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A question on Photons


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Say i was standing on the Moon looking back at Earth,and someone on Earth swiched on a Torch for say 30secs,then switched off.Obviously i,m not going to see that beam of light,because its to dim.

But Photons have left that torch for 30 secs.

My question: If these Photons meet no obstruction on their journey,would they continue forever,or would they iventually Decay,and cease to exist.

I suppose this could apply to any light source glowing on Planet Earth.

Also Radio signals,what keeps them going once the source is switched off.

Daft question maybe.

Mick.

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My question: If these Photons meet no obstruction on their journey,would they continue forever,or would they iventually Decay,and cease to exist.

Thyey would keep going forever.

Also Radio signals,what keeps them going once the source is switched off.

Effectively the same thing that keeps a ball going when you throw it: momentum.

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Radio signals do decay, that's for sure. Well, in the common meaning of sound at least. If they didn't then you would hear your keyboard clacking forever as you are typing your question here on SGL.

I'd take your keyboard back to the shop if it makes audible radio waves. Or sell it to a university for a research project. Or sell it to a military weapons company.

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Radio signals do decay, that's for sure. Well, in the common meaning of sound at least. If they didn't then you would hear your keyboard clacking forever as you are typing your question here on SGL.

Keyboards clacking produce sound, not radio.

Radio waves decay in strength as a function of distance (by an inverse-square law) because they are spread out over an ever larger surface area. If nothing absorbs them, the individual photons in the radio signal will not decay. They just get further and further apart, diluting the signal.

Sound decays through friction in the air, absorption by objects, etc.

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If we're talking individual photons then yes, they go on forever, until they impinge on something. However, light beams, behave in the same way as radio transmissions, following the inverse square law...

The chances are you'd not detect any of the photons arriving at the moon, as they'd been absorbed in the atmosphere, but if we assume that some get through, then you should be able to detect them on the moon.

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Hi Mick.

Not a daft question at all.

Photons move through free space at the speed of light. They do not deviate from a straight line, or slow or stop, unless something interferes with their progress.

Photons do not need a push from behind to keep going. When you turn off your torch, the photons already on the way continue.

Radio waves are exactly the same stuff as light waves, but longer wavelength. Radio wavelengths run from thousands of metres (long wave band) down to millimetres (extreme end of microwave). Between radio and visible light is infra red.

When you look up at the night sky you see photons in the 'visible' light range from distant objects. The visible range just happens to be a set of wavelengths that pass easily through our atmosphere. So our eyes have evolved to make use of this. There is nothing special about these wavelengths. Some of those stars could have 'gone out' or exploded many years ago but we view the photons emitted before then.

We perceive photons as weakening or dimming on their path because they spread out. Suppose your torch emits 1 million photons per second and these are in a cone shape path, like for most torches. If you stand close to the torch, you will pick up most of the photons. As you move further way, you intercept less of the photons. You perceive this as dim light. The energy in each photon is the same, it's wavelength is the same. But as there are less per second getting into your eye, so you see the torch as dimmer.

If you have a perfectly parallel light beam, then the intensity does not reduce with distance. A laser pointer is an example of something that nearly achieves this.

A simple (hopefully) explanation. I make no apology for not discussing space time curvature and, photon speed in different media, gravitational effects and anything else that adds to the complications!

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If you have a perfectly parallel light beam, then the intensity does not reduce with distance. A laser pointer is an example of something that nearly achieves this.

Thanks for the comprehensive explanation.

However, I don't agree with this point. All light beam (including laser) suffers diffraction, so any beam will diverge over distance and reduce in intensity even if they started as a perfectly parallel beam. The diffraction is a function of the aperture / beam width and wave length, a narrow beam will spread faster than a broad one.

It is the same reason why resolution in our telescopes are limited.

Edited by E621Keith
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Thanks for the comprehensive explanation.

However, I don't agree with this point. All light beam (including laser) suffers diffraction, so any beam will diverge over distance and reduce in intensity even if they started as a perfectly parallel beam. The diffraction is a function of the aperture / beam width and wave length, a narrow beam will spread faster than a broad one.

It is the same reason why resolution in our telescopes are limited.

precisely. only an infinitly wide beam gets no wider :D

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Wow,what a fantastic response.Thanks to everyone who replied,and between you all, you have answered my question,and much more.Physics and the Universe,are are so entertwined it sets the mind racing.

Shame we cant hitch a ride on a bunch of Photons.

Well not yet anyway.

Regards.

Mick.

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Something I have been trying to find out for a while and have as yet not been able to confirm, so I guess you could call them my thesis

1. A photon when created is already travelling at the speed of light, there is no acceleration.

2. A photon that has been travelling for say 180,000 light years when detected has not aged since its creation.

I get so many of these little questions pop up that I've started a book to jot them down in. :rolleyes:

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Yes - photons created are already travelling at the speed of light. It implies infinite acceleration - except because they have no mass this makes F=ma a bit redundant.

There is no way to measure the age of a photon, so its sort of a meaningless question. As far as the photon is concerned, I think its born and dies in the same instant, even if it cross the universe.

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