# Michelson Morley experiment

AhmedHesham
In Michelson Morley experiment we use a moving
mirror to reflect light and then we find out that light
moves with c regardless of the speed of the mirror.
My question is why do we need a theory to replace
the galilian composition of velocities?i mean this
can be explained by saying that light moves with c
if we have an accelerating charge regardless of the
way it accelerates .We don't need the change the
Galilean law because it has nothing to do with this.

Mentor
In Michelson Morley experiment we use a moving
mirror to reflect light and then we find out that light
moves with c regardless of the speed of the mirror.

No, that's not what the M-M experiment does. The mirrors are all at rest relative to each other. What changes is the state of motion of the Earth, which is carrying the experimental apparatus along, relative to the rest of the universe. Michelson and Morley thought that the changing state of motion of the Earth would cause the speed of light to change. They found that it didn't.

why do we need a theory to replace
the galilian composition of velocities?

Because if Galilean composition of velocities is correct, light will not move at speed c in the Michelson-Morley experiment regardless of the state of motion of the Earth.

this
can be explained by saying that light moves with c
if we have an accelerating charge regardless of the
way it accelerates

What does an accelerating charge have to do with the Michelson-Morley experiment?

AhmedHesham
No, that's not what the M-M experiment does. The mirrors are all at rest relative to each other. What changes is the state of motion of the Earth, which is carrying the experimental apparatus along, relative to the rest of the universe. Michelson and Morley thought that the changing state of motion of the Earth would cause the speed of light to change. They found that it didn't.

Because if Galilean composition of velocities is correct, light will not move at speed c in the Michelson-Morley experiment regardless of the state of motion of the Earth.

What does an accelerating charge have to do with the Michelson-Morley experiment?
The Galilean law describes how things move relative to each other. Michelson Morley experiment says light moves with c regardless of the motion of the mirror that reflects that light. Why is it said that Galilean law doesn't fit with the experiment?

2022 Award
The Galilean law describes how things move relative to each other. Michelson Morley experiment says light moves with c regardless of the motion of the mirror that reflects that light. Why is it said that Galilean law doesn't fit with the experiment?
As @PeterDonis points out, that's not what Michelson and Morley found. They found that the speed of light is always c regardless of the state of motion of the entire experiment. Imagine doing a similar experiment with sound. You would get different results depending on whether or not you were moving with respect to the air. Michelson and Morley expected that they would find a similar effect as the Earth moved through the ether, the supposed medium that supported electromagnetic waves. But they found nothing. The speed of light was always the same in all directions at all times of the year.

This is a strong indication that the speed of light is the same in all frames.

AhmedHesham
Mentor
My question is why do we need a theory to replace
the galilian composition of velocities?
We need a theory to replace the Galilean composition of velocities because there are more experiments than just the MMX that need to be explained. Robertson (Rev. of Mod. Phys. 21, pg 378 (1949)) showed that the Lorentz transform could be deduced from three key experiments: Michelson Morely, Kennedy Thorndike, and Ives Stillwell.

https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.21.378

AhmedHesham
We need a theory to replace the Galilean composition of velocities because there are more experiments than just the MMX that need to be explained. Robertson (Rev. of Mod. Phys. 21, pg 378 (1949)) showed that the Lorentz transform could be deduced from three key experiments: Michelson Morely, Kennedy Thorndike, and Ives Stillwell.

https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.21.378
Do all of them involve light. If so they are all equivalent to Michelson Morley experiment

Mentor
Do all of them involve light. If so they are all equivalent to Michelson Morley experiment
Did you read the paper? What did it say?

And no, not all experiments using light are equivalent to MMX. Not even all interferometer experiments are equivalent.

AhmedHesham
Did you read the paper? What did it say?

And no, not all experiments using light are equivalent to MMX. Not even all interferometer experiments are equivalent.
I mean all results are light have the same speed regardless of the motion of the source or the observer.

Staff Emeritus

You really do want to be reading the answers - including answers pointing you to a paper. If you don't, people are likely to stop providing them to you.

Dale
Mentor
The Galilean law describes how things move relative to each other.

Yes, including light. Which means the Galilean law says that light moves at different speeds relative to things in different states of motion. But as you yourself point out:

all results are light have the same speed regardless of the motion of the source or the observer.

Do you see the issue?

AhmedHesham

You really do want to be reading the answers - including answers pointing you to a paper. If you don't, people are likely to stop providing them to you.
I am sorry. Forgive me I was busy that time so I could not read it. Thanks

Staff Emeritus
I was busy that time so I could not read it.

Maybe you should read the answers you already received before asking more questions. If you make a habit of not reading the answers you get, people may draw the conclusion that writing them is just a waste of time.

AhmedHesham
AhmedHesham
Maybe you should read the answers you already received before asking more questions. If you make a habit of not reading the answers you get, people may draw the conclusion that writing them is just a waste of time.
OK ,thanks.

Gold Member
The Galilean law describes how things move relative to each other.

Sure, but that description doesn't match what's observed. If it did we'd be accelerating particles to light speed on a desk top. As it is, we have accelerators that are larger than some cities and they cannot and never will get particles to reach the speed of light.

All of this is well understood. Your original question seems predicated on a misunderstanding of the Michaelson-Morley experiments. Best to sort that out before moving on. There have been lots of other experiments like it, and many many others unlike it since the famous 1889 version. It really is ancient history.

SiennaTheGr8
I think the details of the MM experiment are sometimes overemphasized in introductory SR. For my money a simpler way to reveal an inconsistency between Maxwell and Newton is via dynamics: if mass is additive—and if energy and momentum are indeed conserved—then there's no way that something massless like light can carry momentum (radiation pressure was detected and widely accepted before 1905).

That becomes abundantly clear when you try to make sense of the thought experiment in Einstein's original ##E=mc^2## paper without SR. If a body at rest emits two equally energetic light waves simultaneously in opposite directions so that it remains at rest (momentum conservation), then it ostensibly loses no mass in the process (because light is massless and mass is ostensibly additive), but then an observer in any other rest frame runs into a contradiction: on the one hand, neither the body's velocity nor (ostensibly) its mass changes, which means its momentum likewise (ostensibly) doesn't change; on the other hand, Galilean relativity and/or the Doppler effect guarantee that the light waves' momentum vectors don't cancel for the moving observer, which means the body's momentum must change.

Of course, the results of experiments like MM are cited to justify Einstein's second postulate, but the point is that you don't need to consider the whole interferometer setup to see that something is rotten in the state of Denmark.

Dale
Homework Helper
Gold Member
I think the details of the MM experiment are sometimes overemphasized in introductory SR. For my money a simpler way to reveal an inconsistency between Maxwell and Newton is via dynamics: if mass is additive—and if energy and momentum are indeed conserved—then there's no way that something massless like light can carry momentum (radiation pressure was detected and widely accepted before 1905).

While I agree that the "details of the MM experiment are sometimes overemphasized", I don't agree that "a simpler way... is via dynamics".
Essentially, the MM experiment is a pair of arbitrarily oriented light-clocks whose round-trip times are compared.
Since we didn't have high-resolution clocks, it was clever to study an interference pattern... so the optical properties of the apparatus are featured in textbooks.

In my opinion, we don't need concepts of mass, energy, and momentum to see that the postulates of SR are in conflict with Galilean/Newtonian physics... kinematics is sufficient.