think you can split it in two parts actually, one is the wave description, but that one falls apart at the moment of annihilation to me, as that is a 'photon' interaction to me. To me it has to be, as it is as local as can be, and to get to a wave picture you need to introduce frames of reference communicating this wave you describe.

So locally I would define the communication as carried by 'photons'. How do they do it?
=

To add to 'spin'. Normally we define it as being of two possible values only, 'up or down', but is that correct or is it a result of the way we do the experiments? If it's possible for this spin to have more values, would that add to the information? And if so, how does it then get read by the sink? Should be possible, if so, to construct a experiment testing it, shouldn't it?
=

There is also naturally the aspect of synthesizing out a information over time, by our brains. This one assumes a constant called the 'arrow of time', locally equivalent for anything inside the observable universe we define. As long as this arrow is a (local) constant shared throughout a universe you could assume that it is not the single piece of information described by each photon that makes up the 'frozen image' your brain construct but a 'string of photons' giving you that still image that your brain, using the arrow once again, then further strings together into a moving picture.

Maybe

[yor_on]

That would make seeing into a logical construct, synthesized by the brain (using time), from yet another underlying construct, consisting of synthesizing information from a string of photons annihilating. So what you see is a illusion, created by your brain under a arrow, or, your reality.

[yor_on]

Hi Everyone!

I just finished reading that entangled photons communicate in a way not currently understood; and transmit information to each other 100k times faster than the speed of light!

Ken

That's just not possible. Information has to be at light speed. If you find a way to transmit (useful) information faster than the speed of light you're invalidating 'c', or possibly making it into a subcategory of some entirely different theory. This sort of statements have a ability of appearing on the net now and then, but as far as I know, none validated.

[jccc]

Mike - of course the number of photon once it slips below a threshold will lead to image loss.  As will red-shift - look a the equation I posted above - at longer wavelengths less angular resolution is possible for the same size light gatherer.  And please stop postulating about the sensitivities of inanimate massless particles (they hate it) and it is pure speculation - and please do not take this thread o/t by arguing that it is good physics

May I ask what massless particle are you talking about? Bosons, gluon or photon? Thanks.