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Comment by flurdy | Hacker News Reader

I am always baffled by the conundrum of the futility of launching a spacecraft to go far away by our own ability to invent faster engines.

Say it takes 300 years to get to Alpha Centauri. If you launch it you have 150 years to invent a spacecraft that goes twice the speed of the first one. Very likely. If you launch that one it will overtake the other one. Now you have 75 years to find an even faster engine. Launch it and you got 37 years. etc.

Quite likely you can keep doing this until you can reach it in e.g. a year. So why not hold all launches until then....

Does this have a name? Or original source? Did I get this from Futurama?

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The irony is that without the practical experience of those intermediate steps, you might never reach the point where the diminishing returns become negligible.

You're basically describing the reverse of Zeno's paradox - the "why bother" paradox.

This is a fairly common trope in SciFi settings which have Generation Ships.

Some time after the ship gets launched (centuries, even), FTL gets developed. Then they either arrive and find a civilization at their destination, or they are considered to be off-limits and not to be messed with, depending on the setting.

That assumes such improvements are even possible.

At least for non-magical propulsion, we don't really need "faster" engines. Take whatever engine you want, let's say it can accelerate at a constant 1g. In less than 4 years(from an observer standpoint), you are at 97% of lightspeed. From your point of view, that would take less than two years.

If you can have an engine that can accelerate at a constant 1g for years (fueling it is left as an exercise for the reader), then you are all set. You can go anywhere in the galaxy inside a human lifespan – of course, assuming you don't care for the people left behind.

If you want to accelerate at more than 1g, there may be physiological issues.

I personally believe that it is easier to put human/whatever consciousness in a computer than it is to achieve FTL.

Not that the former is easy, but the latter is basically impossible. After the former is achieved, we can basically coast on sleep for eternities.

What would that point of that be?

Speaking as someone whose body is gradually breaking down, if I could wake up tomorrow in a computer headed for the stars, sign me up!

As in most cases, problems left for the reader are usually the hard ones!

The rocket equation tells us that if one somehow managed to build a spacecraft containing an infinite power source that weighed a single kilogram, and incorporated the most efficient engine designs known to us (electrostatic ion thrusters), it would still need to be fueled with a reaction mass of xenon equivalent to approximately two billion times the estimated mass of the observable universe to achieve even 10% of the speed of light.

Please add a little more info how this is calculated because my expectation is quite different.

The rocket equation is dv = isp * g0 * ln(m0/mf) [1]. For dv we assume 10% the speed of light, isp of a ion thruster is say 19300 s [2], for g0 I will use 9.80665m/s^2 and mf is 1kg. For m0 this gives us m0 = mf * exp(0.1 c / (isp*g0)) = 6.2e69 kg. The (baryonic mass) of the observable universe is something like 1.5e53 kg [3]. Or in other words about a factor 4e15 (much more than the 2e9 the grand parent poster claimed) less then the necessary fuel.

1: https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation 2: https://en.wikipedia.org/wiki/Ion_thruster#Comparisons 3: https://en.wikipedia.org/wiki/Observable_universe

Wasn't project Orion about using successive nuclear explosions to get up to 25% the speed of light?

The crucial point is that to get to (low) relativistic speeds, you also need a relativistic exhaust velocity or the reaction mass just becomes untenable. Current ion thrusters are not up to the job.

For example, if you had a vehicle weighing 10t empty (but including energy), and a thruster capable of accelerating ions to 1% the speed of light, you'll only need 17t of reaction mass to achieve a speed delta of 1% the speed of light.

This ratio holds up as you scale exhaust velocity and delta V, so the exact same reaction mass would be needed for a 10% light speed delta V using a thruster shooting ions at 10% c. As a rule of thumb, you always need a reaction mass about 170% of the vehicle's dry mass when your thruster exactly matches your target velocity.

Now, to illustrate how critical exhaust velocity is, imagine you make thrusters with exhaust velocities 10x that of the target delta V. In those cases, you'll only need a reaction mass of roughly 10%. However, if your exhaust velocity is 0.1x the target delta V, you'll need a 2200000% reaction mass!

One fairly decent argument against waiting until we have the technology is that it cuts against how we develop the necessary technology in the first place. We can't improve our propulsion/spacefaring tech without designing, building and launching spacecraft. There's absolutely no reason to assume that we will have invented a faster engine if we've just been sitting around for 100 years not building anything. Hell, we're having trouble rebuilding a moon program fifty years after Apollo 11, inspite of all the advances that have happened in the meantime.

Oh, I am not saying we shouldn't launch, it is just an interesting conundrum.

I think if you do launch then you have a clear target to improve on.

And the next one does not have to go to the same destination.

It is covered in this Wikipedia article. It is called 'the incessant obsolescence postulate'. https://en.wikipedia.org/wiki/Interstellar_travel

"It has been argued that an interstellar mission that cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity and not yet having reached the limit, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion (the incessant obsolescence postulate)"

>assuming that a civilization is still on an increasing curve of propulsion system velocity

I think this is the error

You can't get faster engine without having slower one first.

How do they even know what to improve upon without building it and experiment with it?

Also simulation is not a replacement for real world testing. They should launch stuff into space to figure out if it works and their simulation is making the correct assumptions.

Also I'd imagine these launches are sending back valuable data that they can use to make adjustment and improvement for future engines.

Based on what we seem to be learning about how the body responds to the hostile environment of space, it seems reasonable to think we should wait for the faster engines. Send drones if you must.

https://en.wikipedia.org/wiki/Interstellar_travel#Wait_calcu...

"Why should I buy a computer today, when a computer in 10 years can do a ton more for less money?"

Say you built a computer in late 2009 with AMD Radeon HD 5870, Intel Core i7 975, Intel's X25-M SSD and 16 GB RAM.

That computer would still feel pretty snappy today for desktop tasks, running dual 2560x1440 screens. Although new AAA games would probably have issues, either low frame rates or not running at all. Most indie games would work just fine.

I think the change will be even less in next 10 years. 2019 hardware will perform just fine in 2029.

Unless something dramatically different pops up, we're in the era of diminishing returns.

That's more or less the result of having already waited. There was a time, not very long ago in the grand scheme of things, when the computer you wanted would cost $5000 and be essentially obsoleted by the $5000 computer coming in 60-90 days. (Perspective: PC review magazines of the day would usually include the time for a Gaussian blur on a 1MP image as a meaningful test.) When you were talking about cutting minutes off of a large spreadsheet recalc, the immediate gain from upgrading today had to be measured carefully against the even greater gains possible next quarter.

If you need to run a computation that is going to take 100 years with today’s computers, you probably should wait a bit to start it with faster computers.

Unless you can buy both, in which case you should start now and continue with the newer computer.

But then you've spent more money. You could have just bought more hardware in the future.

For a fixed budget, assuming that Moore's Law still holds (dubious, these days), you should wait until the computation will take 26 months on hardware purchased within that budget [0].

[0] https://arxiv.org/abs/astro-ph/9912202

More like "why start walking now when I can wait for a bus." In the case of a trip to the corner store, like, Saturn, get walking. If you're crossing the city, like, Alpha Centauri, wait for the faster transport

> Quite likely you can keep doing this until you can reach it in e.g. a year

Given that Alpha Centauri is 4 light-years away, this is a fairly bold speculation.

In fact is it possible most of the energy it got is not from human but from inter planet force? Hence we need someone to keep on calculating when is the best time to send another one. Not that we have sent any ...

It has been 47 years since a person walked on the moon, so it might just be a good idea to do something when you can and not wait for it to be possibly easier to do in the future.

This is why I didn't invest in solar yet. Even with subsidies, it looked to me like they would take far more than double the time to pay for themselves than it would for the price to cut itself in half based on current trends. Might as well wait a few years.

I think it might be time to look again? Cost of panels is extraordinarily low and cost of installation (e.g humans) is not on a similar curve. I looked at price/watt and went ahead and installed. The payoff period is ~11 years, 1 1/2 years in. Warranted to 20. You could take panel price to zero and it wouldn't have changed the math much.

I believe it's called the "wait calculation".

Unfortunately we have not sent any for 40 years ... the speed is not but the data collection power may be improved exponentially until recently.

We cant reach it in a year because it is more than 4 light years away.

Innovator’s dilemma

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