[Ron Hughes (OP)]

I've looked online but have been unable to find the answer.

[graham.d]

We covered this a lot in the discussion:

http://www.thenakedscientists.com/forum/index.php?topic=28617.0

a topic also started by you, Ron.

It seems it is a subject that is quite deep and not wholly understood. I did a fair bit of research (well, as much as time allowed) and noted that there was quite a bit of disagreement amongst physicists on the subject.

[Ron Hughes (OP)]

I am not referring to an accelerated particle. I'm talking about a charged particle traveling near C with respect to an observer.

[Soul Surfer]

In truly empty space no, a particle does not lose any energy by emitting radiation however, in our universe the cosmic microwave background represents a potential source of interactions and energy loss if the particle has a high enough energy.  This sets a predicted upper limit on the energy of cosmic rays that could travel a long distance across the universe because they would lose energy in their interactions with CMB photons.  This limiting energy is way above the highest energies that are ever likely to be achieved by man made accelerators.

[Ron Hughes (OP)]

Since there is no place in the Universe where radiation does not exist then charged particles in motion with respect to an observer probably do lose some energy as very long wavelength radiation. As you already know, a charged particle will emit radiation wither it loses or gains energy with respect to an observer.

[graham.d]

Sorry Ron. I opened your post but then forgot the heading which was your question.

A note to the web designers: it would be useful to still show the topic header after you open the topic.

To reply to your question, I agree that without interactions with em fields, or other fields, there would be no radiation - at least I can't think of a mechanism.