Gravity Deflects Light & Massless Particles: Einstein & Susskind

[jeremyfiennes]

In his GR youtube talk ( , starting 24:30), Susskind shows that a light photon on straight path in a stationary frame has a curved path in an accelerated frame. Concluding, as did also Einstein, that gravity deflects photons. But exactly the same argument applies to massless particles. Meaning that these too should be subject to gravity, which they are not. Zero marks for logical reasoning, both Einstein and Susskind?

 

[PeroK]

In his GR youtube talk ( , starting 24:30), Susskind shows that a light photon on straight path in a stationary frame has a curved path in an accelerated frame. Concluding, as did also Einstein, that gravity deflects photons. But exactly the same argument applies to massless particles. Meaning that these too should be subject to gravity, which they are not. Zero marks for logical reasoning, both Einstein and Susskind?

Zero marks for Jeremy Fiennes, I'm afraid!

In Newton's gravity, only objects with mass are affected by gravity. In GR, as the theory goes, light and any other massless particles are affected by gravity. This is, then, a key test of GR against Newton.

The first test of this was during a total eclipse of the Sun. Light that passed close to the Sun (normally we wouldn't see this light) was indeed deflected and the stars behind the Sun appeared out of position. That was a big moment for GR in terms of its acceptance.

 

[Ibix]

But exactly the same argument applies to massless particles. Meaning that these too should be subject to gravity, which they are not. Zero marks for logical reasoning, both Einstein and Susskind?

You are aware that photons are massless particles, right?

 

[Orodruin]

In Newton's gravity, only objects with mass are affected by gravity.

Even this is debatable. In the limit of mass going to zero for particles in classical mechanics, acceleration remains constant.

 

[Dale]

Zero marks for logical reasoning, both Einstein and Susskind?

That is a little harsh. Even if you think they are wrong (which they are not), I would think that they would get a decent amount of partial credit for working through the problem so carefully.

That said, you are definitely in the wrong regarding GR, and although it is not as clear as in GR I also believe that you are in the wrong regarding Newtonian gravity.

 

[jeremyfiennes]

I maintain: zero marks for both. The argument is purely geometrical. And therefore applies to anything, real or hypothetical, massful or massless, traveling in a straight line in the stationary reference frame. One must therefore conclude from it that anything, real or hypothetical, massful or massless, is subject to gravity. Which is nonsense.

 

[Ibix]

I maintain: zero marks for both.

Arrogant, much?

The argument is purely geometrical. And therefore applies to anything, real or hypothetical, massful or massless, traveling in a straight line in the stationary reference frame. One must therefore conclude from it that anything, real or hypothetical, massful or massless, is subject to gravity. Which is nonsense.

Why do you think it's nonsense? The bit about "hypothetical" is neither here nor there; if it's a hypothetical something it's not real and therefore not detectable. Thus this is an unscientific untestable claim. But everything else is affected by gravity, massive or massless. As @PeroK pointed out at length above, we've tested it.

 

[Dale]

One must therefore conclude from it that anything, real or hypothetical, massful or massless, is subject to gravity. Which is nonsense.

That is correct. Anything with or without mass is subject to gravity. Not only is it not nonsense it has been experimentally demonstrated. The universe agrees with Einstein/Susskind and gives the failing marks to you (although I expect that the universe is kind and won't give you a 0).

Please be advised, this site is for learning mainstream physics, not for asserting personal theories, particularly not ones that have been experimentally contradicted.

 

[PeterDonis]

One must therefore conclude from it that anything, real or hypothetical, massful or massless, is subject to gravity. Which is nonsense.

No, it isn't. Light is subject to gravity. Light bending by massive objects has been directly measured.

The OP is based on a mistaken belief about actual experimental evidence. Thread closed.