I'm making a fairly hard Sci-Fi story, and I was wondering what the most scientifically plausible 'energy shield' (for ships and stations) would be. My world tries to remain true to basic physics, but I'm not researching every law and limit to get it perfect. If it's not possible by our current understanding of physics, what should I change to minimize the number or obviousness of the flaws in its plausibility?
Same as Earth's, which serves us well: magnetism.
A superconductor can be charged with a huge current that then acts as an enormous permanent magnet.
A ship may need to handle dust and gas moving at high speed relative to the ship, but that material is not charged. So charge it: spray electrons to charge dust (like a room ionizer does) or use a laser or tuned microwaves to cause gas to become ionized. Also, might say that the corona effect magnifies the results and sweeps away more than you explicitly ionized (though it's not obvious that that would be the case in a near vacuum).
physical swarm controlled via flux pinning
If you want something to affect material near the ship to serve as protection, you are asking to change the momentum of that stuff. What could do that without contact is electromagnetism or gravity. That is, an "energy shield" is electromagnetic, with no other reasonable choice.
With "contact", you can still avoid walls by spraying small particles out. The corona effect uses electric charges to accelerate charged particles which then smash into other particles, pushing them away.
The idea of shielding that's not simple walls is useful for constant renewal rather than wearing out, and being stronger than mere matter. Something that is really cool to see demonstrated is the use of superconductors to “pin” an object in a definite position in space relative to each other, with a sizable gap between but acting like a solid object.
So, have a swarm of small magnets surround the ship, some distance from each other but with enough layers that the swarm is opaque. An incoming rock will hit one of the plates and not make it through the swarm. Each magnetic plate is covered with spongy material, so the hit will accelerate the plate without (typically) breaking it. The plates are far enough apart so it won't knock into another. But, the displaced chunk is loose and did not fracture a large normal object like a plain matter shield. After absorbing the energy, the superconductor takes on the energy while slowing the motion, then returns it to its proper position.
The plates can be made with weak seams crossing it where it will break in a controlled manner if hit too hard; and each piece is still controllable as part of the swarm.
The swarm can be dynamically reconfigured, to beef up the direction where hits are expected, such as the direction of travel or where debris have been spotted.
I've seen the superconductor station-keeping explained as possible for a structural component that won't "break" but can be returned to its proper position after being over-stressed. Using a swarm for shielding is an original idea.
Fans of the RI Christmas Lectures will recognise this demonstration. Here is an overview which touches all the bases. I could not find a video of the particular demonstration that inspired me illustrating the strength of such bonds for structural elements in space construction.
The problem with electron beam welding, however, is that it needs to be done in a vacuum. This requirement is quite inconvenient, because it means creating a vacuum box that may be as big as an entire room.
Dr. Herschcovitch invented the plasma window to solve this problem. Only 3 feet high and less than 1 foot in diameter, the plasma window heats gas to 12,000°F, creating a plasma that is trapped by electric and magnetic fields. These particles exert pressure, as in any gas, which prevents air from rushing into the vacuum chamber, thus separating air from the vacuum. (When one uses argon gas in the plasma window, it glows blue, like the force field in Star Trek.)
But can the plasma window be used as an impenetrable shield? Can it withstand a blast from a cannon? In the future, one can imagine a plasma window of much greater power and temperature, sufficient to damage or vaporize incoming projectiles. But to create a more realistic force field, like that found in science fiction, one would need a combination of several technologies stacked in layers. Each layer might not be strong enough alone to stop a cannon ball, but the combination might suffice.
The outer layer could be a supercharged plasma window, heated to temperatures high enough to vaporize metals. A second layer could be a curtain of high-energy laser beams. This curtain, containing thousands of crisscrossing laser beams, would create a lattice that would heat up objects that passed through it, effectively vaporizing them. I will discuss lasers further in the next chapter.
And behind this laser curtain one might envision a lattice made of "carbon nanotubes," tiny tubes made of individual carbon atoms that are one atom thick and that are many times stronger than steel. Although the current world record for a carbon nanotube is only about 15 millimeters long, one can envision a day when we might be able to create carbon nanotubes of arbitrary length. Assuming that carbon nanotubes can be woven into a lattice, they could create a screen of enormous strength, capable of repelling most objects. The screen would be invisible, since each carbon nanotube is atomic in size, but the carbon nanotube lattice would be stronger than any ordinary material.
So, via a combination of plasma window, laser curtain, and carbon nanotube screen, one might imagine creating an invisible wall that would be nearly impenetrable by most means.
Yet even this multilayered shield would not completely fulfill all the properties of a science fiction force field—because it would be transparent and therefore incapable of stopping a laser beam. In a battle with laser cannons, the multilayered shield would be useless.
To stop a laser beam, the shield would also need to possess an advanced form of "photochromatics." This is the process used in sunglasses that darken by themselves upon exposure to UV radiation. Photochromatics are based on molecules that can exist in at least two states. In one state the molecule is transparent. But when it is exposed to UV radiation it instantly changes to the second form, which is opaque.
One day we might be able to use nanotechnology to produce a substance as tough as carbon nanotubes that can change its optical properties when exposed to laser light. In this way, a shield might be able to stop a laser blast as well as a particle beam or cannon fire. At present, however, photochromatics that can stop laser beams do not exist.
Also, metamaterials have been used to create "invisibility cloaks" that deflect light around them, as in the following idealized graphic:
Presently these only work to "cloak" very small objects at very specific non-visible light frequencies, but scientists hope in future to expand the technology to larger objects and a greater range of frequencies. So potentially a technology like this might be adapted to deflect lasers around an object. There is also an analogous idea based on metamaterials called a "universal mirror" which would reflect incoming light beams back in the direction of their source. This article on the "universal mirror" specifically mentions it might have applications in laser shielding:
The metamaterial could also act like a aggressive shield, protecting objects from airplane-based, high-energy laser systems, which are being developed by Boeing, by bouncing the lasers beam back at their source.
Finally, if you want to get into much more far-future technologies, potentially it might be possible to create something like an invisibility cloak by setting up a thin layer of "exotic matter" that would bend spacetime in a way that would deflect all incoming light, as discussed in this paper. This is a purely theoretical idea that would require a civilization to be able to create and control very high densities of exotic matter with properties that present-day physicists aren't sure are even physically possible, in ways that would also allow other far-future technologies like the Alcubierre "warp bubble" that would allow for a form of effectively faster-than-light travel.
Boeing’s proposed system involves using a combination of lasers, electricity and microwaves to rapidly heat up the air between the vehicle and a blast. This heat creates a plasma shield that's denser than the surrounding air and able to deflect or absorb the energy from the incoming shockwave.
University spacecraft research
The most common hard-science sheilding that I've encountered in fiction is ablative armor. It isn't very elegant, but surrounding you ship with multiple layers of hardened foam provides a non-critical surface to catch the particles which get by your outer magnetic fields and inner swarm defenses. As these particles collect, they provide additional layers of impact absorbing material to keep ever bigger particles away from your ship's atmospheric shell and other vital systems.
If really big particles get through, your foam armor can even break off during impact, carrying most of the momentum and mass of the collision away from your hull. Then automatic foam dispensers on the hulls surface can replace the missing layers, preparing your ship for future impacts.
Upon arrival at your destination, you can either vibrate the hull to shake loose the remaining foam or bring the swarm in close to scrape the foam away from the outside in.
Essentially there isn't anything in hard science beyond maybe some of the ideas with magnetism and plasma. Active defenses (interceptor missiles, lasers, etc) are more plausible but still tricky.
Just dodging and getting out of the way is surprisingly effective, there is a lot of space out there and with no atmosphere to carry shockwaves near misses do very little damage.
If you have gravitational control in your universe then it's not hard science but you can see manipulated gravity fields being plausible as a defense mechanism. They could divert incoming fire away from your ship or even suck it into miniature black holes!
Bent space shield.
Gravity is usually considered as a force and so I assert it meets the requirement of this question. Gravity is not a real force but an apparent one - gravity emerges as a consequence of the ability of mass / energy to produce inhomogeneities in space: bent space.
To make the bent space shield, you would use negative matter much in the way it is proposed for use in making the Alcubierre drive.
Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel. Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space without breaking any physical laws.
If you have the tech to make the drive, making an energy shield would be a lot easier. You would bend space around the ship such that energy or matter entering one side of the field would continue on its path, but be routed through bent space around the ship and out the other side. This is slick in that it works equally well for moving particles or electromagnetic radiation. Even very energetic particles moving at relativistic speeds would be effectively rerouted. The energy of the particle or radiation remains as it continues through bent space, out the other side and on away from the ship.
A side effect of this shield is that it would also be a cloaking device: the ship within this shield is invisible. I am not sure what energy emitted from the ship itself does when it hits the inside of the shield.
An interesting thought experiment: what happens if a ship with a field like this rams a larger object? Mass impacting the bent space field is routed through it. Could a ship with a bent space shield fly through solid matter? If it closed up any windows in the shield, could it fly through a star?
Thinking more on the concept - a ship could use the same negative mater for its warp drive and its shields. But the stuff must be rearranged to serve these different purposes. To activate the drive you need to take down the shield and move the negmat to the specific place you will use to generate your drive. If you come out of warp, it will take time for the servos to redistribute the mass around the ship and generate your shields. This helps make ships so equipped less overpowered and godlike - you cannot just invisibly and untouchably warp from place to place.
If your looking for something energy based, you could do a hard light shield by having a cloud of Rubidium around the craft and have weak lasers fire a few photons at a time (or scale up, the research is readily available) to create a hard light construct. Here's the paper on the experiment for generating hard light: http://www.nature.com/nature/journal/v502/n7469/full/nature12512.html And a news article for summary: https://www.sciencedaily.com/releases/2013/09/130925132323.htm