# Could Dark Matter be mass in EM fields?

1. Mar 7, 2008

### warwickphys

Didn't Einstein say that mass and energy were interchangeable quantities?

Electromagnetic waves transport energy - Does this therefore mean they also transport mass?

Could, therefore, all the radiation coming out of every decaying atom in the universe, every star and every accelerating charge (gaining kinetic energy) not cause some large scale gravitational effects that could explain dark matter?

I also thought that this could mean in places where different EM waves from different star systems happened to superimpose there could be greater mass. The amount of dark matter in a galaxy would be proportional to the number of stars in it then I suppose, and things like E=hf mean x-rays would be "heavier" etc...

2. Mar 7, 2008

### warwickphys

12 views? someone say something! even if its just to point out why you think this is a stupid question.

3. Mar 7, 2008

### Wallace

Relativity provides a unified theory for mass and energy as viewed by different observers, but that doesn't mean that E=Mc^2 is a blank cheque on interchanging mass and energy in any arbitrary way.

No.

Radiation does gravitate in the sense that radiation causes curvature of space-time just like any other energy (where energy here includes all types of energy, including mass). However this effect is already included in calculations (and it almost always too small to be significant), so radiation cannot be dark matter.

4. Mar 7, 2008

### warwickphys

alright...i stand corrected...

5. Mar 8, 2008

### pam

Electromagnetic energy is included in the considerations that imply dark matter.

6. Mar 8, 2008

### AstroRoyale

Yes, that is correct, EM fields are considered when addding up the energy budget. The energy density of EM fields is proportional to the square of the fields themselves, for a magnetic field, $$U=\frac{B^2}{2\mu}$$ and for an electric field the energy density is $$U=\frac{\epsilon E^2}{2}$$. In the interstallar medium of the Milky Way and most other "normal" galaxies, the magnetic field strength is around $$1\mu G$$ i.e. really small, so it really is pretty insignificant. On large scales the galaxy is very neutral, so no electric fields are present.

7. Mar 10, 2008

### Chronos

Impressive post AstroRoyale. You appear to be well informed.

8. Mar 10, 2008

### AstroRoyale

Well thank you!