Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Ask Astro: How can a photon be massless?

RELATED TOPICS: PHOTONS | PARTICLE PHYSICS | EINSTEIN
photon
As both a wave and a particle, a photon can have no mass but still carry energy — as long as it has momentum.
Astronomy: Roen Kelly
How can a photon of light carry energy but also be considered massless? Doesn’t Einstein’s E = mc2 make that impossible?
Robert Bobo 
Pullman, Washington 

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 or 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.

Caitlyn Buongiorno 
Associate Editor 
0

JOIN THE DISCUSSION

Read and share your comments on this article
ADVERTISEMENT
FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
Apollo_RightRail
A chronicle of the first steps on the Moon, and what it took to get there.