“E(photon) + E(electron_before) = E(electron_after)
p(photon) + p(electron_before) = p(electron_after)

hc/λ + 0 = γmc^2
h/λ + 0 = γmv”

hc/λ + 0 = γmc^2 should be hc/λ + mc^2 = γmc^2,
so hc/λ + 0 = (γ – 1)mc^2

I suppose an electron can’t absorb a photon in an isolated system consisting of only the electron and the photon. That doesn’t apply to this universe. Solitary charged particles emit radiation when accelerated in a magnetic field – cyclotron radiation – consequently the reverse can also happen! The key is the external field – external to the electron and the photon.

The radiation gets more energetic with stronger fields – so with other charged particles nearby and thus strong fields – the plasma before recombination – the photons emitted and absorbed are of short wavelengths, with thinner matter – the current interstellar or intergalactic plasma – the emissions and absorptions happen mostly (or perhaps only) at longer wavelengths. But it happens! All it takes is something in the surroundings to interact with, even if it’s just the fields of solitary protons passing by now and then.

I have absolutely no idea what the probabilities may be – but obviously they’re very very low, practically zero (or really zero, see previous parenthesis) for visible light in the interstellar medium.

I guess the wavelengths that are absorbed efficiently, increase very rapidly with reduced density, so only a few hundred million years of expansion and clustering was enough to make the difference between ionized gas being opaque or transparent to visible light. Based on the ionospheric cutoff of radio waves from around 10-30m wavelengths – I’d say the present interstellar medium may be opaque from around, oh, I don’t know, a few kilometers wavelength perhaps.

……………………………

Don’t you just love, as Phil would say, the enheadbustingness of the real world?

As for a concept of now, it get’s even a little more interesting when you bring human physiology and psychology into the mix. For example, while it takes about 8 seconds for light to travel from the sun to your eye, it also takes further time for your eye to transmit it to your brain, then for your brain to flip the light image and then interpret the image so that you see the sun and the surrounding image as you do (ie. right-side up), … and yet it may be not quite as others might experience the event.

Groovy, huh!

I do have a question, though. How do we figure out how old a galaxy is? I know some of them are very old, but I’m not sure how we know. Is there some calculation done based on the brightness of light or the type of light? I’d like a relatively simple scientific explanation if you have a moment, please.

“As far as a “center,” scientists all pretty much agree that there isn’t one. To people in that galaxy, the Milky Way would appear to be on the “edge” of the visible universe, along with other galaxies 12.9 billion lightyears away in all directions. ……”

There you go, I still cannot grasp that concept, nor the explanation that the Universe is akin to a forever (?) inflating balloon. (I don’t want to be around when that balloon gets pricked). I remember an analogy used years ago that there were spots on the surface of that balloon, and as it inflated, the spots would move further apart, relative to each other, and usually not mentioned, the fact that the spots themselves, (galaxies), would also expand. I can now appreciate that the solid matter in those galaxies cannot themselves expand, due to the otherwise rigid physical laws, without invoking the usual conservation of energy arguments among others. Shades of super-sized elements come to mind. Or Super Ants as per Joseph’s updated analogy.

Now I know somebody out there will tell me that that is exactly what happens. I cannot grasp that. Nor can I accept that Space is “curved”. Light may curve through space, due to some influence such as gravity from its passing some relatively large body. If the “curved space” idea is taken to a logical end, or at least a good way towards it, then I would have to expect Parallel Universes, or at least “Worm-hole” short cuts, due to the curling-up of the fabric of space wrapping around itself, not unlike some leaves when they dry out, and, familiar to Australians, some Eucalyptus tree bark, as it naturally peals off, curls around itself. I know as a kid it was not a very good idea to light and smoke it. It tasted awful!

Getting back on topic, I also have trouble trying to incorporate the “Nowness” of Now. Yes, I know that the Sunlight that bathes us, that generates the weather, gives life to vegetation etc., is already 8.25 seconds old. The conditions on the Sun’s surface, or even the rest of the Sun, for that matter, haven’t changed for the WHOLE of man’s existence. The gravity it exerts on our planet has been a determinate of us, and we have seen that the Astronauts have worked up means to cope without it.

I can conceive of a “Time”, that accepts ALL things happening now, regardless of the consequences, but as it does not rely on the the velocity of light limitation, can this be related to the “Dark Matter” question, (does light even travel through it, or does something else travel much, much faster through it? – What’s the velocity of Dark?). I wonder what may be happening to those outlying galaxies at this moment, that we can see 13bn years ago. Maybe they have dissipated, as we have seen much closer and younger galaxies change and absorb/get absorbed and generate new galaxies. With dissipation out there, there may not be any more galaxies formed, only slightly foggy space, near the limit as we think we see it.

Ivan.

I wonder if those conditions would have permitted some kind of exotic “life”, in the sense of self-replicating units, to exist in the (much thicker than today’s) interstellar gas clouds at some point. And if so, if intelligence and self-awareness could evolve. And if so, if those life forms could ponder their ultimate long-term fate and discover that the universe would soon become too thin for them to exist anywhere.

And now, here we are, self-aware intelligent organisms, in a different age of the universe, wondering how long WE will be able to continue to exist before (say) the Great Rip.

After billions or trillions of years, when the universe has evolved to the point where carb0n-based life is no longer possible, will the conditions permit yet ANOTHER exotic kind of self-replicating units to evolve intelligence and self-awareness, and ask the same question about their OWN fates?

@icemith: Yep, it’s really hard to picture in the mind’s eye. Because of early inflation, and the dark energy expansion since, to a lesser extent, it appears that the universe stretches far beyond what we an see. In facy, everything seems pretty much homogenous in all directions (wiki WMAP for moar… everything). Basically, if the universe does have an edge, or even some “suburbs” that are more sparse or somehow different then the rest, we can’t see ‘em.

As far as a “center,” scientists all pretty much agree that there isn’t one. To people in that galaxy, the Milky Way would appear to be on the “edge” of the visible universe, along with other galaxies 12.9 billion lightyears away in all directions. They can see galaxies that we can’t see (those further from us than they are) and we can see galaxies they can’t see. The expansion of the universe isn’t caused by galaxies moving out from some central point, it’s caused by the space between the galaxies expanding. One metaphor I like is ants on the surface of an expanding balloon. The distance between them on the surface of the balloon is increasing, but none of them can be said to be “at the center” of the expansion, and the ants themselves aren’t moving across the surface of the balloon (assuming they’re just stopped dead in their tracks wondering where the heck their buddies are going).

I was thinking along similar lines. That we assert we can “see” (almost) all the way back to the “Big Bang”, that means we have determined the position of the actual Event. Or have we?

What are the co-ordinates? Or are we just asserting that because we have determined the oldest found galaxy, by definition, it must be close to that event.

I need to understand that if that event WAS the, shall we say, the center of the Universe as we know it, and it wasn’t hard up against a wall, (Dark Wall?), then the spread would have been in all directions. Yet I have seen diagrams where it is depicted as cone shaped. Is it to do with the spin rates of the photons/protons/electrons ….. or even the elements after they were formed?

My other observation relates to the fact then, that there is another 13.7bn years, approx., of expansion straight past that Universe center out to the far edge. And also in other directions as well. That assumes that WE are at the edge, which obviously we are not. So can we flip our ‘scope around and find that there are still another 13.7bn years of light incoming! Or does it?

At this point I tend to give up. I really don’t want to go back to the much simpler world where the Earth was the center of the Universe!

Ivan.

A proton is a hydrogen ion.
A hydrogen ion isn’t necessarily a proton.

In a small minority of cases a proton can combine with a neutron or two, making deuterium or tritium, respectively. Sans electrons they’d be hydrogen ions, but a proton stuck to a neutron isn’t a free proton.

Conservation of energy always applied, check the 1st law of thermodynamics. Unless we are dealing with times less than h/dE (from Heisenberg Uncertainty principle) which we aren’t.

Woof!