Only Newton's 1/r2
gravitational field gives closed orbits.
Presence of Sun modifies geometry of space making Mercury orbit visibly
(0.48'' for century) open.
The following programme (cortesy of dott. Mauro Brunato) shows how orbits
get open for fields other than 1/r2.
Youa can load zip or exe version, together with example of input parameters.
General relativity predics also that light rays are cuvred in
presence of mass
(by a factor of two more that it would result from Newton's theory).
Therefore, light from distant stars is curved and/or doubled by
massive galaxies,
like on this photo from Hubble telescope
A similar effect is common as you see in these photos:
1. a special "magic" eye
2. a simple ruler
3. infinity of "fashionable" gadgets from window shops
Trento
Milano
Verona
Paris Elementary particles Electron is a lepton - light particle,
like e Greek "small" euro coin
Positron seems to be the same as electron - just its reverse,
but maybe is slightly different? (find the difference)
Lepton tau is still called "light" but
it is heavier that proton and neutron, which are called "hadrons", i.e. heavy
Proton, neutron and antiproton are 1837 times heavier than electron (2
zł coin is like 1 euro) Proton and neutron are made of quarks: we know that three are inside but nobody can
extract them
proton (quarks uud), neutron
(quarks udd), antyproton (anti-quarks
uud) - see the "tails" of quarks "up" and
"down"
Quarks differ by mass, electrical charge,
flavour (strange, beatiful) and color; then we have anti-quarks
They do not stay still inside nucleons but spin so quickly that seem to be white
This is so called "Standard Model" of elementary particles. But, maybe,
proton is more complex, like this cube?
ruler
and neutron 
are made of quarks: we know that three are inside but nobody can
extract them
inside nucleons but spin so quickly that seem to be white 
