🚀 go-pugleaf

>Date:>>>Tue, 26 Apr 1994 17:35:40 EDT
>Reply-To: Sharing resources for high school physics <PHYSHARE@PSUVM.BITNET>
>Sender: Sharing resources for high school physics <PHYSHARE@PSUVM.BITNET>
>From: PHYS2SHS@uriacc.uri.edu

>>1. If light is a particle, how can it travel at the speed of light
>>without becoming an infinite mass?

>The particle of light (the photon) has a particle wave duality.  It is
>like a particle in that it is a "clump" ("pluse") of light waves transmitted
>together.  It particle nature is not one of mass.  The photon has a 0 rest mass
>and therefore can not be applied to the revelativistic equation.  Remember that
>general relativity was structured arround light having the ultimate speed there
>fore there is some technicalities in applying light to the equations.  (also,
>in General Relativity Einsein defined light as massless and a wave)

That photons have mass zero is a consequence of Einstein's theory of
relativity. However this is not the only possibility. De Broglie has a
detailed theory of light using what is called a "pilot wave" where
photons have finite mass. De Broglie estimates this mass to be less
than 10�(-65) kg. If you want to know the details of de Broglie's
theory there is a book called "Me'canique Ondulatoire du Photon et
The'orie  Quantique des Champs" by de Broglie. I think that Bohm,
Hiley and Vigier worked on this too, but I may be mistaken.

>>2. If light is a particle, when it leaves a flashlight why doesn't it
>>have enough momentum to push the flashlight backward?

>No mass, no momentum, no recoil.

Zero rest mass in no way implies zero momentum. There is indeed a
recoil, but it is too weak to be felt. BTW, that is also a classical
effect and you do not need to have photons to have a recoil. A
textbook on electromagnetic theory will show how to compute what is
called "pressure of light". In fact, since the classical
electromagnetic field has a physical existance, it has both momentum
and energy. Talking about recoil, I remember that a very striking
experiment I saw was when a very strong N2 laser was used to bombard a
piece of paper. Since it was a pulsed laser at each pulse we could see
the paper moving due to light pressure.

>>3. relativity states that to the observer, the mover's time always goes
>>slower. Then how can one person's clock be ahead of the other's,
>>considering that either one could be considered the mover or the observer?

>Einstein gave a long explaination of this, but I try to state the main idea.
>>- Each one looking at the other would seen the other's clock move slower.
>so therefore neither would appear to be ahead of the other.  They can only
>determine whose clock is ahead if they both stop and are brought together.
>(Hope this is understandable)

It should be pointed out that in order to measure such discrepancies
on the clock measurements between two distinct observer they'd have to
travell with an enormous speed, something like at least 0.1 times the
speed of light (which is 300,000 km/s, or 188,000 miles/s). Another
important point is that to bring the two observer toguether you would
have to accelerate them (like making a U-turn) and then the special
theory of relativity would not apply anymore (you'd have to use
general relativity).

>>4. If you could stand in a black hole and shine a light straight up,
>>would the light escape, turn around and come straight back down, or orbit
>>to the right or left?

>If you stood at the center of a black hole (yeah, right) and could shine a
>light out of the hole.  You couldn't really do it the light would never be
>able to get out of the source due to the gravational effect.

>If however you could stand at the black hole boundary (called the Swartzchild
>Radius) and shine a light it would orbit in a beam arould the black hole for
>ever.

>>5. If we could travel at speeds greater than the speed of light, could we
>>travel back in time?
>>

Nobody knows.

>No. If we could travel at an infinite speed we could only stay were we were in
>time.  We can never return to the past.  :-(

Well, there is no consensus about that, as far as I know. For example, there
are a lot of models for the universe (the most famous one being the
Goedel model) where it is possible to travel back in time.


>For a good explaination of some of this stuff read Stephen Hawking's "A Brief
>History of Time".
                         -Gregory R. Snow (phys2shs@uriacc.uri.edu)
                          Smithfield High School Physics 2 Class


I'd strongly recomend Prof. Edwin F. Taylor and Prof. J. A. Wheeler's book
"Spacetime Physics", W. H. Freeman, NY 1992. It is an excelent and
clear introductory book and it is not difficult to read.

Jose Acacio.