I am a relative beginner with a question.

There are many descriptions of the train experiment and two flashes of light.

The Michelson Morley experiment tells us that the speed of light is independent of the frame of reference.

However if an observer in "fixed space" looked at the Michelson Morley experiment would the observer see a light pulse moving in the opposite direction to the equipment located on the earth the same way that the train experiment happens for the "statuonary" observer, i.e.the light pulse reaches one end of the equipment before the other?

1 hour ago, Caruthers said:

The Michelson Morley experiment tells us that the speed of light is independent of the frame of reference.

My understanding is that Michelson-Morley determined that the aether does not affect the motion of anything propagating through it ... as if it wasn't even there.
So the aether was discarded.

As for the frame independence of the speed of light, since the 1600s, one of the tenets of Galilean relativity is that no experiment performed in an inertial frame can determine uniform motion; you can only determine accelerated motion.
Einstein's thought experiment pictured himself sitting on a beam of light and holding a mirror in front of his face.
If the speed of light was frame dependent, his reflection would not be able to 'catch up' to the mirror and reflect back to his eyes; he would see a 'black' mirror and would be able to tell he was moving at the speed of light; a violation of Galilean invariance.
To preserve Galilean relativity, he must be able to see his reflection, which means the speed of light must be the same for all observers, or frame independent.

( I have tried to keep this explanation at the 'beginner' level )

Edited August 25Aug 25 by MigL

Thank you. I understand those points. My question, though was specifically about the Michelson Morley experiment. An interferometer consisting of two light beams at r8ght angles to each were used. This, of course, was located on the Earth's surface so any observer had to be located in that frame of reference.

An observer in "fixed space" would view the light beam aligned with his eye direction as being simillar to what is referred to as a "light clock" and the other perpendicular as simillar to a passing train.

My question is about how to understand that picture. Clearly the interference pattern shows a reality, but "thought experiments" would say something differen occurs and that different parts of the equipment are experiencing time dilation in different ways.

3 hours ago, Caruthers said:

An observer in "fixed space" would view the light beam aligned with his eye direction as being simillar to what is referred to as a "light clock" and the other perpendicular as simillar to a passing train.

Good Morning and how have you been since 2021 ?

What please is this 'fixed space' you speak of ?

Remember what was said back in 2021 ?

On 11/12/2021 at 12:49 PM, studiot said:

For instance take a piece of squared graph paper.
It is a continuum. A geometric continuum.
A finite continuum, since the paper has edges, but a continuum nonetheless.

The graph paper has no 'origin'.
We can impose an origin at any point by imposing a pair of axes.
And impose scales marked on the paper.
This then gives a particular coordinate system to the grid.
It also gives us a large number of possible different coordinate systems.
 

I recognize that "fixed space" is relatively meaningless but trying to refer to some observer who is not on our planet and views the earth rotating in front of him such thst the experimental equipment passes hm by. Just trying to describe an observer not in the frame of reference of the experiment.

12 hours ago, Caruthers said:

However if an observer in "fixed space" looked at the Michelson Morley experiment would the observer see a light pulse moving in the opposite direction to the equipment located on the earth the same way that the train experiment happens for the "statuonary" observer, i.e.the light pulse reaches one end of the equipment before the other?

If the equipment moves, the mirrors are moving away from the source on one pass, but toward the source as it returns to the detector (or the opposite). The perpendicular arm has a different path, too.

The train experiment is about the simultaneity of events, and light hitting the mirrors would not be simultaneous for that observer, but nothing happens to the interference pattern

6 hours ago, Caruthers said:

Thank you. I understand those points.

I'm going to have to assume you don't.
If the speed of light is frame independent, as you have agreed, so what difference would be observed from a different inertial frame ?
All frames are valid; there is no universal frame such as 'fixed space'.

Maybe I don't then, but I am asking and trying to understand. I thougnt that the fundamental point of relativity is thst observers in different inertial frames will observe different things, so I do not understand then why you say - what difference would be observed from a different inertial frame - as that was my original, if admittedly poorly phrased, question.

My cboice of the words "fixed space" was clearly a poor choice as s9me of the replies focussed on that. Let me try a different way of describing what I mean. Consider an observer who is not moving relative to a far off galaxy such that the earth is spinning in front of him. Call that observer "Andromeda". The equipment is located on the earth. They are in relative motion.

As Swansont says above the light hitting the mirrors would not be simultaneous for "Andromeda", like the train,, though the interferometer says it is and as relativity tells us they will see different things such as simultaneity.

4 hours ago, Caruthers said:

Maybe I don't then, but I am asking and trying to understand. I thougnt that the fundamental point of relativity is thst observers in different inertial frames will observe different things, so I do not understand then why you say - what difference would be observed from a different inertial frame - as that was my original, if admittedly poorly phrased, question.

My cboice of the words "fixed space" was clearly a poor choice as s9me of the replies focussed on that. Let me try a different way of describing what I mean. Consider an observer who is not moving relative to a far off galaxy such that the earth is spinning in front of him. Call that observer "Andromeda". The equipment is located on the earth. They are in relative motion.

As Swansont says above the light hitting the mirrors would not be simultaneous for "Andromeda", like the train,, though the interferometer says it is and as relativity tells us they will see different things such as simultaneity.

Gosh you get up early, but this last post is really good (+1) as it pinpoints your misunderstanding nicely.

You may remember back to school where you learned about 'relative velocity', velocity parallelograms, velocity triangles and the like.

But all velocity is relative (to something). Velocity on its own is meaningless. The school stuff relied on a background earth that wasn't 'going anywhere'.

Fast forward to einstinian relativity. This has the same thing.

In Einsteins day the fastest known artificial things were trains, so it is no surprise that the background was taken as the tracks.

We now call this 'The Laboratory Frame@

Note I have slipped in the use of the word Frame.

But we say that everything is stationary in its own frame, meaning that the relative velocity of any body to itself is zero.

Looking forward a bit here is a question.

Two spaceships are approaching each other, one at 0.1c and the other at 0.8c.
What is their closing velocity or relative velocity?

Can you see what is wrong with this question, considering what I have just said before ?

You also mention ' the fundamental point of relativity' also called 'The principle of Relativity'

We went through this in your previous thread, but it requires that the form of the physical laws or equations are the same in all systems (GR) or the same in all inertial systems (SR).

What is does not say is that the numbers yielded by these equations will be the same, apart form the speed of light which is the second postulate.

Does this help ?

As studiot points out, some quantities are relative, such as velocity and kinetic energy.

Some are not - these are invariant quantities. This can be because of being inherently the same, or because any dependence on relative quantities cancels out, such as with the invariant interval (the variation in the spatial component cancels with the time component)

Events can’t happen in one frame but not happen in another. If two objects collide in one frame, they collide in all frames. If you don’t get a fringe shift with a M-M apparatus in one frame, that event will not differ if observed from another.

8 hours ago, studiot said:

Gosh you get up early, but this last post is really good (+1) as it pinpoints your misunderstanding nicely.

You may remember back to school where you learned about 'relative velocity', velocity parallelograms, velocity triangles and the like.

But all velocity is relative (to something). Velocity on its own is meaningless. The school stuff relied on a background earth that wasn't 'going anywhere'.

Fast forward to einstinian relativity. This has the same thing.

In Einsteins day the fastest known artificial things were trains, so it is no surprise that the background was taken as the tracks.

We now call this 'The Laboratory Frame@

Note I have slipped in the use of the word Frame.

But we say that everything is stationary in its own frame, meaning that the relative velocity of any body to itself is zero.

Looking forward a bit here is a question.

Two spaceships are approaching each other, one at 0.1c and the other at 0.8c.
What is their closing velocity or relative velocity?

Can you see what is wrong with this question, considering what I have just said before ?

You also mention ' the fundamental point of relativity' also called 'The principle of Relativity'

We went through this in your previous thread, but it requires that the form of the physical laws or equations are the same in all systems (GR) or the same in all inertial systems (SR).

What is does not say is that the numbers yielded by these equations will be the same, apart form the speed of light which is the second postulate.

Does this help ?

Yes, thank you. I am still ponderIng though

6 hours ago, swansont said:

As studiot points out, some quantities are relative, such as velocity and kinetic energy.

If velocity and kinetic energy are relative quantities does it follow that mass is also relative since KE =1/2[mv^2] ?

Or can m be viewed as a conversion factor between velocity and KE in a similar way to how c is,I think seen as a conversion factor between space and time?

5 hours ago, swansont said:

, such as velocity and kinetic energy.

7 minutes ago, geordief said:

If velocity and kinetic energy are relative quantities does it follow that mass is also relative since KE =1/2[mv^2] ?

Or can m be viewed as a conversion factor between velocity and KE in a similar way to how c is,I think seen as a conversion factor between space and time?

No. If you transform KE and v between two frames, it only works if the mass is the same.

49 minutes ago, geordief said:

If velocity and kinetic energy are relative quantities does it follow that mass is also relative since KE =1/2[mv^2] ?

Invariant ( rest ) mass is just that ... invariant.
Relativistic mass is dependent on the relative velocity of the observer.

Another way of looking at it is that the rest mass is equivalent to the rest energy via E=mc².
While the relativistic mass, or energy, takes relative motion into account via E²=(mc)²+(pc)².

Oh, and keep your finger off the 'return' key 😄 .

1 hour ago, MigL said:

Oh, and keep your finger off the 'return' key 😄 .

I’ve edited out most of the great void

11 hours ago, geordief said:

If velocity and kinetic energy are relative quantities does it follow that mass is also relative since KE =1/2[mv^2] ?

Or can m be viewed as a conversion factor between velocity and KE in a similar way to how c is,I think seen as a conversion factor between space and time?

Help! How does this apply to my question?

8 hours ago, swansont said:

the great void

+1

😀

Edited August 27Aug 27 by studiot

1 hour ago, Caruthers said:

Help! How does this apply to my question?

I don't think it does. It was just a question that occurred to me about velocity and Kinetic Energy.

I was wrong but it doesn't have much bearing on your question.

2 hours ago, Caruthers said:

Help! How does this apply to my question?

You were inquiring on the topic of relative quantities, and we’ve confirmed that mass is not a relative quantity.

1 hour ago, swansont said:

You were inquiring on the topic of relative quantities, and we’ve confirmed that mass is not a relative quantity.

I've never had to think about what makes a property relative as opposed to invariant, because I've been exposed to this stuff for almost 50 years.
It seems to me that the 'basic' tenet of Einsteinian relativity is the interchangeability/swapping of distance with time, depending on frame.
Any properties/quantities that involve distance, time, motion, etc. would be frame dependent, or relative; energy of a moving mass or charge being one such property.
Properties that are 'intrinsic' ( measured in the rest frame ) and not dependent on distance or time would then be invariant.

Is there a relativistic principle, or other law, that states this idea more clearly ?
( I remember a great discussion about a charge 'falling' in a gravitational field, and whether it radiates or not )

2 hours ago, MigL said:

I've never had to think about what makes a property relative as opposed to invariant, because I've been exposed to this stuff for almost 50 years.
It seems to me that the 'basic' tenet of Einsteinian relativity is the interchangeability/swapping of distance with time, depending on frame.
Any properties/quantities that involve distance, time, motion, etc. would be frame dependent, or relative; energy of a moving mass or charge being one such property.
Properties that are 'intrinsic' ( measured in the rest frame ) and not dependent on distance or time would then be invariant.

Is there a relativistic principle, or other law, that states this idea more clearly ?
( I remember a great discussion about a charge 'falling' in a gravitational field, and whether it radiates or not )

It is so easy to confuse yourself when pondering the subject of Relativity in general.

Most properties are measured relative to a datum or standard.

The results of measurement may also depend upon the method of measurement and/ or the circumstances of measurement.

Galilean and Einstinian relativity are about whether motion affects these measurements.

So consider an approaching meteorite, made of silicon, oxygen and iron. Its colour will change depending upon the speed of approach.

However the atomic numbers of its constituents will not, as will its overall composition not change.

However should we be able to remotely measure atomic weight (mass) this will depend upon the speed of approach.

Does this help ?

@Caruthers

Is there a problem we can help you with ?

4 hours ago, studiot said:

Does this help ?

No. Perhaps you misunderstood my question.

Galilean relatrivity is based on the notion of absolute ( background stage ) of both space and time.
Newton's laws operate in this 'domain'.

Einsteinian relativity discards this notion of absolute space and time, there is no 'preferred' frame or background stage, due to the fluid interdependence of time and space.
This does not only affect time and distances, but also properties dependent on them, such as velocities, kinetic energies, etc.

6 hours ago, MigL said:

Is there a relativistic principle, or other law, that states this idea more clearly ?

Is my question.
As it is often difficult explaining why mass is invariant, but kinetic energy is relativistic, or frame dependent.
Or why energy radiated by a moving object differs from that radiated by an object at rest ( in your frame ), although made up of the exact same atoms.

A follow up question, if I may, on

"Einstein's thought experiment pictured himself sitting on a beam of light and holding a mirror in front of his face. If the speed of light was frame dependent, his reflection would not be able to 'catch up' to the mirror and reflect back to his eyes". We can agree it does as everything is in the same frame of reference

I am struggling to describe the next thougnt, but let's say an observer in a different frame was sitting on a beam of light travelling in the opposite direction. Each believes himself to be stationary. What would he see about that situation. I think that they both see each other travelling at c, because they are. But what does the second guy see about the motion of Einstein's image and reflectiom.

Thank you for any help.

19 minutes ago, Caruthers said:

A follow up question, if I may, on

"Einstein's thought experiment pictured himself sitting on a beam of light and holding a mirror in front of his face. If the speed of light was frame dependent, his reflection would not be able to 'catch up' to the mirror and reflect back to his eyes". We can agree it does as everything is in the same frame of reference

I am struggling to describe the next thougnt, but let's say an observer in a different frame was sitting on a beam of light travelling in the opposite direction. Each believes himself to be stationary. What would he see about that situation. I think that they both see each other travelling at c, because they are. But what does the second guy see about the motion of Einstein's image and reflectiom.

Thank you for any help.

I do believe I asked you a similar question a couple of posts back.

Are going to attempt an answer ?

1 hour ago, MigL said:

No. Perhaps you misunderstood my question.

Galilean relatrivity is based on the notion of absolute ( background stage ) of both space and time.
Newton's laws operate in this 'domain'.

Einsteinian relativity discards this notion of absolute space and time, there is no 'preferred' frame or background stage, due to the fluid interdependence of time and space.
This does not only affect time and distances, but also properties dependent on them, such as velocities, kinetic energies, etc.

Is my question.
As it is often difficult explaining why mass is invariant, but kinetic energy is relativistic, or frame dependent.
Or why energy radiated by a moving object differs from that radiated by an object at rest ( in your frame ), although made up of the exact same atoms.

Sorry if it was not clear.

Yes I am well aware of the difference between Galilean relativity and Einstinian R.

But don't they both satisfy the principle of relativity in that they expect the laws of physics ( as they then knew them) to operate the same in different moving frames ?

Of course they did not know about the Doppler effect, let alone the relativistic Doppler effect

Is a background stage not a different way of providing compliance within their knowledge ?

But Lorenz transformations are not needed in this system.

It is the system I called the schoolboy system for the OP.

The speed of light is not invariant in this system.

In Einstinian relativity -

Swansont has already pointed out that rest mass is invariant, but relativistic mass is not.

If you had three bodies ( say A , B, and C) moving relative to each other would you not choose the frame of one of them as the rest frame or the laboratory frame ?

So if you chose C then you could calculate the velocity of A relative to B and find it was exactly equal and opposite to the velocity of B relative to A.

Edited August 27Aug 27 by studiot

"Einstein's thought experiment pictured himself sitting on a beam of light and holding a mirror in front of his face"

I have a follow up question on this. I think we (I) understand this as everythiing is in the same frame.

His mirror would/could also act as a rear view mirror in whivh he could see an observertravelling in the opposite direction such that he sees Einstein travelling at the speed of light. How would that oberver appear to Einstein in the rear view mirror