Einstein's asertion that no signal can propagate faster than light, together with some experimental confirmations is a major reason why it is considered by informed physicists that interstellar travel is impossible. However, some people have some problem in understanding that Einstein's idea of relative time is incompartible with the kind of interstellar travel the also desire. They go on accepting that time is relative and that they can instantaneously move from any point to any other point in the universe!

You won't even go through spiritual means such as telepathy!! It has something to do with the way Einstein link slowing clocks to the slowing of time itself!! To instantaneously communicate with an andromedan, there is no short cut to both you and the andromedan sharing the same moment, making the simultaneity between you and the andromedan absolute, violating Einstein's relativity! The only sensible way out is to reject Einstein's and hence Minkowsky's interpratation of Lorentz transform. This means to simply reject everything that was fascinating about relativity and to make both Einstein and Minkowsky's interpratation irrelevant. Indead, Hendrik Lorentz had already explained everything physically relevant together with formulating all the relevant mathematics.

At the dawn of 20th centuary scientists were about to understand that the relativistic effects together with electromagnetic forces are natural accidence that occur due to the fact that we leave inside some medium exibiting strange properties. Einstein's entry to the scenario terminated this perfectly sensible line of thinking. Einstein says that relativistic effects are some inexplicable essence of nature that occur in principle even in absolute vacuum. This required a lot of indoctrinations of the next generation of physicists to swallow such! Nowadays almost everyone think it is normal for narture to be that way. But infact Einstein's claim is such far fetched one that even a die hard beleiver  has to constantly remind himself of it! Nothingness necesarily lacks properties and thus can never limit anything. Absolutely, a void cannot bend, munch, shrink, dilate swallow or twist things. Absolutely, it cannot slow clocks down. Anything achieving such acrobatics must be SOMETHING in an otherwise void and we only need  to figure out how to suck this thing out and then the drama disapears!

To illustrate the problem, let us use the following inductive reasoning. Imagine a beam of light emerging from diamond, then it enters a piece of glass, then it enters the transparent perspex, then clear water, then parafin, then petrol, then carbondioxide, the nitrogen, then, oxygen, then hydrogen, bla bla bla. The speed of light increases steadily as it pass through all those mediums. How can a thing that is clearly variable from medium to meadium holus bolus morphs into unsurmountable constant by entering some magical medium a human being DEFINE by fiat and call it 'vacuume? We can see from those experiments that speed of light depends on the amount of concentration of something and makes no sense to talk of 'least possible density'.

When you are trying to evacuate a container, you can only completely remove things of a certain size. No matter how perfect technology is, it ismpossible to make an idealy air tight container, let alone a pumbing machine. So you cannot even remove the giant molecules of air off a region in space, let alone the smaller buzzing subatomic particles. So we can't reasonably claim  to describe a condition of vacuum  . If we have trouble even removing air off a region, what sense does it mean to say that Einstein's theories have be tested over and over? Consider for instance attempt to accelerate particles in CERN to the speed of light. how sure are we that the difficulty in attaining the speed of light is due to the property of vacuum and not the simple inevitable collision between the accelerating particles and the particles buzzing around in 'void'? If it is impossible to remove these other particles, then Einstein's theory CANNOT be tested!

This is even worsen by the lightness of the hadrons accelerated. You donnot even need quantum mechanics nor equations to understand this. Take a piece of feather and a stone and try to race them downwards due to gravity. You realise that making a thing light does not necesarily make it easier to accelerate it, if a region of space in which you are performing the experiment is not a perfect vacuum. Of course physicists understand such. But to avoid being embarassed by a 12 year old, they have a fancier way of saying it. It is called HIGGS MECHANISM. So we are baptised in some ocean. But in case you notice a feather parachuting slowly but still wish to say space is void you can say talk of some magical mass donor called higgs. So like you might give a pair of pants to someone, this beast gives 'mass' to particles! But of course you should be smart enough to see that the particles are simply bumping on some particles buzzing around and thus looking heavier.

FINE STRUCTURE CONSTANT

I can also make my case in experimental, not just ontologically. Astrophysicists are reporting cases where the so called fine structure 'constant' infact varies from point to point in the universe! While this might buffle the modern physicist, it is pretty straight foward if you understand space as to be filled with a medium as a matter accidence and not essentially like I have explained above. So the understanding is a nobrainer, who after all espect the density of the medium to be constant everywhere? Then light of course vary from medium to medium. Remember that fine structure relates speed of light this way;

A=q2/hc

A=fine structure constant

q=charge of an electron

h=Planck's constant

c=Speed of light

This equation is deduced by considering forces acting on the electron from 'vacume' which means the 'vacume' is a medium. Why should there be a thing such as 'vacume' that acts upon things but unlike anything else it is not subject to the rules of any other thing such as the cabability of being, at least in principle, sucked off a region. Once you accept this dose of common sense, then ask yourself why some other regions of cosmos should not have this stuff in less density and thus the speed of light is much higher in those regions? This neatly explains the variable fine structure constant observed alongside some other things. So if we can in principle make the speed of light as high as we wish by sufficiently reducing the density of the medium in space, what place do Einstein's physics have in reality? Non! We remain with the insights that early 20th century physicists had!

QUANTUM PHYSICS

You can ask, if the speed of light is constant contingent to what is in principle removable from space and not the property of space, might we at times see light shoot much faster? The answer is a big yes! In QM, a single photon can shoot at ANY speed!! Only statistic ensemble of photons that moves slowly. So the constancy of the speed of light in 'vacume' is indead very contingent. It is NOT a fundamental law! The quantum fluctuation of c is easily explain by the fluctuation of the density of the medium just as the speed of sound is never espected to be constant in air. It depends on tones of factors from temperature to humidity.