# The Naked Scientists Forum

### Author Topic: Could there be drag in a vacuum?  (Read 4227 times)

#### Supercryptid

• Hero Member
• Posts: 606
##### Could there be drag in a vacuum?
« on: 12/03/2009 04:12:46 »
Classical wisdom holds that a vacuum offers no resistance to the movement of objects travelling through it. Afterall, if it contains no air, water, or solid particles, then why should it?

Then I thought of a possible scenario where movement in a vacuum might produce drag. The Casimir Effect may give insight into this:

In view of quantum mechanics, the vacuum is portrayed as being filled with virtual particles that flicker into and out of existence in fractions of a second. In Figure 1, these vacuum flucuations exert a pressure on a stationary metal plate. However, since the pressure on both sides of the plate are equal, there is no net force acting on the plate.

In Figure 2, we have two metal plates that are placed closely together. The vacuum pressure on the outside of the plates remains normal. Between the plates, things are different. The fact that there is a limited amount of space between the plates means that fewer virtual particles exist between the plates than on the outside of them. Thus, the pressure between the plates is lower than that on the outside of them and they are pushed together. This is called the Casimir Effect. The existence of this effect proves that vacuum fluctuations can exert a net force when one part of the vacuum contains more energy than another part of it.

Finally, we have Figure 3. In this figure, a metal plate is moving forward through a vacuum at some ungiven speed. This is where the Doppler Effect comes into play. From the view of the metal plate, the virtual particles in front of it will appear to be blue-shifted because it is moving towards them. The virtual particles behind it will appear to be red-shifted because it is moving away from them. When a particle appears blue-shifted, it will appear to have a shorter wavelength and therefore more energy. The opposite is true of red-shifted particles.

Since the virtual particles in front of the plate have more energy than the particles behind the plate, then they should produce a net force acting against the direction of movement (i.e. drag).

If this is the case, then why hasn't it been discovered yet? Perhaps the effects of vacuum drag are so small that our instruments are not sensitive enough to detect them? Afterall, the Casimir Effect is exceedingly weak on our every day scale of things.

Or perhaps vacuum drag has been discovered.

The spacecraft Pioneer 10 and Pioneer 11 have been measured as slowing down faster than they should be taking known forces acting on the craft into consideration (such as the Sun's gravity). This has been dubbed the Pioneer Anomaly. Could this slow-down be caused by vacuum drag?
« Last Edit: 12/03/2009 04:15:58 by Supercryptid »

#### Vern

• Neilep Level Member
• Posts: 2072
##### Could there be drag in a vacuum?
« Reply #1 on: 12/03/2009 12:55:52 »
Quote from: Supercryptid
In Figure 2, we have two metal plates that are placed closely together. The vacuum pressure on the outside of the plates remains normal. Between the plates, things are different. The fact that there is a limited amount of space between the plates means that fewer virtual particles exist between the plates than on the outside of them. Thus, the pressure between the plates is lower than that on the outside of them and they are pushed together. This is called the Casimir Effect. The existence of this effect proves that vacuum fluctuations can exert a net force when one part of the vacuum contains more energy than another part of it.

Yes; I suspect there could be a slight drag in the vacuum. However I would not attribute it so quickly to the Casimir Effect. Your statement that the existence of the effect proves the vacuum fluctuations concept is a little assumptive. It is evidence for the fluctuations; it shows that there is something operating there. But empty space is full of electromagnetic forces. We can detect some of that force; but nowhere near all of it. So I'm withholding judgement; maybe the vacuum fluctuations are simply electromagnetic phenomena.

Edit: Maybe I should have said that the drag in space could be attributed to the Casimir effect; it is just that the root cause of the Casimir effect need not be vacuum fluctuations.
« Last Edit: 12/03/2009 12:58:51 by Vern »

#### The Naked Scientists Forum

##### Could there be drag in a vacuum?
« Reply #1 on: 12/03/2009 12:55:52 »