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 Click here to return to the Physics Archives

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