Reddit Q&A

 Hello all,

As mentioned in an earlier post, on March 9 there was some activity on /r/science on the press release for my warp drive paper (https://www.reddit.com/r/science/comments/m1gyyi/breaking_the_warp_barrier_for_fasterthanlight/). Several questions  came up that I thought required some follow-up. So instead of outing myself on Reddit, I will address here those questions that I can answer quickly. Others might appear in their own posts in the future.

1) Phonons as a source of negative mass:

Theories of phonons (essentially density waves in a medium) can be described as an effective (quasi-)particle field with negative mass in the sense that they are repelled by gravitational fields of massive bodies such as the Earth. Examples of phonons include the sound waves in air, or most any other pressure/density waves over a massive, elastic, and nearly homogeneous medium. (https://en.wikipedia.org/wiki/Phonon). However, phonons are not likely to provide the "exotic matter" needed by the Alcubierre and other earlier warp drive models as the total mass density of the medium plus phonons will still be positive.

The jury is still out on warp plasma in general.

2) & 3) Creating the soliton and in-space collisions:
Two great questions here. Not sure if I can ELI5, but I will do my best to be understandable.

The paper does not cover how to create/accelerate one of these solitons/bubbles. They are 'constructed' mathematically to exist ad infinitum, meaning forever. Answering the question of how these solitons are created and accelerated will be crucial to forming experiments and addressing the horizon problem. 
Note that the means of acceleration will still need to obey the relativity version of momentum conservation (stress-energy-momentum conservation).

For the second question, gas, dust, and other objects impacted by the bubble has been discussed in the literature, at least for the Alcubierre drive. One study (https://arxiv.org/abs/1202.5708v1) shows that objects are given a boost in speed in the direction of the soliton. For warp bubbles moving faster than light, these objects can become "time locked" in the shell of a constant velocity bubble where they are stuck in the leading edge. I may write another post on this once I have a better sense for how colliding objects will  behave in this new class of warp bubble.

4) Time dilation and trip duration:
Okay, so you kind of got me on a technicality here: the trip time as viewed from the ship traveling through space-time at near light speed can be shortened to a human lifetime by dilation effects. An earlier version of the press release actually accounted for this possibility, and indicated that it was more desirable for both the ship crew and their friends and family back on Earth to be alive for a reunion on the ship's return. This was altered by the editors to the third paragraph (https://www.uni-goettingen.de/en/3240.html?id=6192). Personally I would rather be able to come home to my family and friends.

In any case, even if you could find a rocket propellant with a high enough specific impulse to accelerate a rocket for years on end (https://en.wikipedia.org/wiki/Specific_impulse), you would still be faced with the conundrum: Is it worth it to take the trip if the technology that would surpass you is developed while you are en route?

5) Warp bubble miniaturization:
This is actually the first step of several of the past energy reduction techniques ( see https://arxiv.org/abs/gr-qc/9905084v5 and https://arxiv.org/abs/gr-qc/0207057v3). The equation for the total energy required scales as

E_total ~ R2/w,

implying that by scaling down the radius (R) of the probe while keeping the bubble radius to shell width ratio (R/w) constant, that the energy scales linearly with bubble size. If we were to shrink the bubble from R=100m to R=1mm, the size of our miniature robotic ship. The energy would reduce by a factor of 100,000. This is still on the order of some large fraction of the Earth's mass. Plus, you would still have to find a way to put all that energy into such a confined space. Probably best to wait until the energy needs are reduced by some tens of orders of magnitude.


Have a good Thursday.

Erik

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