I refer you to a reprint of Relativity, the Special and General Theory by Albert Einstein, translated by Robert W. Lawson, University of Sheffield, England, fifteenth edition by Bonanza Books, New York, 1961.
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Einstein claimed that space and time are relative and actually are different for different observers. He also claimed that gravitational force and inertial force are equivalent.

He offers several "thought experiments", e.g., a stone dropped from a passing train and two simultaneous lightening strikes, to prove that space and time can vary according to the location of the observer. The problem is that observation does not and cannot change reality. Relativity only exists in the eye of the observer.

He also provides a "thought experiment" consisting of a man in an elevator to prove that gravitational force and inertial force cannot be distinguished from each other. In reality, a very simple experiment can prove otherwise.

But then he extrapolates these observations into his sense of reality that space can be bent, time can be traveled and the speed of light is not traversable.
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Let's look at each of these.

First, a man riding aboard a train drops a stone from a window to the ground. A man standing on the embankment observes this. The man on the train sees the stone fall in a straight line. The man on the embankment sees the stone fall in a parabolic path. In this "thought experiment", Einstein assigned a "reality" to both observations disregarding the physical or real path taken by the stone. In fact, the path taken by the stone does describe a parabola.

What Einstein ignored in his "thought experiment" was the momentum provided by the moving train. The man on the train, possessing the identical forward momentum, simply didn't observe it. By insisting on two separate "realities", Einstein was led to the conclusion that space can in fact be different for two different observations. In actuality, only the observations and not the reality differ.

The second one is interesting. A man is traveling aboard a train and another man stands on the embankment. Two simultaneous lightening bolts strike, one at point "A" behind the train and another at point "B" equidistant ahead of the train. Einstein, relying on DeSitter's observations, concedes that the propagation of light is constant. He then observes that, since the man on the train is moving toward point "B", and away from point "A", he will see the flash from point "B" before he sees the flash from point "A". Meanwhile, the man on the embankment will see both flashes simultaneously. His conclusion is that events simultaneous with reference to the embankment are not simultaneous with respect to the train.

Again he confuses observation with reality. He should also have observed a slight blue shift in the strike at point "B" and a slight red shift in the strike at point "A" due to the Doppler Effect.

The error in his third "thought experiment" surprises me. This experiment purports to prove that a man standing in a closed elevator cannot tell the difference if he were motionless on the surface of the Earth or if he were accelerating in an "upward" motion in free space without the influence of gravity. If you were in such a situation and held two identical weights in outstretched hands, you could instantly tell the difference. When dropped while on the surface of the Earth, the weights will fall toward the center of mass and not in parallel paths. In space under the influence of inertia, the weights will fall in absolute parallel paths.

This led Einstein to the wrong conclusion that gravitational mass and inertial mass were equivalent. This led further to his conclusion that speed faster than light, or 299,792.458 km/sec, is impossible because inertial "mass" increases with speed and becomes theoretically infinite at that speed. I enclosed mass in quotation marks because inertia confers no mass to an object. What inertia does do, however, is to store energy. A bullet, weighing only a few ounces in the hand, can strike its target with the energy of a ton because the inertia of the bullet, at the point of impact, releases that stored energy. The bullet, at impact, still weighs only a few ounces if measuring its actual mass.

In conclusion, I don't want to detract from Einstein's achievements, but merely point out what I believe to be flaws in his theories that have led us on some merry chases. For example; time travel is possible while travel at speeds faster than light is not, that space is malleable and someday we'll find worm holes. I think that we've wasted too much time, effort and resources on these concepts.
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What do I believe; what do I think is possible?

I believe that someday we will get to the stars at speeds greater than that of light. It'll probably be in some form of "generation" ship that will accelerate halfway towards its destination and decelerate the rest of the way. The human body can stand only so much inertial force - just ask a fighter pilot.

I believe that time travel is impossible simply because there is no such thing as "time". That's in the physical sense. The term "hour" has no more meaning than "mile" or "gallon" when not in the context of some discussion. Time is only a construct we've invented so we can plant wheat and build skyscrapers. It's no more than a system of measurement like linear distance or volume.

I believe that the red shift observed by astronomers does not necessarily explain an expanding universe because there are too many other explanations for the red shift. The "tired wave" theory is the most logical and the one I prefer. The "tired wave" theory is simple to understand. Toss a stone onto the surface of a still pond and you will see the waves spreading from the point of impact become longer and lower in amplitude. If these waves were electromagnetic and within the visible light spectrum, they would acquire a red shift and become weaker as the energy is expended in the propagation process. Therefore, I see no reason to invent a "Big Bang" to explain what I think is an observation that I think has been misinterpreted.

I believe that the accretion disk that formed Sol, the Earth and the other planets is not unique. The laws of probability rule. If we confine ourselves to looking for single stars of second and subsequent generation systems, we'll eventually find habitable planets. First generation systems lack the heavy elements and multiple star systems are just too unstable. Had our accretion disk contained a lesser amount of heavy elements, Jupiter, and perhaps Saturn, may have become stars and we wouldn't be here discussing this matter.

I believe that there are other intelligent life-forms in the universe. I also believe that there are far fewer than anticipated by the Drake formula. There are just too many variables to consider. Those forms will undoubtedly be carbon based because the next best candidate, silicon, just cannot form strong enough chemical bonds to build the long, durable molecular chains needed for complex and intelligent life forms.

I believe that it is unlikely that we will hear from aliens any time soon. I refer you back to the "tired wave" theory as applied to electromagnetic radiation. Nor will they hear from us. We'll just have to get out there ourselves or else send one-way, deep space probes advertising our presence.

I believe that the universe is infinite but that the matter contained within it is finite and constantly rearranging itself. Therefore, I rule out parallel universes and all sorts of fiction that may be inspired by that concept.
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In short, there's a whole universe out there. And we'll get there when we discard some of this chaff that we've accumulated in our sciences. I think we will; it'll just take time.