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Weighing Sunlight
Name: Dylan
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
Hi, I saw this claim on TV that says the suns rays have a
weight of 2 pounds per 1 square mile on the earth. I was wondering if it really
was possible to weigh sunlight.
I saw this while my sister was watching YTV, and they have odd little facts
sometimes, and this happened to be one of them. I am not entirely sure what
the exact fact was but this was pretty much what it said. " sunlight has a
weight of 2 pounds per 1 square mile on the earth.
Replies:
There are a couple of reasons to be confused about this. First, we have been
taught that photons have no mass, therefore we would guess that they shouldn't
exhibit weight. Photons do have zero "rest mass", but as they always travel at
the speed of light they are never at rest. Photons do have energy and momentum,
despite being "massless", and when they interact with an object (either absorbed
or reflected) they impart a minuscule amount of energy, proven over 100 years
ago by Pyotr Lebedev. We know photons have energy, defined by E=hf, and we know
E=mc ,or m=E/c which tells us the magnitude of this pseudo-mass (previously
known "relativistic mass", a term that has fallen out of favor).
Coincidentally, this "radiation pressure" due to the pseudo-mass is precisely
what makes "solar sails" possible (spacecraft propelled by the "pressure" of
sunlight). But again, sunlight does not have "weight" in the way we normally
think of it. If we were able to stop sunlight from moving, its perceived
"weight" would vanish.
The other thing that can be confusing is the Crookes Radiometer which spins
magically in the presence of sunlight, and might seem to support the idea that
sunlight has weight. Many people (like myself) have been told that striking
photons caused the movement. However, in reality the Crookes Radiometer is
driven by thermodynamic processes.
P. Bridges
Because light has energy, it can impart a force on particles. The optical
properties of certain materials allow them to be moved or held using light.
This effect is the basis for instruments like 'optical tweezers'. Typically
when we talk about 'weight', though, we mean the effect of a field like
gravity on a mass. The force that light can produce does not work the way
gravity and mass produce a weight. I am not sure how this calculation was
performed, but perhaps it was based on the "Yarkovsky Effect", which has to
do with the effect on celestial bodies (also not due to mass/gravity).
Hope this helps,
Burr Zimmerman
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Update: June 2012
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