# Perfect vacume ?

1. Apr 27, 2012

### robert nies

Is it possible to create a perfect vacume, or is it like trying to attain the speed of light,mainly that the amount of energy required to propel you a little faster as you approach that speed grows expotentially as your gain in speed grows less. To draw an analogy like the the carrot and the stick.

2. Apr 27, 2012

### Drakkith

Staff Emeritus
It is possible for a perfect vacuum of finite volume to exist, but unless you are out in deep space the distance between particles is still very very small. Man made "perfect" vacuums are effectively impossible except on exceedingly small scales.

3. Apr 27, 2012

### jambaugh

Not possible since you'd have to get the volume to absolute zero. (Any temperature gives you a "photon gas" and even a wee bit of other particles created in particle +anti-particle pairs.

One can however get effectively close enough not to matter, so that the probability of the number of particles being greater than zero is arbitrarily large. But at this scale of extremes one must work in terms of probabilities and one is stuck with the same sort of asymptotic limit.

Note that Drakkith's comment on "exceedingly small scales" has its own problems because of quantum uncertainties.

This is one of those cases where Zeno's infinite steps really is relevant. To reduce the amount by a half requires twice the effort (at least) so the infinite fractional sequence can't be consolidated into an accomplishable finite step.

4. Apr 27, 2012

### Drakkith

Staff Emeritus
Just to be clear, by "vacuum" I am referring to an area of space devoid of just matter, so photons wouldn't matter in this context.

Last edited: Apr 27, 2012
5. Apr 28, 2012

### jambaugh

Where you have photons in an open ended thermal spectrum you can have pair creation and annihilation, i.e. honest to goodness fermionic gas.

Thus you could never state that there is exactly zero probability of finding an electron (or a positron) inside a box. But I'm pushing at extremes with that point. A more realistic issue is that you cannot bound a region without either extreme fields which may invoke pair production or using conventional matter off which the thermal photons might kick a few electrons.

But why exclude photons? They carry energy and will contribute to the net mass of the box they're within. They exert pressure. I think you're being a "statist" discriminating against particles on the basis of their statistics! (Or maybe your a "spinist"?) Don't make me call the ACLU!

6. Apr 28, 2012

### Drakkith

Staff Emeritus
Lol. I just don't want to bog the OP down considering that he probably knows very little about pair creation and annihilation and all that.

7. Apr 28, 2012

### Redbelly98

Staff Emeritus
Every material has a vapor pressure, and a vacuum chamber must be made of some material. So no, a perfect vacuum is not possible.

8. Apr 28, 2012

### mrspeedybob

It's not like trying to attain the speed of light though. Attaining the speed of light is impossible in principal. A perfect vacuum is impossible for now in practice, but possible in principal.

If the universe continues expanding matter and energy will become more and more diffuse. Eventually every elementary particle will be alone in it's causal patch. At this point there will be large volumes of absolute vacuum.

9. Apr 28, 2012