John asked in Science & MathematicsPhysics · 4 weeks ago

# If light has zero mass why is a light beam deflected when it passes close to a large body?

If light has zero mass why is a light beam bent when it passes close to a large body?

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• 2 weeks ago

I think the most effective way to understand it is how does Einstein think about it..

I will refer to his "Elevator thought experiment"..

Imagine you are in an elevator, going upward in a constant acceleration.. if the speed of the elevator is, lets say, up to 50% the speed of light..

We already know what it feels like when an elevator goes up, right? it feels like there is a (1) FORCE PULLING DOWN, and you know inside the elevator, you have (2) STATIC FRAME OF REFERENCE (you are at rest)..

Einstein try to imagine, if you have a flash light in your hand, and you turned it on perpendicularly to the direction of the elevator, we would EXPECT that the (3) LIGHT WAVES WILL BEND due to the elevator moving up to 50% of its own speed (light speed)..

imparting the 1st postulate of special relativity, it states that

"in all frame of reference, the laws of physics are the same.."

As we analyze the thought experiment, it does follow the laws of physics. Then, Einstein theorize (if we focus on your frame of reference inside the elevator) it is the same as you are standing in a surface of a planet (you have a static frame of reference) the gravity you experience is like the FORCE PULLING DOWN like in an elevator. Therefore, we would also expect that the light waves will bend due to the presence of gravity.

The space-time fabric idea, (im not sure, because it was not stated on Special Relativity) were theorize from his General Theory of Relativity...

Thats all I know.. :)

• D g
Lv 7
4 weeks ago

Light has 0 rest mass because it never stops it actually has relativistic mass because of its velocity

• 4 weeks ago

The light does not bend, the space through which the light is passing, bends due to gravity.

• Jim
Lv 7
4 weeks ago

Light does a lot of strange things, why be surprised when it follows a curved space-time?

Did you know if you orbit Earth, your clock is different to mine??? Time changes. Light bends in a field, lots of things we can see with accurate experiments.

• 4 weeks ago

Here's a thought:  General relativity says that the curvature of spacetime (gravity) depends on the distribution of mass and energy. Perhaps we can say that energy (like mass) also responds to gravity.

• 4 weeks ago

Everything is affected by gravity. Gravity is a curvature of space. Light follows a geodesic thru curved space.

• 4 weeks ago

Another way to look at it is that while light has no rest mass, it does have inertial mass - mass equivalent to the velocity of the light.  That is also the reason why a "Light Sail" works.

• NCS
Lv 7
4 weeks ago

Don't think of gravity per se but rather the bending of the space-time fabric caused by massive objects. Light simply follows these bends.

See here: