Expert: Philip Stahl - 11/26/2006

Question
Has anyone ever actually seen an atom or a subatomic particle, neutron, electron, or proton and are there photographs of the physical reality of them or are these theoretical constructs based on mathematical necessity?
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The text above is a follow-up to ...

-----Question-----
If everything in the universe is moving and there is no fixed point of reference, if we can detect no movement of the earth (particularly rotating at 1,000mph), and if most of the observational explanations seem to show that the earth is not moving then how can we know for sure that the earth is rotating around the sun rather than the entire universe rotating around a fixed earth?
-----Answer-----
Hello,


What you are really getting at when you ask about some "universal motion" or speed (or absence thereof) is the *absolute motion of the earth* in the universe. This has indeed already been settled, and was the object of the Michelson-Morley experiment (you can google this, to learn more about it).

The experiment, to make a long story short, gave null results. This led to a fundamental postulate of special relativity theory, namely that it is not possible to define an absolute coordinate system with respect to the absolute speed of an object in space.

Such an absolute coordinate system would be based on the universe at large, but we have no way of ever doing that. Not the way we can impose a latitude, longitude coordinate system on Earth (or galactic longitude, latitude on the galaxy).

Since we can't define such a system, there's no way to ascertain the Earth's (or any other body's) resultant motion within it! The most we can do is to assay and compile the separate motions, but cannot ascertain what "resultant" these have in respect to the cosmos as a whole!

However, this inability to define or use a universal reference frame has nothing to do with establishing LOCAL, LIMITED inertial reference frames - and assessing the individual motions therein.

Since the Earth's motion about the Sun is part of a local inertial frame (with the Sun the center of mass, essentially) it IS possible to define a relative fixed point-body about which others revolve.

We know for sure we can do this, because careful measurements of the Sun's position relative to fixed points on the celestial sphere (the projection of Earth's coordinates into space, except latitude is now 'declination' and longitude is 'right ascension') allows this.

More to the point, IF Earth were not revolving around the Sun there would be no change of seasons, or differences marked in the Sun's altitude (OR azimuth) through the year - from various locations. Rather, the Sun would be essentially fixed in position as seen everywhere on Earth.

That this is not observed, and NO evidence exists to support it - shows that in the limited reference frame of the solar system Earth is indeed revolving about the Sun!


Answer
Hello,

Direct observation of atomic - subatomic particles is mostly precluded by their small scale size - however, this does not mean they are merely "constructs" possessing no physical reality!

In the case of atoms, field-ion microscopy can reveal (under favorable circumstances) images on a fluorescent screen corresponding to individual atoms. A number of college physics texts bear such images on their front covers (e.g. 'Introduction to Modern Physics' by C.H. Blanchard et al)

Apart from this, we *deduce* the presence and existence of atoms by the fact that relatively simple chemical reactions always occur in certain ways such that:

a) the proportion of reactant A to reactant B (in a compound) is constant

b) If A and B form several different compounds, (AB, AB_2, A_2B_3 etc) the relative amounts of B which combine with a given amount of A will be in the same ratio as the number of atoms of B per atom of A in the formula - and thus will be related by simple numerical ratios.

These sort of chemical reactions, yielding certain compounds in certain, discrete ratios of the reactants simply would not exist if atoms didn't truly exist!

In the case of the electron, its existence was deduced by careful experiments (e.g. by J.J. Thompson) which yielded a precise ratio of its charge to its mass. NO other particle fits this ratio - which is unique - and we directly associate this (q/m) with the electron.

Thus, (q/m) for electron = (-1.758796 +/- 0.000019) x 10^11 Coulombs/ kg

Thus, the experiments of Thompson (and many others since) are consistent with the existence of a negatively charged particle with a charge to mass ratio that is independent of the experimental conditions applied (In the experiment, a cathode ray tube was used, in which a beam of particles was passed between two electrically charged plates)

WIth the advent of modern quantum mechanics, of course, the wave nature of the electron was also predicted, according to which:

L =  h/ p

that is, its wavelength would equal the Planck constant (h = 6.62 x 10^-34 Joule-sec) divided by its momentum (p = mv)

The Davisson-Germer experiment in 1929 finally validated the above - using heated nickel crystals, which disclosed the phenomenon of electron diffraction upon examination.

Thus, the entity with the specific charge-mass ratio also was found to have a matter wavelength consistent with previous data.

Similar cathode ray deflection experiments can also disclose the proton, with its own particular charge-to-mass ratio, exposing a new entity with roughly 1830 times more mass than the electron. We call this the proton.

The positron - the negative (or anti-matter) counterpart to the electron, was discovered in 1932 by Carl Anderson. Anderson had been examining the tracks made by particles in a cloud chamber (using cosmic rays) which was placed in a magnetic field.

He found that a certain species of particle deviated exactly opposite to the expected path of electrons, corresponding to a positively charged particle. The degree of devation was also consistent with a similar mass. Thus, the charge to mass ratio was equal in magnitude but opposite in sign to the electron. He named the particle associated with these paths, "positron".

Similar indirect measurements have consistently identified a particle of zero electronic charge, with slightly more mass than the proton, which we call the "neutron".

Apart from the indirect measurements, we are assured of the existence of all the various subatomic particles by the fact that they are consistently identified as either products or reactants in defined nuclear reactions, that always yield the same amounts of energy, Q.

where Q = initial rest energy - final rest energy

Q = [{(m(A) + m(B)}  - {m(C) + m(D)}] c^2

where the masses in the first parenthesis represent the reactants in some nuclear reaction, and the masses in the second represent the products.

Note that the Q will yield an anergy amount consistent with Einstein's mass-energy equation:  E = mc^2

E.g.

H1  +  H1  ->   D2 + energy

two protons combine to yield deuterium + energy

More exactly given by:

H1  + H1  + (e-) ->  D2 + v  + 1.44 MeV

two protons fuse to yield heavy hydrogen with a positron and neutrino (v) emitted along with 1.44 MeV of energy released. (Note, writing (e-) on the reactant side is equivalent to the *release* of the postiron (e+) on the product side)

This can be validated by substituting the requisite masses in the 'Q equation' given previously.

Finally, we are convinced of the reality of atoms, electrons by the fact that specific transitions of electrons (between defined energy levels) yield a specific pattern of spectral lines unique to the element.

As a general relation we can write for this energy defined by energy level :

E(n) =  - 13.6 (Z^2)/ delta n^2    eV

where Z is the atomic number, and 'delta n' denotes the energy level difference.

For hydrogen, Z = 1, and

let delta n  =  (1/n2^2  -  1/n1^2)

with n2 = 3 (e.g. 3rd energy level) and n1 = 2 (second energy level)

Then

E(n) =  - 13.6/ (delta n)  =   1.889 eV

which will correspond to what we call the second Balmer line for hydrogen (which can actually be seen as an absorption line in the Sun and other stars.)

Now, this is not intended to be comprehensive by any means, but only to show by way of a number of discrete examples, how actual empirical data are used to identify the nature of atomic and subatomic entities.

These are not merely abstract "constructs" - since they all have bases in actual, first hand observations and experiments (e.g. cloud chamber, cathode ray deflection etc. not to mention modern day linear accelerators) that provide a composite view of each entity that is consistent.

If the subatomic particles such as electrons and neutrons were merely "constructs" we would hardly expect to see repeated nuclear reactions which yield predictable energy, or see spectral lines from the respective atoms - when their electrons make predictable transitions between energy levels.

Hopefully, this foray helps to illuminate some of the issues you raised.