THANK YOU!

I released an article on Time Dilation.
http://www.aliceinphysics.com/#web/mathematics/en/tin_soldiers1.html

Han Erim

[[***“In order for human-alien reproduction to be possible, both species would have to have”***

None of these are impossible, nor is any combination of them. Do they sound implausible? Yes, but only because you strongly weighted your descriptions to make them sound that way.

Do you want a quantitative estimate? For all conditions to be met is implausible, pal. Low-probability.

[[Honestly, I’m not inclined to believe it myself,]]

But you couldn’t resist a chance to yank my chain anyway, right?

[[ but that doesn’t make it bad physics or even bad biology. I could even be wrong. After all, I have no idea what I’m talking about. ]]

I have to agree with you there.

[[A key point here (and the one I made earlier) is that there are no known examples of alien life. This makes even the probabilities impossible to determine.]]

No, it does not. We may not be able to determine them exactly, but we can make pretty darn good order of magnitude estimates.

[[Consequently, Carl Sagan and Larry Niven don’t have the slightest clue what they’re talking about, and neither do you.]]

Don’t get me confused with you. You have no idea what you’re talking about. I do have an idea what I’m talking about. See the difference?

“In order for human-alien reproduction to be possible, both species would have to have”

None of these are impossible, nor is any combination of them. Do they sound implausible? Yes, but only because you strongly weighted your descriptions to make them sound that way.

Honestly, I’m not inclined to believe it myself, but that doesn’t make it bad physics or even bad biology. I could even be wrong. After all, I have no idea what I’m talking about.

A key point here (and the one I made earlier) is that there are no known examples of alien life. This makes even the probabilities impossible to determine.

Consequently, Carl Sagan and Larry Niven don’t have the slightest clue what they’re talking about, and neither do you.

I think that worrying about sound in space is as pointless as wondering where that orchestra playing the rousing battle theme is at.

[[Point #5 is a matter for exobiology and, like that fledgling field, remains entirely hypothetical. Easy human-alien reproduction SEEMS quite implausible, but is by no means impossible, especially PHYSICALLY.]]

In order for human-alien reproduction to be possible, both species would have to have

1) The same set of nucleic acids. Note that over 1,000 varieties of RNA have been found.

2) The same set of amino acids. We use 20 out of several times that number.

3) The same optical activity for three types of chemicals — nucleic acids, amino acids, sugars. We have left-handed aminos and right-handed sugars. I forget which chirality nucleic acids have.

4) The same genetic code. Not all organisms have exactly the same genetic code even on Earth.

5) The same mitochondrial genetic code. See above. If the mitochondria don’t work, the embryo will be a cancerous mass, not a viable descendant.

6) The same or a close number of chromosomes. Human-ape interbreeding is impossibly, and one reason for that is that great apes have 48 chromosomes and humans have 46. Donkeys have 62 and horses have 64, but although they can reproduce, it only works for one generation. Mules and hinneys are sterile.

7) The same sex-determination mechanism (XX-XY). Many other types are known; e.g. birds and lizards have ZZ-ZW, where the heterozygote is male. Spiders have systems involving up to five chromosomes (e.g. XXYYY).
The same mode of pregnancy — placental pregnancy as opposed to marsupial pregnancy, cleidoic eggs or amphibian eggs.

9) Roughly the same genital pH. If the alien babe has a vaginal pH of 5, Captain Kirk’s sperm are going to get denatured before they get anywhere useful.

10) That neither partner will become allergic to the alien proteins of the other. That would abort the pregnancy pretty fast.

I’ll leave out similar methods of copulation, since we can assume artificial insemination. That might take care of objection #9, as well.

Human beings can’t reproduce with chimpanzees, and we share 95% of our genome with chimpanzees. To expect that we could reproduce with someone who came from a planet where evolution went on for billions of years out of touch with the Earth is just not very likely.

Carl Sagan: “Such matings are as reasonable as the mating of a man with a petunia.”

Larry Niven: “LL could more likely mate with an ear of corn than with Kal-El.”

As for Bespin, Hoth and all that – arn’t they just stories?

The standard retcon is that the Falcon is equipped with an incredibly-slow emergency hyperdrive, that can get them to the nearby Bespin in a matter of hours, but would take a long while to get anywhere else.

How I think Lucas envisioned it is that the planets of Hoth and Bespin are part of a close-knit cluster (possibly a single system. Lucas’ characters are often quite loose with their astronomical terminology) of stars and that it is therefore possibly to travel between them in a matter of hours using the Falcon’s (very fast) sublight drive. It’s just phenomenal luck that Lando somehow ended up in the same neighborhood.

That depends on how you define it. By the definition used by the sci-fi community at large, FTL need not violate relativity.

Let’s be clear here. By your definition, there is NO FTL (defined as normal matter exceeding the speed of light in local frames of reference) in popular film or televised sci-fi. It just doesn’t happen.

The same definition that defines warp-drive travel, hyperspace travel, or tachyon-conversion travel as FTL necessarily defines wormhole travel as FTL. This is the definition used by the SF community at large.

Ah, so it got fat on potato chips and freeze-dried ice cream, then. I happily stand corrected. Thanks for taking the time.

Regarding sound in space, I actually don’t have a problem with it to the extent that we are being told a story, not being given witness to events as they actually happen. If the omniscient POV switches to outside the space ship, and it blows up with a loud kaboom, I’m actually fine with that. It’s no different than having someone tell the story verbally with a saliva-spattering “Kaboom!” when he got to the explosion. It’s merely a story-telling embellishment.

But if the characters in the film are watching a video feed of the same explosion, and *they* hear the loud kaboom, that’s just wrong.

Even so, I think Firefly’s depiction of space events without sound is very exciting.

Also, mating with aliens is easy. Don’t ask me how I know.

Sigh – OK, I’ll say it again. I agree that the Alcubierre drive does not violate relativity in the slightest – I agree that it needs exotic matter which may not be physically feasible. At all points in the space time, all local velocities are less than light (it seems that some people have a difficult problem with this, but it means all velocites are time-like and sit inside light cones).

BUT! – the Alcubierre drive can get between earth and Alpha Cen in an arbitrary short time – less than 4 year, less than 1 year, less than 1 minutes

There is no time dilation, so the time for the journey as measured on earth, Alpha Cen and in the drive it self are all exactly the same.

I would suggest you look at Hartle’s (excellent) relativity textbook on this.

The main point is that a star N light-years away can be reached by the crew of the spaceship in M years ship time where M < N, but it takes light N years to reach the star.

“The key point is that Alpha Centauri is four light-years away without your FTL device (the wormhole, in this case) and that (with your FTL device) it didn’t take you four years to get there. That’s all there is to it. Wormholes permit FTL travel.”

If that’s an example of FTL then the classic Twin paradox (travelling close to the speed of light) is also FTL. Alpha Centauri is four light-years away and it only took you 1 year to get there ship time. Like I said, it all depends on what your definitions are.

Velocity = distance / time

Who measured the distance? (which reference frame)

Whose clock measured the time? (which reference frame)

Certain people who claim certain questions are stupid never specify the answers to these questions. Instead, they assume them and they assume their answers to these questions are the only correct ones.

Wormholes are not FTL because you never go Faster Than Light (FTL, get it?) If you want to demonstrate FTL for wormholes you have to synchronize two clocks that are a large distance apart. This is not possible (read Max Jammer if you don’t believe me). That is why we can only talk in a meaningful way about local velocities. Locally, you never go faster than light through a wormhole.

But I have the feeling I am repeating myself…

In Star Wars, the solution is called the hyperdrive. In The Empire Strikes Back, it is an important plot point that the Millenium Falcon’s hyperdrive is broken–that is why they can’t escape from Hoth cleanly and why they go to Bespin (where Vader is waiting, etc.). But how the hell do they get there with a busted hyperdrive? We see the Falcon out in the middle of space in the asteroid field, and then suddenly they’re landing on a planet. Even if we assume they can travel at 99.9999% of the speed of light without the hyperdrive, it should take them *years* to get to any other star system.

This is just so dumb, and so unnecessary, as it could have been handled with just an extra line (“We can rig the hyperdrive to make one jump! Let’s make sure we pick a safe system where we can get help! I know, my buddy Lando! He’ll have some cold Colt .45 for us!”). I realize this is far from the only nitpick one can make on the Star Wars series, but it’s the main one I know where it actually affects the whole plot and makes nonsense of it.

Wormholes are a type of FTL travel, just like warp drives, hyperdrives, and tachyon-conversion engines. (none of which violate relativity.)

The key point is that Alpha Centauri is four light-years away without your FTL device (the wormhole, in this case) and that (with your FTL device) it didn’t take you four years to get there. That’s all there is to it. Wormholes permit FTL travel.

[[You’re completely ignoring one of the basic axioms of general relativity, which is that all frames of reference are equally valid. If there exists any frame of reference which could observe FTL travel to have occurred, then you’ve lost either relativity or causality.]]

I looked at Aldeberan. Then I turned in place 360 degrees. Using my head as the frame of reference, Aldebaran traveled many times faster than light. Does that mean I’ve lost either relativity or causality?

Faster than c with respect to what? There is no such thing as local velocity.

That’s exactly backwards; 4-vectors are fundamentally local objects.

You’re completely ignoring one of the basic axioms of general relativity, which is that all frames of reference are equally valid. If there exists any frame of reference which could observe FTL travel to have occurred, then you’ve lost either relativity or causality.

You’ve missed my point entirely. In no frame of reference is pretending the shortest path between two points doesn’t exist a valid method of calculating travel speeds. An observer who wasn’t aware of the existence of a nearby wormhole might conclude that a traveller had achieved FTL speeds, but that observer would simply be wrong, as wrong as someone who observed that I made it from Edinburgh to Glasgow in 20 minutes and concluded that I was travelling at 12,000 mph. No, I just took a shorter path.