At the Science and Future Editorial Board meeting, in our Translation Collective and Editorial Board's e-mail groups, and in conversations among ourselves, we conveyed to Kerem Çankoçak the questions that fall within the field of physics, among the questions about the Interstellar movie that we collectively created, and we are presenting his written answers.
Kerem Cankoçak:
First of all, let me give some general information. Physicist Kip Thorne, the film's science consultant (and also one of its producers), wrote a book explaining the scientific background of the Interstellar movie. The book, which was released at the same time as the movie, is currently being translated and will soon be published by Alfa Publications under the name The Science of Interstellar . Some of the answers in this written interview were brief due to space constraints, but there are much longer explanations in the book. I tried to answer the questions with Kip Thorne's explanation; Also, the images are taken from Kip Thorne's book.
Levels of scientific theory: Facts, to be proven, and speculations
– Is the wormhole used in the movie to go to a completely different corner of the universe a phenomenon that we are sure of, like stars, nebulae, galaxies? Or is it an assumption on paper? Or is it speculation?
– Scientific speculation. Physicist Kip Thorne, the film's science advisor (and also one of its producers), divides scientific theories into three in his book The Science of Interstellar : The first is proven scientific facts; relativity theory, quantum theory, etc. like. The second is those that are considered certain to be proven even if they have not been proven yet: For example, although we have not been able to send humans to Mars yet, it is certain that we will send them to Mars in the near future. The third type of scientific theories are theories that do not contradict other scientific theories, but have not yet been proven; string theories, 5 or 11 dimensional space-time, etc. like. We have no evidence that these theories will be confirmed. However, since they are compatible with other theories, we cannot regard them as fantasy or fantasy. Maybe they will be falsified in the future and replaced by other theories; But right now, there is no harm in trying to explain some facts about the universe by using them.
What is a wormhole?
– Let's say we have enough technology, if we want to create a wormhole like in the movie, what do we need to do for this?
Since this is not currently technologically possible, the answers will remain at the science fiction level. But if we look at what a wormhole is, maybe we'll get some idea of what we'll need technologically. A wormhole is a passage that directly connects one point of the space-time plane to another point in a completely separate region. It can be visualized simply as when you take a piece of paper and fold it so that its two edges touch, you can travel between the ends that touch each other .
Kip Thorne is a physicist who has worked for years on this wormhole idea he used in the movie. The original idea goes back a long way. In 1916, Ludwig Flamm discovered in Vienna a solution to Einstein's equations depicting wormholes. Today we know that general relativity allows for a wide variety of types of wormholes. For years, physicists remained indifferent to Flamm's improbable solution to Einstein's equations, the wormhole. Then in 1935, Einstein himself and his physicist friend Nathan Rosen, unaware of Flamm's work, rediscovered Flamm's solution, found its properties, and speculated about its real-world significance. Other physicists unaware of Flamm's work also began calling his wormhole the "Einstein-Rosen Bridge." Kip Thorne discovered that in order for this bridge to remain open, a strange substance called exotic matter must exist.
Exotic matter is a very strange thing; because it has negative energy and negative mass! Therefore, it has an anti-gravitational effect. For example, if the earth had negative mass, objects on the earth would accelerate upward. Even more interesting, if you want to hit a tennis ball with negative mass, you do not swing your racket towards the ball, but rather in the opposite direction! Thanks to such an exotic substance that we cannot imagine with our common sense, the wormhole does not collapse in on itself. Undoubtedly, such a substance has not been discovered yet, these are just theories on paper; But they are consistent with other tested, observed theories, so they are scientific theories. No matter how strange exotic matter is, it does not contradict current physics theories. So if you want to make a wormhole, you first need to find exotic matter. You must create negative energy in this. Actually, maybe it's not that difficult. Quantum physics tells us that we can create negative energy in a vacuum, in a very small portion of space-time. We just don't know what technology we can use to do this. Just like in Jules Verne's time, we knew that going to the Moon was theoretically possible, but we were technologically inadequate.
Continuing from the wormhole, for an ant walking on an apple, the surface of the apple is the entire universe. If a worm has crawled through the apple, there are two ways for the ant to reach from top to bottom: going around from the outside (going through the ant's universe) or going down the wormhole. The shortest way to go from one end to the other in the ant's universe is the wormhole .
In this example, the internal structure of the apple through which the wormhole passes is not part of the ant's universe. Likewise, in our universe, the wormhole is located in the bulk . However, we have not yet been able to detect the existence of such a mass (the 4th space dimension).
Kip Thorne describes the life of the wormhole with these words:
“Initially in picture (a) our universe has two singularities. As time progresses, singularities extend towards each other through the stack and meet, creating the wormhole (b). The perimeter of the wormhole expands; (c) and (d). Then it narrows and breaks off where it thins (e). It leaves two singularities behind (f). Birth, expansion, contraction and rupture occur so quickly that nothing, not even light, has time to pass through the wormhole and reach from one side to the other. Anyone or anything that tries to reach, or ventures into the wormhole, will perish at the breaking point! This prediction is inevitable. If the universe somehow created a spherical wormhole that contained no gravitational matter, that's how that wormhole would behave. Einstein's laws of relativity dictated this.”
Let's say you have a wormhole that doesn't break. Light entering the wormhole converges (its cross-sectional area decreases) and diverges (its area increases) as it leaves the wormhole. A wormhole bends light rays outward, like diverging lenses.
However, gravitational objects such as the Sun or a black hole bend the rays inward. They cannot bend the rays outward. In order to bend the rays outward, an object must have negative mass (or, equivalently, negative energy). Based on this fundamental fact, Kip Thorne concluded that a spherical wormhole must be lined with some kind of material that has negative energy. At least the energy of the material had to be negative as seen by the light ray or anything or anyone passing through the wormhole at near the speed of light. He called such materials “exotic matter.” It was later realized that, according to Einstein's laws of relativity, all wormholes can only be passed through if they are lined with exotic matter. This is the requirement of the theorem proven by Dennis Gannon of the University of California at Davis in 1975.
Although exotic matter has not yet been proven, it must exist due to the laws of quantum physics. Physicists have even managed to create tiny amounts of exotic matter in the laboratory between two electrically conductive plates positioned closely together . This is called the Casimir effect. But no one is yet sure whether a wormhole could contain enough exotic matter to remain open.
– Who creates the wormhole in the movie and puts it there? "Who are they? Is Cooper one of “them”?
– Now, I think this is actually a paradox. If I understand the movie correctly, “they” are Cooper's past. So the past interferes with the future. This is the famous grandparent paradox. This paradox exists in almost every time travel (towards the past) movie or novel. If you're following the same timeline, there's no way to avoid this paradox. However, if you enter another timeline when you go to the past, and therefore actually go to a completely different universe, you can escape this paradox. But there is nothing like this in the movie. So they don't go to another universe. They're in the same universe and the same timeline. Therefore, the film could not escape the paradox, it covered it up.
Was gravity created in Endurance ? Inside this spacecraft, people moved by stepping on the ground. How can gravity be created in the vacuum of space, inside a spacecraft?
As Endurance rotates around itself, it creates centripetal acceleration, which adds weight to the astronauts, just like the acceleration of the earth's gravity. By the way, let us remind you that the diameter of Endurance is 64 m (1 percent of the wormhole) and that it is manufactured with technology that can resist the tidal effects of the black hole.
Gargantua black hole and Miller's planet
– Due to the gravity exerted by the black hole called Gargantua on Miller's planet , the planet is under the influence of a very large tidal force. We understand the effect of this force from the giant waves that occur, but when we consider the possible magnitude of the force, does it seem possible for the planet to maintain its structural integrity? Also, can it be so easy to take off with a spacecraft from a planet where gravity is so strong?
– To understand this issue, it is necessary to explain general relativity a little. In 1915, Einstein realized that time must be curved by the masses of heavy objects (such as the earth or a black hole), and that gravity occurs due to this bending. Kip Thorne defines this phenomenon as “Einstein's law of time warps”: “We can express it verbally as follows: ' Everything likes to live where it ages the slowest, and gravity pulls it there.' “The greater the slowing down of time, the stronger the pull of gravity.”
Einstein's laws of relativity say that when near a black hole, planets, stars, and underpowered spacecraft will move along the straightest paths allowed by the hole's warped space and time.
A few years ago, Kip Thorne and his students discovered a new perspective on these planetary paths. In Einstein's theory of relativity, there is a mathematical quantity called the Riemann tensor. Describes details of the bending of space and time. Thorne and his team found lines of force hidden in the mathematics of this Riemann tensor that squeeze some planetary paths together and stretch others apart. They called these "tendex lines", based on the Latin word tendere , meaning "to stretch".
A black hole is not the only object that produces stretching and compression forces. It also produces stars, planets and moons. In 1687, Isaac Newton discovered their existence in his theory of gravity and used it to explain ocean tides. In Newton's reasoning, the Moon's gravity pulls the side closer to the Earth more strongly than the side farther away. On the sides of the Earth, the direction of the pull is a little more inward, because it is in a slightly different direction on both sides of the Earth, towards the center of the Moon.
The Earth, on the other hand , does not feel the average of these gravitational pulls because it is falling freely along its orbit. (This is similar to how the Endurance crew do not feel Gargantua's gravitational pull inside the Endurance as the ship remains in orbit above the black hole. They only feel the centrifugal force from the Endurance's rotation.) What the Earth feels is the averaged -out lunar gravity, shown by the red arrows in the left half of Figure 9; in other words, a stretch is felt towards and away from the Moon, and a compression is felt from the sides . Qualitatively the same thing happens around the black hole.
These felt forces stretch the ocean away from the Earth's surface on its Moonward and Moonward sides, creating sea level rise there. And the felt forces compress the oceans towards the earth's surface on the sides of the earth, creating lower sea levels there as well. As the Earth makes a complete rotation around its axis, one every 24 hours, we see two rises and two falls in ocean level. Because of their role in ocean tides, these gravitational tension and compression forces ( forces felt by the earth ) were called tidal forces. These tidal forces, measured using Newton's laws of gravity, coincide with extremely high precision with those we calculated using Einstein's laws of relativity. The result should also be the same, because the laws of relativity and Newton's laws always have the same predictions when gravity is weak and objects move at speeds much slower than light.
In the relativity depiction of lunar tides , tidal forces are created by blue tendex lines compressing the sides of the Earth and red tendex lines stretching them toward and away from the Moon. This is exactly like the tendex lines of a black hole . The Moon's tendex lines are the visual embodiment of the Moon's bending of space and time. It is remarkable that such a small bend can create forces large enough to cause ocean tides!
On Miller's planet, tidal forces are tremendously great and are the cause of the giant waves that Cooper and his crew encounter. However, this is not a force that affects astronauts or spaceships on the planet.
– Miller's planet was very close to the black hole Gargantua. How far away from the event horizon must a planet be to maintain its form and establish a stable orbit around a black hole? Could a planet be sheltered this close to a black hole?
First, we should take a look at the basic facts about black holes. Black holes consist of warped space and warped time. Nothing else, any substance etc. There is no. Kip Thorne illustrates as follows: Let's imagine that you are an ant and live on a child's trampoline, which consists of elastic fabric stretched between long legs. Since you are a blind ant, you can see neither the poles nor the stone nor the bent rubber sheet. But you are a clever ant. The rubber sheet is your entire universe, and you suspect it is bent. To determine its shape, you walk in a circle at the top and measure the length of the circle, then walk from one side to the other, passing through the center of the circle, and measure its diameter. If your universe were flat, the circle length would be π = 3.14159… times the diameter. But you discover that the circle length is much smaller than the diameter. You conclude that your universe is highly bent!
The space around a non-spinning black hole has the same twist as a trampoline: Take an equatorial slice through the black hole. This is a two-dimensional surface. As seen from the pile, this surface is bent in the same way as the trampoline.
A singularity is a very small region where the surface forms a point and is therefore “infinitely bent,” and where the tidal gravitational forces are apparently infinitely intense, so that matter as we know it ceases to exist from stretching and compression. However, it should be kept in mind that Gargantua's singularity is a little different than this.
Whatever falls into a black hole, as soon as it passes through the hole's event horizon , it is inexorably pulled downwards into the hole's singularity, and once it passes the horizon, no one above the horizon can see the signals being sent from within. This confinement actually occurs due to time warping of the hole.
Inside the event horizon, time is bent so extreme that it flows in a direction that you would think is spatial: It flows downwards towards the singularity. This downward flow is actually why nothing can escape a black hole. Everything is pulled inexorably towards the future, and since inside the hole the future is downwards away from the horizon, nothing can escape upwards beyond the horizon.
But outside the event horizon the situation is different. Miller's planet is as close as possible to Gargantua, as far as its survival is concerned. We know this because the crew's excessive waste of time can only occur when they are very close to Gargantua. At such a close distance, Gargantua's tidal gravitational forces are particularly intense. They stretch Miller's planet toward and away from Gargantua and compress the planet's sides.
The intensity of this stretching and compression is inversely proportional to the square of Gargantua's mass. Because the greater the mass of Gargantua, the larger its ring length, the more gravitational forces of Gargantua are similar in various parts of the planet, resulting in weaker tidal forces. To create such a situation, Kip Thorne's calculations showed that Gargantua's mass would have to be at least 100 million times greater than the mass of the Sun. If Gargantua were less massive than this, it would shatter Miller's planet!
The circumferential length of a black hole's event horizon is proportional to the mass of the hole. When the circumferential length of the horizon is calculated for Gargantua's 100 million solar masses, the result is close to the Earth's orbit around the Sun: approximately 1 billion km.
Physicists consider the radius of a black hole to be the circular length of its horizon/2π (about 6.28). Because of the extreme space warping inside the black hole, this is not the actual radius of the hole. It is not the actual distance from the horizon to the center of the hole, as measured in our universe. However, as of the measurement made inside the stack, it is the radius (half the diameter) of the event horizon. The radius of Gargantua in this sense is about 150 million km, the same as the radius of the Earth's orbit around the Sun.
Can there be light near a black hole, and can life develop there?
– How can Miller's planet be bright when near Gargantua? Don't black holes absorb light?
– As explained above, black holes absorb light only when it crosses the event horizon. In addition, there is no darkness around black holes as a result of the lensing effect and other effects. As explained at length in the book, black holes have an accretion disk and their eruptions emit radiation. In fact, this radiation is intense enough to fry anyone who comes near it. Such disks can be found around black holes that have not destroyed a star in the millions of years they have left behind and have not been "fed" for a long time. The magnetic field, normally confined to the disk's plasma, may have largely leaked out. The eruption, which was previously powered by the magnetic field, may have also ended. Gargantua's disk is one such disk: eruption-free, thin, and relatively safe for humans.
– Is it possible for life to develop on a planet so close to a black hole? Can the molecules necessary for life escape Gargantua's gravity and reach a planet this close to the black hole?
– The elements necessary for life were already present when the planet was forming. The emergence of life is a slightly different problem. Kip Thorne points out a scientific mistake in the movie in his book:
In Interstellar , after Miller's planet is revealed to be unsuitable for human life, Amelia Brand advocates going to Edmunds' planet, which is far away from Gargantua, instead of the closer Mann's planet: "Chance is the first building block of evolution," she tells Cooper. “But when you're in orbit around a black hole, there can't be enough going on; it swallows up asteroids and comets and other events that might otherwise reach you. We have to go further away, out of his playing field.”
A typical orbit has the shape of Pulled by Gargantua's intense gravity, the object moves inward. But before it reaches the horizon, centrifugal forces increase sufficiently to blow the object outward again. This happens over and over again, almost endlessly.
The only thing that might interfere here is a chance close contact with another massive object (a small black hole, star or planet). The object is thrown around the other object, following a slingshot path, and thus settles into a new orbit around Gargantua, with its angular momentum changed. The new orbit almost always has a large angular momentum and centrifugal forces that protect the object from Gargantua, just like the old one. Very rarely, the new orbit carries the object almost directly towards Gargantua, with an angular momentum too small for centrifugal forces to overcome, causing the object to sink into Gargantua's horizon.
Astrophysicists have simulated the simultaneous orbital motions of millions of stars orbiting giant black holes like Gargantua. Sling shots gradually change all the orbits and in this way change the density of the stars (how many stars are in a certain volume). The density of stars near Gargantua will increase, not decrease. Random bombardments of asteroids and comets will become more frequent, not less frequent. For individual life forms, including humans, the environment near Gargantua will become more dangerous, encouraging faster evolution if enough individuals survive.
– The black hole in the movie was a black hole rotating around itself. What is the effect of the rotation of the black hole on the gravitational force it creates?
– Just as the Earth rotates around itself, black holes can also rotate. A rotating hole drags the space around it into a vortex-type rotation. Like air in a tornado, space rotates fastest at the center of the hole, and the rotation slows as one moves outward, away from the hole. Everything that falls towards the horizon of the hole is dragged into a perpetual rotation around the hole by the rotation of space, like trash caught and carried away by the wind of a tornado. There is no way to protect oneself against the drag of this rotation on the edge of the horizon.
If we knew the mass of a black hole and the speed at which it was spinning, we could deduce all the other properties of the hole from Einstein's laws of relativity: its size, the intensity of its gravitational pull, how far its event horizon is stretched outward by centrifugal forces near the equator, the details of the gravitational lensing of objects behind it. Everything.
As shown in Interstellar, a physicist who knows Einstein's laws of relativity can extract the mass and spin of Gargantua from the properties of Miller's planet and from there obtain all the properties about it . Now let's see how this happens.
If Miller's planet was to be as close to Gargantua as possible without falling into it, Gargantua would have to spin incredibly fast for the seven-year time slowdown in the hour predicted in the scenario to be possible.
Black holes have a maximum spin rate. If they rotate faster than this maximum value, the horizon disappears from sight and the entire universe can freely see the singularity within; in other words, the singularity becomes bare , and the laws of physics probably prohibit this. With Gargantua's spin near its maximum possible limit, TARS' orbital completion time, viewed from a distance, was approximately one hour. TARS completes this distance in 1 hour, which means that its speed measured from afar is approximately one billion kilometers per hour, which is almost equal to the speed of light! If Gargantua rotated faster than the maximum, it would orbit TARS faster than the speed of light, breaking Einstein's speed limit.
Gargantua's fire shell
– Can the event horizon of a black hole be approached so easily? Even if we ignore the fact that they are spaghetti due to gravity, don't they encounter celestial objects that are frequently found in the vortex around the black hole and move at very high speeds?
Several specific locations of Gargantua in Interstellar on the equatorial plane: Gargantua's event horizon (bottom circle), the critical orbit Cooper and TARS were on before falling into Gargantua near the end of the movie (third from the bottom circle), the orbit of Miller's planet, the orbit in which Endurance was parked while the crew visited Miller's planet (top circle), and a section from the non-equatorial orbit of Mann's planet dropped into the equatorial plane. The outer part of Mann's planet's orbit is so far from Gargantua (600 times Gargantua's radius or more) that it is not included in this image.
These places are determined according to the scenario. For example, Cooper describes the parked orbit in the film as follows: "So we follow a wider orbit around Gargantua, parallel to Miller's planet but a little further out from it." By this he wants it to be far enough away from Gargantua to be “outside the time shift”, in other words, at a distance (five Gargantua radii away) where the slowing of time is quite modest compared to Earth. Also suitable is the 2.5 hours it takes for Ranger to travel from this parked orbit to Miller's planet.
Near Gargantua, gravity is so intense and space and time are so warped that light can be trapped in orbits beyond the horizon, looping around the hole many times before escaping. These trapped orbitals are unstable because photons eventually always escape from them . Kip Thorne called this temporarily trapped light "a shell of fire." This shell of fire plays a key role in the computer simulations underlying Gargantua's visual form in Interstellar .
If a black hole is spinning, its fiery shell spreads inwards and outwards, so that it ceases to be just the surface of a sphere and occupies a finite volume. In Gargantua, which has a giant spin, the fire shell in the equatorial plane expands from the lower fire shell circle to the upper fire shell circle in Figure 17. The fiery shell has expanded to include Miller's planet, critical orbit, and much, much more! The lower ring of fiery shell is a beam of light (a photon orbit) that rotates repeatedly in the same direction ( forward ) as Gargantua's spin . The upper ring of fiery crust is a photon orbit moving in the opposite direction ( backwards ) to Gargantua's spin . Obviously the rotation of space allows forward light to be much closer to the horizon without falling, compared to backward light.
To survive, a planet, a star, or a spacecraft must counter Gargantua's massive gravity with a comparably large centrifugal force. This means it has to move at very high speed. Therefore, it must approach the speed of light. While visiting Miller's planet, the Endurance crew is parked within a radius of ten Gargantuas, traveling at a speed of one-third the speed of light. Miller's planet also moves at a speed that is 55 percent of the speed of light: 0.55 c .
The only way to achieve the huge speed changes of c /3 required in Interstellar is gravitational slings, which are used in the film. The gravity-assisted maneuver is called a “gravity slingshot” and is frequently used by NASA in the Solar System.
To change speeds by c /3 or c /4, Ranger would need to get close enough to the small black hole and neutron star to feel their intense gravity. At these close ranges, if the deflector is a neutron star or black hole with a radius of less than 10,000 kilometers, humans and the Ranger will be torn apart by tidal forces. For Ranger and humans to survive, the deflector must be a black hole at least 10,000 kilometers across (about the size of the earth).
There are black holes of this size in nature . These are called intermediate-mass black holes or IMBHs (intermediate-mass black holes), and despite their large size, they remain very small compared to Gargantua; Gargantua is ten thousand times larger than them. However, Chris Nolan chooses the neutron star instead of IMBHs for the script in order not to confuse the audience with multiple black holes.
Getting out of a black hole unscathed
– Is it possible to escape from a black hole without any damage while preserving physical integrity?
- Yes it is possible. Of course, if you use a special black hole model. Since no one knows what is inside a black hole, it is not clear which model will prevail. But according to one model, it is possible to enter a black hole and remain unscathed. Let's expand a bit:
In 1991, Eric Poisson and Werner Israel discovered a second type of singularity while working on Einstein equations. This singularity grew as the black hole aged. The reason was the extraordinary aging of time inside the black hole. If you fall into a black hole like Gargantua, gas, dust, light, etc. will be taken with you. Many other things also come into play, such as: For an outside observer, it would take billions of years for all of this to enter the black hole. But for someone inside the black hole, that's less than a second. Therefore, if you enter such a black hole, you will see all this matter falling towards you in a thin layer at a speed close to the speed of light. This layer creates intense tidal gravitational forces that distort space-time. Tidal forces form a singularity as it grows infinitely. As a result, an “inward singularity” occurs .
Because tidal forces pull and compress, once you reach the singularity the net force is finite rather than infinite, and you may have a chance at survival .
In 2012, Donald Marolf and Amos Ori discovered a third type of singularity. Before you are gas, dust, light, gravitational waves, etc. falling into the black hole. This singularity, called the “outward singularity” created by things like this, also grows as the black hole ages. A small part of them is reflected towards you as a result of space and time warps inside the black hole. This reflection is compressed like a shock front due to time slowing down. Again, it creates tidal forces and these grow towards infinity and form a singularity. But this time it is about the “outward singularity”. You have a chance to survive in this type of singularity.
Falling into a black hole and changing size...
Do existing black hole hypotheses say that black holes can cause size changes?
These properties of black holes that we mentioned were deduced from Einstein's equations by many physicists such as Karl Schwarzschild, Roy Kerr and Stephen Hawking. In 1915, Schwarzschild discovered the details of the warped space-time around a non-rotating black hole. In the jargon of physicists, these details are called "Schwarzschild metrics". In 1963 Kerr (a New Zealand mathematician) did the same for a rotating black hole: He deduced the “Kerr metric” of the rotating hole. And in the early 1970s, Stephen Hawking and others deduced a set of laws that black holes must obey when they swallow stars, collide and merge, and sense the tidal forces of other objects. It is certain that black holes exist. Einstein's laws of relativity say that when a massive star uses up the nuclear fuel that keeps it warm, the star must implode. In 1939, J. Robert Oppenheimer and his student Hartland Snyder used Einstein's laws to discover that if the implosion was completely spherical, the imploding star would have to create a black hole around itself, then create a singularity at the center of the hole, and then be sucked into the singularity. . There was no matter left behind. The resulting black hole consisted entirely of warped space and time. In the years since 1939, using Einstein's laws, physicists have shown that an imploding star will produce a black hole, albeit one that is deformed and spinning. After all, black holes are about gravity.
On the other hand, "dimension changing" is actually a somewhat independent phenomenon. However, they do agree on one thing: quantum gravity. No one has any doubts about the warping of space-time. But this twist can happen in two ways:
1) There is no other dimension than the 4-dimensional space-time we live in, and the warping is the warping of space-time itself. The expansion of the universe since the Big Bang also occurs as the expansion of all space-time. Just like a balloon, but there is nothing but a balloon. This is the classical answer, and this answer was the common view of all physicists until recently.
2) However, there is another explanation: especially after the 1980s, with the diversification of quantum gravity theories, string theories are being adopted by popular theories such as M-theory. And that is this: The space-time we live in is bent in a fifth dimension called the bulk (let's think of all other dimensions as the 5th dimension). So, according to this explanation, “What is our universe expanding into?” The answer to the question will be, “It is expanding within the stack or within the 5th dimension.” According to these new theories, the universe we live in is also a brane within this stack.
Moreover, according to these theories, all forces except gravity (electromagnetism, weak and strong nuclear forces) are trapped within our membrane. Only gravity can transfer between dimensions.
The movie Interstellar is based on the assumption that the bulk exists. On the other hand, if there is a pile, then according to theory it must be "twisted". Technically speaking, if the bulk (5th dimension) were not bent, gravity would behave according to the inverse-cube law, not the inverse-square law. In other words, the gravitational force between the Sun and the planets would be inversely proportional to the cube of the distance, and in this case, the planets would disperse into space instead of revolving around the Sun.
Now let's reduce the dimensions of our 4-space-time dimensional membrane, which we showed in 2-dimensions, to 1-dimension (North-South) and express it with the thick line in the middle.
Physicists trying to understand quantum gravity think that the extra dimensions are microscopic and fold in on themselves. This prevents gravity from spreading too quickly. However, in the movie Interstellar , another speculative step was taken and it was assumed that at least one of these dimensions did not fold in on itself. The reason for this is to make room for the hero of the movie. At the end of the movie, Cooper falls into a 4-dimensional cube called a tesseract.
– What is a tesseract?
– As I mentioned in the previous question, if, as some physicists claim, our 3-dimensional space is bent within a higher dimensional hyperspace called the "bulk", which is not a part of our universe, this tesseract is a 4-dimensional is a cube.
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