# Is there anything such as antigravity?

1. Oct 2, 2004

### Aki

Is there anything such as antigravity?

2. Oct 2, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums Aki!

It rather depends on what you mean by 'anti-gravity'.

In one sense, bouyancy could be thought of as anti-gravity.

If you are thinking of an analogy with electrical charges (like charges repel, unlike charge attract), or a bar magnet, then the answer is no; there is no experimental or observational result which suggests that there are forms of mass which can repel each other.

You might think that atoms of anti-matter would show anti-gravity? (for example, anti-hydrogen, made of an anti-proton and a positron, would 'fall up' in a vaccuum tube here on Earth) Well, no; although this hasn't been observed yet (anti-hydrogen is very difficult to make!), the limited results on the gravitational effect of anti-matter are consistent with the theory - anti-matter 'falls down'.

3. Oct 2, 2004

### geometer

Special Relativity predicts a kind of anti-gravity. Special Relativity requires that the vaccuum have a negative pressure equation of state, given this, a positive cosmological constant will act to cause a large scale repulsion.

4. Oct 2, 2004

### Gonzolo

I think you mean General Relativity.

5. Oct 2, 2004

oops - yes!

6. Oct 2, 2004

### Mk

An exotic form of matter would, in the spaceitme-is-a-sheet analogy be on top of a hill instead of in a little hole. How is there negative pressure in a vacuum?

7. Oct 4, 2004

### Aki

what is negative pressure? I've always had trouble understanding it

8. Oct 4, 2004

### Nereid

Staff Emeritus

9. Oct 5, 2004

### Aki

thanks for ths site. Um...I already have a question on the first sentence; "Recently two different groups have measured the apparent brightness of supernovae with redshifts near z = 1." What is that "z"?

10. Oct 5, 2004

### Nereid

Staff Emeritus
redshift; z = (observed wavelength - 'source' wavelength)/('source' wavelength).
http://www.wordiq.com/definition/Redshift

Last edited by a moderator: Apr 21, 2017