Thursday, 1 December 2011

100,000 year old light!

How 'old' is the light from the sun?  I mean the light that you can see when you go outside today.

You might be surprised to know that the light took something like 100,000 years to get from the centre of the sun where it was first created by the process of nuclear fusion to the surface.  Then took only about 8 minutes to get from the surface to the earth.

Most of the fusion takes place close to the centre of the sun where the pressure and density are high enough.  Photons that are created in this process can't escape as they are surrounded by hot plasma.  Every time the photon 'hits' another nucleus or electron it is reflected in a random direction, and you can use some simple mathematical physics - known as the 'random walk' - to estimate how long it takes a photon (traveling at the speed of light of course) to get away from the core.

This is almost analogous to trying to get out from the centre of crowd of people. You couldn't just run straight through because you would be pushed back by people and be bounced around. You might have to go back to the centre to get past a tightly packed area. The other people represent the nuclei and electrons that make up the sun.

Incidentally - you might remember reading my post Indecisive Neutrinos about the speed of neutrinos measured from the 1987A supernova. I mentioned that they arrived 3 hours ahead of the first light from the event, having traveled for 168,000 years and I half-explained that the reason for the three hour difference was because the neutrinos hardly interact with matter but that the light does.

'In the light' of what you have just read, that comment might make more sense now.


Pruthvi said...

yes it makes more sense. What about the surface of the sun ? Are no photons generated from the surface ? Are all photons generated from the core ?

Hilary said...

Actually I've read estimates of anywhere from 10,000 years to 100,000 years to get from the centre out...but of course this makes one gigantic assumtion, which is that it IS fusion that powers the Sun and not graviational collapse or some other process...the jury is still out on this one...the only reason they assume it is fusion is because they believe the Sun is nearly 5 billion years old and the only known process that can account for such age is fusion...but if it isn't 5 billion years old....

RosaRubicondior said...


Of course individual photons will vary about a mean, as you would expect of a Normal probability distribution. In fact, this is an example of a 'law' derived from mass action which applies not at the lower level of individual particles but at the higher level of mass action - 'structure' emerging inevitably from chaos. See also the Gas Laws.

If it were only gravitational collapse, what would prevent the sun collapsing to a black hole?

There is, of course, no such assumption since fusion can be observed by spectroscopic analysis and the fact that the reality exactly matches the theory in terms of energy output. This is how science validates its theories to ensure they are not merely 'gigantic assumptions'.

In fact, the centripetal pressure of photons is balancing the centrifugal pull of gravity. Gravitational collapse is what forces hydrogen nuclei together to form helium nuclei with the release of energy (photons), so the final size of the sun is a function of the initial mass and the amount of hydrogen.

I understand that Sir Arthur Edington, who initially said the sun proved a young universe because it would have run out of fuel by now, recanted when fusion was discovered to be the source of energy, not combustion as he had assumed.

Hilary said...

Actually noone can 'see' into the core of the sun and I am sure you know that there are many possible ways to have the same end is an assumption, and the assumption is based on a belief that the sun is the age it is. The because the results appear to validate those assumptions then it is continued to be assumed that fusion is the way the sun is powered.

There have been many many assumptions about how things work over the centuries and many results have 'proved' the assumptions to be correct...until it was finally realised that actually it wasn't. Archimedes assumed Aristarchus to be completely wrong in the 3 century BC because Archimedes could not see parallax. However, Archimedes had made incorrect assumptions which it took 1800 years to realise. Archimedes was a big shot, so everyone believed him! Aristarchus was ignored.

I can give you many more of such examples...theories of how the Moon came to be...even the latest one is slowing hitting the dust...all have been taught as fact at one time or other...

Science can never be totally sure of its interpretations.

Anonymous said...

Balderdash. The only jury still out is in the courtroom of your mind. There is absolutely no contention regarding what powers stars, except among those whom cling to 19th century science. Science improves its understanding with new evidence and observation. It does not make the claim "to know everything", which is the bar all "goddidits" aspire. If you have a requirement for absolute knowledge, then science isn't your field.

Reasonable Sanity

Darckilo said...


I am not an astrophysicist but I think I have absorbed enough knowledge to understand the very basics of light and the speed at which it travels. From comments you made I am assuming that you believe the universe is less than ten thousand years old and that everything was created in six days. If our galaxy (The Milky Way) is 100,000 light years across how do you explain photons reaching us two thousand years ago from distances that are far greater than ten thousand light years? The closest galaxy is more than 80,000 light years away and we can see it.