Isn't the observed effect of gravity proof that light has mass?

Question

Example: A "Black Hole" has enough gravity to keep light from escaping?

Forgive my ignorance but I think I may now have a better understanding please advise IF I have it right. Space is not just empty so much as it is like a fluid or jello and objects in that fluid displace it causing a rippling effect. anything caught in the ripple is pulled towards the source of the ripple. So in my simple mind Space is to the Universe what Water is to a Fish.

Answer 1

You are defining gravity as an attractive force between two massive objects. This definition comes from Newtonian physics. If we assume this definition and accept that gravity is a force between two massive objects, then gravity's effect on light certainly would suggest that light has mass.

However, we now know not to define gravity that way. In fact, some suggest that we shouldn't even use the word "gravity." "Gravity" is the name of the force that Newton described. Because we now know that force does not exist, it may be best if we don't talk about "gravity" at all.

Newtonian physics does a decent job explaining gravity as an attractive force between two massive objects. However, there are a number of problems with the idea.

For one, this model assumes that if one massive object suddenly disappears or moves away very quickly, its gravity will change instantaneously. For example, imagine that the moon suddenly disappeared. The light bouncing off of the moon just before it disappeared would still take a small amount of time to get to the earth, so for a few moments everyone on earth would still think the moon was there. However, since the moon's gravity immediately disappeared with it, the tides (which are caused by the moon's gravitational tug) would immediately react. In other words, we'd still SEE the moon in the sky over the ocean; however, the ocean underneath it would rush in (or out) to return to wherever it would be without the moon's gravity. There is something that is displeasing about this idea. It is as if "information" from the moon reached the tides via gravity FASTER than light. This just doesn't seem right.

Similarly, in Newtonian physics we expect a certain orbit around the sun due to the tug between the sun and the earth. Keep in mind that that tug is partly determined by the distance between the earth and the sun. As the earth goes around the sun, the distance between the two of them changes, and Newtonian physics assumes that as the distance changes, the gravity between them immediately changes. However, while the earth travels roughly as Newtonian physics suggests, there are slight deviations that simply cannot be explained with Newton's model.

And so what general relativity does is to say that gravity is NOT a force between two objects. Instead, it is a change in the space (and time) around objects due to the presence of mass. Even if an object has no mass (like light) if the space is deformed around it, it will still feel an impact of that change in curvature.

Now, this interpretation solves all of our problems. Now there's no need to think about an instantaneously changing force. If we just put everything into a context where they float through a lumpy spacetime that gets deformed by massive objects, it explains all of our observations. If an object disappears in space, the deformation in that space is removed, which causes ripples in the space around it. In other words, it takes time for a change in mass in one place to have an impact on another place. It is this effect that properly models the deviations from Newtonian physics that we observe.

So now that we have abandoned the idea that gravity is a force, then the observation of its effect on light does *NOT* suggest that light has mass. In fact, it suggests that the space around the light has been deformed thus forcing the light to travel a different path.

Answer 2

The equation:
gravitational force = Fg = (G)(m1)(m2) / (r^2)
(where G is a constant,
m1 & m2 are masses of objects 1 & 2,
and r is distance between objects 1 & 2)
is only an approximation of the real story, which is actually a more complex equation.

When you want to approximate the Fg (gravitational force) on light, you use the light's mass equivalent to plug into the above equation.

Light's mass equivalent can be calculated using Einstein's equation E = mc^2, where m would be the mass equivalent.

No,
the bending of light by objects with mass does not prove that light has mass. The force of gravity is not necessarily a function of mass.

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A previous answerer (nObody) had used the analogy of light travelling along a line. It is sort of right.

Light travels at the maximum speed of anything we know (3.0 x 10^8 m/s) because it takes the shortest path. This path is curved when an object has curved the space around it. Things that travel at a speed less than that of light, must take a path longer than that of light.

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Answer 3

Not really. It's not the gravitational force as such that prevents light from leaving the blackhole, but the curvature of the spacetime around the singularity - the light always travels a "straight line", but massive objects affect the geometry of the surrounding space such that these "straight lines" bend slightly. Around a blackhole though, they are bent so much, that loop right back into the singularity - and that is why photons, following those lines, get "sucked" back, and cannot escape, not because of the gravitational interaction

Answer 4

nope it isn't a proof

mass curves space. objects continue to follow "straight lines", i.e. shortest paths possible, only in a curved space (or more exactly, spacetime).

if the curvature is large enough, those lines are closed circles and nothing can escape, irrespective of mass.

this is one of the great findings of Einstein's General Relativity: gravity can influence the paths of even massless objects. whereas in Newtonian Gravity, massless objects should not see their paths curved.

experiments have clearly proved Einstein right

Answer 5

It depends on your interpretation of what is gravity. According to the theory of general relativity, gravity is curved space...not some magical force that attracts objects over great distances...therefor, even masses objects like light can be affected by curved space...hence the bending of light by the sun.

Answer 6

Gravity curves space and light travels in a straight direction trough space. If space is curved, the straight path is curved.
ie: if we lived in a 2d space universe on the surface of a cylender we would not know our straight path is curved when we go around the cylender.

Answer 7

Possibly. No one really knows how gravity works though, except that it's related to mass. The formula that expresses the amount of force exerted due to gravity [ (G*m1*m2)/r^2 ] is just a mathematical representation - it doesn't mean that's all there is to the story.

Answer 8

Not exactly. Energy has no mass (since mass is measure of matter) but energy is affected by gravity and other energy waves cannot escape black hole either

Answer 9

"the observed effect of gravity"
all stellar bodies that have mass also have an electromagnetic field. how do we know it's gravity bending light not electromagnetics?

Answer 10

The photon has mass, but not rest mass.