Is Light Heavy?

Question

I remember being told at school that light was a wave ( a form or energy) and so does not have mass like particles do.

I know that light can be bent by gravity but gravity can only effect anything that has a mass. So does that mean that light too has an associated mass, however minuscule?

Answer 1

it does not have a weight in the classical sense... but we know it can transfer power... (see lightsails) there seems to be a logical construct called relativistic mass, as opposed to ballistic mass... which seems to indicate that at lightspeed, the practically weightless photon, does in fact weigh a LITTLE something... see if you can make anything of it...

http://www.newton.dep.anl.gov/askasci/phy00/phy00332.htm

Answer 2

EXCELLENT QUESTION! The short answer is that nobody can give you a fully satisfactory answer. NOBODY! If they can, they are sitting on a guaranteed Nobel Prize in physics!

Read on to see why!


The problem most people run into when thinking about this is making a sharp distinction between mass and energy. On the most fundamental level they are exactly the same animal.

It is possible to set up an experiment where photons (light) are directly converted into other massive particles, and vice versa.

You can calculate the equivalent mass of a photon from it's energy mc^2 = hv (where m = mass, c = speed of light, h = planck's constant, v = frequency of light) Solving for m will give you the equivalent rest mass.


Now it turns out that this is not the framework that physicists use to think about bending of light by gravity. We like to imagine that light bends because it is just following space (which is distorted by the presence of a graviational field). The more you think about this, the more you realize that "space" is a rather abstract concept. In reality, the presence of a graviational potential/field does something to light passing through it... but what!?

The million dollar question is why do we think about these things in two different ways? Well, that's been bugging physicists for the past fifty+ years. We've been looking for a way to unify our understanding of Gravity, Electromagnetism, Mechanics (Relavitivty), and Quantum Mechanics into a more fundamental theory.

So far... no dice...

Answer 3

No, light does not have mass, otherwise it would not be able to travel at lightspeed. Einstein's equation shows how to convert *momentum* into *mass* - E=mc^2 is only relevant to a particle *at rest*, which a photon can never be (as the speed of light is the same in every reference frame).

Relativistic mass is an outmoded and somewhat confusing way of explaining this - I'd try to avoid any references to this as it makes things a lot more complicated.

As someone above explained, light is bent in large gravitational fields because (according to theory) spacetime is curved in the vicinity of such fields, and the light follows the curves, not because it undergoes any kind of gravitational pull.

Answer 4

Yes the particle of light has the densest mass of the Universe.And makes up all other mass architectures of the Universe. Very vely simple isnt it?and I is no Physisit.So light has to follow all the same rules of gravity.
When a mass is set in motion we say it has energy.So everything that moves experiences the Energy phenomena.

Answer 5

i will let some one else answer this fully, but the basic idea is that gravity does not pull on light, and light has no mass..

The thing that gravity does is it bends space... thus the shortest distance between two point around an area of gravity is a curved path, and that is the path that light takes.

(think of gravity as a bowling ball on a bed sheet ... causes the sheet to flex and it is no longer flat... well that is what gravity does (but in three dimmentions))

Answer 6

Light can be converted into mass and vice versa. So, more light you have, more mass you can get by converting it.
Now coming to your question about gravity and light, if you have ever heard of something called Black hole. It has got such a huge force of gravity that it even attracts light towards it. Which means light wave does have an associated mass, though it may be considered as zero for most of the theroems.

Answer 7

Light must have an associated mass - they talked years ago about having spacecrafts with large sails, so that the sun could push them into space and accelerate the craft. I remember that the sails would have to be very, very, very large and that it would take an enormous amount of time to accelerate the craft to any reasonable speed - so the force must be miniscule - it must be defined somewhere

Usig the equation E = hf where f is the frequency of light and h is Plank's constant

h = 6.636 x 10 to the power of -34
f = frequency of light is around 400 to 700 nanometres or 400 to 700 times 10 to the power of -9

E would be 4 to 7 times 10 to the power of minus -41 - a very small amount, this is for one photon

Remember also that due to the inverse square law, the farther away the spacecraft, the less number of photons hitting the sails.

Solar sails (also called light sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion using large membrane mirrors. Radiation pressure is small and decreases by the square of the distance from the sun, but unlike rockets, solar sails require no fuel. Although the thrust is small, it continues as long as the sun shines and the sail exists.

Solar collectors, temperature-control panels and sun shades are occasionally used as expedient solar sails, to help ordinary spacecraft and satellites make minor corrections to their attitude and orbit without using fuel. This conserves fuel that would otherwise be used for maneuvering and attitude control. A few have even had small purpose-built solar sails for this use. Some unmanned spacecraft (such as Pioneer 10) have substantially extended their service lives with this practice.

The science of solar sails is well-proven, but the technology to manage large solar sails is still undeveloped. Mission planners are not yet willing to risk multimillion dollar missions on unproven solar sail unfolding and steering mechanisms. This neglect has inspired some enthusiasts to attempt private development of the technology, such as the Cosmos 1.

The concept was first proposed by German astronomer Johannes Kepler in the seventeenth century.[1] It was again proposed by Friedrich Zander in the late 1920s and gradually refined over the decades.

Answer 8

The understanding of light is getting better as time pass by. Light consists of photons that have a mass.

Answer 9

Yes, you are absolutely correct. Energy always has mass. Therefore, photons also carry mass according to much energy they have; the higher the frequency of a photon the more energetic it is and, therefore, the more mass it possesses.

Answer 10

light is produced when the electrons of the atom are excited .
the electons release photons [ light--form of energy ]

the weight of the electrons are very negligble and think about the photon