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| onerock |
 Posted: Fri Mar 20, 2009 11:18 pm Post subject: Does light have Energy? |
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Does light have Energy?
I mean that if we go to the end of the universe (in a not infinite universe) and we look into it, we won´t see the stars from the other side of that universe.
I know light would take time to travel from one side of the universe to the other one, but it should still make it or? Or is there an Energy factor that light posseses and it loses it the more it advances through space (like a kind of fuel)? |
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| MagiMaster |
| Posted: Sat Mar 21, 2009 2:24 am Post subject: |
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| Light does have energy (consider solar panels) but it doesn't lose that energy while traveling through a vacuum. If the universe were a sphere that had been expanding at 1 light-year per year, then from the edge, you'd be able to see all the way to the middle (13.5 billion years ago) but not all the way to the other edge, since that light hasn't had time to reach that far. |
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| Manynames |
| Posted: Sat Mar 21, 2009 4:07 am Post subject: |
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Yes, light has an energy. It may seem strange to think that an energy can have also an energy, but it is true.
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| John Galt |
| Posted: Sat Mar 21, 2009 4:15 am Post subject: |
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| Is it not more accurate to say that light is energy? |
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| Manynames |
| Posted: Sat Mar 21, 2009 4:19 am Post subject: |
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| John Galt wrote: |
| Is it not more accurate to say that light is energy? |
If my memory serves me correctly, i think Newton was the first to propose that light has an energy. If one then says that light is simply energy, then it is open to that interpretation as well.
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| Manynames |
| Posted: Sat Mar 21, 2009 4:26 am Post subject: |
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| Manynames wrote: |
| John Galt wrote: |
| Is it not more accurate to say that light is energy? |
If my memory serves me correctly, i think Newton was the first to propose that light has an energy. If one then says that light is simply energy, then it is open to that interpretation as well. |
Needless to say, the best way i have come to accept the photon energy, is simply being a kinetic energy when taking into consideration  where the mass is reduced to zero. So the energy of a photon is purely kinetic.
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| Numsgil |
| Posted: Sat Mar 21, 2009 6:53 am Post subject: Re: Does light have Energy? |
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| onerock wrote: |
Does light have Energy?
I mean that if we go to the end of the universe (in a not infinite universe) and we look into it, we won´t see the stars from the other side of that universe.
I know light would take time to travel from one side of the universe to the other one, but it should still make it or? Or is there an Energy factor that light posseses and it loses it the more it advances through space (like a kind of fuel)? |
The visible universe is something like 13.7 billion light years in radius. Coincidentally the age of the universe. So the even more distant portions of the universe haven't had a chance to reach us. Which tells us the universe is probably either infinite or really, really big. Current data strongly suggests infinite (a flat curvature).
Light doesn't ordinarily lose energy as it travels, however our universe is expanding (the distance between any two points in the universe increases over time). Light compensates for this by bleeding off energy as it travels. So when you're talking about light on the scale of the universe, it does sort of "burn fuel" as it travels. |
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| DrRocket |
| Posted: Sat Mar 21, 2009 2:10 pm Post subject: Re: Does light have Energy? |
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| onerock wrote: |
Does light have Energy?
I mean that if we go to the end of the universe (in a not infinite universe) and we look into it, we won´t see the stars from the other side of that universe.
I know light would take time to travel from one side of the universe to the other one, but it should still make it or? Or is there an Energy factor that light posseses and it loses it the more it advances through space (like a kind of fuel)? |
Mass and energy are one in the same thing. Light is a form of energy. The energy of a photon is descrived quite simply by  where  is energy  is Planck's constant and  is the frequency of the light.
It is really that simple.
Unless the photon interacts with other particles or gravity it maintains its energy and does not loose it simply traveling for a distance, no matter how large. |
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| Jean-Pierre Liégeois |
| Posted: Sun Mar 22, 2009 10:19 am Post subject: Re: Does light have Energy? |
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| Numsgil wrote: |
| onerock wrote: |
Does light have Energy?
I mean that if we go to the end of the universe (in a not infinite universe) and we look into it, we won´t see the stars from the other side of that universe.
I know light would take time to travel from one side of the universe to the other one, but it should still make it or? Or is there an Energy factor that light posseses and it loses it the more it advances through space (like a kind of fuel)? |
The visible universe is something like 13.7 billion light years in radius. Coincidentally the age of the universe. |
Not in my understanding. The universe might be 13.7 or so billion years old, but it's been expanding during all that time, which makes the most distant objects that we can see actually more distant than 13.7 billion years. I seem to recall that the size of the observable universe is more like 47 billion light years.
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| So the even more distant portions of the universe haven't had a chance to reach us. Which tells us the universe is probably either infinite or really, really big. Current data strongly suggests infinite (a flat curvature). |
As far as I know available data suggests that the universe is either flat or has a positive curvature, but since 'flat' would be true if the universe's density was exactly equal to a given value, the prior odds that the universe is flat are strictly zero (which doesn't mean that's impossible). So no, I don't think that current data strongly suggests a flat curvature.
Second, a flat curvature does not imply infinity, you might very well have a flat universe with a multi-connected topology. Flat only means that the universe could be infinite.
And if it so happened that the universe was finite, it might very well be smaller than the observable universe. Current data suggests that it should be bigger than a few billion light years in radius, but that's it. So we can't even tell whether the universe is really, really big.
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| Numsgil |
| Posted: Sun Mar 22, 2009 1:02 pm Post subject: Re: Does light have Energy? |
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| Jean-Pierre Liégeois wrote: |
| Numsgil wrote: |
| onerock wrote: |
Does light have Energy?
I mean that if we go to the end of the universe (in a not infinite universe) and we look into it, we won´t see the stars from the other side of that universe.
I know light would take time to travel from one side of the universe to the other one, but it should still make it or? Or is there an Energy factor that light posseses and it loses it the more it advances through space (like a kind of fuel)? |
The visible universe is something like 13.7 billion light years in radius. Coincidentally the age of the universe. |
Not in my understanding. The universe might be 13.7 or so billion years old, but it's been expanding during all that time, which makes the most distant objects that we can see actually more distant than 13.7 billion years. I seem to recall that the size of the observable universe is more like 47 billion light years.
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Oh that's right. I'd forgotten that 
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| So the even more distant portions of the universe haven't had a chance to reach us. Which tells us the universe is probably either infinite or really, really big. Current data strongly suggests infinite (a flat curvature). |
As far as I know available data suggests that the universe is either flat or has a positive curvature, but since 'flat' would be true if the universe's density was exactly equal to a given value, the prior odds that the universe is flat are strictly zero (which doesn't mean that's impossible). So no, I don't think that current data strongly suggests a flat curvature.
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As the margin of error decreases as our instruments get more exact, 0 is always in the bounds. That strongly suggests 0 to me. Your definition of "strongly suggests" might vary of course. The implication if the curvature is exactly 0 is that there's some balancing factor we probably don't understand. I don't think you can apply probability to something like "odds that the real value is X", since we have no prior experimental data to compare against. Just the one universe. And there's always the biasing factor that we arose in this universe, and maybe the only universes that can support life are ones with 0 curvature for whatever reason.
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Second, a flat curvature does not imply infinity, you might very well have a flat universe with a multi-connected topology. Flat only means that the universe could be infinite.
And if it so happened that the universe was finite, it might very well be smaller than the observable universe. Current data suggests that it should be bigger than a few billion light years in radius, but that's it. So we can't even tell whether the universe is really, really big. |
Oh? As far as I was aware the scientific consensus was that flat curvature = infinite. Do you have a link I can read to brush up on my faulty understanding? 
I don't think it's probable for the universe to be smaller than the observable universe, because if that were so we would expect to see identical galactic clusters/superclusters, etc. on either side of us at our universal antipole (or whatever you want to call it). Of course they'd have a different shape since we'd be viewing it from different angles... But surely we would have figured out by now that we are viewing the same thing from two different directions? I mean, they have done just that to prove gravitational lensing. |
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| KALSTER |
| Posted: Sun Mar 22, 2009 3:00 pm Post subject: |
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AFAIK the universe is not infinite as in being of infinite size or having an infinite amount of energy, but infinite in 3D in the same way the surface of a sphere is infinite in 2D. It is finite and unbounded.
Link
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| Numsgil |
| Posted: Sun Mar 22, 2009 5:04 pm Post subject: |
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| Yeah. It doesn't have to be infinite, but it probably is unbounded. However, if it is finite, it's really, really big. It might also be infinite proper. Nothing really breaks if the universe is flat and infinite. You can still construct the math (like set theory math). It just makes it really hard to picture properly |
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| Manynames |
| Posted: Sun Mar 22, 2009 5:08 pm Post subject: |
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| Numsgil wrote: |
| Yeah. It doesn't have to be infinite, but it probably is unbounded. However, if it is finite, it's really, really big. It might also be infinite proper. Nothing really breaks if the universe is flat and infinite. You can still construct the math (like set theory math). It just makes it really hard to picture properly |
If there is however no boundary point in the future cone (or an end to the universe) then it is always best to envision the expansion as very much being infinite. Infinity is just always one more than now, and if there is no catastrophic depletion of the universe, then the expansion of spacetime certainly seems to be very infinite in nature.
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| Jean-Pierre Liégeois |
| Posted: Mon Mar 23, 2009 12:08 am Post subject: Re: Does light have Energy? |
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| Quote: |
| As the margin of error decreases as our instruments get more exact, 0 is always in the bounds. That strongly suggests 0 to me. Your definition of "strongly suggests" might vary of course. The implication if the curvature is exactly 0 is that there's some balancing factor we probably don't understand. I don't think you can apply probability to something like "odds that the real value is X", since we have no prior experimental data to compare against. Just the one universe. And there's always the biasing factor that we arose in this universe, and maybe the only universes that can support life are ones with 0 curvature for whatever reason. |
Yeah, that's why I said 'prior odds', that is, before data comes in. But you're quite right, using probability theory here is probably going too far.
There might be a good, unavoidable reason why the universe should be strictly flat that future theories might unveil.
But if, on the other hand, flatness only depends on an accidentally determined density value, then flatness would be quite extraordinary indeed, because it would require an infinitely precisely determined density value.
So what you're saying is that data may suggest that there's a law in the universe that set the density to be equally what it takes to make the universe flat. Why not, after all. The thing is, no one can assign a likelihood to this, so it all boils down to hunches and aesthetic considerations.
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| Oh? As far as I was aware the scientific consensus was that flat curvature = infinite. Do you have a link I can read to brush up on my faulty understanding? |
There's a book called 'The wraparound universe' that deals with that kind of stuff. It's a bit expensive in english version, but really interesting. Maybe there are other books discussing this, I don't know, cosmological topology is a rather new field of physics (well, some developments date back from the 30s, but the field stagnated for a long while after that).
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| I don't think it's probable for the universe to be smaller than the observable universe, because if that were so we would expect to see identical galactic clusters/superclusters, etc. on either side of us at our universal antipole (or whatever you want to call it). Of course they'd have a different shape since we'd be viewing it from different angles... But surely we would have figured out by now that we are viewing the same thing from two different directions? I mean, they have done just that to prove gravitational lensing. |
Well, matching different images of identical objects would be straightforward if the universe was only a few million light years in size, but when you start considering sizes around the billion light years it starts getting considerably trickier. The thing is, not only are images of the same object placed at different distances and angles, but they also are images of the same object at very different times. And not many things stay the same in several billion years, not even superclusters apparently.
As far as I know, the lower bound that's been found for the size of the universe is a few billion light years. Past this size, images of the same thing start looking too different for us to reliably tell what they are.
But there are other ways one could tell if the universe was actually smaller than it looked, and one of them is observing the cosmological microwave background. Current data has proved inconclusive with respect to that, but one can hope the Planck satellite, which will be launched next month, will help finding an answer.
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| DrRocket |
| Posted: Mon Mar 23, 2009 9:56 am Post subject: |
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| KALSTER wrote: |
AFAIK the universe is not infinite as in being of infinite size or having an infinite amount of energy, but infinite in 3D in the same way the surface of a sphere is infinite in 2D. It is finite and unbounded.
Link |
Not quite.
It is not know whether the universe is unbounded or not (unbounded is often called infinite and bounded finite but that is a small abuse of language).
It is also not know whether the curvature of the universe, on a large scale, is negative, zero or positive. There are quite a few knowledgable people who think that the curvature is negative and that the universe if unbounded.
The space-tiime manifold of the universe is modeled as a 4-manifold without boundary, but being without boundary has nothing to do with being unbounded. |
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