# Does light travel forever?

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

1. ### drumbeatRegistered Senior Member

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Imagine a star giving off light for a split second of time, and the photons of light from this exact point in time are travelling through the universe in all directions like an expanding bubble.
Is this the correct way of imagining light?

If the light travels 10 million light years away and reaches, for example, our Moon, it would appear brighter than if viewed from another moon a further 100 million LY away. What about a billion LY away? Is there a point when the light becomes completely undetectable?

If I liken if to my computer monitor....
An image taken next to the star has 1,000,000 bright pixels.
An image taken from 10 million LY away has 300,000 bright pixels.
An image taken from 100 million LY away has 50,000 bright pixels.
An image taken from 1 billion LY away has 1,000 bright pixels.
etc

Am I right in thinking there will be a distance from which only 1 pixel is bright, and a distance even further where no pixels are light remaining? Where the photons are so spread out that they are undetectable?

3. ### RhaedasValued Senior Member

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Good question. Light can be thought of as packets of energy, so at what point is there not enough there to qualify as a photon?

This might tie in with Olber's paradox, which assumes that the light would continue on, and if the universe was infinite in age, then the sky would be bright due to all the starlight. Now the universe isn't infinite, but if it was, would we truly see all stars?

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

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What do you mean the universe isn't infinite?

How could the universe not be infinite? Is there a dead end sign at the end of the universe? How do you propose the universe ends, in any direction of travel?

You mean the mass ends and there is just infinite space? Well then, you've made a couple mistakes. The first of which is that the term universe implies all mass and space, period. The term universe suggests everything in (infinite) space. So even if the universe is an object in space, as you imply, that has boundaries, the infinite space that the object resides in is just that,... infinite, and considered the universe as well. The second mistake is that if you consider the universe as a single entity, as a massive object in space, if there were to be another similar object in the infinite space, such as the one you refer to ours as, that object would also be part of THE universe. There isn't TWO or more universes, there is ONE infinite UNIverse.

7. ### AbsaneRocket SurgeonValued Senior Member

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Assuming that a photon never gets absorbed by anything then yes, that photon travels forever. As for living within an infinite universe... I don't know what you mean. As far as we can tell, the universe that we know might simply loop back onto itself much like a 2D universe shaped as a sphere in 3D. A photon, in theory, could traverse the entire surface of the ball forever.

8. ### RhaedasValued Senior Member

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The matter of an open or closed universe is still unknown, as the critical density doesn't point either way.

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

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Ignoring the mass in space, as we know it, would you say that space is infinite?

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

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It's not even open to debate.

There is infinite space, by default.

There is mass, as we know it, that resides in that infinite space.

A finite mass in an infinite space doesn't a finite universe make!

12. ### drumbeatRegistered Senior Member

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375
Thanks.

So does one photon show up as a very, very faint light to the observer?

13. ### OnlyMeValued Senior Member

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There has been some experiment that demonstrate an ability to detect a single photon. Even some suggestion that the human eye is capable of perceiving a single photon. Under most realistic conditions detecting a single photon from the edge of the observable universe would be completely overwhelmed by photons arriving from nearer sources.

Setting aside the issue of whether a single photon from such distances could be detected, there would be no real information that we could obtain from the detection of a single photon other than the ability to detect it.

What we perceive at bright or dim light is not the result of single photons. It is instead a measure of the total number of photons involved in the observation of a specific source. There is no difference in the intensity of a single photon from the edge of the observable universe compared to a single photon of the same wavelength from a nearby source.

14. ### cosmictravelerBe kind to yourself always.Valued Senior Member

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Not unless there's a stop it comes to, or a stop light.

Researchers now able to stop, restart light

She and her team continued to tweak their system until they finally brought light to a complete stop. The light dims as it slows down, so you think that it's being turned out. Then Hau shoots a yellow-orange laser beam into the cloud of atoms, and the light emerges at full speed and intensity.

15. ### OnlyMeValued Senior Member

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Or so it seems...

It has been long known and supported by experiment that the speed of light varies relative to the density of the medium through which it travels, gaseous, liquid or solid. The refractive index of a vacuum is 1 and theory assumes that empty space is equivalent to a vacuum and everywhere uniform, thus the speed of light in empty space or a vacuum is assumed to be a universal constant. That assumption originally asserted by Einstein in 1905 has come to be accepted as supported by observation and therefore gained the status of fact.

It would seem from the limited information the link provides that what has been accomplished is the creation of conditions where the refractive index of an ultra cold collection of atoms is greater than anything we could have previously imagined.

It does raise the question as to whether similar conditions might exist in the depths of intergalactic space, which would similarly affect the velocity of light on its way to "us". A very troubling possibility for General Relativity which relies largely on the universal constantancy of the speed of light to model curved space and time dilation.

It would be interesting to know if theory supports the potential for similar conditions occurring naturally. Empty space could no longer be considered empty as far as photons are concerned...

The implications would be revolutionary.

16. ### AlexGLike nailing Jello to a treeValued Senior Member

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The photons aren't stopped or slowed while in transit. They are absorbed and reemitted by the atoms (actually, the electrons, as light only interacts with charged particles.)

17. ### OnlyMeValued Senior Member

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I am unsure how this would be proven since all materials we can use to test it are composed of charged particles. On the other hand, since the neutrino has no charge there should be nothing to prevent a collision between a photon and a neutrino, other than the probability being less than the probability that a neutrino would interact with ordinary matter.

While we can create and control a beam of light, photons, with what should be a sufficient degree of intensity, we cannot at present say that we can create a directed source of neutrinos with sufficient intensity to even begin to construct a test.

18. ### RJBeeryNatural PhilosopherValued Senior Member

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Yes, mostly, except you're kind of glossing over the quantum nature of photons. You might do better to think of it as an expanding sphere of "points" which are all separating from each other.
Not exactly. Luminosity is related to "photons per second", and you're right that as the photons become so spread apart on our growing sphere, our receptor (eye ball, telescope, etc) can capture fewer and fewer of them which makes what we're looking at appear dimmer. However, we will ALWAYS be able to view a single photon, in theory, from any distance with any suitable receptor (i.e. a receptor sensitive enough to capture a single photon), at any arbitrary point in the future. In other words, a photon travelling through the empty vacuum of space** does not lose energy.

**Ignoring gravity effects for simplicity's sake

19. ### RocksRegistered Member

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Wouldn't this mean that redshift would have to be caused by space dust/etc? I haven't been able to quite grasp some of the explanations of how redshift is supposed to be caused by the expanding Universe. Everything I know about EM waves (which is more than the average bear since I studied remote sensing at undergrad and grad level, but obviously, no expert) suggests to me that light is unchanging in look at speed unless something slows it down, thereby changing the color. I don't get how expansion affects this, although I admit I am used to study near bodies, not one billions of light years away.

20. ### RhaedasValued Senior Member

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As light travels over huge amounts of expanding space, its wavelength gets longer, so it goes towards the red end of the spectrum. Its speed is always c.

21. ### RocksRegistered Member

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Should I think of the expansion as being like refraction, then? I don't really know how else to compare it to anything else. The Wiki wasn't helpful

22. ### BelieveHappy mediumValued Senior Member

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I've understood it to be that the expanding universe stretches out the light wave, increasing its wavelength and causing it to shift towards the red (i.e. longer wavelength)

23. ### siphraRegistered Senior Member

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There is no such thing as 'by default' in science.

Prove your view? Most current theories describe a universe with a finite volume at any time, that essentially loops back on itself.