# Casimir effect

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If the best vacuum that can be created on earth is not truely a vacuum, how can we be sure that the Casimir effect is caused by particles of negative mass and not particles of positive mass striking the outside of the plates?

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I don't think the Casimir effect is supposed to show negative mass, just more energy on the outside of the plates than inside?

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No, my understanding is that is shows particles of negative mass between the plates, thus causing them to move together. I have a hard time getting my head round that!

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I'm pretty sure it shows more energy outside the plates than inside pushing them together.

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According to 'The Science of Dr Who', it causes two metal plates, seperated by just a few millionths of a meter in vacuum to be pulled together slightly by the neggative pressure of exotic matter between them.

But how can they be sure that it's not, as Gaz says, external pressure?

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Did they say the negative pressure was caused by actual negative mass? I've had a quick flick through my books and the difference in pressure was explained by more/ less energy either side of the plates.

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Sorry, negative pressure, not mass. My previous post is almost directly taken from the book.

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My understanding of the Casimir effect is that the close proximity of the two plates excludes certain wavelengths of energy (those too long to fit between them) while outside the plates all wavelengths can apply pressure, hence they have a slight advantage and the plates seek to converge...

But i could be wrong!

Olly

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PS Negative pressure is a boggling concept. Doesn't it makes the universe expand?

Olly.

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Interesting to note that the gas density in all of these beautiful nebula that we image is less than the best vacuum we can produce on Earth.

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• 4 weeks later...

According to 'The Science of Dr Who', it causes two metal plates, seperated by just a few millionths of a meter in vacuum to be pulled together slightly by the neggative pressure of exotic matter between them.

Afraid that's not quite correct -- Exotic matter is something altogether different...

Ok, so firstly vacuum actually contains energy. Everyone assumes vacuum is just nothingness, but actually, according to quantum mechanics, vacuum is full of energy. I think I ranted about that in this thread when I was talking about Hawking Radiation...

The Casimir effect is a manifestation of that energy. Take two big metal plates and hold them close enough together and they'll be drawn together by vacuum energy. In essence, because the metal is conductive it alters the vacuum energy (technically speaking, the "vacuum expectation value") between them.

So in other words, it isn't really negative pressure. Actually it's pressure created by vacuum.

Which IMHO, is an even more mind-bending concept.

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My understanding of the effect is like Olly's - it can be thought of as an external "pressure" because some wavelengths are not allowed between the plates.

Don't know about Dr Who but it was fun to see it showing up in Lost as an explanation for the hatch imploding!

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