# BEC Propulsion?

1. Aug 21, 2007

### Yum Yum

This is kind of a wierd question, so I'll just throw it out there. So I was talking to my mate about BEC a couple of weeks ago and he proposed an interesting idea. I was telling him how BEC was used to slow down light to 38mph, what he suggested was using this concept to move a solar sail. So my question is, would the acceleration gained by light passing through the BEC have a greater impact on the solar sail than the idea of light propulsion already does? I have no idea about things on this kind of level so it would be interesting to know people's opinions on this as BEC is interesting enough as it is.

2. Aug 22, 2007

### lightarrow

That way of moving a solar sail would be less efficient than an absorbing body, which is, in turn, less efficient than a solar sail: in the first case the body acquire the light's momentum, in the second, the double of it. With a BEC, it would acquire almost the momentum of the first case.

3. Aug 22, 2007

### Yum Yum

Hmm, so light traveling at a constant speed would have a greater effect on the sail than light being slowed down by the BEC?

4. Aug 23, 2007

### lightarrow

No, the problem is different: with a solar sail you have light reflected back from the sail, so the momentum gained from the sail is 2p where p is the initial light's momentum: since light is reflected, its momentum change from +p to -p, ; for the momentum conservation's law, the total momentum must be the same before and after reflection, so the sail must acquire a momentum p:

p(sail) + p(light after reflection) = light's momentum before reflection = p
that is:
p(sail) + (-p) = p --> p(sail) = 2p.

In the case of total light's absorption:

p(sail) + p(light after absorption) = light's momentum before absorption = p
that is:
p(sail) + 0 = p --> p(sail) = p

In the case of partial light's absorption, without reflection, so that the other (non absorbed) part of light goes through the semi-transparent body without interacting with it, we have:

p(body) + p(light after partial interaction) = p
that is:
p(body) + x*p = p, where 0 < x < 1 --> p(body) = p(1 - x) < p.

The case you propose, with BEC, should be considered as the last I wrote.

You can yourself compute the case of partial absorption and partial reflection.