For that sort of thing, you are better off focussing the air to a point to break down the air and create a tiny plasma. This would look like a 'pixel' in the air.
> A question I would like the answer to is how long does it take for an atom to radiate a photon after being struck with a photon of sufficient energy to cause reradiation?
This is determined by the uncertainty principle as well. The uncertainty (and thus average time) is determined by the energy. The larger the energy, the larger the uncertainty in the time, and so longer before the atom decays.
You can have one photon cause an atom to excite, then a second photon to cause the atom to excite further. But this is different from two photons acting together to cause a single excitation - the difference between that it doesn't require an inbetween step to be available.
In another reply, someone said that photons 'collide' by decaying into other particles, which then collide with each other.
I got annoyed at the way the photo-electric effect was taught. It had always seemed 'obvious' that if a single photon didn't have enough energy to free an electron, then maybe two photons struck the metal at the same time.
I found out later that my hunch was correct - it's just unlikely for two photons to hit an atom at exactly the same (to within a plancks time) with a low powered laser.
While I'm on the subject of laser, another cool things about high powered lasers is that the photons can collide. If you shine two beams so that they cross paths, some photons will collide with each other and scatter. This has always fascinated me since it shows that the distinction between matter and light is a very fine one indeed.
Another cool thing about this laser is that the pulse is very short. Now because the position is being constrained (since it's a short pulse), it must mean that the momentum is very uncertain. (You cannot know the position and velocity of something at the same time). This in turn means that the laser has a whole range of wavelengths - it does not have a specific wavelength. Which, to me, makes it very un-laser-like. It's not coherent, monochromatic, etc.
>apparently the SilverLight installer is just some shady.exe. As opposed to the rest of the software on your computer that just magically appeared and runs in some otherworldly fashion, I suppose.
In linux, when you install a package, it is digitally signed so that you know that it comes from who it says it does. I think Windows has some sort of digital signing of executables, but I don't know the details.
> Galaxies travelling at > c for the early part of the universe?
Yep. We can see redshift greater than 1 (and hence 'speeds' greater than c) for very distant objects. (That's not entirely circular - there's other ways to measure distance, such as looking at pulsars etc).
There's also the background microwave radiation, whose temperature fits perfectly the predictions for an expanding matter-dominated 13ish billion year universe.
There's also nucleosynthesis. If you treat the early universe as a simple gas, you can work out the expected proportions of all the particles (neutrons, protons, electrons, helium, etc) that would have been created. Each decays at a different rate, so the proportion of these particles would depend on how long after the big bang we are. The theory fits observed data to within 1% for all the proportions for a universe that is 13ish billion years old. Again this model assumes an expanding universe, so would also predict the size to be about 26bn LY or so, currently.
There's also various other methods, but those are the main ones that I can think of. You can do lots of fancy stuff if you know you are looking at a Quasar, pulsar etc. Incoming cosmic rays also tell a story.
Now, don't take this all/too/ seriously. We know that there must be problems with the standard big bang model, as the universe isn't quite expanding at the rate we'd expect. But nucleosythesis and background radiation are so successful that any modification is going to have to be quite a gentle one.
Yes, you'll never see any light that it emits _now_. But you are seeing the light that it emitted 18 billion years ago, a time when it was moving close, but not faster than, the speed of light, relative to us.
It can seem somewhat circular, but pretty much everything in physics is. E.g.:
We know gravity exists because when we drop things, they fall. We know something will fall because gravity will pull it down. and
The electric field strength is defined as proportional to force that would be exerted on a stationary test particle. The force on a test particle is proportional to the field strength.
For cosmology, the key is that there are lots of measurements, and that they all agree with each other. There are several independent ways to measure the age of the universe, and they pretty much all agree with each other. Likewise there are quite a few different ways to measure distance, and so far they've all agreed with each other too.
The spacetime on which it sits is moving 3 or 4 times the speed of light, relative to us. Even if it was moving really quite fast, it wouldn't affect the redshift by all that much.
It means that the galaxy is 12 billion years old. Its current distance would be several times further away - possibly 90 billion light years away or so (just rough estimate, without doing the calculations).
The reason is because while the light has been travelling to us, the spacetime has been expanding, making its distance a lot larger.
In fact, the galaxy is probably moving many times the speed of light away from us currently.
Others have pointed out that the big bang happened 'everywhere'. This is correct, but it's worth mentioning that we are moving relative to it. We are moving at approximately 400km/sec relative to the initial big bang.
And yet again you make a post which contains no substance at all.
My original post was the comparison of Muslims and Christians to resort to violence. I cannot see how you can say that the death count has nothing to do with it.
My original post was stating that Christians tend to resort to violence a lot less than muslims.
You offered some examples where Christians have resorted to violence. A few of your examples were quite a while ago (KKK), and others more recent (Iraq war.)
The Iraq war is terrible, and I'm very much against it. However on the larger scale, the number of muslims killed by soliders in iraq is pretty small compared to the muslim-on-muslim deaths in the last 50 years.
I backed this up with a link, showing the number of deaths. You, of course, ignored the link while offering no evidence at all that it was wrong.
If you want to argue that Christians have killed, in recent years, as many (or even close tO) Muslims for religious reasons, than Muslims have done so to other Muslims, then try to do so with actual facts.
Spending most of your post just insulting doesn't really get your point across.
Well a negative mass is going to violate the equivalence principle anyway. How an object falls is going to depend on whether the object is made of positive or negative mass.
You've made an argument that both the inertial and gravitational mass can't both be negative. You could get perhaps around this by saying that the inertial mass is positive (so acceleration is in the direction of the force) but the gravitational mass is negative (provides a repulsive force).
Slashdot Mirror
It wouldn't be very predictable.
For that sort of thing, you are better off focussing the air to a point to break down the air and create a tiny plasma. This would look like a 'pixel' in the air.
Uh yes, I got my energy and time wrong - like you say.
> A question I would like the answer to is how long does it take for an atom to radiate a photon after being struck with a photon of sufficient energy to cause reradiation?
This is determined by the uncertainty principle as well. The uncertainty (and thus average time) is determined by the energy. The larger the energy, the larger the uncertainty in the time, and so longer before the atom decays.
You can have one photon cause an atom to excite, then a second photon to cause the atom to excite further. But this is different from two photons acting together to cause a single excitation - the difference between that it doesn't require an inbetween step to be available.
In another reply, someone said that photons 'collide' by decaying into other particles, which then collide with each other.
I got annoyed at the way the photo-electric effect was taught. It had always seemed 'obvious' that if a single photon didn't have enough energy to free an electron, then maybe two photons struck the metal at the same time.
I found out later that my hunch was correct - it's just unlikely for two photons to hit an atom at exactly the same (to within a plancks time) with a low powered laser.
While I'm on the subject of laser, another cool things about high powered lasers is that the photons can collide. If you shine two beams so that they cross paths, some photons will collide with each other and scatter. This has always fascinated me since it shows that the distinction between matter and light is a very fine one indeed.
Another cool thing about this laser is that the pulse is very short. Now because the position is being constrained (since it's a short pulse), it must mean that the momentum is very uncertain. (You cannot know the position and velocity of something at the same time). This in turn means that the laser has a whole range of wavelengths - it does not have a specific wavelength. Which, to me, makes it very un-laser-like. It's not coherent, monochromatic, etc.
>apparently the SilverLight installer is just some shady .exe.
As opposed to the rest of the software on your computer that just magically appeared and runs in some otherworldly fashion, I suppose.
In linux, when you install a package, it is digitally signed so that you know that it comes from who it says it does. I think Windows has some sort of digital signing of executables, but I don't know the details.
> Galaxies travelling at > c for the early part of the universe?
/too/ seriously. We know that there must be problems with the standard big bang model, as the universe isn't quite expanding at the rate we'd expect. But nucleosythesis and background radiation are so successful that any modification is going to have to be quite a gentle one.
Yep. We can see redshift greater than 1 (and hence 'speeds' greater than c) for very distant objects. (That's not entirely circular - there's other ways to measure distance, such as looking at pulsars etc).
There's also the background microwave radiation, whose temperature fits perfectly the predictions for an expanding matter-dominated 13ish billion year universe.
There's also nucleosynthesis. If you treat the early universe as a simple gas, you can work out the expected proportions of all the particles (neutrons, protons, electrons, helium, etc) that would have been created. Each decays at a different rate, so the proportion of these particles would depend on how long after the big bang we are. The theory fits observed data to within 1% for all the proportions for a universe that is 13ish billion years old. Again this model assumes an expanding universe, so would also predict the size to be about 26bn LY or so, currently.
There's also various other methods, but those are the main ones that I can think of. You can do lots of fancy stuff if you know you are looking at a Quasar, pulsar etc. Incoming cosmic rays also tell a story.
Now, don't take this all
Yeah, you'd think so. But almost all of the red shift occurs _as it is moving through space_.
The light has a certain wavelength. But then spacetime expands underneath, as it is moving. So the wavelength increases (red shifts).
By the time it reaches us, it's redshift indicates a speed that is pretty much equal to the current speed of the galaxy.
(And yeah, I meant 12.8 billion years)
Not even to mention that a university doesn't subscribe to a lot of the journals - particularly the specialist ones.
Yes, you'll never see any light that it emits _now_. But you are seeing the light that it emitted 18 billion years ago, a time when it was moving close, but not faster than, the speed of light, relative to us.
It can seem somewhat circular, but pretty much everything in physics is. E.g.:
We know gravity exists because when we drop things, they fall. We know something will fall because gravity will pull it down.
and
The electric field strength is defined as proportional to force that would be exerted on a stationary test particle. The force on a test particle is proportional to the field strength.
For cosmology, the key is that there are lots of measurements, and that they all agree with each other. There are several independent ways to measure the age of the universe, and they pretty much all agree with each other.
Likewise there are quite a few different ways to measure distance, and so far they've all agreed with each other too.
Just to be really nitpicky, it is 12 billions year old, but it's probably closer to 100 billion light years away at the moment (if it exists still)
The spacetime on which it sits is moving 3 or 4 times the speed of light, relative to us. Even if it was moving really quite fast, it wouldn't affect the redshift by all that much.
It means that the galaxy is 12 billion years old. Its current distance would be several times further away - possibly 90 billion light years away or so (just rough estimate, without doing the calculations).
The reason is because while the light has been travelling to us, the spacetime has been expanding, making its distance a lot larger.
In fact, the galaxy is probably moving many times the speed of light away from us currently.
Others have pointed out that the big bang happened 'everywhere'. This is correct, but it's worth mentioning that we are moving relative to it. We are moving at approximately 400km/sec relative to the initial big bang.
It's a real effect, as in it's not a con. It's not due to camera trickery or a hidden motor.
Why does that make him a jackass? He's saying that he saw it accelerate, and that as far as he could see there were no tricks etc.
And yet again you make a post which contains no substance at all.
My original post was the comparison of Muslims and Christians to resort to violence. I cannot see how you can say that the death count has nothing to do with it.
My original post was stating that Christians tend to resort to violence a lot less than muslims.
You offered some examples where Christians have resorted to violence. A few of your examples were quite a while ago (KKK), and others more recent (Iraq war.)
The Iraq war is terrible, and I'm very much against it. However on the larger scale, the number of muslims killed by soliders in iraq is pretty small compared to the muslim-on-muslim deaths in the last 50 years.
I backed this up with a link, showing the number of deaths. You, of course, ignored the link while offering no evidence at all that it was wrong.
If you want to argue that Christians have killed, in recent years, as many (or even close tO) Muslims for religious reasons, than Muslims have done so to other Muslims, then try to do so with actual facts.
Spending most of your post just insulting doesn't really get your point across.
Well at least I gave a link to back up what I said. If you feel that it is wrong, please back up your statement.
Your ideas are seriously skewed. The vast vast majority of violent deaths of Muslims is from other Muslims.
http://wasteofmyoxygen.wordpress.com/2007/10/09/staggering-statistics-on-muslims-killing-muslims/ for example for a quick run down.
Go read that, and then realise how silly it sounds to blame the white man in all this.
Well uh yes. A group that doesn't kill in the name of religion is better than a group that does. Or do you disagree with that?
Such as? What Christian specific religious laws are there that are enforced?
The difference being with Christians is that it's only figuratively speaking when you say 'up in arms'.
Hmm, good point.
Well a negative mass is going to violate the equivalence principle anyway. How an object falls is going to depend on whether the object is made of positive or negative mass.
Uh, would NOT be possible. Bah.
You've made an argument that both the inertial and gravitational mass can't both be negative. You could get perhaps around this by saying that the inertial mass is positive (so acceleration is in the direction of the force) but the gravitational mass is negative (provides a repulsive force).