Tachyon Rockets... Could They Even Work?

[Spacescifi]

 

Tachyons are purely theoretical and probably do not exist.

The only interest I have in them is using them as rocket propellant... but I have a few questions... assuming anyone knows.

1. Tachyons would go no slower than light speed and always move faster than light it is said. So if that is the case, what kind of engine would it look like? Not sure if it would even look like a rocket nozzle.

2. I ended up abandoning my idea for FTL photon scifi rockets, since someone elsewhere calculated that if the speed limit of light was ignored and we used newtonian physics to calculate, a photon's energy would also go up... in other words, you still get a death ray exhaust photon rocket even with FTL photons

 I thought I could get by using less FTL photons, but doing that either results in pitiful thrust if too little, or still death ray exhaust if sufficient.

Question? Would tachyon rockets emit all types of deadly cerenkov radiation? Or would the exhaust be invisible and barely interact with anything?

 

Thanks.

 

[Guest]

Tachyons have no place in modern physics. Well, taychonic fields are a thing, but they're not FTL, just really weird theoretical concept that happens to involve imaginary mass. They're useful as far as physics go, but do not have any practical applications, current or speculative (unless you count superconductor theory, which is where they tend to come up).

[kerbiloid]

Spoiler

A tachyon-propelled rocket is a thing when you have a time machine to refuel.

 

[StrandedonEarth]

Spoiler

 

[Shpaget]

If tachyon is always FTL, then you don't really need much energy to accelerate it to stupid fast. Basically, just show it the door and it will be out before you know it. Literally (since, you know FTL and stuff).

Unfortunately, with tachyons being imaginary and all, we don't know how much mass they have and how fast they go, so we can't calculate ISP and thrust. Well, even if we did know their mass and speed, we still have no math capable of calculating the reactive forces of FTL stuff.

Honestily, this entire thing is not very scientific.

[Guest]

Actually, we could attempt to calculate Isp the normal way. The problem is that we'd get imaginary thrust. Basically, the problem is that tachyons, to move faster than light, need to have imaginary mass. This means their mass flow rate will also be imaginary. This would also lead to imaginary delta-V, because dry mass of the ship can't be imaginary (because then the whole thing would be a tachyon). This also propagates into conservation of mass, momentum and so on.

That's the problem with tachyons, in a nutshell. You get a lot of imaginary terms wandering around, which don't really have a physical interpretation. That's not even getting into the fact they would travel backwards in time (actually a problem with any true FTL. Warp drives don't suffer from this, but they mess with spacetime instead of travelling faster than light).

[Shpaget]

So what does acceleration of 321 + 56i m/s² even mean?

[Guest]

Presumably you start accelerating into another dimension.  As I said, that's the problem with tachyons. We don't know what imaginary mass/thrust/acceleration even are. Or if they are anything at all (likely not).

[kerbiloid]

If tachyon rockets existed, we would already have one.

[magnemoe]

tachyons increase their energy going slower.
They are basically an mathematics model of an particle on the other side of the 1/x graph of energy then you is on the other side of light speed. 
Just because something is possible mathematically does not imply its anyway connected to reality. Example an satellite at -300 km attitude.  

Add that we does not know any way it can be theoretically be created.

 

[kerbiloid]

6 hours ago, magnemoe said:

Example an satellite at -300 km attitude.  

A deep geostationary orbit.

[Rakaydos]

On ‎11‎/‎3‎/‎2019 at 3:01 PM, Dragon01 said:

Presumably you start accelerating into another dimension.  As I said, that's the problem with tachyons. We don't know what imaginary mass/thrust/acceleration even are. Or if they are anything at all (likely not).

Cathrine Asaro had a science fiction setting with that as her plot-tech. Because technically, the lighspeed barrier is only a point in complex vectorspace, and so a rocket that can generate an imaginary component to their thrust can accelerate past lightspeed if they do it with a partially imaginary (complex is the mathematical term) velocity. Also they stored fuel in complex-space pockets they called Klein bottles.

[Guest]

Well, if it's well thought-out, then it might be worth a read. Although technically, lightspeed barrier would be a 3D hyperplane in the complex vector space, so getting "around" it by fiddling with the imaginary component seems dubious (although taking shortcuts through it could give such impression).

On 11/3/2019 at 11:49 PM, magnemoe said:

Just because something is possible mathematically does not imply its anyway connected to reality. Example an satellite at -300 km attitude.  

Actually, orbiting within a body is not possible. Barring the obvious problems, gravity inside a massive object is given by a different equation than outside. In particular, shell theorem shows that an object inside a hollow shell experiences no gravitational force from it at all. So it turns out math does have this case covered.  

[kerbiloid]

The Earth radius is ~6371 km.
Below the -300 km there is a lot of mass to let the object orbiting.

An object inside a hollow shell experiences the net gravitational force produced by the shell parts.

Gravity inside a massive object is given by the absolutely same equation as outside, sum(G*dM*m/r²).
Just inside a uniform sphere/ball the part of the sphere/ball outside the orbit produces zero net gravity.

No need in theoremes, it's a trivial integration example.

Edited November 9, 2019 by kerbiloid