Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: quat-fro on 11/12/2013 10:38:01
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Good morning folks,
I've been racking my brain over this one since I was a child but have never had a sufficiently good explanation.
How come a distant star is able to be observed from any direction?
Is it emitting a sufficiently high density of diffuse photons that no matter how slightly one moves, there's always a beam of light going to find itself to my retina, or do lights wavelike properties fill in the 'gaps'?
Or have I been looking at it far too simplistically?
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I've been racking my brain over this one since I was a child but have never had a sufficiently good explanation.
How come a distant star is able to be observed from any direction?
Is it emitting a sufficiently high density of diffuse photons that no matter how slightly one moves, there's always a beam of light going to find itself to my retina, or do lights wavelike properties fill in the 'gaps'?
Photons are not like little bullets that start from a point and travel straight. You have to consider the electromagnetic field, whic is emitted with the same intensity in a spherical wave (if the source is spherically simmetrical). The same would be true for electrons or other particles if their field would be simmetrical as that.Or have I been looking at it far too simplistically?
Yes :)
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lightarrow
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Hi Quat-fro, welcome.
I think one of the problems arises from the fact that it is very difficult for us to visualise wave/particle duality. The convenient way to think of it is that a photon (for example) is both a wave and a particle, depending on how we look at it. Perhaps it would be more accurate to think of it as being neither a wave nor a particle. Why should it ever be either?
Thinking of it in that way makes sense of the idea that this “entity” can take every possible path when travelling from A to B. If it is neither a wave nor a particle, why does it have to take any path? Is the concept of a path something which we impose on it because that is how things work at the scale with which we are familiar?
If every photon that travels from a distant star to my eye takes every possible route, there will be no gaps; I will see the star from any and every angle.
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Hi Quat-fro, welcome.
I think one of the problems arises from the fact that it is very difficult for us to visualise wave/particle duality. The convenient way to think of it is that a photon (for example) is both a wave and a particle, depending on how we look at it. Perhaps it would be more accurate to think of it as being neither a wave nor a particle. Why should it ever be either?
Or maybe, the photon can be EITHER wave or particle depending on it's location relative to external stimuli. This means that the spherical wave produced by the distant star can coalesce it's energy into a distinct particle when confronted by electromagnetic energies or sufficient gravitational forces. Wave transformed into particle via external elemental stimuli.
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Wave transformed into particle via elemental stimuli.
Or a "wavicle" that looks like a particle because of the way in which it is being observed, but still remains a "wavicle"? It could be very difficult to distinguish.
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I think it is simpler to answer this using classical photon "bullets" (although a solution using wavefunctions would produce pretty much the same answer, after some maths...).
- The star emits the same number of photons as are later absorbed elsewhere. (Conservation of Energy)
- Each photon carries a fairly small quantity of energy, so there are an immense number of photons emitted, in all directions.
- Each photon has a fairly independent frequency, direction, and phase, ie it is non-coherent - more like an incandescent light than a laser.
- The intensity of the light decreases as the square of the distance (Inverse Square Law). This means that if you double the distance, the same number of photons per second is spread out over 4 times the area. Equivalently, a quarter of the photons per second will pass through the same area.
- The apparent brightness of the star is measured by its magnitude (http://en.wikipedia.org/wiki/Apparent_magnitude#History): A combination of the star's actual brightness, decreased due to distance, and decreased by absorption in interstellar dust clouds.
- If the star is less than 7th magnitude, the brightness is so low (too few photons per second) that it won't form a detectable image on the retina, even in a dark location on a night with no moon.
- If it is a cloudy night, photons from a bright star will be diffused by the water droplets, so it won't form a detectable image on the retina
- In the dark, the iris opens wide, to collect as many photons as possible - similar to the aperture setting on a digital camera
- In the dark, your retina switches to the more sensitive rod cells (http://en.wikipedia.org/wiki/Photoreceptor_cell).
In the dark, the retina becomes more sensitive - similar to a higher "ISO setting" on a digital camera.
So, in conclusion - for a bright star, there are enough photons striking the same spot on your retina every second to trigger the sensation of light.
But you can't see most stars with the naked eye, because they are too faint. The interval between photons is too great to trigger the sensation of light.
I think the "bullet" analogy is more intuitive than a (non-coherent) wave description because we often imagine quantum waves to be like ocean waves (which are fairly coherent) whose amplitude can become arbitrarily low, and spread out over an arbitrarily large area. However, Einstein showed through the photoelectric effect that the energy of a single photon is indivisible - so a photon either strikes your retina, or it does not. And it strikes your retina in a given second, or it does not.
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This is perhaps my issue,
As a very mechanically minded bloke, I see the world in a Newtonian kind of fashion.
Things do mostly appear to be very linear and the conclusion my own logic was drawing towards is that even a visible star couldn't possibly be filling every slightest perspective with 'bullet' / laser beam photons, there would have to be a practical limit. Though there's obviously the question of scale, my eyeball clearly being a huge magnitude smaller than the light emitting star in question...
Light is of course part of the electromagnetic spectrum so I guess I really shouldn't be surprised that there are infinite viewing angles, yet again I suppose there's a large degree of scale involved, tiny waves coming from vast solar surface areas.
Thanks for shedding a bit of 'light' on the subject folks!