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What Happens To Light


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#1 hazelm

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Posted 22 January 2019 - 04:20 PM

A friend asked me a question that has me wanting to copy the tree in the forest question.  If a star is shining and there is no one there to see it, is there light?

 

That was not her question, of course.  Her question was what happens to light once it passes us.  She is asking about the first light that reaches us - those first photons.  Where does it go?  Good question?  Unlike sound, light is particles (photons).  Even though they are massless, they are something real that is traveling.  Can her question even be answered without the question I asked above - along with its answer?   I am thinking the answer to my own question is "no".  Then her answer would be it goes on forever until there is no one to see it.

 

So, (1) what happens to light once it passes us?

And (2)  what happens to light if there is no one to see it?

 

 

 

 



#2 Moronium

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Posted 22 January 2019 - 06:19 PM

 

And (2)  what happens to light if there is no one to see it?

 

 

With respect to this question, what do you think, Hazel?



#3 exchemist

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Posted 23 January 2019 - 01:49 AM

A friend asked me a question that has me wanting to copy the tree in the forest question.  If a star is shining and there is no one there to see it, is there light?

 

That was not her question, of course.  Her question was what happens to light once it passes us.  She is asking about the first light that reaches us - those first photons.  Where does it go?  Good question?  Unlike sound, light is particles (photons).  Even though they are massless, they are something real that is traveling.  Can her question even be answered without the question I asked above - along with its answer?   I am thinking the answer to my own question is "no".  Then her answer would be it goes on forever until there is no one to see it.

 

So, (1) what happens to light once it passes us?

And (2)  what happens to light if there is no one to see it?

Regarding (1), light goes on until it is absorbed by something. All matter will absorb light at some wavelengths so eventually a photon will encounter something that absorbs it. The energy will go into increasing the energy of electrons in the matter and this energy will then usually get redistributed within the matter as heat.

 

Regarding (2) light is a physical phenomenon, not just a phenomenon of perception (like colour, for example). And so is sound, by the way. 


Edited by exchemist, 23 January 2019 - 01:50 AM.

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#4 hazelm

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Posted 23 January 2019 - 05:26 AM

Regarding (1), light goes on until it is absorbed by something. All matter will absorb light at some wavelengths so eventually a photon will encounter something that absorbs it. The energy will go into increasing the energy of electrons in the matter and this energy will then usually get redistributed within the matter as heat.

 

Regarding (2) light is a physical phenomenon, not just a phenomenon of perception (like colour, for example). And so is sound, by the way. 

Thank you, Exchemist.  Re #2. Meaning the answer is yes - there is still light there?  Of course.  But sound which has to move air to bounce off another property?  I'll think on that later. 

 

Edit:  Reading Moronium's question, I see I misworded #2 but I suspect you knew what I meant from my opening comment. 


Edited by hazelm, 23 January 2019 - 05:58 AM.


#5 hazelm

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Posted 23 January 2019 - 05:55 AM

With respect to this question, what do you think, Hazel?

 

I see I misworded my question.  (2) It should have read "If a star is shining and there is no one to see it, is there light?"

 

My answer to your question:  if I knew the answer, I'd not have asked the question.  And that is not meant to be a sassy answer.   I have never yet settled the tree in the forest question.

 

My friend's question (1):  "what happens to light once it passes us by?" My answer to her was "it just keeps on going" and - reading Exchemist's answer - I see I was only half right.

 

There is still a third question but I'm working on it.  Maybe I'll find the answer myself which I like to  do if I can.



#6 exchemist

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Posted 23 January 2019 - 07:00 AM

Regarding (1), light goes on until it is absorbed by something. All matter will absorb light at some wavelengths so eventually a photon will encounter something that absorbs it. The energy will go into increasing the energy of electrons in the matter and this energy will then usually get redistributed within the matter as heat.

 

Regarding (2) light is a physical phenomenon, not just a phenomenon of perception (like colour, for example). And so is sound, by the way. 

Yes I've never understood this "tree in the forest" business. If a tree falls, it will create sound waves, whether observed or not. Sound is not just a phenomenon of perception, like taste or smell. Hearing is perception, but not sound.

 

Sound, and light, are physical, energy-carrying waves. Nobody asks if the waves on the sea cease to be there if nobody is looking at them.  



#7 hazelm

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Posted 23 January 2019 - 09:47 AM

Yes I've never understood this "tree in the forest" business. If a tree falls, it will create sound waves, whether observed or not. Sound is not just a phenomenon of perception, like taste or smell. Hearing is perception, but not sound.

 

Sound, and light, are physical, energy-carrying waves. Nobody asks if the waves on the sea cease to be there if nobody is looking at them.  

Not even Bishop Berkeley? 


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#8 exchemist

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Posted 23 January 2019 - 10:25 AM

Not even Bishop Berkeley? 

Haha yes indeed, I wondered if you might bring him up. 

 

But then I would rephrase my remarks, to say that light and sound are not in a category any different from the tree in the Oxford limerick:

 

There was a young man who said: "God

Must find it exceedingly odd

When he sees that this tree

Just ceases to be

When there's no one about in the quad"

 

"Dear Sir, Your astonishment's odd.

I am always about in the quad.

And that's why this tree

Continues to be,

Since observed by, yours faithfully, God."


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#9 Moronium

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Posted 23 January 2019 - 10:45 AM

Given the direction that this thread has taken, I can't help but note that Berkeley's "to be is to be perceived" dictum, which is the essence of abject solipsism, has, unfortunately, been warmly embraced in certain areas of scientific theory.

 

Of course I have SR in mind here.  In that theory, the assumption of an independent objective reality is essentially abandoned in favor of the premise that subjective perception is the only "reality."



#10 hazelm

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Posted 23 January 2019 - 11:42 AM

Haha yes indeed, I wondered if you might bring him up. 

 

But then I would rephrase my remarks, to say that light and sound are not in a category any different from the tree in the Oxford limerick:

 

There was a young man who said: "God

Must find it exceedingly odd

When he sees that this tree

Just ceases to be

When there's no one about in the quad"

 

"Dear Sir, Your astonishment's odd.

I am always about in the quad.

And that's why this tree

Continues to be,

Since observed by, yours faithfully, God."

I have a book with brief summaries of what each of the old philosophers taught.  The one for the Bishop left me thinking he didn't really say what people say he said.  But we find a lot of that where those who are dead and gone get misinterpreted and are unable to defend themselves.   There are a lot of people today who actually believe - and preach - what Bishop Berkeley is supposed to have said.  The same can be said of Einstein - and many others.  I think I want my tombstone to say  "I didn't say that."



#11 Moronium

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Posted 23 January 2019 - 12:28 PM

George Berkeley was a brilliant man who made a strong case for solipsism by taking empiricism to it's logical extreme.  His arguments were, given his premises, virtually irrefutable.

 

Boswell once asked Samuel Johnson how he could possibly refute Berkeley.

 

Johnson simply said:  "I refute him thusly..."

 

Then, without saying another word, he kicked a rock.


Edited by Moronium, 23 January 2019 - 12:41 PM.


#12 Moronium

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Posted 23 January 2019 - 12:35 PM

By the way, Hazel, I never said that Einstein said what I said about his theory.  He did, however, come extremely close to it.

 

By the 1920's he had fully repudiated his earlier embrace of the positivism of Mach which he had whole-heartedly endorsed when he invented SR.

 

In an exchange with Heisenberg concerning QM in the 1920's, he condemned the theory for adopting a solipsistic viewpoint.

 

Heisenburg, shocked, replied to the effect that it was Einstein himself who had popularized such an approach when he invented SR.

 

Einstein replied that:  "Perhaps I did subscribe to such nonsense at that time, but it is nonsense, nonetheless."

 

Much later, Einstein also told Karl Popper that his biggest regret was that he had ever adopted positivism as an acceptable philosophy of science to begin with.


Edited by Moronium, 23 January 2019 - 12:37 PM.


#13 hazelm

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Posted 23 January 2019 - 01:47 PM

"

Then, without saying another word, he kicked a rock."

 

Which goes right along with my "Bumping into the corner of that non-existent table still hurts."

 

"By the way, Hazel, I never said that Einstein said what I said about his theory."

 

I never said that you said what Einstein said about what you said.

 

I thought positivism was a philosophy of life.  I do not understand "philosophy of science".    Is it other people's opinions of a scientist's theory?  Those are a dime a hundred.



#14 Moronium

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Posted 23 January 2019 - 02:12 PM

 

I never said that you said what Einstein said about what you said.

 

 

Yes, I know, but that seemed to be the implication of your post, given the context is was made in.



#15 Moronium

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Posted 23 January 2019 - 02:15 PM

 I do not understand "philosophy of science".    Is it other people's opinions of a scientist's theory?  Those are a dime a hundred.

 

 

No, it's not that.  You can read more about it here, if you want:

 

Philosophy of science is a sub-field of philosophy concerned with the foundations, methods, and implications of science. The central questions of this study concern what qualifies as science, the reliability of scientific theories, and the ultimate purpose of science.

 

https://en.wikipedia...ophy_of_science



#16 hazelm

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Posted 23 January 2019 - 02:40 PM

No, it's not that.  You can read more about it here, if you want:

 

 

https://en.wikipedia...ophy_of_science

Good explanation.  Thank you.  I confess I had not tried too hard to find the facts about it. 



#17 Moronium

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Posted 28 January 2019 - 09:18 PM

By the way, Hazel, I never said that Einstein said what I said about his theory.  He did, however, come extremely close to it.

 

By the 1920's he had fully repudiated his earlier embrace of the positivism of Mach which he had whole-heartedly endorsed when he invented SR.

 

In an exchange with Heisenberg concerning QM in the 1920's, he condemned the theory for adopting a solipsistic viewpoint.

 

Heisenburg, shocked, replied to the effect that it was Einstein himself who had popularized such an approach when he invented SR.

 

Einstein replied that:  "Perhaps I did subscribe to such nonsense at that time, but it is nonsense, nonetheless."

 

Much later, Einstein also told Karl Popper that his biggest regret was that he had ever adopted positivism as an acceptable philosophy of science to begin with.

 

 

I just stumbled on to this passage again, so I thought I'd come back and post it here for elaboration.  A lot of people, including (perhaps especially) scientists, have little appreciation for the role that philosophy plays in scientific theory.

 

The impact of Mach’s positivism was transmitted by the young Einstein. But around 1920 Einstein turned away from positivism because he realized with a shock some of its consequences; consequences which the next generations of brilliant physicists (Bohr, Pauli, Heisenberg) not only discovered but enthusiastically embraced: they became subjectivists. But Einstein’s withdrawal came too late. “Physics had become a stronghold of subjectivist philosophy, and it has remained so ever since.” [Popper]. Popper witnessed Einstein’s radical change of opinion about Mach’s philosophy: “Einstein himself was for years a dogmatic positivist ... He later rejected this interpretation: he told me in 1950 that he regretted no mistake he ever made as much as this mistake.”

 

http://blog.hasslber..._Relativity.pdf


Edited by Moronium, 28 January 2019 - 09:39 PM.