Realistic science fiction is based on scientific facts and projected technological developments that can be reasonably inferred from known technological trends. An example of realistic scientific fiction is the movie Deep Impact, which dealt with a comet colliding with planet Earth. Since comets are a scientifically established reality which we know periodically travel around in the solar system, it was not too far-fetched for the film to depict a comet colliding with our planet. There is a small chance that something like that may happen in the next million years.
But what is Interstellar based on? The plot all revolves around the idea of a spacetime wormhole. Science fiction writers like wormholes because with a wormhole you can have a plot line in which astronauts are instantaneously transported from one solar system to another. A wormhole is a hypothetical tunnel between two different points in space, which might allow instantaneous transportation between the two points. If that sounds far out, it is. There is no evidence at all that wormholes exist or can be created in a way that would make interstellar travel easier. Scientists have not even done any “proof of concept” experiment in which some tiny particle was transported from one place to another by traveling through a wormhole.
What does science actually tell us about interstellar travel? Its real message is that interstellar travel is in all likelihood something that can only be very slow, requiring a long time. We know that the distance between stars is so great that it takes light almost five years to travel from one sun to the nearest star. We also know that no spaceship can be accelerated to a speed greater than the speed of light, because that would violate Einstein's Special Theory of Relativity. That means the minimum time to travel from one star to the nearest star is going to be about five years. But that's only if you can travel at the speed of light. There are engineering reasons why it's more likely that no spaceship could travel more than half as fast as that, which leaves you a travel time of over a decade to go one from star to the nearest star.
Wormholes are a conceptual way of completely turning this reality on it head. Instead of having a universe in which interstellar travel is very slow, the idea of wormholes gives the science fiction writer a universe in which heroes can zip around the universe as effortlessly as a New York City commuter would zip around to different parts of Manhattan by using the subway. But for the moment, transporting astronauts through wormholes is science-related fantasy, not realistic science fiction.
Besides the complete lack of observational evidence for their existence, there is reason to believe that wormholes which allow rapid instantaneous travel do not exist and cannot be created. One reason is that if such things did exist or could be created, then interstellar travel would presumably be very easy, and Earth would probably be receiving visitors from all over the galaxy. If wormholes could be created, we would expect that we might have received many interstellar visitors. Almost the only way we can overcome the “where's everybody” issue known as Fermi's Paradox is to reject the speculative idea of wormholes, and assume that interstellar travel is very slow, taking many years for even trips between one star and nearby stars.
From what little we can infer about what wormholes might be like if they existed, we can assume that if a wormhole existed, some incredible technology would be needed to travel through one without getting killed. Traveling through a wormhole would presumably be like nuzzling right up to a black hole – you would be right on the edge of fantastically vast forces that could instantly twist a sun or a planet into a pretzel.
One can perhaps imagine some incredibly advanced extraterrestrial spaceship that travels through wormholes, some spaceship using a technology millions of years more advanced than ours. But in Interstellar, human astronauts travel through a wormhole using an ordinary spaceship built by a version of NASA that is operating in secret (on a “last legs” type of Earth at a time in which humans are living in houses and driving cars that look like today's houses and cars). That stretches credulity, but it gets even worse later in the movie, when a character travels through a wormhole all by himself, without even having a spaceship. That's scientifically realistic science fiction? The same character also ends up bouncing around through time like a ping pong ball on a ping pong table, which isn't terribly realistic.
Let's compare Interstellar to another movie in regard to scientific realism. The other movie is The Wizard of Oz. Both involve characters being transported to distant places by means of exotic things that can be described in scientific language. In Interstellar the transportation occurs by means of a wormhole, something that has never been observed. In The Wizard of Oz the transportation occurs by means of a tornado, a known natural phenomenon that has been observed many times by scientists. So which movie is more scientifically realistic in its central premise? The Wizard of Oz, not Interstellar.
Dorothy's tornado: more scientifically realistic than "Interstellar"