# Can this warp drive concept be used for actual travel?

In this universe, a "warp drive" is more like a "constant linear velocity" drive. Once it is turned on, the craft travels at exactly the same velocity (speed plus direction), in a straight line, that it had initially. There is no changing this velocity during the warp trip, including by gravitational fields.

So assume that a spaceship is in orbit of a planet. It has a velocity at a given time, $$\vec{v}$$, with respect to the sun. Now, it wants to travel to another planet. The problem is that the planets are all moving with respect to the sun. Is it possible to put the spaceship into an orbit so that it can arrive at the orbit of the other planet at the right time, and so that its velocity matches what it would need to enter orbit of that second planet? Is that a practical method of travel in this universe?

• Suppose you have a ship with moving parts...say two modules, connected by a telescoping arm. If the warp drive module is at one end, and the the drive is turned on as the arm is extending, is your warp velocity based on the velocity of the warp module, or the velocity of the center-of-mass of the ship? If the former, you have a reactionless drive. (Big yes!) But be warned... google "Friends Don't Let Friends Use Reactionless Drives In Their Universes".
– Qami
Commented Mar 18, 2021 at 17:16
• Technically speaking, an object at rest does move in a straight line when it falls (graphic explanation). Gravity simply bends space, so that a "straight line" (geodesic) veers off in typically inconvenient directions. Commented Mar 18, 2021 at 17:28
• Maybe I am dense, but don't we already routinely make flight plans which make spacraft arrive at the destination at the right time with the right relative velocity for whatever purpose we have -- landing, entering orbit etc.? Commented Mar 18, 2021 at 18:21
• @MichaelStachowsky You said "straight line" and not affected by gravity. Neither of those make much sense. Under GR, both of those are impossible. Straight lines cannot exist, the universe is curved (try drawing a straight line on a football). Also, gravity curves spacetime, so, a massive blackhole would curve your path for you...
– Aron
Commented Mar 19, 2021 at 3:08
• I don't know how convenient it would be for travel, but there's lots of ways to make quite a handy perpetual motion machine out of it. For example, if it can be used in an atmosphere, just turn it on while landed. The Earth is being pulled by gravity toward the Sun, but the ship isn't, so if you do this at the correct time you'll quickly rise upwards. Then turn off the drive and drop back down using helicopter blades to slow the descent, and use the blades to run a generator. Then repeat for infinite free energy. (I'm sure that much more efficient ways are also possible.) Commented Mar 20, 2021 at 11:29

TL;DR: useful but slow. Probably no good for human colonisation of space. If you can scale it up though, it might be interesting.

Is it possible to put the spaceship into an orbit so that it can arrive at the orbit of the other planet at the right time,

Yes. I won't bother trying to work out the maths here, but it isn't super difficult. You can basically brute force the problem and work out when you'd have to launch in order for your trajectory to intersect the target's orbit at the same time as the target intersects your trajectory.

Note that if you keep the warp drive on you're not in an orbital trajectory, and if you turn it off you'll end up in a heliocentric orbit with a different trajectory.

and so that its velocity matches what it would need to enter orbit of that second planet?

This is slightly more problematic, but not necessarily impossible depending on where you're going. On Earth, for example, you've got your ~30km/s speed around the sun, +- your 8km/s max orbital speed. If you wanted to go to Mars, which has a ~24km/s speed about the sun and max +- 2.5km/s orbital velocity, you can see that it is easy to be going too fast. Conversely, going towards Mercury, you'd need to have 47km/s velocity to inject yorself into its orbit.

For Mars you might be able to aerobrake, but for Mercury you'd need to do some tricky gravitational manoevering to just brush by the planet, turn off your warp and fall into its gravitational field to give you enough speed to warp past it again, and repeat until you've been given enough of a boost. Fiddly, but not impossible, I'd say.

Is that a practical method of travel in this universe?

It'll be faster than a conventional Hohmann transfer going outsystem because speeds are constant, but it'll be less convenient at the end because your velocity vector will be pointing the wrong way and you may be travelling too fast to be gravitationally captured, etc. It may or may not be slower than a transfer insystem (towards the sun) but it'll be much cheaper in terms of delta-V which is always nice.

For flying to large bodies, like Jupiter or the Sun, it'll be quite useful but slow, because you're limited to the orbital speeds of the place you're leaving from which may be much less than the speed of available rockets but at least you're guaranteed to be able to be caught at the other end.

On the upside it enables you to do super gravity assist trajectories... I'm not quite sure how much of a boost you could get here, but dropping in towards the Sun might get you over 100km/s of extra speed. Not enough to fly to another star in good time, but not to be sneezed at.

The most interesting thing here is the size of the warp. If you can use it to boost huge chunks of rock into space from a planetary surface, or change the direction of an asteroid, the fact that the trip will be slow will be much less problematic because you'll have access to vastly more space, shielding and resources than unassisted trips.

# Yes-ish

This will get you from point A to point B. Navigation will require timing things right- aim for where the planet will be (or you will gain the correct velocity to intercept that planet). The rest is just geometry of your system.

This system alone does not guarantee that you will have the proper speed for orbit around a target body. It depends on the mass and speed of the bodies of concern, altitude of orbits, and the spatial relationships between the bodies. In any case, another method of acceleration should be included- which may make a combination approach practical. I would imagine that not having to speed up to escape one body would justify the use of this drive. This holds even if you need to slow down on the other end- the space vessel would just need to slow down less. Speeding up could be an issue, but could be accomplished by a burn in a low gravity area before insertion (as to avoid gravity counteracting the acceleration from engines).

As a note- your frame of reference doesn't matter much here! The spacecraft already has the system's velocity (since it is in orbit above a planet), so it doesn't really matter if you include that in the 'straight line' and 'constant velocity'.

# A Straight Line And Constant Velocity!?

Never mind changing orbits- use this to launch from the ground! A lot of the fuel real-world rockets use is just trying to break through the atmosphere and get to orbit! This assumes, of course, it could work on the surface of the planet and is more efficient than chemical propellants currently used.

If you can impart even a small amount of velocity to your vehicle, you can cruise to the limits of outer space. You may even get into a reasonable orbit with this- you go up and the planet also moves out of the way (depending on your launch site and initial direction).

• I vaguely remember that getting into space with a rocket is pretty easy while gaining the sideways velocity to stay in orbit is the real challenge. Is that incorrect?
– nwp
Commented Mar 19, 2021 at 13:23
• @nwp An object in Low Earth Orbit has an altitude of about 250km and a velocity of about 8 km/s. That makes its kinetic energy about 10 times larger that its potential energy - so yeah. Commented Mar 19, 2021 at 14:54
• @nwp The higher you go, the less energy you need to establish an orbit afterwards. For example the geosynchronous orbit at 35000 km requires only 3 km/s of speed. Commented Mar 19, 2021 at 18:57
• @nwp the link talks about the amount of fuel to get to max q- usually well within the atmosphere. It's on the order of 40-ish percent just to get there! Of course, at that time, they've begun translating a lot of their vertical speed into horizontal speed, so I edited this to encompass both situations- the point is, you use a lot of fuel in earth's atmosphere, so if this this more effective, use it to launch! Commented Mar 22, 2021 at 13:39

# This sounds more like a hyperspace drive than a warp drive

As others have pointed out, the idea of just "going forward in a straight line, at constant velocity" is in obvious conflict with General Relativity, which holds that this is in fact what all planets (and spacecraft, when not actively applying thrust) are already doing. The reason it does not seem that way to naive observers is that they're trying to describe the celestial mechanics within a Newtonian model where time and space are flat (and independent). You want your drive to operate with respect to the Newtonian model of space, rather than the General Relativity one, and also to make your spacecraft immune to gravity, which under GR is due to the shape of spacetime itself.

In essence, you want a non-GR spacetime for your drive to take as reference. I suggest that this is good old "hyperspace", that has been a staple of scifi space travel pretty much forever.

"Turning on the drive" means transporting your spacecraft to hyperspace, which for some reason is not curved, so there is no gravity to affect your spacecraft; whatever velocity you entered with, you will keep as long as you stay. When "turning off the drive", you are transported back. Easy as that. You might be able to manoeuvre in hyperspace, if you eject something to provide delta-v, but this requires that the object you eject carries its own hyperspace drive unit. If you fire regular rocket thrusters, then the exhaust will just get stuck when it hits the hyperspace drive bubble (or perhaps your drive will malfunction).

One difference to the classical hyperspace drive that seems to be implicit in your setting is that your hyperspace is just as large as regular space (which is really big, but we all know that quote); otherwise one popular way of explaining why hyperspace travel is fast used to be that hyperspace is smaller than regular space — jumping to hyperspace doesn't make you go faster on your trip from A to B, but you can get there in a shorter amount of time because in hyperspace the distance from A to B is shorter than in regular space. Hence the traditional need for putting your spaceship on the correct heading in regular space before making the jump to hyperspace.

Making the constant velocity linear travel effect related to a separate but parallel spacetime can also be used to motivate restrictions on the drive, such as not being usable to get out of a black hole: you can only jump to hyperspace if the curvature of regular space is small enough (possibly depending on how much power you feed into your drive), so inside the event horizon it is useless. Maybe likewise it can't be used at a planet surface, so you need other means to get into space to begin with. I seem to recall it being quite common in older scifi that you couldn't even use hyperspace jumps within (the inner parts of) a solar system for just that kind of reason, leading to interstellar voyages having long stretches cruising past the planets of one system before jumping to the next system, beginning a second stretch of travel past the planets there. But you seem to want your drive to be usable for travel between planets of the same system, which the modern reader will probably find more interesting anyway.

It sounds doable.

The ship will be traveling in a straight line with fixed velocity, so you need to aim for the point where the planet will be when the ship will cross its orbit, set the velocity to the orbital velocity needed at the destination planet and then fire the drive at the right moment.

After that enjoy the trip.

However, considering that typical orbital speeds are around tens of km/s, the travel will take some time. If you make the travel short by increasing the speed, you will need to provide means for slowing down, else you will overshoot and miss the orbit.

• Don't forget the larger curving movement of our arm of the galaxy. There are lots of potential variables if you're suddenly moving straight along whatever tangent you were last at. Commented Mar 18, 2021 at 16:38

Be sure to call the graveyard -- Einstein will be turning over, and they may need some precautions against the resulting earth tremors.

## FRAME CHALLENGE

Your drive violates General Relativity. It violates various conservation laws as well, but the big one, here, is that for this drive to work, there is an assumption of a preferred frame of reference. Otherwise, when your velocity vector remains constant, you have to ask "constant relative to what reference frame?

Since that's an unwanswerable question, General Relativity says this drive can't work in our universe.

• Hang on...it's very probable I'm missing something, but why can't you just say "constant relative to all reference frames" ? At the moment when the warp drive was turned on, it had a velocity relative to any and all reference frames...why can't it just keep those velocities, unchanging, for each respective reference frame? Or would that cause some kind of contradiction...?
– Qami
Commented Mar 18, 2021 at 17:58
• @Qami: You mean inertial reference frames, of course. If you really mean "all" reference frames, than this is obviously logically impossible, because the reference frames themselves may be accelerating relative to each other. Commented Mar 18, 2021 at 18:19
• @throx Nope, SR isn't broken at all here. If you are traveling at constant velocity in an inertial frame, a Lorentz boost isn't going to change that fact. Just Lorentz boost the velocity 4 vector if you don't believe me.
– Aron
Commented Mar 19, 2021 at 2:46
• However GR is definitely broken. The reason? The framework OP describes only makes sense in Cartesian/Minkowski space/spacetime. Which is only exists in GR spacetimes with a zero energy density (massless). This gets worse when you throw in QM, where zero point energy would mean that zero energy density isn't even possible in a massless universe...(source, I have a Masters in Physics).
– Aron
Commented Mar 19, 2021 at 2:52
• To be fair, the question does postulate a different universe. Commented Mar 19, 2021 at 14:10

My initial reaction was similar to the frame challenge from Zeiss Ikon.

However, this perhaps instead gives a suitable set of pseudo-science explanations for the functioning of the drive...

The number of possible reference frames are as vast as the universe itself - a reference frame in which you have constant linear velocity relative to the immediate surroundings [unless acted on by a force] is basically the physics we have now (and "the immediate surroundings" changes as you move according to that velocity).

So the drive allows you to change what reference frame you maintain constant velocity relative to e.g.:

• standing still on the surface of a planet, you engage the drive, selecting the centre of the planet on which you are standing as the reference frame. The planet continues to rotate, but no longer affected by its gravity [i.e. no longer tied to the reference frame that is constantly accelerating towards the centre of the planet] you continue travelling tangential to the surface of the planet. The effect from an observer on the planet's surface is that you slowly start to drift upwards, accelerating faster and faster until you disappear from view in the sky.
• from the same place, you instead select the centre of the solar system - as well as the effect from the planet's rotation, you now also have an effect from the planet's orbit. Your apparent acceleration (negating the effect of the planet's own acceleration towards its sun) could be in any direction relative to the planets surface, depending on time of day...
• similarly, if you select the centre of the galaxy, or a different planet or a different star etc. there'll be even more apparent acceleration for local observers. Pick a reference frame that itself is strongly accelerated, and you'll start accelerating very strongly (e.g. even within our own solar system you might consider mercury, or a comet on a close pass to the sun)

With an infinite number of reference frames to choose from, the trick would be to find one that actually accelerates you in the direction you want, and which then will also decelerate you relative to the target planet in time for when you switch the drive off, and end up conveniently in orbit around the target planet. Skilled navigators could switch between multiple observer reference frames mid-flight.

There's a huge amount of hand-waving here of course, but the intention would be that an observer in your chosen reference frame would observe you continuing to move with constant linear velocity, which other observers near to you would perceive as massive acceleration in their own reference frames.

However, unnoticed by the early experiments, which used large planets or stars as the reference frame for a lightweight experimental device, there's an equal and opposite effect on the chosen reference frame. Not only is there a (potentially very large) effective force on you to allow you to be observed with constant linear velocity from your chosen reference frame, but there's an equal an opposite force on the object(s) at the chosen reference frame.

Taking the possible effects to the extreme, consider the case where either you or the chosen "observer reference frame" were near a black hole. As you (or the chosen observer) approach the event horizon, you would still need to be observed as having constant linear velocity by the observer, despite the massive disruption of spacetime in the vicinity of the black hole... and that implies enormous acceleration effects, and probably some changes in time perception, for you or the chosen observer, or possibly both.

... but beyond a certain limit, the energy transmission capacity of the drive itself just breaks. As you pass over an event horizon of a sufficiently large black hole, it's certainly possible that a relatively nearby and relatively lightweight observer could be dragged in with you in order to continue to observe you with constant linear velocity at least for a little while... or that a sufficiently massive and distant observer would "drag you away" from the black hole as you continue on what they observe as a constant linear velocity almost but not quite touching the event horizon, but the energy to do that if you use a more massive "anchor" object (and have a linear trajectory from the observer's perspective that gets too close to the event horizon) would exceed the capacity of the link itself, perhaps having a similar effect as if you'd turned the drive off prematurely; it probably doesn't end well for you in that case.

A differently-dimensioned analogy

Suppose rather than a "constant linear velocity" we instead want the observer and you to maintain a constant distance. The equivalent of this "warp drive" is a rigid rod (or a rope under tension) with you at one end and the "observer" at the other. Until you let go of the rod/rope or it breaks, you'll stay a fixed distance from the observer - any movement by either of you will only be possible if the other moves too, unless the movement happens to be such that the distance stays the same.

The warp drive basically works similarly to this, but rather than fixing distance, it fixes linear velocity, with the acceleration of you and the observer / anchor point tied together in order to maintain this.

The main difficulty to define what it actually means for a distant observer to continue to observe you with constant linear velocity, if the distant observer is in an accelerating reference frame that distorts its perception of the distant stars so that most of the universe does not appear to the observer to have anything like constant linear velocity...