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# A thought experiment about light

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If I have two perfectly balanced boards. I shoot a pinball between the two boards at a perpendicular angle (90°), the pinball will bounce back and forth in the same position between the two boards. I think everyone will have no objection to this point, because this pinball is affected by inertia, and the rotation of the earth has no effect on it.

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Now presume that light is not affected by inertia.

We set up two perfectly balanced mirrors. One faces south and the other faces north. I can set up a light source in them, keep the light being reflected back and forth at a fixed point in the two mirrors. Then the angle of the light source must not be perpendicular to the two mirrors, but slightly to the east. This is because the earth rotates from west to east. If the light is not affected by the earth’s rotation, the light source must be slightly east to catch up the two mirrors in motion.

My question is: If I rotate the mirrors and light source that have been set up above by 180°, the light source will be slightly offset to the west. Will the light be reflected back and forth at those two points?

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First of all, does light have inertia? The most simplest of model would say yes. In fact many books and scientists will tell you it has a mass. This is because it's energy has units of mass so we writ

I never witnessed any experimental evidence personally. So I doubt everything. I believe in photons because I think photon is the lowest unit of light. Light must be composed of something, and it

First, in your "pinball" experiment, you would also need the pinball not affected by gravity or be air resistance.

I am not sure what is meant by "not affected by inertia"

Light does not, strictly speaking, have mass but it does have energy which, by "E= mc^2", is equivalent to energy so light HAS  inertia.

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6 hours ago, HallsofIvy said:

First, in your "pinball" experiment, you would also need the pinball not affected by gravity or be air resistance.

I am not sure what is meant by "not affected by inertia"

Light does not, strictly speaking, have mass but it does have energy which, by "E= mc^2", is equivalent to energy so light HAS  inertia.

"not affected by inertia" I mean  "the speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source".

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Okay, that is true but has nothing to do with inertia.

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On 4/13/2021 at 8:47 AM, HallsofIvy said:

Okay, that is true but has nothing to do with inertia.

Would you mind explain light in the inertia?

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Ignore this thought experiment first now, because I think what you are trying to say is already known in physics, it's called the Shapiro effect. Not only does light take a bit longer coming back from a gravitational source, but when spin is included we find that light also takes a bit longer when moving against a rotating system. So for quick instance, light moving in the flow of the Earth's spin is a little faster than a signal moving against the spin. The reason why is because of a mixture of ideas in physics. The rotation of the Earth does effect the motion of a system, gravity itself effects the time it takes for light to leave a source. You can say the delay caused by rotation arises from a drag effect of gravity that we commonly call frame dragging. Torsion itself, though not very well understood, is subtly linked to the whole idea of frame dragging. While many articles can mislead us into thinking they are two different phenomena, the truth is they are effectively saying the same thing in regards to how mass can twist spacetime around with it. When speaking about the Shapiro effect in terms of signal pulses from a satellite, like a moon, or a distant planet, it's called the gravitational Shapiro effect. When the body is rotating, it's simply called the rotating Shapiro delay.

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Posted (edited)

Just a final thing, while some may claim frame dragging and torsion as two different things, the ability of a rotating mass to twist space with it is so similar it begs the question whether the frame dragging of Earth's gravity could just be a low energy (Newtonian approximation) of torsion.

Edited by Dubbelosix
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Posted (edited)

Here's a nice collection of discussions by people who don't really know what they are talking about:

They have the math right about torsion, but the objections are wrong. Torsion doesn't naturally fall out of GR at all, it's actually a choice of the model. One poster went as far to say there is no torsion in general relativity, when this isn't even true. In fact models which have torsion explain reality as we understand it, just as well without it. Torsion should be part of the Poincare group, so it's largely a myth that people have adopted thinking it vanishes in general relativity. Truth be told, most authors just ignore torsion because it's just a simpler case.

Edited by Dubbelosix
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Posted (edited)

And these things where facts I brought up on science.net with Mordred who agreed with these essential things I highlighted, citing papers as evidence to what I was stating.

From the bivector essays I made, torsion naturally appeared without all the issues of whether it should vanish or not, the torsion was simply there from the geometric parts of the theory.

Edited by Dubbelosix
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20 hours ago, Dubbelosix said:

And these things where facts I brought up on science.net with Mordred who agreed with these essential things I highlighted, citing papers as evidence to what I was stating.

From the bivector essays I made, torsion naturally appeared without all the issues of whether it should vanish or not, the torsion was simply there from the geometric parts of the theory.

Thanks for your information, it is too much to me.

It sounds like somebody has already done this experiment, do you know the name and the result? To me, this experiment can prove light is affected by inertia or not. So I really want to know it.

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Posted (edited)

First of all, does light have inertia? The most simplest of model would say yes. In fact many books and scientists will tell you it has a mass. This is because it's energy has units of mass so we write for the mass of a photon

m = E/c^2

Why though when a photon is commonly said to have no mass at all? It may be faulty, but one school of thought will teach you that light has mass but no matter. There's a simple experiment which may support this. When a photon is absorbed by an atom, or any system, it contributes to the matter of the system. So technically speaking, some distinguish between mass and matter. So keep that under your hat.

On the other hand, the Sagnac effect does not prove directly the photon has inertia. Only that it makes a statement about how the photon is affected by a dynamic medium where gravity stretched space and rotating bodies pulls space in certain directions.

Edited by Dubbelosix
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The light has mass but no matter is further derived from the momentum it has, so we distribute one factor of the velocity and we get

p = mv = E/c(v/c)

With v/c linked to things like the Doppler shift and fine structure. It even has an energy, so we can further write

Energy = E(v^2/c^2)

So we know that light has units of mass indicating it may have inertia. It has evena momentum, so along came Einstein who realised that the best way to approximate, to near accuracy, a relatistic equation which encodes motion of a particle that has (matter)

E =sqrt(M^2c^4 + p^2c^2)

The first term in the parenthesis refers to rast mass, so when the momentum is not large, it approximated for a heavy particle to simply as

E = Mc^2

For a photon, the matter term is set to zero and we get a simple relationship for any radiation as

E = pc

Yet it still is measured in units which have mass encoded in (p). So I'm nit saying this view is right, but it does have some reasonably solid experimental backing thay we need to be careful maybe of whether we should distinguish between the words, mass and matter as non trivial terminology. Still when one talks today about a photon, we say it's massless, but only because it has no rest mass denoted in the notation above with a large M.

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Posted (edited)

When being picky about the issue of mass and matter, I once said it was itself a semantic thing. Because it may still turn out a photon has directly matter, but it has to be so small like 10^-51g. The idea was atleast taken seriously by many capable minds. Debroglie went as far to say by coherence of his own theory, light must have matter. The issue, while one theoretical model has pervaded, has not completely ruled this upper limit out by experiment.

Edited by Dubbelosix
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15 hours ago, Dubbelosix said:

The light has mass but no matter is further derived from the momentum it has, so we distribute one factor of the velocity and we get

p = mv = E/c(v/c)

...

Thanks for your explaintion. But I am a Relativity skeptics.

I admit that my expression is wrong. What I want is some proof that  "the speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source". I think this experiment can prove it or not.

Each time I asked some questions and always got some theory answer. I care about fact more than theory.

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Posted (edited)

Well that's not a good start is it? You wanted evidence of a photon having inertia, you say you're not able too do the math and even in light of evidence from relativity you'd rather ignore that evidence, because "you don't trust relativity"

Well you should.

Edited by Dubbelosix
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13 hours ago, Dubbelosix said:

Well that's not a good start is it? You wanted evidence of a photon having inertia, you say you're not able too do the math and even in light of evidence from relativity you'd rather ignore that evidence, because "you don't trust relativity"

Well you should.

I don't trust lativity, and I don't trust anything.

I want to know "the speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source" is correct or not. To me, facts are more persuasive then theory.

Is there any factto support it?

I think this experiment can prove it or not easily, it is easier than Michelson–Morley experiment to do. I am curious if a similar experiment has been done.

Based on your understanding of the theory of relativity, can you tell me what the result of this experiment will be?

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On 4/16/2021 at 3:59 AM, Dubbelosix said:

So we know that light has units of mass indicating it may have inertia. It has evena momentum, so along came Einstein who realised that the best way to approximate, to near accuracy, a relatistic equation which encodes motion of a particle that has (matter)

If photon has momentum, could that be possible:

When two photons collide, part of the momentum of one is transferred to the other?

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