# Safe Alcubierre drive

I want my world to have an Alcubierre drive for slower-than-light-travel. One third the speed of light will do. Since there is no FTL we don't have to worry about causality paradoxes. However the drive still has problems! They are about radiation.

Hawking radiation inside the bubble will heat the ship and cook everyone inside it. Also on arrival the ship will release infinitely blueshifted radiation and cook everyone at the destination. Is there any scientificaly plausible way to fix these problems and make Alcubierre safe both for ship and its destination?

Maybe it is possible by adding fictional negative energy exotic matter in the story? Maybe some specific spacetime curvature would do? Maybe the problems don't arise at all if we travel at sublight speeds.

Upd.: abovementioned wikipedia article says that there is no problem with Hawking radiation if the bubble is subluminal. But there is still dangerous blueshifted radiation at the destination if I understood correctly: https://arxiv.org/pdf/1202.5708.pdf Or maybe I'm wrong? Any ideas besides careful navigation?

• @Goodies Alcubierre is more a maths thing than a science thing. Commented May 9, 2022 at 15:55
• What does this bit mean: "(beside possible causality paradoxes if it is FTL-equivalent - let it be "not-FTL"-like)" Do you mean you want to build an Albububbierre drive that goes at sublight speeds? Commented May 9, 2022 at 15:56
• @NervousCat Okay I have edited the question to clear things up. You can hit the Edit button if you don't like any of the changes. You can also see how to embed a link. Commented May 9, 2022 at 16:16
• Destination pulses can be avoided just by carefully navigation... aim to the left of the destination, stop short, and travel in from there conventionally. The rest, is that just a heat/radiator problem, or does the Hawking radiation cause more problems than that? Commented May 9, 2022 at 16:17
• @Skek Tek Alcubierre bubbles moving faster-than-light can actually lead to causality paradoxes, for example, there are still possible situations when you can see spaceship arrival before it left using telescope, to sent message to your past or to return before you left. Ability to arrive at the destination faster than light is automatically means possibility of such situations. From the article mentioned above: "he [Alcubierre] writes: "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)". Commented May 10, 2022 at 5:37

The warped space is supposed to create a flat region in the center, so any blue shifts going through the warped space should be nullified by complementary redshifts, I believe.

However, there is still a blue shift for your cruise velocity.

You're looking for the doppler equation for light :

$${{\lambda} \over {\lambda_0}} = {{v} \over {c}}$$ which, in your case $$= 0.3$$

To get the energies of these photons

$$E = {{hc} \over {\lambda}}$$

Putting them together $${{E} \over {E_0}} = {{\lambda} \over {\lambda_0}} = {{c} \over {v}}$$ which, in this particular case $$= 3$$

The energy of each photon is what matters, and for water the absorption depth for ~37% of the incident light the be redirected follows a linear logarithmic relaitionship. You can see a graph here.

Since the relationship to frequency is logarithmic, tripling the photon energy does not change the amount of shielding material your ship will need as much as your might expect. You will need 1.5x times more radiation shielding.

According to the answer to this question, that baseline amount of shielding is about 1 meter (1 yard, or about 3 feet) thick. You might have "sci-fi" technology that reduces that amount.

Not a Problem

I do not understand the maths behind the Alison Cubert Warp Drive. But I am VERY good at reading Wikipedia articles. On a good day I can also read the Abstract and Summary of an academic paper.$$^{1}$$

EAT THIS!

Of course, all the aforementioned problems disappear when the bubble remains subluminal. In that case no horizons form, no Hawking radiation is created, and neither strong temperature nor white horizon instability is found. The only remaining problem is that one would still need the presence of some amount of exotic matter to maintain the subluminal drive.

This is from Semiclassical instability of dynamical warp drives. The paper cited in this bit of Wikipedia:

See that [4]? That's the paper I'm talking about.

The take home is that for a subluminal drive there might be no radiation problems. There might not even be a bubble!

The take home is that you can sometimes answer your own questions by scrolling down on Wikipedia and following some of the citation links. The power is yours.

$$1.$$ Just the one though. Then I need a little lie down.

• Be careful. If you scroll too fast in Wikipedia, you might stir up enough exotic matter to cook lunch before you make breakfast. 😁 Commented May 10, 2022 at 13:15