Dalo

Why is the sky blue on Earth

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Just now, Strange said:

And you expect a prism to behave differently but a spectrometer not to? What is the theoretical justification for that? 

 

I think you should be the one to explain how a prism is the same thing as a spectrometer.

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4 minutes ago, Dalo said:

I think you should be the one to explain how a prism is the same thing as a spectrometer.

Some spectrometers use prisms. Others use diffraction gratings. They all use optical dispersion. There is no reason to think that electromagnetic waves behave differently on different planets.

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"edit: like I said, spectrometers do not tell you anything about the atmosphere, only about the elements themselves. They cannot therefore be used as an argument. "

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2 minutes ago, Dalo said:

"edit: like I said, spectrometers do not tell you anything about the atmosphere, only about the elements themselves. They cannot therefore be used as an argument. "

But they tell you the same thing about the elements on Mars as they do on Earth. Therefore they behave the same way.

But I don't understand what you expect to be different (or why) so I have no idea if the fact that a prism behaves the same way on Earth and Mars actually answers your question as to whether a prism would behave the same way on Earth and Mars ....

 

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Just now, Strange said:

But they tell you the same thing about the elements on Mars as they do on Earth. Therefore they behave the same way.

But I don't understand what you expect to be different (or why) so I have no idea if the fact that a prism behaves the same way on Earth and Mars actually answers your question as to whether a prism would behave the same way on Earth and Mars ....

If prisms behave the same way on Mars as they do on Earth, that would be a very strong indication that sunlight, wherever it is captured, is composed of the spectrum as we see it through a prism on Earth. That would be another confirmation of the Newtonian assumption that the colors he saw did not come from the prism itself and/or the air particles in his room.

That is a central assumption in Physics, just like the idea of universal gravity which is simply a theory and which got an extra empirical confirmation on the Moon by Armstrong.

Therefore, either nothing will change for the theory of light, or, if prisms on Mars give another spectrum, a new theory will have to be devised.

You may think that we already know the answer and I cannot blame your conviction, but that is still not a proof.

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2 minutes ago, Dalo said:

That is a central assumption in Physics

It is not an assumption, it is a conclusion from the evidence. It could, of course, turn out that the conclusion is wrong but that would require evidence. 

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, just like the idea of universal gravity which is simply a theory

It is a theory, in that it is supported by lots of evidence. (Phrases like "just a theory" always ring alarm bells, BTW.)

It could be that it is wrong. And, of course, many people have tried to use the evidence from galactic rotation curves, for example, to support alternative theories gravity. So far, none of the alternatives work as well as the accepted theories. 

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Therefore, either nothing will change for the theory of light, or, if prisms on Mars give another spectrum, a new theory will have to be devised.

While you are right to say that our theories should be continually tested (as they are) there are probably more significant concerns than whether refractive dispersion depends on which planet you are on. After all, if you could be shown that one of the spectrometers on Mars used a prism (and it behaved as expected) then you could just say "but what about Mercury? Or Venus? Jupiter? Saturn? All of the exoplanets? The planets we haven't found yet? Other galaxies?" 

At some point, you have to accept that we have a lot of evidence and a theory to explain the evidence. Yes, science is always provisional, but you seem to be taking that doubt to extreme degrees.

You might ask what Newton had for breakfast when he did his experiment. Would that make a difference? Unless we test it, we won't know. And what day of the week was it? Which socks was he wearing? Was it before or after he invented the cat flap?

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Just now, Strange said:

At some point, you have to accept that we have a lot of evidence and a theory to explain the evidence. Yes, science is always provisional, but you seem to be taking that doubt to extreme degrees.

Of course, but I do not think that my doubts are extreme. They would be if prisms on Mars gave the same spectrum as on earth, and I would use the argument that it has not been tested on all stars or planets. Mars and Earth and different enough to make additional proofs superfluous.

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2 hours ago, Dalo said:

so is the interstellar space, or vacuum.

Yes, indeed. So what is the source of your confusion about scattering being responsible for the sky being blue on earth?

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Just now, swansont said:

Yes, indeed. So what is the source of your confusion about scattering being responsible for the sky being blue on earth?

Allow me to refer you to the discussion that followed.

 

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1 hour ago, Dalo said:

or there is nothing to be illuminated.

Let us take all potential observers away, and put a light sensor among the 'scatterers". We can say by looking at the sensor afterwards whether it has received light or not. That does not depend on our vision organ and its idiosyncrasies. We could also have the sensor indicate the wavelength of light received, and then imagine that all particles in the air are such sensors. All the sensors, I think, would tell us the same story, that they have received light, and that it was blue.

This way we will have confirmed that the sky is blue without once having appealed to scattering or vision.

Am I missing something here?

The sky looks blue if you are in an airplane, or if you move from place to place. Taken pictures. We've done the experiment. That doesn't tell you that the sky is inherently blue, since scattering can be taking place everywhere.

But if you look for light coming from some volume of nitrogen or oxygen in a lab, it doesn't have the spectrum that we observe from the atmosphere.

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Just now, swansont said:

But if you look for light coming from some volume of nitrogen or oxygen in a lab, it doesn't have the spectrum that we observe from the atmosphere.

that seems to reinforce the idea that the spectrum on Mars would look different than on Earth. But I suppose that you think that this wouldn't prove anything new?

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14 minutes ago, Dalo said:

Of course, but I do not think that my doubts are extreme. They would be if prisms on Mars gave the same spectrum as on earth, and I would use the argument that it has not been tested on all stars or planets. Mars and Earth and different enough to make additional proofs superfluous.

Yes, that is actually extreme.

You are demanding specific experiments, rather than considering other ones that show the same thing. But you don't give any valid reason for not considering these other experiments. That's not how science works. 

3 minutes ago, Dalo said:

that seems to reinforce the idea that the spectrum on Mars would look different than on Earth. But I suppose that you think that this wouldn't prove anything new?

Which spectrum? There are at least three spectra in this discussion.

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Just now, swansont said:

You are demanding specific experiments, rather than considering other ones that show the same thing. But you don't give any valid reason for not considering these other experiments

I was not aware of any relevant experiments I refused to consider. Could you be more specific?

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8 minutes ago, Dalo said:

I was not aware of any relevant experiments I refused to consider. Could you be more specific?

Every example of refraction on Earth, in space, on the Moon, on Mars and probably elsewhere. There is nothing magic about prisms. They just exploit refraction and dispersion.

"Oh yes, they may all give the same results, but what about doing the same experiment on a wet Saturday while wearing Dangermouse underpants?"

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Just now, Strange said:

"Oh yes, they may all give the same results, but what about doing the same experiment on a wet Saturday while wearing Dangermouse underpants?"

You should really see a therapist. You don't seem to be able to control yourself.

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4 minutes ago, John Cuthber said:

This is interesting

https://mars.nasa.gov/mer/mission/spacecraft_instru_calibr.html

it tells you how they will know what colour things  are on Mars.

Yes it is. 

5 hours ago, StringJunky said:

It's rather a long way away, so, it's effectively pretty small relative to the Earth;  most of the incident rays are diverging just before they enter its atmosphere.

I find your reply fascinating.

1) It seems to imply that the sun, because of its distance from the earth, becomes actually smaller. Is that what you are saying?

2) Are the incident rays diverging before or after they enter the atmosphere?

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18 minutes ago, Dalo said:

Yes it is. 

I find your reply fascinating.

1) It seems to imply that the sun, because of its distance from the earth, becomes actually smaller. Is that what you are saying?

2) Are the incident rays diverging before or after they enter the atmosphere?

Before. They'll be doing both once in the atmosphere.

Edited by StringJunky

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1 minute ago, Dalo said:

what about the first question?

Yes. Not actually smaller but effectively.

Edited by StringJunky

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Just now, StringJunky said:

Yes.

This assumption seems the only possibility for the laws of Optics to be correct.

It does imply a non-conventional concept of space.

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2 minutes ago, Dalo said:

This assumption seems the only possibility for the laws of Optics to be correct.

It does imply a non-conventional concept of space.

Why? 

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28 minutes ago, Dalo said:

1) It seems to imply that the sun, because of its distance from the earth, becomes actually smaller. Is that what you are saying?

It is called "perspective".

6 minutes ago, Dalo said:

It does imply a non-conventional concept of space.

Perspective is very conventional.

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Just now, Strange said:

It is called "perspective".

Which is, allegedly, something that has to do with our vision. The idea that objects effectively, "objectively", become smaller or larger with distance is certainly metaphysically interesting.

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