# Does light travel forever?

Discussion in 'Physics & Math' started by drumbeat, Mar 28, 2011.

1. ### PeteIt's not rocket surgeryModerator

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10,166
That is the *current* idea.
The doppler effect for light is the same as the doppler effect for sound in the special case of the receiver being at rest with the sound's medium.

The equations are usually presented a little differently, because the usual doppler equation for sound ignores time dilation. But if you take the low speed approximation for light doppler, or you include time dilation in sound doppler, then the equations are identical (if the sound receiver is at rest in the medium).

You do know that the speed of sound is not affected by the motion of its source, right?

What problems need explaining?

Last edited: Mar 29, 2011

3. ### siphraRegistered Senior Member

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Strictly speaking, that is but one theory, and not even a leading one. In any universe where travel in one direction leads back to the point of origin, it is possible to travel for an infinite amount of time, however; this doesn't mean the universe is infinite in volume, in fact for a simple example if you have a spherical universe you can estimate the volume by the fact that the diameter of the universe == the distance you have to travel to get back to your point of origin.
Since most leading theories hold this to be the case, the universe volume is finite at any time. And approaches infinite but if you know how long its been since the big bang, you can calculate it's volume.

5. ### RJBeeryNatural PhilosopherValued Senior Member

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Well having alternative theories is different from claiming not to understand red-shifting, which we cannot simply declare doesn't exist just because train lights don't exhibit the effect enough for us to notice. Have you ever read about the red-shift / blue-shift effect of orbiting binary stars? Would your theory suggest that, as one star is orbiting away from our sight and is red-shifted, its light reaching us is being refracted through more space dust than when it soon after is blue-shifted while orbiting toward us?

Also, refractive effects are corrected after the medium has been passed so unless you're suggesting that our orbiting satellites are sitting in fields of space dust particles I think your idea might be misconceived (but interesting!). Anyway, that being said, cosmologists do have some unresolved problems.

Here's a question I've never seen raised and maybe someone can answer: if we must postulate a cosmological term representing a repulsive force at extremely large distances to account for red-shifting, why is the light subject to the increasing distances involved (causing the red-shift) but not the acceleration itself (causing BLUE shift at further distances)? "Anti-gravity" should affect light just as it does mass, analogous to gravity affecting both!

7. ### Janus58Valued Senior Member

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Yes it does. It's a matter of conservation of momentum. Just like a bullet will have more momentum relative to you if it is fired at you from a car moving towards you, a photon emitted towards you from an object moving towards you has more momentum.

In the case of the bullet, this extra momentum is expressed as an increase in speed. So for example, if it was a machine gun firing at you, not only would the spacing between bullet decrease, but the speed and momentum of the individual bullets would increase.

Light, however has a constant speed with respect to any observer, so the photon must use another means to express this extra momentum.

For a photon momentum is found by:

$P= \frac{h}{\lambda}$

Where $\lambda$ is the wavelength of the photon and h is Planck's constant. So for a photon to express the extra momentum, it must decrease its wavelength.

Thus for light coming at you from an approaching source, not are the photons spaced closer together, but the momentum of each individual photon must increase with a coresponding decrease in wavelength.

8. ### Janus58Valued Senior Member

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The M&M experiments were not looking for a Doppler shift( nor would they expect to find one even if there were an aether, as the components of the experiment had no relative motion with respect to each other). What they were looking for was a phase shift caused by the two perpendicular beams taking different amounts of time to make their individual trips.

9. ### OnlyMeValued Senior Member

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True.

However, just as even a small change in velocity would have been detectable, so would have a redshift. Here is why...

The interferometer used a single beam of light that was split and sent in two directions. One direction in the line of the Earth's rotation and orbital motion and the other 90% to that motion. The experiment then recombined the two beams of light and watched for a change in the interference pattern. Had a change in velocity due to the Earth's motion through the ether been present it should have been detectable it was not. The experiment has been repeated.

Now , if the light moving with or against the direction of earth's motion had been red shifted it would also been detectable as the wave lengths of the two beams of light would have been different when recombined and the interference pattern would have been affected. Since a change in the interference was all they were looking at a redshift would have shown up.

They were not looking for it but it would have been detected.

10. ### RocksRegistered Member

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I didn't claim anywhere to have a "theory" and at most, I was hypothesizing with the intent of understanding the information being given to me and what is known. With a lot of basic things about physics across space, it seems I'd have to go back all over and start yet another degree right from the beginning to make heads or tails out of even the basics. That's not going to happen any time soon.

Anyway, how are refractive effects corrected? (i know I should just give up on trying to understand this... I've read about it, also blue shift and i just don't get it... give me ENVI and some pics of the moon and I'm good to go...)

11. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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So you are defining the universe as a single entity, then. You are defining "the universe" as you do, say, "the earth," as an object. So you define the universe as an object of mass in space, but disregard the space it resides in?

Space is infinite, you are describing the universe as a finite object, which it is not. The term universe implies mass residing in (infinite) space.

Last edited: Mar 29, 2011
12. ### OnlyMeValued Senior Member

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This represents one of the alternate views. The problem occurs when considering that this effect is time dependent. It requires that the emission of a single photon occurs over time, sufficient that the motion of the emitting atom can affect the photon's wavelength.

Think about this, atoms vibrate, they move around all of the time. That motion is small by everyday standards but very big by comparison to a photon. The wavelength and velocity of light, photons, emitted by an atom are not changed by that constant motion of the atom itself. Individual photons have to be considered to be emitted outside of any reference to time.

So long as the emission of a photon is not time dependent, the velocity of a phton's source cannot affect either its velocity or wavelength.

13. ### OnlyMeValued Senior Member

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The refractive index of light is normally associated with the density of the material that light travels through. Since currently accepted theory GR assumes that space is empty and uniform, its refractive index is constant at 1 and the speed of light is constant and the same everywhere in space.

If light travels through a gas and dust cloud in space it could be assumed to have a refractive index greater than 1 and slow down the light as it passes through. However, when the photons leave the cloud it speeds back up to c the speed of light in vacuum. When broad spectrum light moves through a prism, the change in refractive index separates the different wavelengths in direction not in wavelength.

By the way never give up. You may sometimes have to struggle through enept attempts to explain something. That is more a reflection on those doing the explaining, including me, than any reflection on you.

14. ### EmilValued Senior Member

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Yes, I think so.
Especially in certain conditions such as the fhoton sphere.

15. ### RJBeeryNatural PhilosopherValued Senior Member

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Well, I would make a poor diplomat because I think I tend to be abrasive in my wording even when I don't mean to be. In any event, light entering a refractive medium is essentially slowed down, causing the refraction, but when it exits the medium it resumes its normal velocity. If light enters and exits the medium perpendicular to its surface then there is no net effect whatsoever (except a slight initial wave front travel time delay). You might be thinking of the Dark Side of the Moon prism effect, but that is simply separating the white light, it isn't changing its cumulative color, and this is because of the angle between the incident light beam and the angle of the prism face.

If passing through a refractive material had a net red-shifting effect then water wouldn't be "clear".

16. ### OnlyMeValued Senior Member

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Currently accepted theory here is general relativity. The conclusions drawn by general relativity depend in part on the assuption that the speed of light is a universal constant in vacuum and empty space. Empty space being assumed to be a vacuum and uniform everywhere. If empty space is not uniformly empty as far as light is concerned and due to the presence of gas and dust its refractive index is not constant the speed of light over galactic distances would be variable proportionally to its changing refractive index. If the speed of light is not a universal constant the curved space and time dilation as defined by general relativity are not supported by observation.

From the perspective of general relativity curved space acts like the refractive index of a medium of varying density. If space is not uniform gravitational lensing and redshifts cannot be attributed to the curvature of space or the direct interaction of photons with the proximity of a strong gravitational source. This does not mean that they aren't affected by the curvature of space or gravitational forces. It means only that those cannot be isolated as "the" cause, or only cause.

A universally constant speed of light is a critical element of the predictions made by general relativity.

Side note: even if it were at some point proven that the density of space is variable and responsible for astronomically observed phenomena.., and the speed of light is not constant throughout the universe..., as far as local conditions are concerned it would not change the predictions of GR. Einstein's field equations are not in conflict with Newtonian dynamics in a "local" frame of reference. And the speed of light could still be considered as a constant locally.

But this is getting way off what is currently accepted and into "what ifs" and speculations.

17. ### Janus58Valued Senior Member

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Yes, the wavewlength is changed by the motion of the atoms. It leads to an effect known as spectral broadening.

The spectral lines for any given element are very thin. However, if you you look at the spectrum of a given object, the spectral lines can be fairly thick. This is because the various motions of the individual atoms in the sample cause the light emission from them to Doppler shift by various amounts causing the visible spectral lines to "smear out" and appear broader.

This spectral broadening can be used to judge the temperature of the sample. The hotter the sample, the greater the velocities of the individual atoms and the more the Doppler shifts from these atoms will smear and broaden the spectral lines.

18. ### RocksRegistered Member

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No worries. I didn't take any offense to it, I'm just really particular about the word "theory" and feel it's overused. When most people say "I have a theory" they really have a hypothesis. To me, a theory is something more robust than "I think ____" and has undergone some degree of rigorous examination and continues to appear plausible. In other words, I generally cringe when I hear the word theory in everyday usage

I'm learning a lot from what you folks are sharing and the other things I've read and definitely have a better understanding about the behavior of light than I did at this time yesterday, but I just realized that everything that makes sense to me now still doesn't translate to a total understanding of how the expansion causes redshift

Actually, that was a very helpful answer. I won't give up, but sometimes I ask myself why I drive myself crazy trying to figure things out that don't really have any effect on my life. I've always naturally been very curious, which can be great, but also maddening when I get off on some tangent!

19. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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:worship:
Thank Eris for someone who thinks that way.

Imagine a piece of (elastic) string held between your hands that's vibrating at a particular wavelength. Now move your hands apart. That string will still vibrate but its wavelength (and therefore frequency) will change due to the increased distance it covers.
If that helps.

20. ### RJBeeryNatural PhilosopherValued Senior Member

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Excellent! I learned something, weeeee

21. ### OnlyMeValued Senior Member

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This represents one of the other difficulties. It is always difficult to address issues that have some basis for consideration from both the perspective of general relativity and quantum theory. Both represent viable theories and yet do not agree in many respects.

General relativity cannot explain gravitational force at subatomic scales and quantum theory does not address gravity at all. Still should the infinities that arise from applying either Newtonian Dynamics or Einstein's field equations at subatomic scales be resolved or a quantum theory of gravity as successful as GR has been be developed, disagreements of this type should also be reduced to a single perspective.

Spectral broadening is described as a phenomena of quantum theory. General relativity describes space as dynamic, while to quantum theory it appears flat. A similar difference in perspective is seen with how each deals with time.

I guess which perspective is a true representation, must wait for a time when we, or someone, has resolved the inherent conflicts that arise between the two.

22. ### EmilValued Senior Member

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An observation:

General relativity is a theory of gravitation developed by Einstein.
Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.

Special relativity is based on two postulates which are contradictory in classical mechanics:
1) The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity),
2) The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.

23. ### OnlyMeValued Senior Member

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I am not a particle physicist. Though it is my belief that gravity is in fact a quantum phenomena, I tend to see things from a classical view of relativity. So this is a serious question.

Though the variation of wavelength described as "spectral broadening" has been described as a Doppler or redshift, could it not also be the result of the atoms constant change in energy potential as it vibrates? The change in energy potential could easily be associated with changes in the wavelength of light emitted. Assuming that the atom's vibration requires accelerations associated with the changes in direction and that it is not instantaneous.