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Name: Steve H.
Status: student
Age: 30s
Location: N/A
Country: N/A
Date: 1/9/2004


Question:
What is the life expectancy of light?

We are told in Astronomy classes and such, that light emitted from the stars and reflected light off of planets takes many years to reach us.This would suggest that light has a very long life span yet the light in my home dies immediately after the source is extinguished.

Does this have a bearing on the radiated lights power source? Such as my 60 watt incandescent light bulb compared to the terra watt (much more I am sure) power of a star?

It would also hold true that if we were to travel closer to an extra terrestrial light source, we would be traveling back in time so to speak. At any given point in our journey we would be seeing the light as it was some time ago (just as it is on Earth.)


Replies:
Steve

Light has an infinite lifetime unless it is absorbed by something. Think of light as a stream of photons -- regardless of whether they come from a distant star or a light bulb. All these photons of various energies (colors) bounce around being absorbed, reflected, scattered -- all the things light does. When you turn off the light bulb, the radiated photons do not die, they simply are no longer being emitted. It is the PRODUCTION that stops, not the individual photons (or waves) that have already been generated. One of the properties of light that sometimes causes confusion is that even though it may have traveled billions of miles, it does not "slow down" (run out of gas so to speak). That is what is observed for golf balls, cars, etc., but not light because light does not experience any sort of "friction" to slow it down.

Vince Calder


Steve,

This response to your questions ought to be quite different from those you may receive from other NEWTON scientists. I wrote it for the love of my life many years ago. With nothing to absorb it, or otherwise impede its passage, light goes on forever. Indeed, looking at starlight is rather equivalent to looking back in time ...

------------
Time Travel

In all of three-space, unbounded by the crowded plane, I wander, vainly seeking such art as is hidden inside my science.

The flat, blind eye scans time and regions incomprehensible. In my search, I am first wedged between then stretched across endless hours.

I cannot touch you, love. Void of your light; the terrain before me is a busy, lonely place.

So I would climb a nearby oak that I may escape to take the longer view. And from a lofty perch, I spy two lovers on the grass.

Embraced, concealed by distance; their image is in the dispassionate, analytical eye, just light in reflection ... nothing more.

But more. I think I understand. It passes me and as it goes ... Light-speeding; apace, the lovers' moment together goes with it,

Gone, but not before I subtract a share for memory's sake.

What curious substance is this fleeing shaft? Photons all? Just energy? A part of Einstein's scheme?

An observer below, nearer to the ground, would see them sooner.

Could I in secret watch once more, these sweethearts as they beam, or see the same beam twice, thrice, or ad infinitum from ever-greater heights above the tree?

Might I see them again from another point higher still?

Why not?

This vision of our time; it is just a ray of photons passed. And if I could, I would race to chase it, catch it, pass it ... then look again ... and again.

In yet another light I, illumined, understand still more. It is we who are the two lovers on the grass. Forever yearning to hold you, love; it is our reflection I long to see again. And more; I seek the holy light, the radiance and energy of your blue eyes.

Through the purity of your flawless flint, crowned, magnetoelectric waves of space-time, now bended, favor me. From the engine of your soul, I am given perfect gifts: Energesis to empower, pluck to embolden my inner quill. Refract, reflect, transmit ... absorb me and I am, with you, made whole.

Shadows now in retreat; at last the veil is drawn; you are the earthly lamp unto my blinded, hungry eye. This promise that I see in you is the vision of an answered prayer.

Ought I fear?
Are you real?
Or have I just climbed another, higher tree?

Regards,
ProfHoff 758


Steve,

When light makes contact with a surface, some does not reflect off. It is absorbed, usually heating the surface. Sometimes it causes chemical reactions to occur, such as photosynthesis in green plants. Consider the fact that light travels at 300,000,000 meters per second, or the length of more than three million football fields in one second. The light from a bulb in your living room bounces around billions of times in less than a second. Each time it hits the wall, part of it goes into the wall's molecules and part bounces off. In a time too short to even start to measure, all has either been absorbed by the walls or passed out of the room through walls and doors.

Black paint absorbs almost all the light that hits it. White paint reflects almost all the light that hits it. This is why the same light bulb makes a white room much brighter than a black room. This is why on a summer day people in black clothes feel so much hotter than people in white clothes. In outer space, the light does not hit anything. It does not get used for anything on its way to the Earth. It just keeps traveling at three million football fields per second until it gets absorbed by something.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


Light is a form of energy. It does not have a "life expectancy" in the sense you are thinking. A photon of light will continue traveling through space until it strikes an object that absorbs it. In an enclosure such as a room this happens very quickly (it takes only a few nanoseconds for the light to reach a wall in a 10 X 10 foot room). While the light may be reflected a few times it will eventually strike something that will absorb it. Your eyes cannot resolve events shorter than a few milliseconds. A given photon might strike a wall up to a 100 thousand times (reflecting each time) in this time period. Even if only 0.1% of the light was absorbed (99.9% reflected -- such a s for a pretty good household mirror) at each wall interaction it would be down to undetectable levels long before it reached a 100 thousand reflections.

In deep space there are very few atoms or molecules to interact with and these individual atoms or molecules may not absorb the particular wavelength of light being transmitted so the light propagates "forever".

The power of the light source does not impact this result under normal conditions. The field of non-linear optics includes a study of the interaction of matter with VERY high light intensities.

Greg Bradburn



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