Recently several labs have achieved the lowest possible temperature.
IIRC even "light" or photons came to rest. Did they have "rest mass"
under those conditions?
..............................................
Web discussion:
What is the mass of a photon?
This question falls into two parts:
Does the photon have mass? After all, it has energy and energy is
equivalent to mass.
Photons are traditionally said to be massless. This is a figure of speech
that physicists use to describe something about how a photon's particle-like
properties are described by the language of special relativity.
The logic can be constructed in many ways, and the following is one such.
Take an isolated system (called a "particle") and accelerate it to some
velocity v (a vector). Newton defined the "momentum" p of this particle
(also a vector), such that p behaves in a simple way when the particle is
accelerated, or when it's involved in a collision. For this simple
behaviour to hold, it turns out that p must be proportional to v. The
proportionality constant is called the particle's "mass" m, so that p = mv.
In special relativity, it turns out that we are still able to define a
particle's momentum p such that it behaves in well-defined ways that are an
extension of the newtonian case. Although p and v still point in the same
direction, it turns out that they are no longer proportional; the best we
can do is relate them via the particle's "relativistic mass".......snip
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html
Gerry
--------------------------------------------
From: tyler
Photons have energy and momentum, but they don't have the sort of "rest
mass" that makes a non-moving object experience gravity. The concept of
mass doesn't really apply to objects that cannot be at rest. Photons are
not tiny classical mechanical particles traveling quickly, but have
totally different behavior. Depending on the context, it's generally
more useful to think of them as a discrete quanta of energy, rather than
as a particle like an electron.
I'm not sure about the black cloth experiment you're talking about,
perhaps someone else on here knows about it? Because mass and energy and
fundamentally equivalent, it is possible for the energy in a photon to
increase the mass of an object when absorbed, without the photon itself
having mass.
Tyler
Bill R wrote:
No argument here, but light does seem to have weight. The study was done
many years ago [I was in HS when I heard about it] using a black cloth
subjected to intense light in a controlled environment, and the cloth
gained
a [very small] amount of weight during the process. It was part of our
class on light in HS physics - would have been about 1965. That is my only
reference to it, so, once more, scientific types needed ... is this true?
BillR
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