Quick update today; just wanted you to know that some qualitative discussion of energy and momentum appears in this new article, to which I will add a discussion of mass over the next day or two. The article also has a bit more about Emmy Noether, the mathematician whom I wrote about yesterday.
Mass and Energy Article Taking Shape
POSTED BY Matt Strassler
ON 03/28/2012
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3 Responses
Thanks Professor Strassler. Much appreciated.
Professor Strassler,
I appreciate this public project. I am a lay person, but have many questions about particle physics. I don’t use facebook or twitter. Where can I ask them? Can I do so on this website or do you have an email address?
Three questions I have are:
what is the difference between a real photon and a virtual photon?
how fast do quarks move?
Are photons and gluons the only particles that are massless?
You can ask questions here, though the website already will answer many of your questions if you look around a bit.
For example, there is a whole article on “virtual particles”: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/
How fast do quarks move? As with electrons or any other massive particle, they can move at any speed as long as it is slower than the Einstein’s universal speed limit (often called `the speed of light’.)
The only *known* particles which are believed to be massless are photons and gluons, and (possibly but unlikely) one of the three types of neutrinos and anti-neutrinos. Gravitons (the particles which are to the gravitational force as photons are to the electromagnetic force) are widely *believed* to exist, though it may be centuries before they are observed. They too (if they exist) are massless. It is possible that there are other types of massless particles in nature that interact so weakly with ordinary matter that we have not yet discovered them yet.
I should add that because gluons are only observed inside of hadrons (composite objects made from gluons, quarks and anti-quarks), verifying that gluons are strictly massless is experimentally impossible; all we can say for certain is that they are very much lighter than the proton mass, that we have no reason to think they are massive at all, and that we have a very good theoretical reason to think they are massless. But experimentally it would be very hard to tell the difference.