An atom in space

[pHoToN_gUrL]

I think they mean this, correct me if i'm wrong:

You put the atom in space. There are also photons in space, photons are packets of energy. Photons can exerct a force (known as raditation pressure) on the atom. Depending on the original motion of the atom and detuning of the laser with respect to resonance (what does detuning of the laser with respect to resonance mean??) the atom can either speed up or slow down. But there is also back ground radiation which will affect the motion of the atom.

 

so question is:

if the atom is originally motion less (as TimbaLanD said), and if the atom can only speed up or slow down depending on it's original motion, then, does it mean that the atom will speed up since originally it was at rest?

also, what happens after the atoms speeds up? will the velocity of the atom continue to speed up and slow down based on the instanteneous velocities? will the atom ever come to rest again (may be at equilibrium) ?

[swansont]

I think they mean this' date=' correct me if i'm wrong:

You put the atom in space. There are also photons in space, photons are packets of energy. Photons can exerct a force (known as raditation pressure) on the atom. Depending on the original motion of the atom and detuning of the laser with respect to resonance (what does [u']detuning of the laser with respect to resonance[/u] mean??) the atom can either speed up or slow down. But there is also back ground radiation which will affect the motion of the atom.

 

so question is:

if the atom is originally motion less (as TimbaLanD said), and if the atom can only speed up or slow down depending on it's original motion, then, does it mean that the atom will speed up since originally it was at rest?

also, what happens after the atoms speeds up? will the velocity of the atom continue to speed up and slow down based on the instanteneous velocities? will the atom ever come to rest again (may be at equilibrium) ?

 

Actually two scenarios are described, since background radiation is a continuum and lasers are generally in a very narrow frequency band. An atom at rest will start moving once it scatters a photon, just as any collision would. The laser detuning issues are more geared toward specific manipulation of atoms; you can get them to do some pretty interesting things under specific conditions (like the Bose-Einstein condensate that Rocket Man mentioed)

 

"detuning of the laser with respect to resonance" means that instead of describing the photon with an absolute frequency, which would be some very large number, the frequency is being described relative to some absorption peak, which has a maximum (or resonance) at some value. The absolute frequency in this case, at ~780.24 nm, is around 3.85 x 10¹⁴ Hz, which makes it awkward to discuss changes on the level of 1 MHz or so.

 

It's also often experimentally difficult to measure/generate the frequency directly like that, and much easier to set up a laser to be offset from the absorption feature by several MHz. You can tune a laser until a sample of material absorbs the light (and set it up so you are at the peak of that absorption), which tells you that you are on resonance without measuring the exact value. There are some distinct differences in electronics that drive circuitry in various frequency ranges, so frequencies corresponding roughly to radio (MHz), microwave (GHz) millimeter (THz) and visible waves (100's of THz) all have different issues, and generally more difficulty as the frequency gets higher. You also have issues of trying to change the frequency of light by those various amounts. So mixing in a MHz-sized signal is a lot easier than directly creating a precise one in the 100 THz range.

[DaveC426913]

You will not practically be able to place the atom in space with no motion. It will have kinetic energy while you are holding on to it. The only way you could remove the kinetic energy would be to cool it to 0K, which is impossible.

[Neil9327]

Question: In deep space (between stars but within the milky way, but away from stellar clouds etc) I guess there are a few scattered atoms. What is the average distance between atoms there.

[insane_alien]

a metre or two

[5614]

Question: In deep space (between stars but within the milky way, but away from stellar clouds etc) I guess there are a few scattered atoms. What is the average distance between atoms there.

Well, the critical density of the universe is about five hydrogen atoms for every cubic meter*.

 

Now whether the average density is above, below or exactly that figure is still unknown, but it's not going to be vastly vastly different. So it gives you an idea.

 

However this is an average for the entire universe. Within a planet it is, relatively, very dense. Even a solar system is, relative to the universe, very dense.

 

"Using the average density of the Earth, or the solar system, or even the Milky Way galaxy as an indicator [of density] for that of the whole universe would be like using Bill Gate's net worth as an indicator of the average earthling's finances... there is a lot of nearly empty space between the galaxies that drastically lowers the overal average matter density"*

 

So back to your question. You ask between stars but within the Milky Way. So I'd say it's about critical density, as a massive guess! So about 5 atoms per cubic meter.

 

Within a solar system with many plannets it is much more. Between galaxies it is much, much less. Hope that answers the question, approximately!

 

*source and quote; The Elegant Universe by Brian Greene

[Neil9327]

No sorry I don't think I phrased my question correctly. I am asking what is the density actually at a random location in deep space, not including the stars in the calculation. So if you take a meter cube in deep space, how many atoms will there be in it on average?

 

Of course since nothing man-made has been outside the solar system I guess this is hard to know. But I would have thought the figure would have been lower for space between stars than for space in our solar system. And higher than between galaxies.

 

Or to pose another question: In the most empty part of the universe, how far away is the nearest atom?

[Klaynos]

Of course since nothing man-made has been outside the solar system I guess this is hard to know. But I would have thought the figure would have been lower for space between stars than for space in our solar system. And higher than between galaxies.

 

IIRC this is supposed to be true.

 

insane_alien's answer sounds about right too...

[DaveC426913]

"An increasingly large number of observations consistently reveal the existence of a much larger amount of intergalactic matter than presently accepted. Radio signals coming from directions between galaxies is discussed. An average density of matter in space of about 0.01 atom/cm3 is derived."

http://www.newtonphysics.on.ca/UNIVERSE/Universe.html

 

That's about 10 atoms per litre.

[Neil9327]

"An increasingly large number of observations consistently reveal the existence of a much larger amount of intergalactic matter than presently accepted. Radio signals coming from directions between galaxies is discussed. An average density of matter in space of about 0.01 atom/cm3 is derived."

http://www.newtonphysics.on.ca/UNIVERSE/Universe.html

 

That's about 10 atoms per litre.

 

So if mankind does find itself able to produce a spaceship that can travel at, say, a quarter of the speed of light, I wonder what effect these atoms would have on drag - there would need to be a constant power applied to the craft to overcome this.

 

Also since these are unlikely to be still, I wonder whether they could be used to propel craft along? probably not.

[5614]

That's about 10 atoms per litre

1) This is not certain, it is an estimate. If the actual average density of the universe was known then the future of the universe (ie. big crush vs expand forever) would be known. However we do not know the average density of the universe, although there are many estimates and calculations which seem to vary between 5 and 10 atoms per meter cubed.

 

However in a star you're getting billions and billions of atoms per m³, so in deep space the density must be significantly lower to average out the relatively massive density of the stars/plannets.

 

If you think that there are [math]10^{23}[/math] atoms in just 12 grams of carbon-12, the density in deep space must be very, very low.