Physics is full of concepts that are counterintuitive. Inertia is an early example of one such concept. But ideas are adopted based on their explanatory power, and one way you get places is with an ethos called 'shut up and calculate'. I.e., don't think too hard about the 'counterintuitive' nature of what the theories predict, crunch the numbers and figure out what the predictions are and leave the implications to the philosophers.
Once you accept that the speed of light is constant in all reference frames, relativity falls out from that. That in itself is a bit hard to accept, though, which was why it was some number of years between the Michelson-Morley experiment in 1887 and Einstein's paper on SR in 1905. GR basically also comes from 'shut up and calculate' after accepting the principles behind SR but adapting them to non-inertial reference frames (and also the insight behind gravity being a fictitious force), but the mathematics is much more complicated than SR, which I assume is why it took Einstein over a decade to work it out.
> Physics is full of concepts that are counterintuitive. Inertia is an early example of one such concept.
Huh?!? Try to start a car by pushing it -- or stopping it, even on flat ground, after it's gathered momentum rolling down some slope -- a few times, and inertia becomes utterly intuitive. I'd imagine that worked for medieval oxcarts, too. Or for moving blocks of stone for the pyramids, or Stonehenge.
Perhaps, but prior to Galileo (or perhaps Avicenna?), Aristotelian physics dominated and motion was explained by 'impetus' (this didn't originate from Aristotle, but instead came long after him). Aristotle distinguished between 'natural' motion, which is the motion of falling objects and 'violent' or 'unnatural' motion, which is caused by some agent (a human throwing a stone, for instance). In the theory of impetus, the agent imbues an object with some impetus, and this impetus is only ever temporary, the object stopping once its impetus is exhausted.
Today, sure, we know that friction is a force that acts against the momentum of a moving object, but these were alien concepts a thousand years ago.
The very idea of using the 1887 experiment from Stone Age to defend GR seems ridiculous. I understand why you are doing that: there are no modern experiments, which you can use to defend your point, but still. Moreover, interpretation of Michelson-Morley experiment as «it proves that physical vacuum does not exist» is frivolous. Let me quote Wikipedia:
> The Michelson–Morley experiment of 1887 had suggested that the hypothetical luminiferous aether, if it existed, was completely dragged by the Earth. To test this hypothesis, Oliver Lodge in 1897 proposed that a giant ring interferometer be constructed to measure the rotation of the Earth; a similar suggestion was made by Albert Abraham Michelson in 1904.
> the first interferometry experiment aimed at observing the correlation of angular velocity and phase-shift was performed by the French scientist Georges Sagnac in 1913. Its purpose was to detect "the effect of the relative motion of the ether".
> In 1926, an ambitious ring interferometry experiment was set up by Albert Michelson and Henry Gale. The aim was to find out whether the rotation of the Earth has an effect on the propagation of light in the vicinity of the Earth. The Michelson–Gale–Pearson experiment was a very large ring interferometer, (a perimeter of 1.9 kilometer), large enough to detect the angular velocity of the Earth. The outcome of the experiment was that the angular velocity of the Earth as measured by astronomy was confirmed to within measuring accuracy. The ring interferometer of the Michelson–Gale experiment was not calibrated by comparison with an outside reference (which was not possible, because the setup was fixed to the Earth). From its design it could be deduced where the central interference fringe ought to be if there would be zero shift. The measured shift was 230 parts in 1000, with an accuracy of 5 parts in 1000. The predicted shift was 237 parts in 1000.
So, Michelson-Morley experiment confirmed that physical vacuum is dragged with Earth, and then Michelson–Gale–Pearson experiment confirmed that it is not completely dragged by Earth. It far from «physical vacuum doesn't exist» mantra from «shutup and calculate» guys.
In this century,
* the existence of Higgs boson, which gives mass, and Higgs field, which exists at every point of our Universe, was proved in 2012;
* the existence of gravitational waves in physical vacuum is proved by LIGO/Virgo interferometers.
Thus, today we have Higgs field, which is presented at every point, which can be used as 0 point, but we cannot detect it. However, we can use CMB (light from distant galaxies with red shift z=1000) as 0 point.
> The very idea of using the 1887 experiment from Stone Age to defend GR seems ridiculous.
Previous comment about SR, although GR is a natural extension of SR for non-inertial reference frames.
> Moreover, interpretation of Michelson-Morley experiment as «it proves that physical vacuum does not exist» is frivolous.
The interpretation of it is that it proves the constancy of the speed of light. This is the case whether SR is assumed or Lorentz ether theory. The two theories result in identical predictions, except LET assumes the existence of an ether but without any way of detecting it.
> So, Michelson-Morley experiment confirmed that physical vacuum is dragged with Earth, and then Michelson–Gale–Pearson experiment confirmed that it is not completely dragged by Earth.
Michelson-Morley rules out a stationary ether, at least without length contraction which Lorentz added to attempt to repair the theory. Michelson-Gale-Pearson rules out ether drag. SR and LET are compatible with both experiments. Whether or not the ether is real is philosophical quibbling without a method for separating LET from SR.
> * the existence of gravitational waves in physical vacuum is proved by LIGO/Virgo interferometers.
This is one of the predictions of GR, so I'm not sure what your point here is.
> Thus, today we have Higgs field, which is presented at every point, which can be used as 0 point, but we cannot detect it. However, we can use CMB (light from distant galaxies with red shift z=1000) as 0 point.
> IMHO, relativity is not necessary at this point.
The Higgs field, just like other fields in quantum field theory, is a relativistic field; its Lagrangian is Lorentz-invariant.
> Previous comment about SR, although GR is a natural extension of SR for non-inertial reference frames.
Sorry, but SR is a special case of GR now, so not a big deal.
> The interpretation of it is that it proves the constancy of the speed of light. This is the case whether SR is assumed or Lorentz ether theory. The two theories result in identical predictions, except LET assumes the existence of an ether but without any way of detecting it.
Now we have LIGO and Virgo interferometers, which clearly shows that speed of light in pure vacuum is not constant.
Now we know that if we will put an interferometer into space, it will detect deviations in speed of light. (We know it as gravitational lensing).
Now we know that gravitational waves exist and are conducted by a medium (Higgs field/Higgs boson).
Now we know that gravitation is a bit faster than light, so photons are not flying on 100% of C, thus they are not immortal, so they are losing energy with time (Red Shift).
If you say that GR/SR is still valid, despite opposing results from experiments, then those experiments are irrelevant for GR/SR at all. Regardless of what experiment will found, scientists will be able to find a way to explain results in terms of GR. With enough number of constants, a formula can make paintings or generate music after all.
> Michelson-Morley rules out a stationary ether, at least without length contraction which Lorentz added to attempt to repair the theory. Michelson-Gale-Pearson rules out ether drag. SR and LET are compatible with both experiments. Whether or not the ether is real is philosophical quibbling without a method for separating LET from SR.
If we put a sail into a box, to test the existence of a wind, then we may find that there is no wind in the box, if the box is tight enough. It can be interpreted as «there is no wind, so no atmosphere, we are on a Moon», or it can be interpreted as «there is no wind in the box». IMHO, the second variant is valid.
For me, Michelson-Gale-Pearson experiment doesn't rule out drag. Fizeau shows that drag exists, but it works in heavy mediums. Physical vacuum is too lightweight to change the course of photon just by itself. It will work at the scale of the Solar system.
> The Higgs field, just like other fields in quantum field theory, is a relativistic field; its Lagrangian is Lorentz-invariant.
Nobody says that Higgs field is stationary. It expected that it will move and behave like any other field, with currents and gradients of stength. However, the average of Higgs field of our visible Universe can be used as a reference frame. I interpret CMB as light of distant galaxies with z=1000, so it can be used as representation of this average. For example, the Sun moves at a cumulative 369 km/s relative to the Cosmic Microwave Background (CMB) in direction 264°l, 48°b.
No, they say that arms are expanded and contracted, but this means that measured speed of light must c±delta, while measured speed of light is in c...c-delta range. AFAIK.
Moreover, if arms are just expanded/contracted, i.e. it's density wave, then gravitational waves and light from distant mergers should arrive at the same time ± few milliseconds, while light arrived 1.7 seconds later, i.e. light is slower by 1E-15.
Moreover, somebody told me that density wave is not possible for gravitation, because gravitons should be massless(of course) spin-2 particles. IMHO it a wave of flips or deflections.
Anyway, Higgs «field», which gives mass, exists, so a medium, which exists at any point of our Univers, exists, so gravitational waves are waves in a g-medium, which is connected to the Higgs medium somehow, thus, for gravitation, c is speed of gravitation in the medium.
For light, it should be same. I see no reason to say that c is the speed of gravitation in g-medium, while light propagates in pure nothing and c is the top speed in the pure nothing. Nonsense.
I agree, that for most calculations those details are unnecessary and can be avoided. However, existence of something is not depended on it necessity for us.
Moreover, AFAIK (I'm not a real scientist, so beware), Higgs «field» and medium for gravitational waves (g-medium?) are two different mediums, because Higgs boson is spin-0 and it gives mass, while graviton is spin-2 and is massless.
Once you accept that the speed of light is constant in all reference frames, relativity falls out from that. That in itself is a bit hard to accept, though, which was why it was some number of years between the Michelson-Morley experiment in 1887 and Einstein's paper on SR in 1905. GR basically also comes from 'shut up and calculate' after accepting the principles behind SR but adapting them to non-inertial reference frames (and also the insight behind gravity being a fictitious force), but the mathematics is much more complicated than SR, which I assume is why it took Einstein over a decade to work it out.