Your idea of repeating the Michelson-Morley experiment in space in the absence of gravity can be a good idea. The MM experiment had the primary purpose to verify or to falsify the existence of ether. The experiment was repeated numerous times and the outcome was always the same: ether, meant like matter that fills the whole universe and that is is essential for light propagation, doesn't exist.  I think also in space in the absence of gravity the outcome will be always the same: ether and absolute reference frame don't exist. It doesn't mean nevertheless the Lorentz invariance (or deformation) is correct. The Theory of Reference Frames, based on the concept of non-absolute privileged reference frame, is another way for explaining the outcome of the MM experiment and outcomes of other experiments.

Your idea of repeating the Michelson-Morley experiment in space in the absence of gravity can be a good idea. The MM experiment had the primary purpose to verify or to falsify the existence of ether. The experiment was repeated numerous times and the outcome was always the same: ether, meant like matter that fills the whole universe and that is is essential for light propagation, doesn't exist.  I think also in space in the absence of gravity the outcome will be always the same: ether and absolute reference frame don't exist. It doesn't mean nevertheless the Lorentz invariance (or deformation) is correct. The Theory of Reference Frames, based on the concept of non-absolute privileged reference frame, is another way for explaining the outcome of the MM experiment and outcomes of other experiments.

Thanks Daniele for your answer. But if you look at the original paper of Michelson-Morley in science journal, when they deriving the equation they assumed that ether is stationary and whole of apparatus is moving. In a case that we assume light speed is independent of speed of light source, their apparatus could be used as a way to measure moving speed of apparatus at space. It is in fact not related to ether wind or existing of ether. So logically we expected that if we repeat the test at space, by measuring fringe change we can measure speed of apparatus.

My problem with your question is your implicite doubt about constancy of speed of light. RT is proved by many experiments and facts from space physics. So the non existence of ether is also a physical fact. If you will really have problems, please think about dark matter as a possible influence.

Dear Vahid,

The further question is: "With respect to which reference frame do we can measure the speed of the apparatus in space?". In the Theory of Reference Frames (TR) I have proved the (empty) space is a medium that is characterized by important physical properties, but the empty space isn't a reference frame. The assumption then the speed of light is independent of source is only a hypothesis accepted in Special Relativity in order to explain the outcome of the MM experiment. But, as I wrote in my preceding comment, TR provides with another explanation, based on the concept that in every physical situation there is a non-absolute privileged reference frame. In the event of the MM experiment the privileged reference frame is the reference frame of apparatus, whether in the classical experiment or in the hypothetical experiment in space.

Dear Hanno,

you give a postmodern interpretation of physics and you consider the speed of light is a constant that is independent of all, but that conclusion is outside time. There isn't need of strange experiments in order to prove the speed of light cannot be an absolute constant.

Dear Hanno

It is certainly true that Einstein relativity is able to describe many physical phenomenon and verified by experiments. However, as everybody admit the relativity theory uses many perplexing thing without any deep knowledge about them. For instance we know that concept of space is very complex and still we do not have deep knowledge about it.  Or similarity concept of time which is completely abstract concept and is not physical thing is also very complex. Apart from both of these, two postulations that Relativity employed are questionable and there is not deep argument about them, there are just assumptions. So as you can see, relativity employed many concepts that we are not sure about them. Just we can defend them by saying that our intuition is unable to grasp the true reality!

The situation for me is similar to few centuries ago when most of observation confirm that earth is center of universe except some few disagreement!

For instant according to relativity if right now we travel with 99% speed of light is space, we could not notice that and still light travel with speed of light relative to us. How is it possible?!

Dear Daniele 

It is true that the concept of relativity and speed is quite deep topic. We need a reference to determine the speed. In a case that ether is the medium of light, then we can say that relative to ether. But the question that if ether moving itself in absolute space moving with what velocity return no answer.

Einstein imagination and though experiment is in fact true in absolute space where there is no ether. However, we could not find any absolute space as we know that according to quantum mechanics, space is full of energy (vacuum energy) .

So, my question is how we can really assured that Relativity could be really fundamental theory?!

In addition according to General Relativity 68% of universe is dark energy and 27% dark matter. But we do not have any clue about these exotic thing. Why we should not doubt about relativity? 

@ Remi: No problem: the equivalence between reference frames in uniform motion with respect to each other means just this: that any experiment set up in one frame, will give the same result as the corresponding experiment set up in the other frame. Thus we may call ``absolute'' any frame we happen to like. As inhabitants of the Solar System, its rest frame might deserve that name from our parochial viewpoint. The existence of frames which are, for some purposes, preferred, does not mean that there is a way to distinguish between inertial frames.

However, even with respect to this, I am not quite convinced of your formulation: the Big Bang is a cosmic phenomenon, and I somehow do not think it can be described locally and without some reference to curved space. In this case, of course, the entire issue of reference frames becomes irrelevant.

If you are moving together with the CBR, you see no Doppler shift in the CBR. If you are moving with the Sun, you see no Doppler shift from the Sun. In my own reference frame, I see no Doppler shift from the tip of my nose. Whereas some of these three objects may be less grandiose than others, they are all individual objects, and their properties are, of course, frame dependent.

@ Remi: Fair enough. My cavil about FTL information transmission is that it links---very directly---to sending information backwards in time. And then I am not so sure which is battier, many-universes or FTL messages. But I am not strongly committed to any specific view on the subject.

Dear Vahid,

Myself have many doubts about the Theory of Relativity, while I don't have doubts about the Principle of Relativity.

Before repeating the experiment in space, we must really understand it. I am not sure that the action of a semi-silvered mirror is really understood in the light of quantum mechanics. How does a semi-silvered mirror act on photons? Does it let through / deflect photons with a probability of 1/2 ? If so, what does the interference still mean, after unifying the fascicles at the end? Or does it split photons? If so, then from where does the supplimentary energy come? Or admit that it does split photons and somehow correctly converts energy from somewhere. Then is it possible that the pairs of (let through / deflected) photons are entangled and their interference acts differently as expected. Also the totally silvered mirrors: are the reflected photons the same photons or new photons? Are they absorbed and reemitted? Do they loose time in the air/glass/silver/glass/air interface? /// Independently of those thoughts - and I just wish to get good answers - it would be important to make some experiments in imponderability. It is ridiculous to work for a unified theory but to make all experiments under the same gravity conditions. What if gravity make us difficult to detect susy partners? /// But again, I think that before someone repeats the Michelson Morley experiment in space, we would better design other experiments, and we would better try to use less black boxes. (this means, less phenomena that we do not really understand and whose behaviour is only predicted in a simplist way, that cannot be so easily checked).