# An atom in space

## Recommended Posts

What will happen if you drop an atom in space? Will it stay put or start moving?

##### Share on other sites

Unless it was at absolute zero atoms move don't they?

##### Share on other sites

I would imagine the chances of you placing it in absolute equilibrium are almost zero.

##### Share on other sites

well it'd be acted on by some gravity....

##### Share on other sites

Yeah, and as long as that gravity force isn't countered by an exactly opposite force, it will move.

##### Share on other sites

What will happen if you drop an atom in space? Will it stay put or start moving?

Objects in motion tend to stay in motion. You are 'dropping' the atom, so I assume the atom would start with some motion. Since space is relatively frictionless, the atom would continue to move.

##### Share on other sites

Objects in motion tend to stay in motion. You are 'dropping' the atom, so I assume the atom would start with some motion. Since space is relatively frictionless, the atom would continue to move.

Until it hit something, like another atom or a photon. Then it would move with a different velocity.

##### Share on other sites

So, it can’t really float in space?? Say we place it in space with no initial motion to begin with. Will it start “accelerating” toward where it can find gravity?

##### Share on other sites

For it to accelerate there must be some force present, with out a force it won't accelerate, n since the original velocity is zero, assuming u placed it in space with no initial motion, the atom would just stay in it's place until there is a force. The atom would accelerate if the force of gravity is present. But since u r asking "will it accelerate toward where it can find gravity?", then i suppose u mean the force that causes the initial motion n furthur acceleration is not gravity. I think it is possible as long as some other force would act on the atom...like electromagnetic

##### Share on other sites

what is absolute zero?

##### Share on other sites

Unless it was at absolute zero atoms move don't they?

Temperature and the other thermodynamical quantities are defined in the limit of a huge amount of particles. The construct called Thermodynamics doesn´t apply to single atoms with definite position and velocity.

As for the question: The atom will move according to the movement equations. If there´s no force, then it will remain in its current state of movement (for example at rest). If there´s any kind of forces working on it (like a cloud of gas giving gravitational attraction), then it will accelerate as a result of the force.

##### Share on other sites

Temperature and the other thermodynamical quantities are defined in the limit of a huge amount of particles. The construct called Thermodynamics doesn´t apply to single atoms with definite position and velocity.

As for the question: The atom will move according to the movement equations. If there´s no force' date=' then it will remain in its current state of movement (for example at rest). If there´s any kind of forces working on it (like a cloud of gas giving gravitational attraction), then it will accelerate as a result of the force.[/quote']

Oh, I see. Also, isn't there always background radiation that would cause the atom to move some?

##### Share on other sites

if you have an atom sitting in space, you will know its velocity to be 0...

if you know it's velocity, then the uncertainty principle will say that you can't know its position.

the background radiation keeps space at 2-3 degrees kelvin.

so it would heat multiple particles to 2-3 K, but how does this effect single particle systems? if it moves, you could take a different reference frame and remove all heat from it.

or does the uncertainty principle help in this matter.

##### Share on other sites

if you have an atom sitting in space' date=' you will know its velocity to be 0...

if you know it's velocity, then the uncertainty principle will say that you can't know its position.

the background radiation keeps space at 2-3 degrees kelvin.

so it would heat multiple particles to 2-3 K, but how does this effect single particle systems? if it moves, you could take a different reference frame and remove all heat from it.

or does the uncertainty principle help in this matter.[/quote']

You got me!

##### Share on other sites

if you have an atom sitting in space' date=' you will know its velocity to be 0...

if you know it's velocity, then the uncertainty principle will say that you can't know its position.

the background radiation keeps space at 2-3 degrees kelvin.

so it would heat multiple particles to 2-3 K, but how does this effect single particle systems? if it moves, you could take a different reference frame and remove all heat from it.

or does the uncertainty principle help in this matter.[/quote']

The atom would end up in thermodynamic equilibrium with the photons, and be moving with some speed. That atom itself won't have a temperature. You could get a prediction of the speed by knowing the Maxwell-Boltzmann distribution, but all that will tell you is the probability of having a particular speed at any given time.

##### Share on other sites

The atom would end up in thermodynamic equilibrium with the photons, and be moving with some speed. That atom itself won't have a temperature. You could get a prediction of the speed by knowing the Maxwell-Boltzmann distribution, but all that will tell you is the probability of having a particular speed at any given time.

hmm...

a generally defined speed in any given direction...

could the velocity of the objects creating the background radiation give a "relative" to have veloticty to?

i would otherwise have difficulty defining the velocity, even if it's a probability.

this comes to mind after doing some research on the bose condensate, it was acheived using minute doppler shifitng on lazer light, any velocity the particles had would shift the light infront to a higher energy relative to it, so the radiation pressure applies a net hindering force no matter which direction it travels at.

so if the radiation is coming in from all directions with an even spread (ideal situation) the doppler effect would slow the particle to an almost complete stop. im guessing that a real scenario would have the particle move away from the most intense light source to hit an equilibrium with the wavelengths and intensities. (likely to be quite fast within the galaxy)

##### Share on other sites

hmm...

a generally defined speed in any given direction...

could the velocity of the objects creating the background radiation give a "relative" to have veloticty to?

i would otherwise have difficulty defining the velocity' date=' even if it's a probability.[/quote']

The velocity always has to be measured against some reference system. You can use the rest frame of the CMB, or some other frame.

this comes to mind after doing some research on the bose condensate' date=' it was acheived using minute doppler shifitng on lazer light, any velocity the particles had would shift the light infront to a higher energy relative to it, so the radiation pressure applies a net hindering force no matter which direction it travels at.

so if the radiation is coming in from all directions with an even spread (ideal situation) the doppler effect would slow the particle to an almost complete stop. im guessing that a real scenario would have the particle move away from the most intense light source to hit an equilibrium with the wavelengths and intensities. (likely to be quite fast within the galaxy)[/quote']

But the Doppler cooling relies on the light being detuned from a resonance, on the "red," or lower energy, side. If you are blue detuned, you induce heating. Interactions with the CMB are going to be off-resonant interactions anyway, so this isn't going to apply.

And also, Doppler cooling only gets you down to temperatures that are typically a reasonable fraction of a milliKelvin. Polarization gradient cooling gets you colder (it takes advantage if the structure of the electron states) and the final step of BEC is evaporative coolong in a magnetic trap.

##### Share on other sites

surely you could do much the same thing with blue light, i think they only chose red because rubidum doesnt interact well with wavelengths around a small band of red light.

the system would be very ineffective with randomly chosen wavelengths.

radiation pressure would be a net force. it would have an equilibrium at some velocity.

howeverthe background radiation would give a very random input to the equation

##### Share on other sites

surely you could do much the same thing with blue light' date=' i think they only chose red because rubidum doesnt interact well with wavelengths around a small band of red light.

the system would be very ineffective with randomly chosen wavelengths.

radiation pressure would be a net force. it would have an equilibrium at some velocity.

howeverthe background radiation would give a very random input to the equation[/quote']

Red and blue in my context is relative, just telling you on which side of the resonance you are. "Tuned to the red" (or blue) is standard jargon for this. When trapping atoms (molasses or MOT trapping), you typically tune a linewidth or two away from resonance; in the case of Rb, the linewidth is ~5 MHz, which represents a very small fraction of a nm change in wavelength. However, if you were to tune the light a linewidth away from resonance in the opposite direction, it would cause heating; as the atom speeds up (over some range of speeds), the light gets Doppler shifted closer to resonance, which increases the scattering rate. The light is still red/infrared, in the case of Rb (~780 nm).

##### Share on other sites

ah, right, so a red-shift could apply more force than a blue-shift depending on the atom because it has a sort of response curve (forgive the acoustic terminology)

so basically, there is no way you can acheive equilibrium because the velocity will determine it's response to certain wavelengths, when it changes velocity it responds much differently to the same light. interesting...

thanks for clarifying.

##### Share on other sites

ah' date=' right, so a red-shift could apply more force than a blue-shift depending on the atom because it has a sort of response curve (forgive the acoustic terminology)

so basically, there is no way you can acheive equilibrium because the velocity will determine it's response to certain wavelengths, when it changes velocity it responds much differently to the same light. interesting...

thanks for clarifying.[/quote']

The force is the same for the same magnitude of detuning, since the response curve is (locally) symmetric (a Lorentzian). But the slope is opposite on the other side of the resonance, so the sign of the feedback changes. Instead of negative feedback, damping the speed to zero, you get positive feedback, speeding the particle up.

##### Share on other sites

and in english please... come on, one of you should be able to put this in simple terms...

##### Share on other sites

and in english please... come on, one of you should be able to put this in simple terms...

Ummm, those are simple terms. Resonance, feedback. You'll have to be a little more specific about what you don't understand, because I don't have the time to start at physics 101 and go from there.

Photons carry momentum. So scattering photons (absorb from one direction, ejecting them symmetrically) exerts a force (radiation pressure) on atoms. Their ability to absorb atoms, or the rate at which they absorb them, depends on the response curve (absorption vs wavelength). As I explained, you can either slow atoms or speed them up (cool or heat) depending on the original motion of the atoms and the detuning of the laser with respect to resonance.

## Create an account

Register a new account