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On the other hand, there are hard limits in physics and the speed of light is only the start. In general, Exponential curves of progress or growth are not unbounded, and are really just the lower part of a sigmoid curve: https://en.wikipedia.org/wiki/Sigmoid_function

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In that vein, Newton and Kelvin both "proved" that heavier-than-air flying machines were physically impossible. That belief held for hundreds of years, and likely put many off even trying.

Einstein said that nuclear fission was impossible, before Fermi promptly proved him dead wrong.

I'm not saying that we're wrong about c, but we don't know what we don't know.

Which is why Stross ended with:

> Anyway, the point I'd like you to take away from this is that while it's really hard to say "sending an interstellar probe is absolutely impossible", the smart money says that it's extremely difficult to do it using any technology currently existing or in development. We'd need a whole raft of breathroughs, including radiation shielding techniques to kick the interstellar medium out of the way of the probe as well as some sort of beam propulsion system and then some way of getting data back home across interstellar distances ...

You wrote: "Einstein said that nuclear fission was impossible".

Do you have a reference for that? The closest account I know is described at http://www.doug-long.com/einstein.htm :

> Among those concerned were physicists Leo Szilard and Eugene Wigner. But Szilard and Wigner had no influence with those in power. So in July 1939 they explained the problem to someone who did: Albert Einstein. According to Szilard, Einstein said the possibility of a chain reaction "never occurred to me", altho Einstein was quick to understand the concept (Clark, pg. 669+; Spencer Weart & Gertrud Weiss Szilard, eds., "Leo Szilard: His Version of the Facts", pg. 83).

but that wasn't about nuclear fission per se but of a chain reaction.

Also, how was Fermi involved? Fission was "was discovered on December 17, 1938 by German Otto Hahn and his assistant Fritz Strassmann, and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch." - https://en.wikipedia.org/wiki/Nuclear_fission .

Fermi supervised the first demonstration of fission [1].

[1] https://en.wikipedia.org/wiki/Chicago_Pile-1

Yes. But as that link point out, it was years after (and a consequence of) the Einstein–Szilard letter. When did Einstein say nuclear fission was impossible, and how was it Fermi who "promptly proved him dead wrong", as madaxe_again wrote?

Edit: jsnathan at https://news.ycombinator.com/item?id=12326618 found a 1934 quote by Einstein expressing an opinion that nuclear power wasn't possible. This is after fission was already demonstrated.

Fission was discovered in 1938. Fermi observed fission but drew the wrong conclusions, he thought he saw creation of transuranic elements.

> Newton and Kelvin both "proved" that heavier-than-air flying machines were physically impossible.

Can we please stop bringing this up? Same with the "faster than sound flight is impossible" quote. Kelvin was well aware that birds were heavier than air and before supersonic flight it was well known how fast a cannon ball can fly.

These people were talking about engineering and economic realities of their time, not physical laws. In contrast, we currently know of no natural phenomena that would hint at the possibility of faster-than-light travel and in fact have good reasons to believe that it is impossible.

> we currently know of no natural phenomena that would hint at the possibility of faster-than-light travel and in fact have good reasons to believe that it is impossible

Is this true? I thought that if my ship is capable, I can go as fast as I like. I just won't appear to be going faster than light to somebody else.

You are being downvoted for your misunderstanding, which is too bad.

In a nutshell, no matter how much energy you pump into your system, you will never go faster than the speed of light, relative to any other viewpoint.

That is, you would never appear to go faster than the speed of light to anyone - including somebody travelling at the same speed as you in the opposite direction.

The universe outside your window won't be passing by faster than the speed of light either. But that's a bit moot, since Lorentz contraction will make the distances in your line of travel shorter. Also time will pass more slowly for you. So subjectively, it's true that you can get anywhere in an arbitrarily small amount of time if you have arbitrarily large amounts of energy, but the rest of the universe will still see you only approaching the speed of light.

No, b6 has it right, you can travel as fast as you want. The universe will get squished along your direction of travel, effectively increasing the speed of light (relative to the distance you're traveling) as your speed increases.

Relative being the key word. With a sufficiently fast ship you could fly across the universe in your lifetime - but you'd get home to find the sun a cinder and earth long gone.

This is not true. Even in your frame of reference, you cannot go faster than the speed of light.

You certainly cannot travel as fast as you want. The speed of light is the speed limit of the universe, to all observers.

You can't travel as fast as you want because it will take ever larger amounts of energy to accelerate each fraction of a percent closer to the speed of light. You're effectively bounded above by the speed of light.

Maybe I need to clarify? A certain amount of energy must be expended to reach a particular velocity. For a relativistically relevant velocity, kinetic energy is given as: mass * c^2 * (1/sqrt (1 - v^2/c^2) - 1); it's clear that v will never reach c.

For an example of how things can escalate: an object massing 1 kiloton traveling at 90% the speed of light has 4x the kinetic energy as the same mass at 65% (for context, a 1kton object at 0.65c has ~50x the energy as was used in 2013). From 90% -> 99% it's a ~5x increase. And from 99% -> 99.9% it's 3.5x. So just a 0.9% increase in speed has almost as much impact on energy as the 25% increase at a lower velocity.

Relativity means that there's no difference between "I'm going at speed V forwards" and "That object is going at speed V backwards" (assuming neither of you is accelerating). In fact, the only way to tell your own speed is to look at the speed of the world around you, and reverse the direction.

Since it's impossible for you to observe anything else going faster than the speed of light, you can never conclude that you are going faster than the speed of light. (Unless you wanted to define everything in the universe as moving faster than the speed of light in the same direction, but that just makes the math stupid for no reason.)

If you start accelerating, you will find that the amount of energy required to go faster (or, equivalently, to push other objects away from you faster) increases to infinity as you approach the speed of light.

I am afraid I am not in the flight for “aerial navigation”. I was greatly interested in your work with kites; but I have not the smallest molecule of faith in aerial navigation other than ballooning or of expectation of good results from any of the trials we hear of. So you will understand that I would not care to be a member of the aëronautical Society.

What precisely is Stross talking about other than the engineering and economic realities of our time? No physical law prohibits interstellar travel.

When did I say faster-than-light travel?

The person you replied to was talking about the physical limits, and you replied "in the same vein".

You said, "proved that heavier-than-air flying machines were physically impossible". That's rather an extreme statement that isn't supported by the passage you quoted or, it seems, what you're saying now.

How about the Alcubierre drive (https://en.wikipedia.org/wiki/Alcubierre_drive)? Yes, there are many unknowns and in filling in those gaps, we might discover warp is impossible after all. But we could also discover the opposite. The reality is that there are potential work arounds that fit our current understanding of physics -- potentially -- and our current understanding of physics is incomplete. In filling in the gaps we might discover more workarounds. Or we might discover that our existing workarounds were duds.

The truth is we won't know until we try. And trying requires the firm belief that we could succeed.

It doesn't do us any good to be pessimistic and announce that we know the physical laws and the physical laws make this impossible. Instead, we should be hopeful and say "Well, here are a dozen things that may or may not work. Let's take a moonshot and see if we can make one of them fly."

Anyone have an estimate of the amount of energy needed for the drive?

never mind the energy requirements - it relies on the existence of exotic matter with negative mass. don't hold your breath. it's a cool idea though.

Not sure, I haven't done all that much physics since undergrad (it was my major). But with regard to the negative matter issue mentioned by another commenter, there is this in the wikipedia article:

> However, at the close of his original paper[2] Alcubierre argued (following an argument developed by physicists analyzing traversable wormholes[3][4]) that the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive.

The Alcubierre drive can be summed up as "If I have one thing that's impossible (negative matter), I can do something else that's impossible (FTL)." (And, no, negative matter is not antimatter.) It's an intellectual exercise, nothing more.

It's all very well and good to say "don't be pessimistic" but there has to be an open mathematically and physically valid path that makes it possible or all you're doing is wishful thinking. And right now, there isn't even a hint of a practical path that we can follow, either theoretically or experimentally, that enables the possibility of FTL.

You don't need warp drive to go to the stars. You need suspended animation, extreme life extension, brain uploading, sentient AI, or some combination thereof.

At 10% the speed of light it is maybe 50 years to the Centauri system including time to accelerate and decelerate. We already know one physically plausible propulsion system that could do it: thermonuclear Orion. Fusion rockets could as well but the physics is not as totally proven out as e.g. H bombs.

50 years is fast. It only seems slow because we are very short lived and very fragile. Biology, not propulsion, is the primary barrier to interstellar flight. Even if it were possible, fixing biology would probably be a ton easier than a warp drive.

But see also: Fermi paradox.

There's also the EmDrive (https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster), which is controversial because it seems to violate all known physical laws. But there are prototypes that seem to work and have been replicated a handful of times by responsible and credible research teams. In each case, there are possible sources of error, but the fact that it appears to be replicated and we can't (yet) rule out the idea that it does work is very exciting. It should be aggressively pursued, because if it does work it's a very big deal.

Instead of looking at things like this with jaded skepticism, we should look at them with excitement and pursue them aggressively! The moment we get overconfident in our own knowledge is the moment we cease to discover new things.

I think the EmDrive is probably fallacious and doesn't actually do anything, and we absolutely need to keep testing it and try to prove that, because if I'm wrong it would be amazing.

Even if we take it as a given that it's nonsense and doesn't actually do anything, we'd still need to test it further just to understand why it does appear to be doing something.

Scientific skepticism, not jaded skepticism: http://theness.com/neurologicablog/index.php/despite-headlin...

>Einstein said that nuclear fission was impossible, before Fermi promptly proved him dead wrong.

Reference please.

And was this before or after the famous Einstein-Szilard letter to Roosevelt, which not only motivated the Manhattan Project but also specifically referenced Fermi's unfinished theoretical research of a possible chain reaction in uranium?

Google found me this [1].

"On 29 December 1934, Albert Einstein was quoted in the Pittsburgh Post-Gazette as saying, “There is not the slightest indication that [nuclear energy] will ever be obtainable. It would mean that the atom would have to be shattered at will.” This followed the discovery that year by Enrico Fermi that if you bombard uranium with neutrons, the uranium atoms split up into lighter elements, releasing energy."

This would be before the 1939 letter.

[1]: https://www.newscientist.com/article/dn13556-10-impossibilit...

Do not confuse "no indication that [nuclear energy] will ever be obtainable" (there is no evidence that it is an achievable engineering feat) with "it is impossible" (it is explicitly not permitted by physics as we understand it)

You're probably right about Einstein not suggesting there was a hard physical limit preventing nuclear energy. But I don't think that hard physical limits are a good argument anyway.

There are FTL proposals that do not violate known physics, such as distorting spacetime using some perhaps yet unknown mechanism.

Saying that there are hard limits in physics here, as you did, suggests that research in this direction is not worth our time.

I believe that is misleading.

That seems way bogus to me. Fermi and the physics world believed that he had found transuranic elements. It was Hahn, Meitner and Strassman who said it was fission in late 1938 (a couple of months after Fermi got the Nobel citing that discovery, but no worries, he had done plenty of Nobel-worthy things).

> That seems way bogus to me.

I'm sorry, I'm not sure exactly which part you're objecting to?

In 1934 everybody thought that Fermi had discovered transuranic elements, that it was fission wasn't clear until Hahn and Strassman's experiment in late 1938.

If Einstein said that, I'd say he was wrong at the time, because already in the 1920s it seemed probable that nuclear fusion powers the stars: https://en.wikipedia.org/wiki/Stellar_nucleosynthesis#Histor...

Of course that's a long way from practical engineering, but it is evidence that nuclear power may be obtainable.

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