Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: barneyboy on 06/12/2013 21:50:13
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if light speed is a constant, never slowing, what powers it? light coming from the farthest reaches of the universe eventually reaches us, each photon having travelled immense distances over billions of years. it must of passed through gas clouds, got dragged about by the universal forces of magnetism and gravity and yet never slowed or never speeded up. on the same vein of thought a photon released by turning on a torch instantly accelerates to light speed as the light from the beginning of time was but I doubt that it will travel as far or for as long so how does it end? does it just cease to be? one Minuit travelling at light speed the next "puff" gone.
If we take light as a wave traveling through space, rather than a collection of photons, then again how does it maintain its energy, its wave form. it must expel some energy. if it expels energy does that not act as a brake trying to slow its velocity. ?
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Your questions are not easy to answer. I personally believe that a photon is slowed down as it travels across the Universe and yes when it reaches us it does go puff and is lost forever. Is the photon dragged about, I think the answer is yes and it.s due to the second law of thermodynamics. I will let you look this up and form your own opinion.
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if light speed is a constant, never slowing, what powers it?
Nothing is required to keep anybody in motion so why would you expect something to keep light in motion?
… light coming from the farthest reaches of the universe eventually reaches us, each photon having travelled immense distances over billions of years. it must of passed through gas clouds, got dragged about by the universal forces of magnetism and gravity
Magnetism, no. Gravity, yes.
and yet never slowed or never speeded up. on the same vein of thought a photon released by turning on a torch instantly accelerates to light speed..
Photons are never accelerated (at least not in GR). They are moving at the speed of light the moment they’re created.
If we take light as a wave traveling through space, rather than a collection of photons, then again how does it maintain its energy, its wave form.
A typical wave diverges as it passes through space. The amplitude thus decreases with time as it travels through space
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I personally believe that a photon is slowed down as it travels across the Universe
Evidence seems to indicate that the velocity of light (in a vacuum) is constant, however, it is difficult to read the one-way speed of light, but only the speed reflected back to the source.
However, while the velocity of light appears to be constant across all wavelengths, the energy isn't.
Redshifting is a change in color of the light due to a slow loss of energy with distance traveled. However, with conservation of energy, that energy must go somewhere.
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how does it end? does it just cease to be? one Minute travelling at light speed the next "puff" gone.
Light of the right wavelength, striking an atom of the right kind can have all its energy absorbed, and used to kick an electron into a higher orbit.
It rarely makes an audible "puff", but yes, the photon ceases to exist. The energy of the photon is not lost, as it can register as light in a digital camera sensor, be turned into heat, re-radiated as a photon of the same energy, or a series of photons of lower energy.
it must of passed through gas clouds ... and yet never slowed or never speeded up.
In a perfect vacuum, light will travel at a specific velocity which we call "c", about 3x108 m/s. According to the Theory of Relativity, light can't travel at any other speed under these circumstances (ignoring virtual photons).
However, if light enters a denser medium, such as an interstellar gas cloud, Earth's atmosphere, or a piece of glass, it will slow down, by a ratio called the index of refraction (http://en.wikipedia.org/wiki/Refractive_index). However, once it returns to a perfect vacuum, the speed returns to c.
This seems counter-intuitive, but the energy of a photon (http://en.wikipedia.org/wiki/Planck_constant) is based on its wavelength/frequency, not on its velocity.
If we take light as a wave traveling through space...how does it maintain its energy, its wave form. it must expel some energy. if it expels energy does that not act as a brake trying to slow its velocity?
The mechanical systems with which we are familiar have a kinetic energy which is proportional to the velocity squared; friction robs some of that velocity, so the energy is sapped, over time.
Light does not experience friction like this, although there are some mechanisms which can sap the energy of a photon:
- High-frequency light (like gamma rays) can bounce off an electron (http://en.wikipedia.org/wiki/Compton_effect), transferring some of its momentum to the electron. The gamma rays then have a lower energy, but still travel at c.
- Optical light of a certain frequency, when observed by a distant astronomer will be seen to be red-shifted (http://en.wikipedia.org/wiki/Red_shift). The received light has a lower frequency than when it was emitted, and hence a lower energy. But the astronomer still sees it traveling at c.
- Optical light of a certain frequency, when observed by an astronomer outside the gravitational well in which it was produced, will see the light to have a lower frequency, an effect predicted by Einstein (http://en.wikipedia.org/wiki/Gravitational_redshift). But the astronomer still sees it traveling at c.
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Thank you for you explanation evan :)
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Evidence seems to indicate that the velocity of light (in a vacuum) is constant,
In inertial frames of reference and in the absense of a gravitational field.
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Evidence seems to indicate that the velocity of light (in a vacuum) is constant,
In inertial frames of reference and in the absense of a gravitational field.
Or the "local" speed of light is constant.
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To CliffordK. The energy is lost to it,s surrounding due to the fact that the photons path from a distant galaxy is bent by other galaxies in it,s vicinity, it is bent away from them to colder regions (thermodynamics) resulting in a red shift.
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Or the "local" speed of light is constant.
Yep!
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I should have mentioned that the energy is lost to it's surroundings by an extremely low frequency electromagnetic wave, the action of thermodynamics is to even out the universes background temperature it wants to make it all equal. I had to do a simple experiment using a laser pen and a light bulb plus some mirrors to convince myself that it was correct. Switching on the light bulb and waiting for it to heat up, the beam did deflect away from it and the mirrors amplified the result. Don't repeat this experiment without using sunglasses it's far to easy to get an unwanted reflection.
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I should have mentioned that the energy is lost to it's surroundings..
A photon moving through a vacuum does not lose energy to its surrounding. The onne way to look as it travels through space the wavelength gets stretched out as the universe expands.
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I should have mentioned that the energy is lost to it's surroundings by an extremely low frequency electromagnetic wave, the action of thermodynamics is to even out the universes background temperature it wants to make it all equal. I had to do a simple experiment using a laser pen and a light bulb plus some mirrors to convince myself that it was correct. Switching on the light bulb and waiting for it to heat up, the beam did deflect away from it and the mirrors amplified the result. Don't repeat this experiment without using sunglasses it's far to easy to get an unwanted reflection.
Was this experiment done in vacuo or in air?
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The experiment was done in air. I cannot accept the current theory that a photon will travel forever without losing radiation, this would give it an infinite lifetime, nothing lasts forever.
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As has been pointed out in this discussion, an electromagnetic wave disperses as it travels, and photons undergo a gravitational red shift in travelling through the universe. Current theory seems to be consistent with observation. However your light bulb and laser experiment seems to me to be more like a Schlieren effect.
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So, if we want, we might argue that the loss of energy of light gets compensated by the 'accelerating expansion', in some distant time resulting in no light surviving any 'propagation' possibly :)
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Now, what would such a universe do to the definition of 'c'? It must still exist in it, won't you agree? And if it does, so does 'light', although no longer measurable. And what would happen to matter? Indeterminacy rules :)
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This is in fact a rather misleading argument, as we still will have a 'slower' expansion than the speed of light in a vacuum locally. To get to it we would need a real acceleration existing and growing in each frame of space, but what we have is a addition of 'new frames' of space uppwelling, creating a 'ftl' effect without any discernible motion.
The funny thing here is that, assuming that galaxies communicate/updates positions relative gravity, this empty vacuum of no resistance still is able to 'push' those galaxies aside from each other, ignoring the gravitational relations between them. How does it do it? I can argue that this is the way the solarsystem keeps its heavenly body's relative distances versus each other, but I can't argue the same over longer intergalactic stretches?
Why?
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What such a reasoning seem to lead me too should be an assumption of a 'force' from the accelerating expansion, able to counteract the gravitational influences by those galaxies interacting? Which is a weird thought as there still is nothing new in those new patches, just a kept 'equilibrium' of SpaceTime energy as I understands it? Unless we somehow add the way light behave?
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Could you use geometry solely to define it? If you for example define it from a 'plane' of some sort, instead of four dimensions? Although that will inevitably lead a 'projective reality', as real as can be for us eh, 'flatlanders' :) but questionable theoretically?
Either there is a 'force' acting, and that concept do not rhyme to me with a vacuum, defined by a equilibrium/virtual particles/indeterminism. Or the mechanism for it hasn't to do with 'forces' at all as it seems to me?
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The Schlieren effect is colour defects in lenses. There is no lens in the experiment, the laser beam didn't go through the bulb it was positioned as close to the bulb without touching it. The bulb was used as a heat source to produce a temperatue gradient. I didn't explain it well enought.
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Schlieren (from German; singular "Schliere", meaning "streak") are optical inhomogeneities in transparent material not necessarily visible to the human eye.
Schlieren imaging is used to show, among other things, variations in refractive index in gases. It's particularly useful in wind tunnels but also good fun for demonstrating convection currents in air. Which is what you have done.
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The experiment was done in air. I cannot accept the current theory that a photon will travel forever without losing radiation, this would give it an infinite lifetime, nothing lasts forever.
Our sensibilities our often wrong when it comes to understanding nature. You might not accept that a photon will last forever but there’s nothing in the laws of nature that prevents it. Light consists of photons. As the light travels through air some of them will get absorbed by atoms and reemitted later. This diminishes the intensity of the beam of light (the intensity of a beam of light is directly proportional to the photon density).
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The bulb was used as a heat source to produce a temperature gradient.
Air at a higher temperature has a lower density, and a lower refractive index.
A temperature gradient produces a gradient in the refractive index of air, which will bend light.
This effect seems consistent with classical physics.
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I agree with you, but does it also occur in a vacuum without any matter present. I originally worked it out electically and then started looking for known laws of nature to describe what was happening, hit upon thermodynamics and it's correct a hot body does bend light away from it.
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I agree with you, but does it also occur in a vacuum without any matter present.
No.
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Destructive criticism. Cite a reference.
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I originally worked it out electically and then started looking for known laws of nature to describe what was happening, hit upon thermodynamics and it's correct a hot body does bend light away from it.
Please show your working.
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nothing lasts forever.
How could you possibly know that?
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A temperature gradient produces a gradient in the refractive index of air, which will bend light.
I agree with you, but does it also occur in a vacuum without any matter present.
How would you propose creating a temperature gradient in a vacuum?
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I've connected an aerial to a spectrum analyser and placed it near the bulb what did I find?
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I suspect "a spectrum", but I'm willing to be surprised.
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nothing lasts forever.
How could you possibly know that?
Right. There's no reason to assume that any given atom or molecule won't last forever if left by itself.
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