> Interstellar travel will consist of information travelling at the speed of light. Pretty much nothing else is practical.

What's your reasoning for this? You would only have to accelerate at 1g for 3 years to hit 99.5% of the speed of light. You can use an electromagnetic shield for dust. It seems conceivable that this will be possible eventually, even if warp drives are not possible.

Forgiving the problem of time dilation and the consequences it has on the expectations of conscious things...

You can't just accelerate 1g for 3 years. What are you burning? A chemical rocket can easily achieve 1g. For about 5 minutes. A nuclear rocket is great, you can get 1g easy enough, but you're shoving hot hydrogen out the back end to get that thrust.

The Rocket Equation teaches us that to get arbitrarily close to c, you need an exponential amount of your ship's mass to be propellant.

I recommend this website to everyone in this thread that hasn't read it yet

http://www.projectrho.com/public_html/rocket/

Edit: moreover, the problem of an EM deflector, or any kind of deflector, is that no matter what, no matter what, you must deal with the kinetic energy of the thing that just hit your deflector. It will (a) slow your ship down, and (b) cost you more energy, e.g. from your generator, to deflect it than it imparted on your vehicle. The energy numbers here are staggering. We have trouble theorizing how a relatively uncontrolled terawatt rocket might work, but imagine a terawatt reactor on your ship somehow powering a terawatt EMfield in front on your ship.

And then you have to worry about dust that's not charged and thus goes right through EM fields.

Indeed, and the benefit of traveling at 0.995 c is that a container of information doesn't suffer from degradation by the inverse square law as it travels over a large distance. It might actually cost less energy to transport information to far flung destinations that way, interstellar snail mail if you will. Some amount of information has to accompany the robot anyway, at the very least, its initial operating instructions.

Of course whoever can get there at 0.996 c can take advantage of a market arbitrage. ;-)

> a container of information doesn't suffer from degradation by the inverse square law

Neither does a well-focused laser?

EDIT: apparently it does, unless you manage to focus your laser very far away. Not sure how practical that is.

1kg matter at 0.1c has a kinetic energy of 4.5x10^14 J

It is certainly very expensive to spend this amount of energy. But is it also extremely challenging to even extract this out of an engine that has to accelerate itself.

1kg matter at 0.995c has a kinetic energy of 8x10^17 J

This is the amount of enery you get if you convert 9kg in pure enery. That seems impossible to reach, even with an anti matter drive.

A 9:1 ratio of fuel to mass doesn't sound particularly bad. Are there known theoretical limits to extracting the full E=mc^2 worth of energy from an object?

No, you can do this with an antimatter rocket.

Good luck capturing and containing antimatter and feeding/containing the explosion though. It's going to be practically impossible to get a 90% fuel ratio from an antimatter reactor, since literally 45% of your ship will be antimatter, by mass.

You can also do this by feeding a black hole with ordinary matter and using Hawking radiation for propulsion. That is probably more practical than carrying around large quantities of antimatter.

https://en.wikipedia.org/wiki/Black_hole_starship

If you watch PBS Spacetime on the 'Kugelblitz' idea, it seems that one way to go about it (over the course of a couple hundred years) would be to convert Mercury into a solar energy collecting swarm by use of Von Neumann probes and essentially harvest much of what the sun has to give. That kind of energy apparently would suffice to create tiny black holes (simply by redirecting enough energy to a focused point), which is I think the theoretically most efficient battery (or star system destroying bomb, depending on how you go about it...).

I... can't.... Half of me recoils in horror at the impracticality, but the other half of me has a good enough armchair sense for the practicality of a high fuel ratio antimatter rocket, so I can't say you're wrong :P

You need to accelerate the fuel as well. And for a reaction engine, you need to carry reaction mass that you accelerate. And at some point you want to decelerate again.

Would be interesting to see whether this is at least theoretically feasible.

How would the economics change if we could only communicate at light speed, but had a functional Alcubierre drive (or other lightspeed technology) and could travel at FTL speeds?

Presumably you would just load up the your Alcubierre station wagon with a bunch of backup tapes and send it down the interstellar highway. You could restore your backups before the original data is written!

> you would just load up the your Alcubierre station wagon with a bunch of backup tapes

Tapes have mass. Unless we learn to suppress inertia, it would be easier to encase the message photons in an Alcubièrre field.

Not suppressing inertia is indeed a problem, unless the message the kind you intend to be understood primarily by virtue of its infinitely inverse ratio of complexity to fidelity.

The secret to time travel is to drive backwards.

The secret to time travel is to forget that you're moving forwards with time, so you'll float instead. Many people have paid other people to surprise them into forgetting that they're moving forwards in time, so they'll float instead.

Do you consider FTL plausible? If so, why?

[warning: serious layman imagination ahead]

FTL === back in time, and maybe we don’t observe it since it doesn’t happen often. Like in iPhones that don’t spontaneously in cosmos every 2^30 years. Another cue is QM entanglement that somehow resolves the inconsistencies by out-of-timeline ‘decisions’ of what ‘really’ happened once you checked the outcome. If nature works in mysterious ways, why couldn’t it resolve the fact of time travel by the same logic: yeah, an object appeared from nowhere, but it disappeared from somewhere, no brainer. Our puzzle is to create serious enough paradox for the nature for it to resolve it in a desired way. First experiments may resemble our first ‘friendship’ with the energy of atom, but then who knows.

Nature isn't commonly held to work in mysterious ways; it's generally held that nature is knowable.

Being able to travel faster than light, or send messages faster than light, would not involve objects popping in and out of existence. That's not even a particularly well-formed concept. You and everything else you know are already moving through time and space, and the rules for how that happens also suggest that if you bend the geometry of the universe sufficiently, you can move to a place in less time than it would take for light to cross it. If you then return, you can end up arriving at your origin before you left it. Relativity does not allow the universe to spontaneously warp itself into the kind of pretzel which would allow for things to travel instantaneously back in time, and if it did, the effects on anything else nearby would likely be catastrophic beyond ready imagination.

Much of your speculation is rooted in misunderstanding. It's more difficult to explain things because your conceptions don't map well to the actual physics. There's a series on YouTube by PBS called Space Time, which I would strongly recommend as an accessible guide to relativity and physics in general.

I know it, that’s in disclaimer!

Unfortunately so to my imagination. But I still hope that there is many orders of magnitude more quirks that could obsolete current well-proven models and open the world of possibilities. Honestly, modern physics restrictions are boring to death.

There's an austere beauty to the laws, but physics restrictions do take all the fun out of speculation, it's true. And it does place the stars out of reach of the individual, at least until we cure aging. Empirical truth has a heavy price.

Makes sense to me.

And even better, if you can exploit something at the destination to build your stuff. As in Carl Sagan's Contact and Peter Watts' Echopraxia.