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Increase in weight if an object is illuminated

  1. Jan 3, 2012 #1
    Increase in weight if an object is illuminated

    Are there experiments which show that the weight of a body, with initial mass between 1 g and 1 kg, really increases if illuminated for a certain time?

    Photons have energy E = hf but no mass. If the weight of a body gets bigger after being radiated with light it means that energy indeed converts to mass.
  2. jcsd
  3. Jan 3, 2012 #2


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    Heating the body up would have more effect than simply illuminating. However I doubt if the weight gain cold be measured.
  4. Jan 3, 2012 #3


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    I saw what you did there!

    Are you wearing a Freudian slip? :approve:
  5. Jan 3, 2012 #4
    Cavendish's equipment was remarkably sensitive for its time. The force involved in twisting the torsion balance was very small, 1.74 x 10^–7 N, about 1/50,000,000 of the weight of the small balls or roughly the weight of a large grain of sand.

    Raising the temperature of an object (increasing its heat energy) increases its mass. For example, consider the world's primary mass standard for the kilogram, made of platinum/iridium. If its temperature is allowed to change by 1°C, its mass will change by 1.5 picograms (1 pg = 1 × 10^−12 g).

    F = 1.74 * 10^–7 N is the weight of a mass m = 1.77 * 10^–8 g = 17.7 ng (measured by Cavendish in 1797).

    1 kg heated to 1000 degree, using laser light, will increase its mass by 1.5 ng (I guess).

    If Cavendish was able to detect forces as small as 17.7 ngf, more than 200 years ago, maybe an increase in mass of 1.5 ng can be measured today with enough precision. I do not know, this is just a question.
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