Does light have mass?

Question

If it even has the smallest mass like 0.0000000000000000000000000000000000000000000001 of AMU. THen we could put flashlights on the back of our cars and go at the speed of light right? Because Newton's First Law says that for every action there is a reaction. So instead of fuel to push the rocket, we could put light beams right?

Answer 1

No, absolutely zero on the mass. Light photons are totally massless, both at speed and at rest.
The photon is a type of particle called a boson. Particles with mass are fermions.


They do have energy and momentum however, and they are bent by gravitational fields. That bending happens due to the bending of space by gravitational masses, not because of any property of the photon itself.

However, you can propell a spacecraft with lightbeams and that has been experimented with at NASA, and such craft have been proposed. The momentum property of the photons are what allow you to do that, and as you point out, Newton takes over and does the rest. Acceleration would be slow but the process could go on for a long time so it is a useful prospect.

Also, using a solar reflector is a viable concept that has received some attention, again using the momentum of a (solar) photon against a big space sail, making use of the "radiation pressure" (that idea goes back to Maxwell).

Answer 2

Photons are not considered to have mass, but they do have "momentum," and that amounts to the same thing for the purposes of exerting a force.

So, yes, if you strap a flashlight to the back of your car, it WILL create a slight push on your car when you turn the flashlight on.

But it WON'T push your car at the speed of light. Newton's 3rd Law actually doesn't say anything about the relative SPEEDS of the action and reaction. For example, the speed at which a rocket goes is actually quite a bit slower than the speed at which the gases shoot out the back of the engine.

So, although it's technically true that a flashlight will push on your car, the amount of the push is much, much smaller than, say, somebody breathing on your back bumper.

Answer 3

A photon has zero rest mass, but it does have mass associated with its energy, and it does have momentum. Because light has momentum, it can be used for propulsion. Tail lights do exert a tiny forward force on a car.

For an ordinary lightbulb pushing a car, the effect is so small as to be useless. For a much brighter light pushing on a spacecraft, however, the force is large enough to be useful. Light sails, driven either by sunlight or by lasers, are a familiar feature of engineering-oriented science fiction. "Solar sailing is a method of converting light energy from the sun into a source of propulsion for spacecraft. In essence, a solar sail is a giant mirror that reflects sunlight in order to transfer the momentum from light particles (photons) to the object one is interested in propelling."
http://solarsail.jpl.nasa.gov/introduction/index.html

However, light sails and laser drives cannot enable a spacecraft to go at the speed of light. They still exert a finite amount force, and it takes an infinite amount of force times distance to accelerate an object to the speed of light (i.e. you can't).

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Edit:

Googling the question turns up various sources apparently claiming that a photon does not have relativistic mass. Certainly the fact that they're moving doesn't mean they have intrinsic mass (aka rest mass, aka proper mass). However, consider these statements from wikipedia:

"Relativistic mass and rest mass (invariant mass) are both traditionally used concepts in physics. However, with the development of Minkowski four-vector notation and general relativity, physicists gradually concluded that the invariant mass is the more fundamental quantity in the theory of relativity, and that the relativistic mass is just a redundant expression for total energy."
http://en.wikipedia.org/wiki/Mass_in_special_relativity

"Relativistic mass" is a redundant term for total energy, and photons have energy, ergo they have relativistic mass. It's just that "relativistic mass" is a deprecated term, so it's not entirely proper in current jargon to say anything about it at all. But the physicists who made up the old term weren't just being stupid; it does convey an important fact:

"The energy of a system that emits a photon is decreased by the energy E of the photon as measured in the rest frame of the emitting system, which may result in a reduction in mass in the amount E / c2. Similarly, the mass of a system that absorbs a photon is increased by a corresponding amount."
http://en.wikipedia.org/wiki/Photon#Contributions_to_the_mass_of_a_system

Answer 4

The 'rest mass` of a photon is zero.
The energy it carries does give it some momentum.
There must be some 'thrust` from a flashlight, but it's too small to measure.
Even at 100% efficiency, how much acceleration do you think the output of a flashlight battery could impart to an automobile?
Theoretically, you could build a 'light rocket`.
The exhaust could do a lot of damage.

Answer 5

E = M x C squared This statement says that energy and mass are equivalent. Therefore a photon travelling with some momentum (energy) has mass. A photon at rest has zero mass. There has been some confusion about this in previous answers. The photon has zero rest mass but photons are never at 'rest'.

Answer 6

a PURE LIGHT has no weight, its totally energy.

Pure Light = any object moving at the speed of light
( this object now is 100% transformed into energy. no mass, zero mass. )

A Pure Light can be slow down by gravity. When it is not moving at the C, (ex: 0.99 C) it now has mass.

Light = Energy = E = mc^2 (when C = C, no mass)

Answer 7

A photon of light does NOT have mass but it DOES have momentum.

The energy of a photon is E=hf ; where f is the frequency of light. where h is very small constant (Planck's constant 10^-27)
so its momentum will also be small due to this constant being so small.

Answer 8

Others have answered correctly.I want to point out an omission.For every action,there is an opposite BUT EQUAL reaction.that is important.Going along with photons have mass(they don't,but just for thought) it would take a mass of photons greater than that of your hypothetical car and your body weight to move the car and overcome friction.

Answer 9

Photons of light have *rest mass,* but photons under normal conditions are never at rest so are considered to be massless.

Answer 10

no