To Saturn at (nearly) the speed of light

[ghwellsjr]

Yes, good point; the word "see" is ambiguous. Unfortunately there doesn't seem to be better terminology for this in ordinary English; you have to use technical jargon.

I like the word "establish" because it has just the right double-meaning to include the need specify a frame and the need to determine the parameter of interest. Of course, I have no expectation that it will ever supplant the word "see". But if I'm not sure what the questioner means, I assume he means "actually see" and make sure my answer makes that clear or else I will answer both ways as I did in this case. I believe in full disclosure.

 

[PeterDonis]

I like the word "establish" because it has just the right double-meaning to include the need specify a frame and the need to determine the parameter of interest.

+1 from me on this; it's a word that doesn't have other undesirable connotations already, and as you say it gets across the fact that there's more than just direct perception involved.

Of course, I have no expectation that it will ever supplant the word "see".

Unfortunately, I don't think so either. But at least we can try starting a trend here on PF.

if I'm not sure what the questioner means, I assume he means "actually see" and make sure my answer makes that clear or else I will answer both ways as I did in this case. I believe in full disclosure.

This seems to me to be a good policy.

 

[Baggins101]

The Earth has not been human habitable for 4 billion years. I'll grant you maybe a significant fraction of 1 billion years but even then, where are these "many earth-sized planets in the habitable zone"? Even one?

How many planets smaller than Jupiter had been found 5 years ago? How many planets of any description had been found 20 years ago? To dismiss any possibility of future interstellar migration on the basis that we have only found one possible candidate beyond our own to date is disingenuous as it suggests they either don't exist or that we will never be able to detect them.

The point about the window of habitability has already been made: tens of thousands of years are nothing on a geologocal time scale.

Are you American? I ask because the only motivation for branching out into the Universe you consider is financial gain. Our own solar system will be plundered for financial gain but that will never be a major motivation for permanent migration and separation from Earth. Motivations are likely to include wanting to create a better life for your descendents than living on an over-crowded resource poor Earth, and the native human desire to explore. How many people signed up for Mars 1?

Humanity will migrate to the stars, of that I can see little doubt. I suggest, however that it will not happen until the issue of energy has been resolved, possibly through nuclear fusion.

 

[Baggins101]

Sure. But that correct point has nothing to do with the incorrect claim that time dilation and length contraction make FTL possible. Just don't "oversell" the effects as FTL and you will be fine.

I do realize time dilation and length contraction are not the same as ftl travel, however I'm sure that you can see that from my viewpoint, as a traveller intent on a one way trip to settle on a distant planet, the effect is the same as ftl travel: i can get to a planet 100 light years away from the point of view of both Earth and my destination planet, in considerably less than 100 years. That Earth has aged 100 years in that time or that my new planet is 100 years older is of no consequence to me.

 

[PeterDonis]

Our own solar system will be plundered for financial gain but that will never be a major motivation for permanent migration and separation from Earth.

I'm not sure that's necessarily true on a long enough time scale. Suppose, for example, a longer-term future in which we have robotic ships that can travel at near light speed to nearby star systems, mine them, and send the resources back at near light speed. Say that means a ten-year turnaround time for getting resources from the Alpha Centauri system to our solar system. That's not too different from the turnaround time for, say, companies in Europe getting resources from East Asia during the Renaissance. On a long enough time scale, such activities are profitable; so far enough in the future, we can expect our descendants to engage in them.

That's not to say profit will be the only motivation; the other motivations you mention will certainly be present as well. But consider, for example, that there are already private companies talking about mining resources from Mars and the asteroid belt over the next few decades, whereas nobody, as far as I know, thinks we will be able to have significant human habitation in either of those places in that time frame. I think it's very hard to predict which set of motivations will get there first in any particular case.

 

[Dale]

I do realize time dilation and length contraction are not the same as ftl travel, however I'm sure that you can see that from my viewpoint, as a traveller intent on a one way trip to settle on a distant planet, the effect is the same as ftl travel: i can get to a planet 100 light years away from the point of view of both Earth and my destination planet, in considerably less than 100 years. That Earth has aged 100 years in that time or that my new planet is 100 years older is of no consequence to me.

None of that makes it the same effect as FTL travel. Please don't push this issue further.

As a traveller intent on a one way trip it doesn't matter if it is FTL or not, as long as you can live long enough to get there. The fact that you can live long enough in no way makes it FTL. I.e. your purposes or intentions do not change the fact that you never traveled FTL.

 

[PeterDonis]

from my viewpoint, as a traveller intent on a one way trip to settle on a distant planet, the effect is the same as ftl travel: i can get to a planet 100 light years away from the point of view of both Earth and my destination planet, in considerably less than 100 years.

Considerably less than 100 years according to your clock; or perhaps a better way to put it would be, while you age considerably less than 100 years. That makes it clear what "effect" you consider important, without inviting potentially confusing or misleading inferences about FTL travel that, as DaleSpam has pointed out, are not valid.

 

[russ_watters]

No one responded directly this:

...as a traveller intent on a one way trip to settle on a distant planet, the effect is the same as ftl travel: i can get to a planet 100 light years away from the point of view of both Earth and my destination planet, in considerably less than 100 years.

You said that last part wrong, though it may be accidental: from the point of view of observers on both Earth and the destination planet, you did not get there in less than 100 years, you only did that from your point of view.

It would be fair to say that to the traveler there is a similarity with FTL travel in that you can travel to distant places that from your point of view before you left appeared unreachable. But the fact that you can watch light travel and never pass it clearly indicates that you are not traveling FTL. And the duration of the trip from the point of view of the people who watched it (not participated) is clear indicator that it wasn't either.

The fact that you get to in effect skip the distance but not the time means that at best it is half similar to FTL travel.

 

[.Scott]

Are you sure about that? I thought parts of the known universe were traveling away from us at faster than the speed of light and will be forever out of reach.

That's true. There may be parts of the universe that are either behind an event horizon or would move behind that horizon before you reached them. But my main point was only to demonstrate the scale of the known universe - not to actually plan a trip.

 

[mfb]

That's true. There may be parts of the universe that are either behind an event horizon or would move behind that horizon before you reached them. But my main point was only to demonstrate the scale of the known universe - not to actually plan a trip.

Actually, we know those objects exist (assuming the expansion of the universe continues to accelerate according to our models, otherwise there could be no horizon) - we can see them today as very old, and now very far away galaxies. So far away that light from us today will never reach them.

 

[PeterDonis]

There may be parts of the universe that are either behind an event horizon or would move behind that horizon before you reached them.

Moving behind an event horizon doesn't make them unreachable to us; it just makes us unreachable to them. The accelerating expansion of the universe (assuming that our current model of that is correct) makes things unreachable both ways, so to speak.

 

[.Scott]

Moving behind an event horizon doesn't make them unreachable to us; it just makes us unreachable to them. The accelerating expansion of the universe (assuming that our current model of that is correct) makes things unreachable both ways, so to speak.

Right. It works both ways. Our event horizon (as seen from them) keeps us from getting to them and their event horizon (as seen by us) keeps them from getting to us.

 

[PeterDonis]

Right. It works both ways. Our event horizon (as seen from them) keeps us from getting to them and their event horizon (as seen by us) keeps them from getting to us.

This amounts to saying that we and they are each inside a black hole with respect to the other. That's not correct. The cosmological horizon involved here is not the same as a black hole's event horizon. For one thing, its location in space changes with time. For another, its location in spacetime (i.e., which surface in spacetime it is) is different for different observers. Neither of those things are true for a black hole's event horizon.

 

[.Scott]

This amounts to saying that we and they are each inside a black hole with respect to the other. That's not correct. The cosmological horizon involved here is not the same as a black hole's event horizon. For one thing, its location in space changes with time. For another, its location in spacetime (i.e., which surface in spacetime it is) is different for different observers. Neither of those things are true for a black hole's event horizon.

There are several ways to create an event horizon. One is with a black hole. Another is with simple continuous acceleration. Another is with expansion of space.
They all have the same characteristics - extreme time dilation, Hawking radiation.

I wasn't saying anything about black holes.

 

[Dale]

I think that any null surface can be considered an event horizon. Once you cross it you cannot send signals back to the other side.