weight of light?

Question

Einstien said that nothing with mass could acheive or exceed the speed of light. So that implies that light does not have a mass. Also it has been proved that a black hole has a gravity so strong that not even light can escape it. So how is it that light has a mass (from the black hole example) yet it can travel at the speed of light?

Answer 1

aw geez girl! What do you think we are? Nuclear physicists?
I think scientists are still trying to figure that one out!
Good question...........deeep.

Answer 2

Although somenathsengupta has it right, let me expand on his answer a bit.

All object with mass curve space time. That goes for you, me, the smallest grain of sand, or the entire planet or solar system.

The more massive the object, the more it curves space.

Right? Well, yes and no. . .

Lets say that the stronger the gravitational field, the more it curves space.

But the more massive, the stronger the field, right.

Not necessarly. You have to remember the formula for gravitational force, Fg = G(m1*m2)/d^2. Now, we can eliminate m2 from the equation becuase m1 is so large that m2 does not count. (Don't worry, we did not hurt m2's feelings, in fact it rather enjoyed feeling skinny.)

But we can not ignore d^2! d^2 is the distance between the objects (from center to center). And that is the whole ball of wax when taking about black wholes. . . Uhh, I mean holes.

As d^2 gets smaller, Fg gets larger, the smaller d^2 gets, the larger Fg gets even if you do not change the mass.

The stronger Fg gets, the more it curves space. Are you following this? Since you can't answer while I am typing (besides, that would be rude), I am going to assume you do.

OK, let's plug in the mass and radius of the earth and see what we get just as a check.

The mass of the earth is 5.97E+24 KG
The diameter of the earth is 12,756.30 KM but since we are only concerned about from the cetner to the surface, we use half of that or 6378.15 KM.
G (the Universal Gravitational Constant) is 6.67E-11 m^3/kg*sec^2.

Now, putting them all into a calculator (and minding our units) we find that Fg is equal to 9.7954587925 m/sec^2 (ok, I cheated, I have all the figures in a spreadsheet already calculated out for me).

Does that figure look familer. It is very close to g, or gravitation acceleration on earth (very close because earth is not a perfect sphere).

Can you see where this is going? If not, hold on for just a bit more.

Now, lets imagine something with the mass of. . . oh, five suns, but with a radius of only 1 mm (a blackhole can be more massive, and has less of a radius).

The figure for Fg would be 6.6363E+23 m/sec^2. That's some pull baby!

And that is some curvature of space! Notthing that come close to it will ever be able to climb out of the hole it creates. Notice, I said close to it. The further away, the greater d becomes and the less the curvature.

Anyway, light, when it get falls down that hole and can not escape.

I could get into Schwarzschild's radius and event horizons, but I think we have had enough on black holes for now.

A pretty heavy subject I think. . .

Answer 3

Yes understood it exactly light does not have any mass the rest mass of the photons is zero but you gone wrong in the second part!!! try to understand this the universe is balanced on the 4-d
or the space-time continnum its just nothing like a ball thorwn on the cloth the cloth is the space -time and the ball is the universe so heavier the ball the more deep it goes thats exactly whats happening near the black hole because light travels on the space time as a particle it goes into the curvature of the black hole and gets trapped but this does not say light has mass.

Answer 4

Ahem, sorry. Unless I'm strongly mistaken, I sort of understand that the main notion underlying a black hole is not its strong, possibly infinite gravity. Its main property (so far as our current model is correct) is a black hole warps space around it so strongly that it captures ANYTHING in our universe, be it light (or any other radiation) or objects having a physical mass. In other words, should black holes really exist (which some bright boys around are beginning to doubt), they would be a singularity in space/time structure, and not only in gravity alone. An object of faith among physicist is there is but ONE unitarian field, and they're doing their best to prove it at the moment. And gravity is only one of these fields, radiation is another. Hence the fact that an object without mass can be captured by a black hole.

Answer 5

Good question, and I've yet to find a good answer.
Since light is energy, and energy is made up of electrons etc. it should have a measurable mass. It could be demonstrated by the fact that light bends around strong gravity fields. But how do you figure it out?
So far, science has yet to find light that travels faster then the speed of light. Sceintists have been able to slow light for a fraction of a second.

Answer 6

The difference between a matter and light energy is that the former has some rest mass where as light energy has no rest mass.
An electron is a matter. It has a mass 9x10^ (-31) kg
which is the same as 8.1 x10^ (-14) joule of energy

If the same electron is made to move with the speed of 0.9 C, its mass becomes 20.6x10^ (-31) kg which is the same as 18.6x10^ (-14) J.

Increase in mass does not imply that some matter is included to the moving object. Increase in mass implies its kinetic energy is increased.

Thus there is no difference between increase in mass and the kinetic energy of matter.

If a force of 1 newton is acting on a mass of 1kg, we can as well say that a force of 1 newton is acting on 9x10^16 Joule, as per theory of relativity

The light is motion of energy or we can say motion of (equivalent) mass. The photons which has no rest mass, has an energy of 'hn' where n is the frequency of the wave.

‘hn’ joule of energy is equivalent to a mass of
“[hn/ 9x10^16]” kilogram.

The gravitational force between two masses implies the force on some energy because there is do difference between a mass and its equivalent energy.

Some times we use the term mass and some time we use the term energy depending upon the context.

Answer 7

The light consists of particles called photons and they are material. ‘material’ means mass or energy or both as E=mc^2, where c is light’s speed, E is energy, m is mass; as light is also an electromagnetic wave with frequency v, then energy of a photon E=hv, where h is a constant of Plank (I don’t remember how much); thus mass of a photon m = hv/c^2; now you put it in general gravity equation F=G*M*m/r^2 and see that if a photon is flying at a distance r from a black hole M, then it should be attracted with force F. and this is also Einstein!

Answer 8

That light cannot escape black hole does not imply that light has mass.The infinite gravity of black hole causes infinite curvature of space-time around the black hole due to which light gets trapped.

Answer 9

Light does have mass, as any object object approaches the speed of light it has infinite mass and infinite NO mass.

It's a paradox, - but light has to have some residual micro mass there.

Answer 10

let me see.

light does not possess mass, but one thing we can say it has is momentum..
but a better way to approach this is to rely on General Relativity..
Einstein showed that a large gravitational field can and does bend light, or at least appear to bend it. what it actually does do, is warp space in such a way that light follows a straight path, but when space itself is bent, light can't simply travels in a straight line, conisitent with the space is traveling..

does that make sense?.. its kind of weird.

anothe way to see it is this..

take a large bed sheet, stretch it flat, then place a rock in the middle of the sheet, what do you see?.. the sheet sinks in the center. anything traveling near it, will tend to change its path accordingly. light behaves in a similar way.
light traveling near this warp space field, will follow a bent space field as well..

Answer 11

Black holes are collapsed stars, thus they have mass.



Heres a nice detailed sight about black holes:
http://www.crystalinks.com/black_holes.html


Hope this helps!