Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Quasars on 23/03/2010 20:30:02
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Quasars asked the Naked Scientists:
Exactly, what is meant when it is said that "It takes the Sun's rays eight minutes to reach the earth"?
How can that be when the sun's rays are continuously shining on some part of the earth at all times and have been since the sun's birth?
When a location on the earth rotates from darkness into light, are the inhabitant of that location witnessing an illusion of some kind, or perhaps, the individual photons from the interior of the sun actually take eight minutes after leaving the sun's surface, independent of the overall sunlight?
Could you please explain?
What do you think?
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Yes it is true that the light from the sun takes about eight minutes to reach the earth. the best way to visualise this in the context that the light from the sun is continuous and reasonably steady is to say that if some sudden event such as a solar flare happened on the sun we would not see it until eight minutes later.
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It's like you being under a rain cloud. When that rain stops it will still take some time for those last drops to reach you. Look at it this way, that rain started before the first drops reaching you, then they had to 'travel'. But what's really interesting is explaining where the photons are in a classical 'nothing' as we also can describe space as, depending on our point of view. Can we prove them to exist, except in their 'inter actions'?
Whatever you think there they still obey the time-distance relations.
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Just another little point to add, just to clarify. It doesn't take photon's 8 minutes to travel from the interior of the sun to earth. It in fact takes millions of years for the photons from the core to reach the surface of the sun due to electromagnetic fields,reactions with atoms in the sun and other bits. Although once the photons escape the surface of the sun, then it takes 8 minutes. It is fascinating to think that the light hitting our skin in the day has been pent up inside the core of the sun for millions of years, then has just been recently free'd of its trap.
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It doesn't take 8 minutes, it takes 8 minutes and 20 seconds actually [:)]
The OP is not, instead, amazed from the fact that light coming from another star (instead of the Sun) could take millions of years to reach us?
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Just another little point to add, just to clarify. It doesn't take photon's 8 minutes to travel from the interior of the sun to earth. It in fact takes millions of years for the photons from the core to reach the surface of the sun due to electromagnetic fields,reactions with atoms in the sun and other bits. Although once the photons escape the surface of the sun, then it takes 8 minutes. It is fascinating to think that the light hitting our skin in the day has been pent up inside the core of the sun for millions of years, then has just been recently free'd of its trap.
I wouldn't destroy this...poetic image, but what you say is a common misconception (I made it too in the past). Photons don't preserve their individuality after interaction with matter: a photon is emitted from a subatomic particle, then it's absorbed by another, then a new photon is emitted again and so on...
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Very true, although the point I was making is the journey of the photon from the core, including being absorbed, then emitted and so on would take millions of years for the precise reason you just mentioned. I guess I meant more in that the quanta of energy that falls on the earth does not come from a straight simple 8 minute path but has been constantly changing and interacting for millions of years before it finally reaches us in its current form.
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I ask you a question: let's say you are able to take a big clump of cold hydrogen (room temperature) as big as the Sun, place it at 149.5 millions of kms from Earth and switch on nuclear fusion in all the body of this clump simultaneously.
After how much time do you expect the light from it will arrive here?
1) 8' 20''.
2) Millions of years.
(hint: I would tend for the first).
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I don't quite see what your getting at? A big clump of cold hydrogen that would have initiated fusion is vastly different from the sun, for one its mass would have to be extremely huge (if the big clump we are talking about is say 50 meters big) to start the fusion in the first place assuming the fusion was not started from anything other that gravitational contraction.
What I am stating with regards to the sun or any main sequence star is accepted astrophysics as I have been taught, in that the sun is not just a ball of burning hydrogen. The sun is comprised of different layers such as the core, photosphere, convection zones and coronasphere these all have vastly different properties. So photons emitted from the CORE (my original point) do not take 8 minutes due to there nature of interactions with the matter in the star and that includes more than just absorption and emission of the photon.
Of course once the photons make there way to the surface, or any emissions from the surface will be largely unhindered then and only then will take about 8minutes 20 seconds (or so).
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I don't quite see what your getting at? A big clump of cold hydrogen that would have initiated fusion is vastly different from the sun, for one its mass would have to be extremely huge (if the big clump we are talking about is say 50 meters big) to start the fusion in the first place assuming the fusion was not started from anything other that gravitational contraction.
Of course it was just a metaphor. Using nuclear fusion with hydrogen or a chemical reaction or what you want, wouldn't change the situation.
What I am stating with regards to the sun or any main sequence star is accepted astrophysics as I have been taught
Could you ask a prof. of physics at university?
, in that the sun is not just a ball of burning hydrogen. The sun is comprised of different layers such as the core, photosphere, convection zones and coronasphere these all have vastly different properties. So photons emitted from the CORE (my original point) do not take 8 minutes due to there nature of interactions with the matter in the star and that includes more than just absorption and emission of the photon.
Of course once the photons make there way to the surface, or any emissions from the surface will be largely unhindered then and only then will take about 8minutes 20 seconds (or so).
It is "the photons make there way to the surface" which is wrong. Photons exchanged in the core are...exchanged in the core and stay exchanged there. The same for those exchanged in the other layers.
Usually the idea of a photon which travels from the core up to the surface is (incorrectly) suggested from the (real) existence of radiation pressure. But a photon is not a particle which individuality is conserved after absorption with matter; it's not as, let's say, a gas molecule scattering with a solid grain of something, then with another grain and so on up to the surface.
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Hmm, fair play. I see what your saying and it definitely makes sense, although it does go directly against the grain of what I have been taught (and assumed to be true). The circumstance laid out in my teachings was that the photons themselves were conserved and that they took millions of years to reach the surface, through being absorbed and emitted as they collided with other atoms. I will have to bring that up as it seem quite a major thing to get wrong. Before I do, I don't suppose I could ask you a little more on what exactly is exchanged between the different layers? Does the photons or the energy they carry not get exchanged in any direction within a star?
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Hmm, fair play. I see what your saying and it definitely makes sense, although it does go directly against the grain of what I have been taught (and assumed to be true). The circumstance laid out in my teachings was that the photons themselves were conserved and that they took millions of years to reach the surface, through being absorbed and emitted as they collided with other atoms. I will have to bring that up as it seem quite a major thing to get wrong. Before I do, I don't suppose I could ask you a little more on what exactly is exchanged between the different layers? Does the photons or the energy they carry not get exchanged in any direction within a star?
The "energy" certainly is. So if a portion of the plasma inside the Sun is hotter than another, it will transfer energy there (it means that, also, the outer layers could in theory heat up the inner layers if these were colder).
Energy however is not owned by photons only but by all the other fermions too. A photon is emitted from a fermion, absorbed by another, which is further from the Sun centre with respect to the first, then this fermion exchange another photon with another fermion still further from the centre and so on. The energy *does* travel from the centre to the photosphere.
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Ah, I see. That in itself is quite interesting in how the energy is distributed within a star, I think I have inherited some more reading material. Thanks for the clarification buddy.