Einstein’s famous mass-energy equivalence equation, or E =
mc2, is actually a special case of a slightly longer formula known as the
energy-momentum relation, which is written out as E2 = p2c2 + m2c4.
This equation relates energy (E) to rest mass (m), the speed
of light (c), and momentum (p), which is the key to how photons can carry
energy but have no mass. When a particle is at rest, it has no momentum and the
equation simplifies to the more familiar E = mc2. But if a particle has no
mass, the equation becomes E = pc.
But wait, you might be asking, how can a particle have
momentum without mass? That’s where light’s duality as both a wave and a
particle comes into play. Unlike a particle, whose momentum is related to its
mass, a wave’s momentum comes solely from its motion, meaning that it can carry
momentum even without mass.
Interestingly, something that has neither mass nor momentum
has no energy, which means it is nothing at all — i.e., it cannot exist. But
photons do exist, so it follows that they can never be at rest. And the only
speed that remains the same in every reference frame is the universal speed
limit (c). Light isn’t the only massless particle, however. Gluons, massless
particles inside atoms, also travel at the speed of light.
Reference:
0 Comments