Does light have mass?

Question

If it does wouldnt it need a large amount of force to propell it? If you had enough light could you push an object?

Answer 1

Light in a vacuum consists of electromagnetic energy moving at a constant speed c. Any amount of that energy, E (in the form of many individual photons of energy h nu_i, nu_i the various frequencies that may be present), can be thought of as being equivalent to mass through Einstein's famous equation E = m c^2.

However, light has no "rest mass," m_0. Nothing that DOES have a non-zero rest-mass can in fact reach the speed c; that WOULD take a very large, indeed an INFINITE amount of force to achieve.

Light is essentially "coasting" at velocity c; once emitted, it needs no "large amount of force to propel it."

Light also has momentum p, related to its energy E via

E = pc.

(Maxwell appreciated this LONG BEFORE Einstein discovered relativity! And indeed, careful experiments show that atoms RECOIL as they emit light, because the momentum light is initially given comes at the expense of giving the same recoil momentum to the emitting source. Likewise, the momentum in the beam can be detected as "radiation pressure" --- see below. And to repeat what this implies --- light carries or has momentum WITHOUT having any rest mass.)

Once again, however, the relationship E = pc can be regarded as the limit of what Einstein discovered more generally, namely that the total energy E of a particle with non-zero rest mass m_0is related to its momentum p and its rest-mass via

E^2 = (m_0)^2 c^4 + p^2 c^2.

[In the slow speed Newtonian limit this gives E = m_0 c^2 + 1/2 m_0 v^2 + ...= "rest mass energy" + Newtonian kinetic energy + ... .

In the limit m_0 --> 0 it yields E = pc.]

Light DOES "push" objects. This effect is called "radiation pressure." You can buy little evacuated globes with circularly arranged vanes, arranged in a horizontal circle, suspended in them. (The vanes are silvered on one side, and black on the other.) These vanes rotate when light is shone on them. The different treatment of the two sides of the vanes results in different amounts of light being reflected by those sides, and that differential asymmetry in the momentum absorbed or reflected is sufficient to rotate the devices. They spin extremely fast in bright sunlight!

Such devices show explicitly that light can "push" objects.

Live long and prosper.

P.S. [Later edit]: The "SOLAR WIND" (see next response) does not refer to light, but rather refers to MASS-LOSS from the Sun in the form of PARTICLES of finite mass --- atoms, molecules, etc. But don't worry about the Sun withering away because of that --- at its present rate of mass-loss, it would take about 10^4 times the current age of the Universe for the Sun to completely dribble off into space. However, the momentum from that solar wind can indeed exert forces on interplanetary dust or probes.

There is no special name other than the "solar radiation pressure" for the analogous effect due to the Sun's light.

The fact that there is both a particulate solar wind AND solar radiation pressure is responsible for what one can see in reasonably good photographs of comets. That is, that there are TWO tails of somewhat different appearances. One of these is due to the solar wind, the other to solar radiation pressure!

In fact, the existence of the solar wind itself was first PREDICTED by a noted German astrophysicist, Ludwig Biermann, from examining the form of comet tails. He understood that radiation pressure would produce one kind of tail, but also realized that radiation pressure alone could not be responsible for the second kind of tail. So from that realization, he postulated and calculated the rate of mass loss from the Sun in particle form! This was a remarkable astrophysical prediction.

It was quite some time before interplanetary probes could be launched that directly DETECTED and MEASURED that mass loss. The results were close to Biermann's predictions.

Answer 2

I think it may have a teeny tiny amt, but keep reading. i may have confused 2 things.

There are areas where light acts like a wave, area where it acts like a particle. so it seems at times as if that particle has mass.

there is something called SOLAR WIND ( honest), and it can push solar sails out where we usually think of space being a vacuum. The sails have to be very large in area- so in that sense it takes a lot of light to make an object move.

Answer 3

Unfortunatly there is not any sparkling explaination. First you may desire to define mass. Does it have weight? the belief of photons is that the ensue in basic terms whilst gentle is modern, yet as quickly as the gentle is off they disappear and have no info of ever latest. - issues do no longer in basic terms dissappear. If photons had mass they might stay after the gentle became off - like water from a hose. incredibly there are some properties to gentle that confirm the colour and brightness. this is like asserting does sound have mass? properly air does, yet does sound? No this is vibrations during the air. in any case, i do no longer think of this is been shown the two way.

Answer 4

According to scientists on 7 news it has mass because gravity can only effect somethin that has mass black holes are strong forces of gravity with radiation light will get sucked into a black hole there for it has mass btw thiae weren't her exact words but thats what she was sayin fun fact only thing that has no mass is dark energy its only thing that wint get sucked into a black that and radiation and white holes so in other words technically yes light does have mass despite to popular belief that it doesnt at least according to that scientists on news

Answer 5

Light cannot have any mass. Anything traveling at the speed of light must be massless. However, light does carry energy and has momentum as defined by quantum laws (Planck's law), so it can interact with matter physically (i.e. push on objects).

Answer 6

Light does not have mass. pure light is a wave and therefore is energy. the light wave is able to carry photons, wich do have mass, but photons are so small that they actually attach themselves to electrons, causing the electron to become hyperactive. I digress, the final answer is, no light has no mass of it's own.

Answer 7

yes