Question

How are solar sails possible? If photons have no mass, how can they push on something?

Asked by: Peeps

Answer

Indeed, photons have no mass. However, they DO have energy and momentum. It turns out that energy and momentum are the requirements that make a solar sail work, not mass.

If we think about a slow massive particle, like a bowling ball, we can help to make sense of this. If the bowling ball is not moving, then it can't make anything move. And a heavy bowling ball can make something move more than a light bowling ball (if we assume both bowling balls are moving with the same speed). To summarize, how much something moves (how much momentum is transferred) depends linearly on both the bowling balls' mass and speed. Momentum (for slow moving massive objects) is defined as p = m*v (p is the momentum, m is the mass, and v is the velocity).

This momentum is what gets transferred when wind hits a normal sail. But photons have momentum without having mass! This is how they can push things around, they transfer momentum. For a photon, E = h*f = mc2 so, p = hf/c. In this case, p is once again momentum, h is Planck's constant (6.62 x 10-34 Joule*seconds), and f is the frequency of the photon. This is how a solar sail works. Photons transfer some of their momentum to the sail, thereby propelling it along very much like a sailboat in the ocean.

Answered by: Andreas Birkedal-Hansen, M.A., Physics Grad Student, UC Berkeley

Search

Loading



Click here to get
a FREE ride with Uber!


Click here to
sign up for Birchbox






Science Quote

'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '

Michio Kaku
(1947-)
Science Sidebar | Science Education Articles
10 Ways to Keep Your Kids Interested In Science

Young children are natural scientists: they ask questions, pick up sticks and bugs outside, and are curious about the world around them. But as they get a bit older, many kids gradually lose their interest in science. They might see it as just another task at school, something that doesn't apply to their lives. Of course nothing could be further from the truth, so here are ten ways you can remind your kids that science is everywhere. Most of these are fun for adults, too! Continue reading ...

Top Selling

Here are our physics & astronomy bestsellers:
Deluxe Water Rocket Set
Mini Plasma Ball
KonusScience 5 Way Microscope Kit
3D Magnetic Field Tube
Scorpion, Ant, Wasp and Flower Bug
Alnico Bar Magnet - 6 inch Long
Solar Radiometer
Weather Station 4M Kit
Periscope
Cherry Wood Levitron

Sponsors

USC University of Southern California Dornsife College Physics and Astronomy Department McMaster University Physics and Astronomy Department