I often read sci-fi and find that writers tend to use physics phenomenon or theories which to the general public are quite obscure and mysterious. They often use them incorrectly or completely violate the rules they're supposed to be using. I thought I would provide a thread that gives some incite into how we determined what the rules are and what some of the actual strange behavior is. The universe is weird, but it's not as mysterious as most people think it is. I'd also like to invite questions or requests for more detail on whatever aspect of physics you wish. As a computer scientist, my expertise is obviously in the quantum mechanics, but I've kept myself in circles of physicists from varying disciplines and read the literature often. Classical Physics Natural Philosophy (Birth of man - 1728) Man has always tried to figure out the universe. Ironically, religion was the first science. They used it to describe the seasons, life, flood... It worked and was consistent with everything that they knew. A long slow trickle of knowledge began when we started writing with people like Imhotep and Archimedes making the first ideas of how things worked, they discovered the five basic machines, but not much else happened until mathematics really took off. Motion, Gravity, and Electromagnetism (1728 - 1915) The modern scientific age can be credited to many people, but since I'm only doing a quick overview, I'll start with Isaac Newton. In 1728 he published what is probably the most important book of all time: Philosophiae Naturalis Principia Mathematica. In it, he described the laws of physics of the day with exact mathematics and unified the falling of objects on Earth to the motions of the heavens. He also codified something that'd been known for a bit longer: relativity. According to Newton (building on Galileo,) all reference frames are equally valid and motions add (if you are moving at x speed towards an objects traveling towards you at y speed, the relative speed to you is x + y.) So gravity was figured out and electromagnetism was next. Lots of people helped with current, Ohms, charge, and it was known that alternating electricity created magnetism. Maxwell discovered and mathematically formulated the opposite. So a disturbance in the electric field created one in the magnetic field, which disturbed the electric field, which disturbed the magnetic field... and he calculated a value for the propagation of the electromagnetic wave (which is light.) Maxwell noticed the problem right away: he'd formulated a velocity that did not depend on the motion of the observer. Newton says velocities must add together, but here was this weird equation where the measured speed must always be the same, regardless of your own motion. Something was rotten in physics. Modern Physics Einstein (1915 - now) Einstein solved this. If everything is relative to your motion, yet somehow this velocity is constant, what's different? Velocity is relative to distance and time, so those must be what's changing, they're stretching This solved the relativity problem, but he wasn't done yet because it didn't work for accelerating objects. Newton's laws of gravity were dependent only on the distance between two objects and their mass, not their motion relative to each other. He didn't need it, his gravity force worked instantly across the universe, so whole new equations had to be worked out to allow the speed of gravity to be equal to light, that took a long time but something incredible happened: gravity fell away, the force completely vanished from the equations, it's just another acceleration. Gravity is an illusion based on curved spacetime. The planets actually travel in straight lines in 4D that look like curves in 3D. Like how if you look at a flat map, the shortest distance between London and NYC looks like its straight across the Atlantic, but in actuality, the straightest path curves up near Greenland. He realize that the speed of light was the translation ratio between motion through space and motion through time. Quantum Mechanics (1905 - now) Around the same time, electromagnetism was also starting to cause more problems. They started to realize that when they hit black bodies with different intensities of light, electrons would pop off. They knew it was because the light was exciting the electrons because the light carried energy. They assumed that as they increased the energy, more would pop off, but it didn't happen. Intensity seemed to have nothing to do with it, only the frequency of the light itself. Planck and Einstein finally figured out that it was because light, and electrons had wave and particle properties. Where waves can have any amount of energy, quantum waves had specific values. This is why electrons would only pop off it specific photons hit them and would ignore others regardless of intensity. It solved the problem of the electrons but opened a whole mess hat took 50 years to figure out as the standard model of Quantum physics. They realized that there were no forces and particles, it was all the same thing and forces acted by matter exchanging other particles. As they kept querying though, things got weirder and weird as things like particles appearing in multiple places at the same time or particles that were connected mathematically, but not in space. Eventually a probability based model of the universe came out of the math and has worked ever since. Particles don't seem to even exist until they interact with another particle, weird, I know, but it's happening. Behaviors are usually described by something called Feynman diagrams which help us visualize. Here we have a possible interaction of two electrons (arrows coming from the bottom) repelling each other by exchanging a photon (wave line.) QM takes all possible interactions like this one and averages them to determine the actual predicted outcome. This encompasses all known forces except for gravity. GR and QM don't play nice - What's next? So we have two models of the universe right now: general relativity and quantum mechanics. They've got some problems which sci-fi writers can capitalize on. First off, they don't use the same kind of math. QM is probabilistic and discreet. Everything has exact, integer values, but has no exact position or state. In GR, everything is smooth, fractions are no problem and everything is fully deterministic. Secondly, they describe different things. Quantum mechanics tells us what happens to particles as they move through space and time. GR describes the spacetime itself. There are other problems like the fact that they have cosmological constants than are orders of magnitude off from both each other and the observed value, but those are the big ones. Third, they describe spacetime differently: GR says it's smooth and can be divided forever, QM says that once you zoom to the tiniest scales, spacetime is chaotic and full of random fluctuations: There are two major competitors to fix the incompatibilities: String (or M) Theory and Loop Quantum Gravity. String Theory (1960s - now) String theory came out of the standard model research. It was original used to describe the nuclear forces, but it failed. They kept up with the math and realized that not only was a gravity particle possible, it was required. Instead of imagining particles like points when they interact and possibility space when not, they picture them as strings vibrating in higher dimensions. QM has the biggest problem with describing the points where particles interact, the points disappear when working with strings because the strings themselves have width so there are no zeros in the math. Those Feynman diagrams with points and lines become more like a bunch of tubes. On the right is a string theory version of the two electrons repelling via a photon (the left side is the original QM version.) Because the diagrams themselves are 3D, you can then "time slice" it however you want to account for relativity. That's a problem still. It unifies gravity and relativity with everything else, but then it leaves space and time as a background player where stuff happens. Loop Quantum Gravity (1990s - now) Loop Quantum Gravity went way outside the box, instead of trying to figure out how to get gravity to fit with the standard model, they decided to figure out how to quantize relativity and describe both whats happening in space and time with spacetime itself. They created a framework in which the interaction of gravity was a property of the spacetime of a specific area. The point interactions of the other models are replaced by regions of spacetime with specific spins and structures. This unifies everything together, stuff doesn't happen in space and time, space and time are woven into what's happening. Once they figured out quantized gravity, the rest of the standard model was formulated by imagining particles as twists or kinks in the spacetime: LQG and string theory's dirty little secret Interesting tidbit that is often overlooked though: neither LQG nor string theory have mathematically described our universe. They are frameworks. Think of simple video game physics in an old 2D scroller. Able to go side to side, jump and have gravity pull you down, bump into objects and have a force between them. That's a framework. Using different parameters you can create any universe from Marios, to Sonics, to Contra, but only one configuration describes each. No one has found a configuration of string theory that actually describes the forces and interactions how we see them, only that it can describe forces and interactions. Likewise, no one has formulated LQG to actually match general relativity, only that GR is within it's range. Usefulness and limits for fiction So here are some common sci-fi tropes and how they match up with different sets of physical laws. Most are expressly forbidden, and usually when I write sci-fi I determine not what's possible given the stringent laws of physics but what's the most minimal thing I'd have to alter in order for my story to work. Time travel - GR, string theory, would make this impossible within two causally connected parts of the universe. In QM, it's possible to formulate interactions so that there appears to be a backwards in time communication, but it's a mathematical illusion, causality is always preserved. In LQG, time gets fuzzy at small scale and at ever smaller scales goes away completely, so it's not a valid question, but in the macro world, it's not possible. Wormholes - GR allows wormholes, but only if they've existed since the beginning of the universe. They can exist, but can to be created or destroyed, spacetime can not tear. String theory and LQG has the theoretical ability to tear spacetime, but the rips appear out of the quantum fuzziness, they aren't manipulable. Warp travel - GR allows it. You can quite easily describe a valid bubble that shoots around the universe unbounded by the speed of causality as long as you are moving forwards in time. It requires an imaginary energy density however, which doesn't seem to have a way to be physically described. Tachyons - GR, LQG, and string theory all forbid it. QM actually requires it, the now-famous Higgs field is actually tachyonic, because it has imaginary mass values. However, no information travels faster than causality and disturbances in the field instantly decay. Subspace communication - All theories forbid it. Lots of people think quantum entanglement will work, but it won't. Entangled particles must be left alone or else their entanglement disappears, so you can't bring them with you anywhere. Negative mass - GR and string theory allow it, but it's not special. It won't move faster than light, and won't respond to forces backwards. In fact, it shouldn't respond to any forces we're aware of at all except gravity. Negative is not the same as imaginary in mathematics. (and imaginary isn't as not-real as it sounds.) QM would not allow it because we understand what makes mass mass. You can't negatively interact with the Higgs field, you either do or do not, it's not like charge that can be positive or negative or energy density, which can be positive or negative. Backwards arrow of time - Since our consciousness only works in one direction, we could never perceive it but GR and string theory allow time to work in either direction and the arrow of time is though to emerge from something else (entropy.) In QM, it's not possible, interactions are one way and destroy information. In LQG, time is an emergent property so the question is nonsensical. Shadow universes - There is no reason to exclude the possibility that there are things we simply don't interact with. Dark matter is one of them (though it is not complex like matter.) If gravity is a boson, then there is no reason that there could be an infinite number more fields with various other things in them, however, if gravity is part of spacetime, everything would be required to interact with it and we'd be able to detect it. Antimatter - It's not sci-fi, this is real and we can make it. It was actually predicted by the laws of QM before it was discovered, and new we understand it very well. it's exactly the same as matter but with opposite charge. It falls in gravity attracts everything else just like matter. We can contain it in magnetic bottles. Mini black hole drives - Very real, tiny black holes put of incredible amounts of energy, but they can not be held in place in any way or subject to any movement. They're also ticking time bombs, if an anti-matter/matter engine fails, just stop feeding it and everything will be fine, if a black hole engine fails, it'll melt down and blow up. A micro black hole will not grow though by pulling everything around it in, it'll simply explode due to Hawking radiation which is a QM affect. Strangelet - often used as a weapon in sci-fi, it's very real. A strange quark is a special type of quark that's more stable than regular quarks and will convert any other quark it touches. All matter is made of quarks at their nuclei, so a single strange quark dropped on a planet, could completely destroy it. It fits with the math of QM, but we've never seen such a thing. Antigravity - GR allows it, it actually requires it. Gravity simply causes space to curve inwards, anything that pushes it outwards (like dark energy) is a form of antigravity. Neutronium - the strongest material in the universe. It's real, but it's only stable under incredible gravitational fields. Neutron stars are these objects, they're gravity is so crazy that atomic nucleii crunch together and form neutrons with then push together until degeneracy pressure stops them. You can't harvest it because once you remove it from the gravitational field, it's no longer stable and will decay. Force fields / light sabres - sort of? You can trap a plasma inside of a magnetic bubble, but they will have energy limitations and radiate massive amounts of heat. You can contain the plasma, but not the heat. Invisibility - Yes, we're already working on that. There are plenty of ways to bend light around an object. Remote sensors - No. QM forbids it, in order to get information about anything, you must interact with it, that requires locality. Teleportation - No, all theories would forbid any type of macro teleportation. You may hear that QM allows particles to teleport. In lay terms, they do, in mathematical terms, they don't. The movement is hidden in the math, but it's there and it's not mysterious. EmDrive - You've probably seen this come out of NASA. Yes, it's real, it does work, and nobody understands why. The most likely explanation is that it's pushing against the quantum foam, there are several other prototypes that do the same thing. I mentioned earlier how GR and QM predict different levels of quantum foam energy. It's very small for GR's, but when calculated with QM it's something like 10^120 times larger, so if that's right empty space still has an insane amount of energy in it. Conclusion / Q&A If you would like a deeper explanation of any of the concepts that I described here or physics in general, please go ahead and ask them here. I only ask that you keep it relevant to science fiction. I could talk for hours about n-branes, but they have no relevance to our universe. If I don't know the answer to something, I probably know someone who does. If you want to see how some of the math works, lemme know what you want to see and what you're level of understand of math is and I'll try to explain it in those terms. If you have a sci-fi cliche that you'd like to know why it would be or would not be possible, list those too.