What is light?

[hewes]

Why do you say that the universe started as nothing but light? According to current theory, there is no time in the history of the universe when there was nothing but light.

I'm just saying that in the time prior to the cosmic inflation and formation of elementary, subatomic particles, the Universe consisted of high density energy. Is that not consistent with "current theory?"

 

[MeteorWayne]

"High density energy" is not light.

 

[SpeedFreek]

I'm just saying that in the time prior to the cosmic inflation and formation of elementary, subatomic particles, the Universe consisted of high density energy. Is that not consistent with "current theory?"

Well, it's a lot more consistent than saying the universe started as "nothing but light". Photons didn't dominate the universe until well after cosmic inflation (at least 10 seconds after!)

 

[hewes]

I was merely countering the assertion that light is nothing without interaction. But thanks for the illuminating comments.

 

[SpeedFreek]

But thanks for the illuminating comments.

:lol:

 

[csmyth3025]

I'm just saying that in the time prior to the cosmic inflation and formation of elementary, subatomic particles, the Universe consisted of high density energy. Is that not consistent with "current theory?"

Well, it's a lot more consistent than saying the universe started as "nothing but light". Photons didn't dominate the universe until well after cosmic inflation (at least 10 seconds after!)

I'm guessing that prior to cosmic inflation the universe was composed of a mixture of high energy photons, paricles (quarks and such), and their anti-particles.

Is there any credible conjecture or widely accepted theory about the relative abundance of these constituants prior to inflation?

Chris

 

[Planet_Lubber]

This is a very "illuminating" discussion all right! I would have accepted the assertion that the early universe was "nothing but light" or more precisely, nothing but wild oscillations of the electromagnetic field and other fields.

Here's another thing I don't understand. In learning special relativity, you have to train yourself to stop thinking in terms of simultaneity. Two events that happen at the same time in one inertial reference frame may happen at different times in another. So any question of what is happening "at this time" has to be refomulated.

But in discussions of cosmology, people commonly refer to "the present Hubble volume" and "the age of the universe when it occupied ... ~1/1000 ... [of] this volume" and "when the universe was around 1000 times smaller than it is today" and "The Hubble parameter at that time" and so on. Now I'm not saying you're wrong to use this terminology. I just want to understand it.

Yes, I'm familiar with the bubble analogy. The bubble is supposed to be a spacelike slice of spacetime. You can imagine it growing, in time. So it's as if some Supreme Being was watching the Universe from outside, and thought of all events in one bubble as occuring at the same time.

But for those of us inside the Universe, how can we uniquely define these spacelike slices? Is the Universe at age 10 seconds just the set of events such that any timelike geodesic going backward in time will take 10 seconds of proper time to get to the Big Bang?

What if different timelike geodesics take different amounts of proper time?

Does it even make sense to define things in terms of timelike geodesics when there were no massive particles to follow them? If not, what exactly do we mean by the Universe at age X?

The singularity theorems of Hawking and Penrose guarantee at least one singularity. But there may be lots of them. Besides, by definition, anything can come out of a singularity. There's no reason to assume that stuff will come out nice and smooth and symmetric.

 

[SpeedFreek]

Have a look at this paper (pdf link on the right side of the page):

The Cosmological Time Function

If you understand it, perhaps you can explain it to me!

 

[Planet_Lubber]

Thanks SpeedFreek, that’s an excellent article! Not easy reading by any means, but at least it’s not encumbered by horrible tensor notation. (Dear God of the Cosmos, deliver me from tensor bundles, and leadeth me not into super Grassman manifolds …)

They seem to be assuming only one initial singularity. That must be either an oversight on the authors’ part, or else maybe there is some well-known (except to me) reason why Lorentzian geometry prohibits multiple initial singularities. Or maybe the results in the paper are actually still valid even with many Big Bangs, and they just didn’t mention it.

The paper does uniquely define a cosmological time function, but it only has nice properties if it satisfies the regularity conditions. Basically, that time since the BB is finite everywhere, and goes to 0 as you go back in time along a timelike geodesic.

So, it is still in question whether our Universe has only one BB, and if so, whether the time function defined in terms of it is regular.

Also, even though the time function is well-defined, how can we Earthlings know what it is? How can we say that, e.g., we are looking at light from a star that was emitted when the Universe was 5 billion years old?

----------------------------------

“That’s no moon.”
-Obi-Wan Kenobi

 

[SpeedFreek]

They take the data from the Cosmic Microwave Background Radiation and combine it with the data for the expansion of the universe to ascertain how much the universe has cooled since the CMBR was released. Then they use that to estimate the elapsed time since the CMBR was released. The rest is extrapolated back beyond the recombination era (when the CMBR was released) using theoretical physics based in high-energy experiments and it gets increasingly speculative as we approach the singularity.

 

[Brusca]

We know it's elecromagnetic radiation.

The question is how does this radiation come out ?

Maybe this electromagnetic radiation and the matter have a common point because of the Einstein relation E=mc^2.

But why cannot this electromangetic radiation become matter ?

 

[benbald]

There are a lot of people on here that know what they're talking about and a lot of people that do not.

Simply put, you may hear that light is a particle, you may hear that it is a wave, you may hear that it is both. But the truth of the matter is that it is neither. If you treat it like a wave, or use wave math, in some situations you'll come out with good predictions. If you treat it like a particle, or use particle math, in other situations you come out with good predictions.

But what is it REALLY? Neither. Until we come up with some sort of model that can give you the right numbers in ALL situations, then the best we can say is that we really don't know.

 

[ramparts]

In fact, we do have a consistent view of what light is - it's part of quantum field theory. The photon is, essentially, a quantization of the electric field. It can be well approximated as either a point particle or a wave, but in reality it's something far more complicated, with elements of both. Unfortunately, that's the best I can do for you without getting into some pretty serious math.

The point is, don't say what physics does or doesn't know unless you know the state of modern physics yourself.

 

[benbald]

That statement is contradictory. First you disagreed and then agreed. With respect, I know modern physics better than most, otherwise what did I do all that research for?

 

[csmyth3025]

I find it interesting (and puzzling) that there can be very high frequency light - such as is postulated to exist in the first seconds after the big bang - which, in its quantitized form (i.e. photons), contains the energy equivalent of an electron or other massive particle. These photons, being massless, always travel at the speed of light. These same photons can, however, spontaneously convert themselves into particle/antiparticle pairs within the confines of the Uncertainty Principle. If these particle/antiparticle pairs are quickly driven apart by further interaction with high energy photons, they can continue to exist, but they can never travel at the speed of light since they are "massive" particles.

I don't disagree with the current thinking on this process, but it's very hard for me to understand the underlying common nature of the "fluid" energy of photons and the "frozen" energy of massive particles.

Chris

 

[origin]

I find it interesting (and puzzling) that there can be very high frequency light - such as is postulated to exist in the first seconds after the big bang - which, in its quantitized form (i.e. photons), contains the energy equivalent of an electron or other massive particle. These photons, being massless, always travel at the speed of light. These same photons can, however, spontaneously convert themselves into particle/antiparticle pairs within the confines of the Uncertainty Principle. If these particle/antiparticle pairs are quickly driven apart by further interaction with high energy photons, they can continue to exist, but they can never travel at the speed of light since they are "massive" particles.

I don't disagree with the current thinking on this process, but it's very hard for me to understand the underlying common nature of the "fluid" energy of photons and the "frozen" energy of massive particles.

Chris

It is rather hard to understand intuitively. That is because all our experience are on the macroscopic level. How do you visualize something that is both a wave and a particle and neither - - you don't really. That is why you describe it mathematically.

 

[ramparts]

Hi Chris,

Maybe this is a useful way to think about it: when particle/anti-particle pairs are created, some of the existing energy goes into their mass (E=mc^2, of course), and only the energy that's left is energy of motion. But since photons have no energy from their mass, all that same energy goes into their motion, allowing them to travel at c.

 

[pberrett]

From what I understand, when you shake an Electron, it generates Photons...

Ah now I understand. I can see a unified theory coming together here.

Photons are Electron dandruff. Light is just a snowstorm of electron dandruff. Your chemist probably has a product that can cure this.

Regards Peter