The Explosive Force of the Braking Alcubierre Drive - What would this look like? The Alcubierre Drive and faster than light travel more generally may be locked away in the realm of fiction forever. That might be depressing to some people but I think their impossibility is really interesting in it's own right.
Here is The Alcubierre Warp Drive: On the Matter of Matter (2012). In it the authors state that a functional Alcubierre Warp drive has the capacity to destroy what is in front of it as it brakes (for lack of a better word). In more detail:

"These results suggest that any ship using an Alcubierre warp drive
carrying people would need shielding to protect them from potential
dangerously blueshifted particles during the journey, and any people
at the destination would be gamma ray and high energy particle blasted
into oblivion due to the extreme blueshifts for P+ region particles."

The paper goes on to explain that P+ represents particles with initial positive velocity that are overtaken by a superluminal bubble.
I would like to know more about this detonation.
Let's handwave FTL being impossible and say a ship is flying through time and space using an Alcubierre Drive. As it goes: what particles are being captured, how much energy could be expected to be released (a quantitative answer/approximation would be amazing) and what would the emission look like?
Note: Equation 15 of the paper may provide insight to an understanding of the relevant blueshifts, however, the authors later write:

Technically the particles in the P+ cannot leave the bubble, and so
there is no value given for this region. However, the time component
of the 4-velocity the particles in this region increases exponentially
for the duration of the time they are caught in the bubble. Hence if
the ship were to ever slow to below the speed of light such that they
could escape and interact with outside observers, they would be
observed to have extremely large energies.

Which has confused me to no end.
 A: It's pretty easy to see why this happens. A trip in the warp bubble can last an arbitrarily small amount of coordinate time, so let's suppose it lasts less than the light travel time across the bubble. Then look at a right-moving wavefront at the start of the trip that reaches the rightmost interior point of the bubble when the bubble arrives at the destination, and another wavefront at the destination that will be at the rightmost part of the bubble's exterior when the bubble arrives, and a bunch of others in between those in the background space. None of these wavefronts can overtake each other, and the leftmost and rightmost ones are separated by the width of the boundary of the bubble when it arrives, so all of the intermediate ones are crammed into the intermediate space. That translates to an average blueshift factor of roughly the length of the trip (light years, probably) divided by the width of the bubble boundary (who knows, but much smaller). The total energy would probably increase by a similar factor.
I question the relevance of this for a variety of reasons.
First, the warp drive already plows through everything in its path between the starting and ending points, so who cares if it also destroys some stuff past the ending point?
Second, they're simply calculating geodesics in the spacetime, meaning they're assuming that their test particles don't interact nongravitationally with the exotic matter. If the exotic matter is opaque then their conclusion is invalidated. It may as well be opaque since it's made-up stuff anyway.
In fact, since the exotic matter vanishes magically into nowhere when its work is done, we may as well suppose that all of the energy of the particles it absorbs disappears with it. It might seem that this violates energy conservation, but it doesn't matter: you can write down a spacetime geometry where the energy isn't there at the end, and plug it into the GR field equation, and then argue that we would get that spacetime geometry if we could just engineer that stress-energy tensor. It's no different from what Alcubierre did, and no less plausible as far as I can tell.
The fundamental problem is that there are no rules restricting warp drive solutions. You can get a stress-energy tensor from any differentiable metric, so anything you write down is fair game. A real theory of warp drives would require an actual physical theory with exotic matter in it, and would probably look nothing like what Alcubierre wrote down.
