# where does light from a torch go?

Discussion in 'Physics & Math' started by emanresU, Apr 13, 2014.

1. ### emanresURegistered Member

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all energy is preserved in this cosmos, it will be recycled....what happens to energy drawn from a battery and send out to light a bulb? What happens to the light?
How will that be "reincarnated" into matter?

Does this mean, when the cosmos is a room, and the energy in it is a heater...that when the room is expanding..it gets colders when the room widens?

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Quite simple: the light is eventually absorbed. It does NOT turn into matter. (Check the laws of thermodynamics.)

5. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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....and converted to heat (thermal energy).

7. ### dav57Extraordinary Thinker ThingyRegistered Senior Member

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Absorbed by what? What if there was nothing to absorb the energy? If there was nothing to absorb the energy, did the energy even exist? Did it even travel if there was nothing to transfer its energy to? Did the energy even begin its journey to nowhere? How can you prove it even started its journey?

8. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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The object it hit.
If it doesn't hit anything, it just keeps traveling forever.

9. ### dav57Extraordinary Thinker ThingyRegistered Senior Member

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how can you prove it even began its travels if it hasn't hit a measurement system?

10. ### OnlyMeValued Senior Member

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You cannot prove it, but it is not an unreasonable conclusion.

Turn on a small light in a large dark room. As you walk in a circle around the light you see it from every location in the circle. From that it is a reasonable conclusion that light was traveling in all directions even when you were not there to see it.

11. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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We don't have to prove it: it's the starting premise of the OP.

12. ### exchemistValued Senior Member

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Quite!

Glad to see there is someone here still with his common sense intact.

13. ### exchemistValued Senior Member

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I don't think it is helpful to think of energy being "recycled". Energy is conserved (at least if you don't consider motion at relativistic speeds), and it changes its form from one to another. But it gradually tends to end up as low temperature heat, due to the entropy increase associated with all spontaneous processes. So it gradually becomes less available to do work. It get degraded, really, not recycled.

In your example, chemical energy in the battery is converted to electrical energy, then to light and heat, and then to heat alone, once the light has finally been absorbed by whatever it interacts with.

P.S. I predict that my use of the "E" word will now trigger a rather incomprehensible contribution from our old friend Wellwisher……...you have been warned……..

14. ### dav57Extraordinary Thinker ThingyRegistered Senior Member

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Conservation of energy means that the energy either stays where it is, or transfers to somewhere else to take another form. Where does the light/energy transfer to from a light source if the light doesn't hit anything or has nothing to connect to in order to transfer energy? It can't dissipate to nothing because that wouldn't obey the law of conservation. And if there is nothing to absorb the emitted energy then where does the light get absorbed?

15. ### OnlyMeValued Senior Member

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This is from Wiki on the conservation of energy.., as it begins...

The conservation of energy laws apply to a closed system, referred to above as an isolated system. In most cases we treat the universe as if it were a closed system, the limits of which are unknown. Until a photon of the light you are speaking of, leaves the universe, it remains in the closed system and is treated as if it will be absorbed or conserved over time. Only should the photon disappear or leave the universe, which is the closed system in this case, would it violate the conservation law.

16. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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Re-asking the question in different ways will not change the answer I provided in post #5. You are over thinking this.

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Pardon me but that is a HIGHLY childish view of our universe. The universe is so HIGHLY filled with matter that practically EVERY single photon will eventually strike something. If it does not, it will retain it's energy (energy is conserved) until it finally does.

Now... can we settle this matter once and for all and get back to being adults here???

18. ### exchemistValued Senior Member

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Then it goes on travelling, of course, which is how it is that we can see far off galaxies. Right?

The light travels until it hits the retina of our eyes or the detector of the camera or whatever, at which point it is absorbed. And for every photon that gets absorbed by our eyes there will be a lot more that hit the ground around where we are standing, or the surface of the earth in general or the surface of other bodies, or the interstellar dust and gas en route. There's no big mystery here. I am not sure why you seem to be labouring this.

19. ### dav57Extraordinary Thinker ThingyRegistered Senior Member

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So, let's consider a light source at the edge of the closed system (or universe, whatever you want to call it). Point the light source outwards where it will never reach anything which can absorb it. The photons can never strike anything.

Where does the energy go? Where and when does it transform or even disappear? Where does that scenario stand in relation to conservation?

Sorry if I'm pushing the boundaries by "over-thinking" this and sorry if i'm asking the question in different ways and sorry if it appears childish but perhaps there's more to what i'm asking than meets the eye.

Also, indeed yes, perhaps every photon DOES strike something, but do the ones that don't strike anything ever exist in the first place? How can you prove they are even emitted? Perhaps you don't see where I'm going with this. Perhaps you see it as irrelevant.

20. ### exchemistValued Senior Member

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If a photon is not absorbed it continues. This has no impact at all on conservation of energy. It is just light energy that remains, for the time being, light energy.

Just as a rock sitting on a ledge has gravitational potential energy that stays in that form.

Why make a big deal out of this?

And yes of course the photons that don't strike anything exist. We can calculate the photon flux from the sun, even though we intercept only a tiny fraction of it. Ditto for all the other stars.

If you are striking the philosophical pose that claims a thing can't be said to exist until we directly detect it, then you are incapable of making any model of the world.

Last edited: Apr 15, 2014
21. ### dav57Extraordinary Thinker ThingyRegistered Senior Member

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How do you know that photons that don't strike anything exist? Please tell me. I want to know. If you don't know the answer, just say so. You can't just assume something you can't measure exists, can you?

22. ### gmilamValued Senior Member

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Why not? Unless you have good reason to doubt it, experience tells us it should be there.

23. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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The answer still hasn't changed from post #5.