I don't understand the Michelson–Morley experiment

[gen x]

I don't understand why people in Michelson-Morley experiment expect that light speed in direction in Earth motion would be light speed + earth speed, if they measure speed from earth frame?
Of course, we will get always light speed if we measure speed from Earth frame(as they did).
If car travel 100km/h and you trow a ball forward at 10km/h inside the car cabine, ball will always have 10km/h if we measure from car, but we need to measure from outside of car ("rest frame") to ball have 110km/h.

So to measure light speed+earth speed we would need somehow measure that speed from that is at rest in relation to earth, but that frame dont exist in universe..

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[Ibix]

Light was, at the time, believed to be a wave travelling in a medium called the ether. Its speed would always be $c$ relative to the medium, and would vary relative to Earth as Earth moved through the medium in different directions. This is exactly how other waves in media, such as sound waves, behave. Their speed is fixed relative to the medium but not the source.

You appear to have in mind a ballistic model where light is a series of little bullets launched by the source. In this case its speed would be fixed relative to the Earth and Michelson and Morley would also return a null result. However, it's ruled out by astronomical observations. If you observe a binary star, light coming from the component coming towards us would be slightly faster than light coming from the component moving away from us, which would cause them to appear to spend more time moving away than moving towards us. This is not observed, so the ballistic model is wrong.

 

[gen x]

Light was, at the time, believed to be a wave travelling in a medium called the ether. Its speed would always be $c$ relative to the medium, and would vary relative to Earth as Earth moved through the medium in different directions. This is exactly how other waves in media, such as sound waves, behave. Their speed is fixed relative to the medium but not the source.

You appear to have in mind a ballistic model where light is a series of little bullets launched by the source. In this case its speed would be fixed relative to the Earth and Michelson and Morley would also return a null result. However, it's ruled out by astronomical observations. If you observe a binary star, light coming from the component coming towards us would be slightly faster than light coming from the component moving away from us, which would cause them to appear to spend more time moving away than moving towards us. This is not observed, so the ballistic model is wrong.

I just want to say that is perfectly clear if you measure light speed from earth frame you will always get same speed.

If they assumed that ether is static, how they now in which direction is earth velocity if earth moves around itself, sun, milky way, other galaxyies, universes etc etc? What if all vectors of all this velocities sum up to zero? Highly unlikely but mathematically possible?

If light speed is not depend on source speed, my logic is because it is massless so source speed can not add "additional energy" to photons. Is that correct?

 

[Ibix]

If they assumed that ether is static, how they now in which direction is earth velocity if earth moves around itself, sun, milky way, other galaxyies, universes etc etc? What if all vectors of all this velocities sum up to zero? Highly unlikely but mathematically possible?

That's why they repeated the experiment multiple times over the course of a year. We could have a zero velocity relative to a static ether at one instant, but not throughout the year.

If light speed is not depend on source speed, my logic is because it is massless so source speed can not add "additional energy" to photons. Is that correct?

No. Light coming from a source approaching you will have higher energy than light from an identical source moving away from you. This is one interpretation of kinematic red and blue shifts. Light from both sources will have the same speed, however.

Photons are entities in relativistic quantum field theory and they do not behave very much like a naive idea of a particle at all. Our general advice is that it is a mistake to try to understand light behaviour in terms of photons until you can study what they actually are. There are times when fudging it with "light is made of particles" will kiind of work (Einstein's explanation of the photoelectric effect is an example) but many times when it won't. Think of light as a wave until you have a solid grasp of special relativity and non-relativistic quantum mechanics. Then you can start learning relativistic quantum mechanics and studying photons.

 

[gen x]

That's why they repeated the experiment multiple times over the course of a year. We could have a zero velocity relative to a static ether at one instant, but not throughout the year.

If source is accelerating toward you, light will comes to you again at speed c?

Our general advice is that it is a mistake to try to understand light behaviour in terms of photons until you can study what they actually are.

Who know what they actually are? They are just math constructions, but what they are?

Is this relative time ,space just math abstraction that somehow produce correct results or reality?
Does theory need to fit reality or only need to fit measurments results?

Why Tesla didnt accept relative time,space ?

 

[Ibix]

If source is accelerating toward you, light will comes to you again at speed c?

Light always travels at $c$ in vacuum.

Does theory need to fit reality or only need to fit measurments results?

How would you tell the difference between a theory that fits reality and a theory that only fits measurements of reality? Note that your answer cannot refer to any observation or measurement of reality, because that opens it to the criticism that your differentiation procedure only matches measurements.

What we do is try to find the theory with the fewest and simplest assumptions that correctly calculates the outcomes of experiments we have done. Then we use it to predict the results of experiments we haven't done. And we use other theories and see what they predict and find experiments where they predict different things and try to do those experiments to eliminate at least one theory. Our fundamental problem at the moment is that, although we strongly suspect our theories aren't completely correct, we haven't managed to find a regime where we can do experiments and in which the theories make measurably incorrect predictions. So we have a lot of very strong constraints on any new theory and very little leverage to test which one is correct.

Why Tesla didnt accept relative time,space ?

You'd have to ask him. Science isn't religion - nobody has to believe anything. The question is, can you make testable numerical predictions with what you do believe? And then, do those predictions match existing measurements? It's that last stage that has beaten all comers except (so far) relativity and quantum field theory.

 

[Nugatory]

If car travel 100km/h and you throw a ball forward at 10km/h inside the car cabine, ball will always have 10km/h if we measure from car, but we need to measure from outside of car ("rest frame") to ball have 110km/h.

Not if we are standing on the back of a flatbed truck so that the air is moving past us at 100 km/hr. The wind changes the speed of the ball relative to us according to which direction we’ve thrown it, and that’s what the MM experiment was doing with light and the aether.

 

[gen x]

Our fundamental problem at the moment is that, although we strongly suspect our theories aren't completely correct, we haven't managed to find a regime where we can do experiments and in which the theories make measurably incorrect predictions. So we have a lot of very strong constraints on any new theory and very little leverage to test which one is correct.

But today we know that Einstein theory dont work at big and micro scale, inside black hole and in quantum mechanics, we must invent dark energy to equations fits universe .
Do we need completly new theory for gravity?

 

[Ibix]

Do we need completly new theory for gravity?

Probably. And some scientists do argue that dark matter and dark energy are evidence of failures of GR (MOND theories attempt to explain galactic rotation curves as a result of modified gravity rather than dark matter, for example, and there are many attempts at a quantum theory of gravity). But they might not be - there might genuinely be kinds of "stuff" that we have only (so far) detected due to their gravitational effect. And so far nobody has managed to put together an alternative gravity theory that does away with the dark sector and also covers that second "matches all known experiments" question.

It isn't that nobody is thinking about this. It's that it is really, really hard to come up with a new theory that is both well enough fleshed out to make predictions and isn't immediately falsified by existing data.

 

[Ibix]

Dragging this back on-topic - if reality does deviate from the invariance of $c$ (or Lorentz covariance more generally) it would have to be very subtle. People do look for evidence of violations of Lorentz covariance and so far have come up empty.

 

[gen x]

Dragging this back on-topic - if reality does deviate from the invariance of $c$ (or Lorentz covariance more generally) it would have to be very subtle. People do look for evidence of violations of Lorentz covariance and so far have come up empty.

I will open new topic

 

[Dale]

Of course, we will get always light speed if we measure speed from Earth frame(as they did).

This is not necessarily true. It is true in ballistic theories of light. It is false in aether theories of light. At the time of the MMX aether theories were preferred. So under the prevailing theory of the time the statement is false.

 

[Dale]

But today we know that Einstein theory dont work at big and micro scale, inside black hole and in quantum mechanics, we must invent dark energy to equations fits universe .
Do we need completly new theory for gravity?

Probably it is best to not hijack your own thread. You already have another thread in this topic. https://www.physicsforums.com/threads/einstein-equations-dont-work-at-large-scale.1086101/ Keep this topic focused on the MMX and the other one focused on the EFE.

 

[Janus]

Why Tesla didnt accept relative time,space ?

Tesla didn't accept that the Moon rotated on its axis either. While he contributed greatly to our understanding of electromagnetics, he was not all knowing.

 

[Dale]

Does theory need to fit reality or only need to fit measurments results?

How can we check if a theory fits reality besides measurement results?

 

[gen x]

How can we check if a theory fits reality besides measurement results?

Theory can be physically incorrect(describe something that not exist at all in reality) but still produce correct results that fits measurements. There are many examples.

 

[PeterDonis]

There are many examples.

Please give some.

 

[Dale]

Theory can be physically incorrect(describe something that not exist at all in reality) but still produce correct results that fits measurements.

How do you know that a theory is physically incorrect?

There are many examples.

Are there? If so, please identify one and explain how we know that it is physically incorrect.