h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

http://www.h2g2.com/A481736

This entry has been submitted in order to explain why we are not yet out there in space colonising the stars, what approaches might be feasible in the future, and some of the difficulties involved in turning these approaches into reality.

CR

I've read through most of this article. It is good.

1. You should have a footnote explaining the reference to the fly in the teleporter.

2. Technically speaking we weren't living in caves 19,000 years ago. Humankind never lived in caves. But we know what you mean.

3. You put "Faster than light travel" as a very small section along with fusion drives, ion drives and so on. It should be a bigger heading, because much of what follows (warp drives etc.) relates directly to it.

4. I don't see any reference to Special Relativity, which states that time slows down in the spaceship as it gets closer to light speed. This has the effect that the person in the spaceship doesn't notice the time passing and can travel vast distances in his or her lifetime, but thousands of years will have passed when he or she gets home. I think this is worth a mention.

Speaking as another lowly researcher I must say this is a good entry--explaining weighty concepts in a clear and concise manner. I did want to note a disagreement with Gnomon, however. Humans *did* take advantage of caves as habitation--why else would archeologists find debris of everyday life, fire pits, garbage, burials, and cave paintings (see Lascaux, et. al.) from Neanderthals, Homo erectus, and Homo sapiens? Your comparison makes a good point of reference to demonstrate how far we've come in a relative amount of time, considering Homo sapiens sapiens have only inhabited this earth for a minute fraction of its existence.

Hello,

It's a nicely done article and gets my approval. I do have some quibbles that I need to make though. Are there any physicists in the house who can fact check this?

1. General relativity established that spacetime is curved around gravitationally massive objects. In fact, it showed that gravity is this curvature (as well as that acceleration is indistinguishable from gravitation). You are correct there. However, this is a separate point from the question of whether spacetime in general (in absence of gravitation) is curved. This is a different question which is related to the development of the universe. If the universe is expanding forever, it will be flat or curved open. If the universe is going to collapse, it will be closed. In fact, it looks like it might be exactly flat, and this is explained as a side effect of Guth's Inflationary Theory, so it's not entirely random. Anyway, I think I'm right, but I'm curious if there are any physicists who know.

2. It's true that time slows down for a spaceship (compared to an outside observer's clock) as it approaches light speed (c). It's also true that the spaceship goes through an apparent Lorenz Contraction to the outside observer. The formula for the time slowdown as related to Earth time is approximately 1/sqrt(1-((v*v)/(c*c))) apparently. Of course, you'd need infinite mass and energy to hit light speed, making it difficult to get too close to light speed. Only the massless particles like the photon can travel at light speed. Even the fastest electron can only go 0.999999 the speed of light. I'm not sure why this is, but you can probably find a good mathematical answer somewhere.

3. For the curious, the best explanation of Special and General Relativity I have seen is in the beginning of the Elegant Universe, the best-selling book about string theory. For those curious about string theory, check out http://www.superstringtheory.com/

4. Now a quick and dirty explanation of why longer burn times (eg, the ionic example) are better. In space, once something's in motion, it stays in motion. Anyway, the velocity in a direction is v=v0+at, initial velocity + acceleration*burn time. So the longer time you can burn your rockets, the less acceleration you need to use for that time (to reach your speed). Why does this matter? Well, F=ma. M is the mass of your ship (if you have lots of chemical propellant, this mass is reduced over time). F is the force put out by your rockets (and pushed against your astronauts). So m usually stays constant, and F has to be below a certain value to keep the astronauts from turning into pulp. So this limits the largest values of a you can have. So if you can burn your rockets for longer, you can get a higher velocity. Any physicists want to check on this and tell me if I'm right or making a mistake here?

5. You can have wormholes potentially. But you're right, even if you could travel through space via wormholes, they are probably small and unstable or the gravitational tidal forces would tear you apart before you could make it through.

Yours,
Jake

I'm a physicist, but i'm not so well informed as you would like. Anyway, i can make some comments...
1- The Boomerang experiment seems to have made a good point that the space-time is flat. The accelerated expansion of the Universe is not above suspect, and some people says it can be a temporary stage.
2- the Time Dilation/Space Contraction effects are not apparent (unless the General Relativity proves to be wrong). No massive particle can reach c because some of the energy used to accelerate it adds to the mass. The fraction added to mass becomes larger as its velocity approaches c, requiring even greater ammounts of energy to obtain a tiny increase in speed. The mass would become infinite if v=c. A electron (or any other particle) can be so close to c as you want, as long you have the energy to accelerate it. The massless particles are always at lightspeed. There are also theoretical particles called tachyons who could not be travelling _slower_ than lightspeed... But actually there was never a hint they exist...
4- You are not incorrect. But the ionic drive should not be burning, I think the idea is to eject charged particles. There are other considerations you put aside, as if the acceleration is constant or no, the time it will take to attain some significant velocity etc.
I think the best advantage of the ionic drive will be the reduction in mass (no fuel to carry).
5- wormholes can exhibit strange effects that would turn it into a hazard to space flight rather than a cutaway.

Comments on the article:
You have talked none of robotic exploration of space, neither of cybernetic modifications to humans, allowing them to need less support, or live longer. Robots probably will be the real explorers (they have reached Mars before us), and when all is safe they can construct the other end of the teleport device...
If you will talk about negative energy, you should include tachyons aswell, about teleportation. Negative energy is a concept more strange and unheard of than tachyons...

The curvature of the universe and its openness or closedness is exactly the same as the curvature of spacetime due to gravitation. The two are the one and the same thing. Each mass in the universe curves the spacetime around it slightly. If the total mass in the universe is enough to curve the whole of spacetime enough so that it closes in on itself, then the universe is closed and finite. If it isn't, then the universe is open and infinite. You can't talk about the curvature of the universe in the absence of gravitation, as there wouldn't be any curvature in this case.

What I don't understand is why everyone is so caught up with traveling at the speed of light. We know it's impossible, so give up on it allready. I mean, shouldn't we just try to travel above the speed of light, thereby overcomming the trouble of traveling at c and also getting home in time for dinner much quicker

Gnomon,

You are correct there. I misspoke. It's true that the curvature of the entire universe is affected by gravitation, but I wanted to distinguish that this is different from local effects of gravitation, and emphasize that currently it seems like the curvature on the macroscopic scale is flat (meaning two parallel beams of light stay parallel). So in the absence of any local gravitational distortion, spacetime is flat.

I guess I just want to clarify the notion of ther being bumps and ripples in the fabric of spacetime that we can tunnel through. In normal travels we are constrained to the surface of this fabric and can't take weird shortcuts through, no matter how bumpy or weirdly shaped it might seem to a higher-dimensional observer. In a simple case, imagine an ant walking on a large sphere vs a large donut. As far as the ant is concerned, he's on a flat surface (ignore the 3-dimensionality of the ant or any gravitational cues). He would have to go to a higher dimension to observe the shape of the space and take a shorter route (say, digging across the sphere to get to the other side). Wormholes don't give us this higher-dimensional perspective, but they give us another route in our dimension.

Anyway, I'm rambling a bit incoherently, but I hope it's a bit more clear anyway.

Yours,
Jake

Well, now I know why I didn't major in Physics in school. It makes me head spin.

Anyway, I've spouted random things enough. It's a great article, and I think it deserves to get into the Edited Guide.

Yours,
Jake

I have planned to use this entry on my University Project on Stars @ A403930 so although comments are fine I would like to ask scouts to ignore this entry as I can get it all sorted quicker through the Uni (Moxon does it as far as know). Thank you, the page is great!

Tango

umm - as far as I know... that's a decision that the author of this entry gets to make, not someone who happens to be in charge of the uni project...

I know I'd be rather peeved if someone went and stole my entry for a uni project without asking me...

And unless I miss my mark, it's not an official project yet anyway... jumping the gun a little aren't you? :-
myre - "Go forth and Peer Review Casting the first stone optional."

http://www.h2g2.com/F49893?thread=90103 The author did indeed make that request - glad that's sorted out... :) This is Peer Review. This thread will terminate at "Project: Stars". Calling at Moxon, Hyperspace, and "Project: Stars". Please mind the closing doors. Recommending for MOVE to http://www.h2g2.com/A403930 myre - "All it takes for evil to triumph is the good man to Peer Review nothing."

Just to clarify, because I am a bit bewildered by this recent commentary around my entry. I put this post in Peer Review because I want it to be an Official Guide Entry as normal. How people use it or link to it is up to them.

The comments to date have been great, thanks, and I don't want them to stop just because of a debate around whether this is a potential Official Guide Entry or a University Entry. From my point of view it is a potential Official Guide Entry, and should be treated as such.

Does this clarify / confuse? All comments and suggestions to my user page please...

CR

Yeliab,

People are caught up with travelling at the speed of light because you can't go faster than any particular speed without first arriving at that speed. Since you can never reach the speed of light, you can never go faster than the speed of light, so travelling around the galaxy will take lifetimes instead of days.

Although hyperspace looks good on Star Trek, we haven't a bull's notion how to acheive it.

Whoops - looks like I haven't made something clear, and this is the result. Sorry.

Entries that are part of University Projects *cannot* be put through Peer Review, though entries that are already in the Edited Guide can of course be linked to from University Projects. This is because whole projects are edited in one go, separately from the normal editing process.

I've added some stuff to the University and Peer Review pages to point this out, and my apologies for not making this clear before. We're still learning how to organise all these schemes concurrently...

Anyway, onto this entry. It has just been picked by a Scout for editing, and it's also not listed on the project page for the Stars project at A403930, so unless anyone has any objections, I suggest we let this one go through Peer Review, and then Tango can link to the edited version from his project if he wants. It's a good entry and worth editing, and I'll make sure it all fits together when I come to edit the project, if that's OK.

Sorry Tango, sorry Crymonal - I hope this doesn't cause any grief.

Mark

Editorial Note: This thread has been moved out of the Peer Review forum because this entry has now been recommended for the Edited Guide.

If they haven't been along already, the Scout who recommended your entry will post here soon, to let you know what happens next. Meanwhile you can find out what will happen to your entry here: http://www.h2g2.com/SubEditors-Process

Congratulations!

Hello, I'll be your in-flight sub-editor for today. If there are any last minute changes you wish to make, any time now would seem to be the best point to let me know!!

- Slash & Burn Editing!

Mark,

Okay, I just wait then!

Tango