I don’t have any objection to saying that the energy produced today in the core of the sun takes x number of thousands of years to reach the surface, due to the random-walk nature of photon-charged particle interactions.
But I do object to the notion that a *a gamma-ray photon * produced by nuclear fusion at the center of the sun and a visible light photon emitted in the photosphere of the sun are the same photon, just ‘x number of thousands of years older’.
Indeed.
[QUOTE=scotandrsn]
Now don’t you wish you’d quit while you were ahead?
[/QUOTE]
An excellent post until it shows that it is not about fighting ignorance but trying to put someone down. I just don’t see the need for this last bit.
FWIW, I think we are really close to a semantics argument.
[QUOTE=scotandrsn]
Now don’t you wish you’d quit while you were ahead?
[/QUOTE]
Sure, but unfortunately, my very first post in this thread was hideously wrong, so there was never a point when I was “ahead” and could quit.
And it’s clear to me now that the OP is entirely justified in saying what he did, since no less of an authority than Bad Astronomer Phil Plait uses almost the precise language that the OP did.
So, in case it wasn’t abundantly clear, I was wrong, and, worse, was wrong in GQ because I spouted off in an area I don’t know much about.
But I still haven’t quite understood why I’m wrong, and am humbly trying to learn.
Both Plait’s language and the other authority you cite, Arnett, use the phrase “absorbed and re-emitted” to refer to what happens to photons in the core of the sun.
Can someone explain in high-school physics terms why we consider that to be the same photon after it’s “absorbed and re-emitted”? As I said, my understanding was that when a photon is emitted, it’s a new photon. So obviously that’s not quite the case. Also, can someone reconcile the statement made about the different kinds of photons - gamma-ray photons vs. visible light photons - in connection with this?
Please educate me.
One additional point: as a Pit thread on the topic has pointed out, my apologies above have not included The Controvert. I apologize as well for taking an unwarrented snarky tone with you. It seems your language was entirely justified, is virtually identical with that used by authorities on the subject, and while I still don’t understand the whys, it’s clear you were right.
So - I hope you’ll accept my apology.
I agree that what **Bricker ** said needs to be dealt with. As great an authority as Plait may be, Feynman is no less an authority. And according to Bricker, Feynman has said that each newly emitted photon did not exist before. So which is correct? Is the same photon random-walking for thousands of years, or are there countless absorptions and emissions of new photons that never existed before?
[QUOTE=Bricker]
Can someone explain in high-school physics terms why we consider that to be the same photon after it’s “absorbed and re-emitted”?
[/QUOTE]
Remember that old saw “Energy can be neither created nor destroyed”, AKA the conservation of energy?
When a photon is absorbed, the energy does not disappear, in fact it comes right back out when it’s re-emitted. It’s the same energy. Since photons are widely regarded as being energy, it’s quite easy to say that it’s the same photon (energy) before and after absorbtion.
An absorbed gamma ray photon can either be emitted as a gamma ray, or say 10 photons of visible light. The only difference between the two scenarios being the shape of the emitted wave function. Yes, that’s a pretty squishy way of looking at the process, but the wave/particle nature of light is that way. You can’t pick out a particular visible photon, and say this is the one that was a gamma ray before it lost its energy in forming these 9 other visible light photons. In the sense that the amount of energy involved is exactly equal, the 10 ‘new’ visible photons are, collectively, the old gamma ray photon.
I think generally in physics the rule is: If we can’t tell one way or the other, the answer doesn’t matter
Is it the same photon in a different state? Or a new photon that just happens to look like the old one ?
To me that’s like asking what happens inside a singularity. We just don’t know so the answer is not part of physics theory.
If I’m wrong and there is an answer it’s gonna be an interesting one.
[QUOTE=Squink]
You can’t pick out a particular visible photon, and say this is the one that was a gamma ray before it lost its energy in forming these 9 other visible light photons. In the sense that the amount of energy involved is exactly equal, the 10 ‘new’ visible photons are, collectively, the old gamma ray photon.
[/QUOTE]
So you would say, then, that even though the ten visible light photons did not exist even a picosecond ago, they are years old because they are the result of a gamma-ray photon that was years old?
That’s … counterintuitive. To me, it would seem more acurate to call those ten photons ‘new.’
Can you explain why we view these two cases (one gamma ray photon vs. ten visible light photons) as being identical? Is it because the energy is identical, and since photons are energy it’s meaningless to make the distinction I’m trying to make?
[QUOTE=Bricker]
Is it because the energy is identical, and since photons are energy it’s meaningless to make the distinction I’m trying to make?
[/QUOTE]
I’d go with something like that. Unless you’re dealing with something like the polarization of entangled photon pairs, it’s just not useful to pick out a single photon and say “this is the photon, and this is its history.” As soon as you look the other way, the thing is going to turn into a wave, and baffle your expectations.
Energy takes 10,000 years or so to diffuse from the core of the sun to its surface, where it’s emitted as light. If you think about energy as photons, you can say that it takes 10,000 years for photons to diffuse from the core to the surface, but when you start talking about a photon in that process you’re just wandering into a semantic cesspool.
[QUOTE=Squink]
In the sense that the amount of energy involved is exactly equal, the 10 ‘new’ visible photons are, collectively, the old gamma ray photon.
[/QUOTE]
Can I then say that the photons emitted from my light bulb are collectively an old gamma ray photon from the sun’s core? It’s the same energy, gone through a few cycles of absorption, storage and emission.
I don’t think it’s useful or instructive to state that 10 visible spectrum photons are in any way the same individual unit as the single high energy photon they were spawned from. Is it really so improper to say that the original photon was absorbed (making it go away) and some new photons were emitted from the stored energy?
Life would cease to exist, but it wouldn’t be in 8.5 minutes. A few years for most of the life we see everyday, a few decades for insects and other smaller forms of terrestrial and aquatic life. Humans would probably be gone by then. The MAIN culprit for their eventual death would be from lack of oxygen, since most of the oxygen produced on our planet is from oceanic algae which would die off without sunlight.
If you were to come back 1000 years post “sun down”, you would probably have a frozen world with life only presents at the depths of the oceans at the hydrothermal vents (they probably wouldn’t be affected at all).
[QUOTE=Cheesesteak]
Is it really so improper to say that the original photon was absorbed (making it go away) and some new photons were emitted from the stored energy?
[/QUOTE]
No, it’s not so improper. It’s just that down near the core the optical opacity is so high that even picking out an individual photon and calling it ‘this one’ is close to meaningless. The mean free path for each photon runs on the order of centimeters, so its lifetime as a discrete entity is on the order of tenths of nanoseconds.
If you want to hurt your head over this issue, see: Two-Dimensional Random Walk of a Photon Emitted from the Solar Core
[QUOTE=gatorman]
Life would cease to exist, but it wouldn’t be in 8.5 minutes. A few years for most of the life we see everyday, a few decades for insects and other smaller forms of terrestrial and aquatic life. Humans would probably be gone by then. The MAIN culprit for their eventual death would be from lack of oxygen, since most of the oxygen produced on our planet is from oceanic algae which would die off without sunlight.
If you were to come back 1000 years post “sun down”, you would probably have a frozen world with life only presents at the depths of the oceans at the hydrothermal vents (they probably wouldn’t be affected at all).
[/QUOTE]
I think you’re WAY off in your estimation of what would happen and how long it would take.
Take away the sun as an energy source, and the earth would cool down rapidly. It would be winter around the world in a few days. Not long after that, it would be 100 below. And colder, and colder. It will snow heavily, as all the moisture condenses out of the atmosphere. Then the CO2 will freeze out of the atmosphere and it will rain dry ice for a while, but we’ll be long dead by then.
The oceans will freeze over within a few weeks. They’re a pretty big heat sink, and as the surface ice builds it would act as an insurator to trap the heat, so it might take a while, but eventually the entire oceans will freeze into a solid block of ice. Liquid water will remain in pockets where there’s enough geothermal activity to keep it going. But there will be no surface water, as it would boil away due to the atmosphere being mostly frozen on the ground.
The earth would wind up being a big frozen ball with ocean ice being hundreds of kilometers thick, and only small pockets of water left deep under the ice.
I suspect all this would happen within a few months, with almost all life being dead after a couple of weeks.
If there was some warning, we might be able to prepare well enough to keep a few thousand people alive for a few years or even a few centuries. Find big caverns all around the world, put nuclear piles in them for heat and light and electricity, build big hydroponic farms, and hope for the best. Areas with natural geothermal heat might be come oases with suitable shelter being hewn from the rock. Depending on how long of an advance warning we had, we might do better than that - it’s hard to tell what we could really accomplish given, say, 100 years to prepare for the big wink-out. Maybe we could dig down a few thousand feet, where there is sufficient interior heat from the mantle to provide energy, and use nuclear bombs to blast out big caverns that we would move into.
Maybe the mass of humanity could move underground - given 100 years to prepare, and the knowledge that the carrying capacity of ‘underground civilization’ might be a tiny fraction of today’s population, people might choose to simply not have children, not wanting to expose them to near-certain death. So we’d have the last big generation, then as we get old and die off the new population would be tiny. In the meantime, we’d be building our homes deep underground, moving our critical infrastructure deep underground, and heavily building nuclear power plants everywhere, of the lowest-maintenance designs possible, all on the same grid with heavy redundancy, etc.
Actually, this would make a pretty cool science fiction story. Imagine a ‘building plan’ for 1000 feet below the surface, where we space out huge nuclear-excavated caverns maybe 5 miles from each other in a huge array, then use tunnel excavating machines like those used to build the English Channel tunnel to connect them all. Imagine several thousand such caverns, built using the world’s nuclear arsenal. We couldn’t connect continents together, so once the ‘big freeze’ happened, we could have entire populations completely cut off from each other except by TV and radio and the internet. These people would know they will never, ever meet. The only import/export goods would be information. If a disaster struck one ‘colony’ all the rest of them could do is watch it unfold.
Here’s another plan: You pack everyone as tightly as you can into a few underground outposts very near the oceans. Then you wait for them to freeze solid, and bore into them for more living space. With nuclear power, you could melt out an entire city inside the ice. However, I don’t know if it would be stable. What would a frozen ocean really be like? As stable as rock? Or would the ice ‘flow’ like glass under pressure and eventually close up any gaps in it? I suspect the latter. But I have no idea how long it would take or whether you could manage the process at all.
[QUOTE=Bricker]
Can someone explain in high-school physics terms why we consider that to be the same photon after it’s “absorbed and re-emitted”? As I said, my understanding was that when a photon is emitted, it’s a new photon. So obviously that’s not quite the case. Also, can someone reconcile the statement made about the different kinds of photons - gamma-ray photons vs. visible light photons - in connection with this?
Please educate me.
[/QUOTE]
Speaking of a photon as being “old” oir “new” is rather tricky; there’s nothing we can fundamentally observe about an individual photon that tells us anything about its age other than by following it from inception to terminus; and the mere act of “observing” a photon means that you will interact with (terminate) it. In quantum electrodynamic (QED) terms, photons exist as force-carriers for the electromagnetic force (though they themselves, of course, have no charge), and so can really only be detected in terms of the result of an exchange between two charged particles–typically, electrons.
Now, an electron–either one bound in the valance shell of an atom, or floating around in a metallic matrix, or bouncing around in a plasma–will absorb a passing photon and increase in energy level if the balance works out right (i.e. it’s just the right energy to hop up to any valance band); this is a statistical quantum process. When we say that they’re “absorbed”, do we mean that the photon is broken down into some kind of “pure energy” and converted into kinetic energy of the electron? Well, yes and no; the photon’s only unique characteristic–it’s frequency or wavelength–disappears, but because it’s QM we can’t just think of it as a car driving a long that just got a little extra gas. The energy packet that is a photon is still stuck “in” the electron (which has its own characteristic mass-energy, charge, and spin) and will eventually be released, either in fractions of its original frequency/wavelength, or in combination with that of other absorbed photons. The electron can’t just make the photon, or energy derived therefrom, part of its own inate structure, so in some sense, at least, the photon “survives” independent of the electron.
When you break a photon down into parts, or combine it together with the packets from other photons, is it still the same photon? shrug Since all photons of a given wavelength are the same, it’s impossible to distinguish between them, or fractions/multiples thereof. And since they don’t decay (subject to the afforementioned unsubstantiated hypotheses) we can’t say anything about the age of an individual photon either. Talking of photons as being “old” or “new” is, in the strictest terms, semantic nonsense. However, we can say that photons produced within the core of the Sun will eventually and inevitably make their way outward, regardless of whether the Sun continues producing additional photons via fusion processes or not; in this sense, whether you consider the photons as being absorbed and re-emitted anew, or “bouncing around” is irrelevent; they’ll continue to worm their way out and be radiated.
With regard to the o.p.'s question and Bricker’s assertion, the photons initially produced by fusion reactions within the core of the Sun are absorbed and reradiated hexillions of times, broken down from being the high frequency gamma photons originally emitted to (mostly) somewhere about the spectrum of visible light, ultimately (as far as the observer is concerned) radiated by the lower levels of the photosphere where the Sun becomes more or less transparent to visible light. The o.p. presupposes “Suppose some magical process causes our sun to stop producing new light photons.” It could be taken to mean, as Bricker has, that the QED processes involving absorption and emission of photons have been frozen; however, doing so would not only pose serious problems with QED, it would also indicate a major violation of the third law of thermodynamics (i.e. bringing a system with a nonzero temperature to a state of no activity). This is, to our understanding, fundamentally impossible, whereas the more conventional interpretation of the o.p.'s condition–that fusion processes within the Sun’s core have just suddenly and inexplicably stopped–merely indicates some heretofore unknown state in which fusion isn’t maintained, requiring no fundamental violations of any natural laws. In this case, it would take between 10k and 190k years the products of photons originally created by the last nuclear reactions to be radiated from the photosphere.
This is all an aside to the o.p.'s intent, however, which was clearly “How long do humans survive if sun stops producing light?”, disregarding by what means light production stops. The answer to this, again, is a few days, save for anyone with the prescience to construct and utilize they type of shelter described by Sam Stone. The estmate of hundreds to thousands of years by the o.p., or even months by others, is grossly optimistic.
Stranger
OK, let me put it this way: When I shine a flashlight at a mirror, and the beam reflects off, is the light that’s reflecting off the same as the light that hit the mirror? The same sort of thing is happening there: Photons are being absorbed, and then immediately re-emitted in a different direction. If that counts as “the same light”, then so does a photon before and after bouncing off of an electron in the Sun’s core. Does it actually count as the same light? That’s a question of interpretation, which doesn’t matter at all for the physics. One can actually construct self-consistent models in which there’s only one electron in the Universe, for instance, which just moves forward and backwards in time so much that it looks like a bunch of them. It’s possible to interpret the light as “bouncing”, and it isn’t really wrong to do so, but it’s also not wrong to interpret it as different, new light.
[QUOTE=Squink]
I’d go with something like that. Unless you’re dealing with something like the polarization of entangled photon pairs, it’s just not useful to pick out a single photon and say “this is the photon, and this is its history.” As soon as you look the other way, the thing is going to turn into a wave, and baffle your expectations.
Energy takes 10,000 years or so to diffuse from the core of the sun to its surface, where it’s emitted as light. If you think about energy as photons, you can say that it takes 10,000 years for photons to diffuse from the core to the surface, but when you start talking about a photon in that process you’re just wandering into a semantic cesspool.
[/QUOTE]
OK, fair enough – except that the OP’s entire premise was that, in fact, we CAN discuss it in that way, wasn’t it? He posits a magical process that stops the production of “new light photons.” But he invites the inference that the ENERGY from the sun will continue to diffuse from the core to the surface, without producing new photons.
I suppose we’re at the point when it’s appropriate to say, “You need better than a twenty-years-out-of-date high-school physics understanding to get this.” I suspect I lack enough foundation to get the concepts.
[QUOTE=Mangetout]
In the movies, funds would be mobilised to send a magic spaceship to the core of the sun to start it going again. In reality, it would be too far in the future to worry about, right up until it was too late to do anything about it.
[/QUOTE]
You mean like this one?
Movie trailers like that drive me nuts. Show me the whole film, why not?
I’m hoping someone will take a whack at Cheesesteak’s question:
This stuff is way over my level, but I had the same thought.
Along the same lines couldn’t you say that all future photon energy was created shortly after the Big Bang and there are no new photons being created at all?
What about looking at it from some sort of information level? If I shoot x number of gamma ray photons at a spot on Chronos’ mirror, and that spot flashes to plasma and radiates away z number of photons spread out over a blackbody spectrum (where z>x), even if I capture all z emitted photons and measure their total energy, is there any way that I can work backward and say that they were originally excited by x gamma ray photons as opposed to, say, y x-ray photons where z>y>x ? So the exact composition of the input energy can’t be derived from the blackbody output, because all a blackbody is concerned with is how hot it is as opposed to how it got that hot in the first place?
I’m hoping that that makes sense to someone…