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Virtual Photon Question



Dear Fermilab (or to whom this may be going to),

Hi. I am 14 years old and I happen to be reading a physics book when I came across something called Virtual Photons and the uncertianty principle. The book does not explain what Virtual Photons are to well, and all I know about Photons is that they could be a wave or matter. If you could help me about what Virtual Photons are and Photons, that would be a great help.

Light is electromagnetic radiation. It is pure energy. It is only part of the electromagnetic radiation spectrum - that which you can see with your eyes. There are other types of electromagnetic radiation with lower energy (such as radio waves) and higher energy (such as X-rays) that humans hae learned to detect with different machines. Photons are what you call light when it acts like a particle. Most of the time you see light acting as a wave. Very high energy light though like X-rays acts more like a particle with a small size. In other words, the photons can bounce (scatter) off each other or different particles like billiard balls for example.

You see it is hard to understand virtual photons if you think of photons as rays of light. Virtual photons pop up when something emits a photon which is almost immediately absorbed by another particle of some kind. This relates to the uncertainty principle as such: Becuase the time of the photon's existence was so short, you really did not have a chance to measure its energy very well. Even though the momentum has to be enough to balance the reaction becuase conservation of momentum has to hold, the energy may not be what you expect. Becuase there is an equation that relates these to the mass for a particle:

Energy*Energy = momentum*momentum + mass*mass

If the energy is different, the mass will be different. You would think from reading your physics book that the photon has zero mass, but if it virtual, it effectively has a mass because of this fluctuation of energy in a short time.

Also uncertianty principle, the book explains it, but not to well. From what I have read the uncertianty principle, means that you can't make a exact determination of energy and time at the same time, because energy keeps changing, it's never the same. Is that right, becuase I am not sure exacltly what the book is saying.

The Heisenberg uncertainty principle applies also to other appropriate pairs of variables (known as canonical variables) such as linear momentum and distance. This may be easier not confusing the use of time in two different ways. You can not make simultaneous measurements of linear momentum and of distance both to arbitrary precision. If the measurements are made at substantially different times or if you are satisfied with only one measurment to arbitrary precision then this is ok. If you can get your teacher to show you a picture of the Dirac delta function and the plane wave (e^(ikx)) as graphs then you could see the difference a probability distribution localized in space (is large only at one point) and one that is spread out over all space.

One last thing, I have theroy that other dimensions are around you all the time, but are on the bottom of the space and time dimension. Like if you keeps making a particle smaller you are still in the space and time dimension, but if you anilate a particle tor maybe some parts of the partcle enter anthor dimension under our dimension that we are in. However, maybe not all the part of the particle that was anihalted were put into the same dimenison, but certin ones were filtered into other dimension. I have heard, that when particles collide, other particles come out of nowhere. I was wondering if particles, like in a atom electrons exchange, exchange for one and other. Also what is between dimensions. Thanks for the help.

Actually, these are great questions and ones I don't think have been answered very well at all. There is a theory called string theory which implies that there are really 10 dimensions but the other 6 (besides 3 space and 1 time) are curled up so tight that they are much smaller than an electron or a nucleus.

I wouldn't say "under" though. The dimensions have both positive(+) and negative (-) values so maybe what you mean is they are very near the zero of the dimension. When particle and antiparticle collide (of the same type) then they annihilate and new particles are formed but they must also be particle-antiparticle pairs unless they are their own antiparticle. If the particles are the same exact kind then yes they exchange. (ie you don't know which one is which when they are close) This actually affects the value of energy levels in the atom for example through the exchange energy contribution as you seem to point out. Dimensions can be conceptualized as straight lines at 90 degrees (right angles) to each other so I don't really think asking if there is anything "between" dimensions makes sense. The annihilation idea is great. Maybe it will inspire someone perhaps yourself to investigate it more and find the answer.

-Bryan Heller

All excellent questions, Brian. Please write back if you have any more or want some better explanations.

Glenn Blanford
Fermilab Public Affairs

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last modified 4/24/1997