• Categories

  • Most Popular Questions

  • Recently Viewed Questions

  • Recent Answers

    How To Make a Digita… on What does a frequency of 100 H…
    Daigrepont on Can an earthquake cause air tu…
    Benedict on How did God come into exi…
    joshua on How does the human body g…
    Ian on How did God come into exi…
  • Recent Questions

  • Blog Stats

    • 2,208,914 hits
  • Visitors since 11-3-08

    counter create hit
  • Terms and Conditions

  • Warning

    We are doing maintenance on this site, so some posts may disappear for a short time. Sorry. Normal service will soon be resumed...
  • Pages

  • February 2008
    M T W T F S S
        Mar »
     123
    45678910
    11121314151617
    18192021222324
    2526272829  
  • Archives

  • Meta

Does light have mass?

Does light have mass?
Charlotte Morgan from West Yorkshire (age 15-24)

Answer

No. But it does carry energy. So it does have a “mass equivalent” given by Einstein’s famous equation.

John Gribbin

Advertisements

One Response

  1. The answer to this really depends on what you consider light and how you think of mass. If you think of light as a photon (a small particle or a quantum of energy), then a photon definitely has no mass in the usual physical meaning.

    Physicists like to think of properties that do not change, and so they talk about an invariant mass which gives a relation between the photon’s energy and its momentum (its movement). In brief, as nothing goes faster than light, all the photons energy is used for its momentum, and so its invariant mass is zero.

    It becomes more interesting when you trap photons in a box. Say you put photons between two perfectly reflecting mirrors. While these photons would still have no mass on their own (moving very fast from one side to the other), the box with the photons is not moving – its momentum is zero, and the lights energy becomes part of the mass of the box. So at least in principle it should be possible to measure a very small increase of the mass of the box if you trap light like this. The trapped light contributes to the mass of the box, but on its own it has no mass.

    Then there is Einstein’s famous equation which relates mass and energy – this gives you a different type of mass, which is usually called the relativistic mass. But the relativistic mass is essentially the same thing than energy (i.e. the mass is the energy divided by a constant value c*c). So physicists prefer to refer to energy in this case. But with the relativistic mass some of the classical formulas can be generalised to still work in these cases. In this sense a photon has a relativistic mass (it’s really a matter of interpretation now).

    If m is the invariant and M is the relativistic mass, E the energy and P the momentum, and v the speed of the particle (e.g. the photon), and c the speed of light, then in formulas this becomes:

    m = sqrt(E^2/c^4 – P^2 / c^2)

    E = Mc^2

    P = Mv

    So for light with v = c, you’ve got m = 0, but M = E/c^2 (and E for light is hf where h is a constant and f its frequency).

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: