# What does a proton look like?

1. ### stonehaven

3
Electrons are not really stationary in an atom, the revolve around the neucleus (protons and neutrons). Heres a question that I would like to ask.

We theorize the structure of atome pretty good, as in, the protons and neutrons bunched togeather in the center with the electrons revolving around. But do we have any ideas on what a proton particle looks like? List any links if you know any, thannks.

### Staff: Mentor

Protons and neutrons are believed to be composite systems of three quarks. http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html

Structure Evidence from Deep Inelastic Scattering
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/scatele.html#c1

It is essentially meaningless to ask "what a proton or neutron looks like", since what something looks like is simply a consequence of what the eye perceives from photons scattering from atoms. At the subatomic level, the boundaries are not necessarily as definite as our eyes perceive in the 'observable' reference frame.

3. ### Hurkyl

16,089
Staff Emeritus
I wanted to object to this. The electron doesn't look like a point particle revolving around the nucleous: it's smeared out across its entire orbital! It really is stationary in the sense that it doesn't change over time. (Sort of like a stationary current)

4. ### Bannon

15
I am pretty sure protons would not emit any visible light due to its high energy level. I believe if any radiation emits from it..the electromagnetic fequency would be to high for the eye to perceive, unless it were the pain of your retna burning up..

5. ### chroot

10,426
Staff Emeritus
Bannon,

For starters, single particles don't emit photons, because they have no energy states. Only composite particles (like atoms, or the nucleus of an atom) have energy states.

Some nuclei definitely do emit electromagnetic radiation -- in the X- or gamma-ray part of the spectrum.

- Warren

### Staff: Mentor

The term X-ray is reserved for the EM radiation coming from electrons in the K or L shells of the atom, that is the photons have sufficient energy to penetrate.

The term gamma-ray is reserved for EM radiation from the nucleus and subatomic particles.

Visible light energy is in the range of 2 - 3 eV.

Brief overview of EM spectrum - http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html

0
There is also evidence that ~5% of magnetic moment of proton results from presence of strange quark--thus it is more complex than (uud) quarks = proton--see these links:http://focus.aps.org/story/v16/st7
http://prl.aps.org/abstract/PRL/v95/i9/e092001

Also is the presence of the "proton sea"--see this from Science:

PARTICLE PHYSICS: Exploring the Proton Sea
Science; 1/22/1999; Watson, Andrew

Recent studies probing deep inside the proton are revealing much more than the expected three quarks and the gluons holding them together. Physicists are finding a churning and bubbling sea of "virtual" particles that pop into existence for an instant, then disappear again, bathing the more enduring components in a quantum flux. The more researchers study this sea, the more surprises it throws up, but charting it is important for future experiments: The world's most powerful particle accelerator, the Large ...

8. ### marlon

THIS is what a proton, or neutron looks like when you operate on them.

You always have three quarks that interact with each other via the strong force by emitting and absorbing gluons. In between these three quarks you have a gazillion of virtual quark/anti-quarkpairs (the socalled dynamical quarks) that pop up and die shortly after.

These quarks are responsible for the fact that the protonmass is BIGGER than the sum of the three quarkmasses. This is contrary to the atomic nuclei masses being smaller than the sum of all proton and neutron masses that are involved because of the negative binding energy.

marlon

9. ### arivero

3,013
The keyword is "glue". It is said that most of the mass of the proton comes from the glue.

10. ### marlon

Well, but, the glue and virtual pairs are directly related to each other, so how can you split up their influence ?

marlon

11. ### Mk

Glue doesn't taste as good as it looks.

12. ### Gokul43201

11,141
Staff Emeritus
That post was by chroot, not Zz.

Spontaneous emission from where ?

### Staff: Mentor

Of what? Gamma-rays? Or other particles.

Like ZapperZ indicated a single particle, e.g. a proton does not emit anything. Even deutrons (pn) do not spontaneously emit gamma-rays, since it is fairly stable, and even alpha particles (2p,2n) are stable, so they don't emit spontaneously. Atomic nuclei (with numerous protons and neutrons) can be 'bumped' into higher energy states, and the excited stated will decay by gamma-emission.

0
But, a "proton" may "decay" via beta (+) or positron decay, whereby a proton is converted into a neutron. A common example is Carbon-11, used in PET scans in medical sciences. The beta (+) emitted from Carbon-11 decay thus joins with an electron (-) from human tissue to yield gamma rays. So, are we saying then that there are not "single protons" within Carbon-11 isotope -- since a single particle does not emit anything ?

15. ### ZapperZ

30,166
Staff Emeritus
Whoa!

I get "credited" for saying something in this thread even when I never participated, till now! Amazing!

:)

Zz.

16. ### Orion1

990

Affirmative, based upon classical nuclear theory, a proton is an extremely dense and spherically symmetric point-like particle.

The proton's spherically symmetric volume geometry is supported by hard nuclear scattering cross sectional experiments.

Average Proton density:
$$\boxed{\rho_p = \frac{3}{4 \pi M_0 r_0^3}}$$
$$M_0 = 6.022 \cdot 10^{26} \; \text{amu} \cdot \text{kg}^{-1}$$ - kilo Avogadro's number
$$r_0 = 1.2 \cdot 10^{-15} \; \text{m}$$ - empirical constant

Imaging the proton with particles or photons with wavelengths of the order $$r_p = \overline \lambda_{\gamma}$$ would produce a 'fuzzy' interference patterned image similar to that of a probability cloud.

Reference:
http://en.wikipedia.org/wiki/Proton

Last edited: Apr 20, 2006
17. ### reilly

1,079
During the late 1950s and early 60s, Robert Hofstadter did a series of brilliant electron scattering experiments with the old Stanford Linear Electron Accelerator. And, his work conclusively showed that protons and neutrons were not point particles, but instead had an internal structure, which was described by the so-called electromagnetic form factors. His work established the rms electric and magnetic radii as 0.7 X 10-13 cm. , and consequently that nucleons were not point particles.

While this is in the same order of magnitude ball park as the nuclear scattering radii, there'e no strong reason to suppose that the two quantities are equal. The scattering circumstances are very different; the electron scattering experiments are at much higher energy than typical nuclear interactions. They involve different forces -- electromagnetic vs. strong.

Nowadays, electron proton scattering is still alive and well at Jefferson Lab in Virginia. A nice summary of current thinking on nucleon structure, including non-spherical shapes can be found in:

http://www.azonano.com/details.asp?ArticleID=409

Regards,
Reilly Atkinson

18. ### Andrew Mason

6,860
Would you consider an electron-positron pair a composite particle? If so, can it be said that this composite particle emits the two gamma rays that result from its annihilation?

AM

19. ### SpaceTiger

2,977
Staff Emeritus
There seem to be a lot of correct but unqualified facts being thrown around here, so maybe we could try stepping back a little. One of the main points of contention seems to be the following statements:

This makes sense if chroot (not ZapperZ ) is referring to free particles that don't change their identity after radiating. A free electron, for example, cannot spontaneously radiate. To see this, just boost to its rest frame and try to conserve both momentum and energy after photon emission. In order to conserve momentum, the electron must begin moving in the direction opposite the emitted photon (zero net momentum). However, a moving electron has more energy than a stationary one, so the initial state (stationary electron) and final state (moving electron plus photon) do not have the same energy. In the absence of an external energy source (i.e. for a free particle), this is not possible.

However, if the particle is composite, then the interactions of its components can have energy. This means that the effective mass of the composite particle (say, a hydrogen atom) can be different after photon emission. The "moving atom plus photon" can, in some circumstances, have the same energy as the initially stationary atom.

This is all well and good, but I'm not sure it explains why a free proton can't radiate. After all, we have already determined that the proton is, in fact, a composite particle (made of quarks and gluons). It is still an open question in the physics community as to whether or not a proton can decay (producing other particles that can decay into photons), but I don't think I've ever heard anyone discuss "energy states" for the proton. Why is this?

20. ### samalkhaiat

1,100

Last edited: Jun 10, 2006