@Rob,

Seen from your perspective, two objects can of course move with a relative speed of near or at 2*c, for example two light rays approaching each other. It is only when you go into the reference frame ("travel with") of one of the objects, that you suddenly realize that the other object, from this vantage point, does not approach you with the same, naively expected, near 2*c, but with only near 1*c.

This is because when you move with one of the objects, the distance travelled by the other object gets length-contracted, and you measure with a different time as well. The details are confusing even when you are doing the maths. Without the maths, it is almost hopeless not to run into contradictions. This is why I recommend Mermin, he manages to get as close to a consistent and correct intuitive picture of these things as I know.

p.s.
Of course you know how to do the math, but I think this is a less stressful way to get an idea how it works.

Perhaps in the far future they'll transmit all the information that makes us us, in a light beam/wave/thingy across the universe, at the speed of light, to a waiting automaton that has done all the travelling for us. :-)

(October 20, 2015 at 7:51 am)Little lunch Wrote: Perhaps in the far future they'll transmit all the information that makes us us, in a light beam/wave/thingy across the universe, at the speed of light, to a waiting automaton that has done all the travelling for us. :-)

Destination star systems => A few thousand to millions of light years away
Travel time as light beam => A few thousand to millions of years

Result => Pointless wastage of time and resources

@Alex: Thanks That sounds similar to something else I heard, that light doesn't actually take X years to travel X light years, because of some weird effect like that. Assuming I heard that right.

(October 20, 2015 at 8:35 am)Aoi Magi Wrote:

(October 20, 2015 at 7:51 am)Little lunch Wrote: Perhaps in the far future they'll transmit all the information that makes us us, in a light beam/wave/thingy across the universe, at the speed of light, to a waiting automaton that has done all the travelling for us. :-)

Destination star systems => A few thousand to millions of light years away
Travel time as light beam => A few thousand to millions of years

Result => Pointless wastage of time and resources

You're missing the zero in the equation. Projecting a consciousness (disregarding the whack for a second) results in a subjective time passage of zero seconds. It's one of the more "feasible" methods of potential colonization. 

As for wasting time and resources, I'd have to argue the "Gwynnite entropic conjecture" which states that such might be the whole purpose of intelligence. 

(October 20, 2015 at 8:41 am)robvalue Wrote: @Alex: Thanks That sounds similar to something else I heard, that light doesn't actually take X years to travel X light years, because of some weird effect like that. Assuming I heard that right.

Yes, so, the time as measured on a spaceship is shortened by the factor sqrt(1-v^2/c^2) with respect to the time measured on earth and the destination.

In the extreme case of light, there is no concept of how much time passes "for" the light itself. This isn't a defined physical quantity because light has no rest frame in which one could define it...

Edit: oops I had it the wrong way around

He's mathing at us, rob. See what you started?

(October 20, 2015 at 1:00 am)vorlon13 Wrote: There are many problems with travel at +99%C.

The background radiation of the universe ahead of you will be extremely blue shifted and extremely dangerous.

Rocks and asteroids are few and far between, the bigger and much more frequent issue would be microscopic grains of dust. They will smack with astonishing, vicious energy and will be extremely dangerous.

Accelerating to that speed is extremely difficult. Even harnessing large nuclear explosives to propel a craft might be good for less than 0.1C. Even antimatter, unless used extremely efficiently won't do it. There is a concept called a Bussard Interstellar ramjet, and it actually gets more efficient the faster it goes, but it is extremely difficult imagining how to build one, let alone piloting it around the galaxy.

The Space Shuttle accelerated for 2-3 Gs for ~8 minutes just to reach 17000 MPH, to reach a high percent of C requires acceleration of 1 G for a year, or 2 Gs for 6 months, or 3 Gs for 4 months, it is difficult to even imagine getting up to those speeds, and then recall you also have to take fuel to decelerate at your craft at the destination.

You're better contemplating large multi-generational ships in the 0.05C range. Then large nukes can work, and actually work well. Preliminary designs and concepts were worked out in the 60's. If you're willing to spend ~400 years building it, a multi-generational craft capable of taking 50,000 people and the materials to provision a viable colony is possible. Refueling and refurbishing (or duplicating) the craft at the destination would take some centuries, and then one or more colonizing attempts could be made, assuming the first destination was well chosen.

That technology could be used to explore and colonize our galaxy in a couple or few million years.

The brussard ramjet is commonly discussed in older hard sci fi. One of the main issues with it is that it relies on a powerful magnetic field. In order to make possible for humans to be on it you have to create a bubble in field around the cab.

(October 20, 2015 at 5:09 am)Alex K Wrote: Concerning the Alcubierre warp bubble, I am still not at all convinced that it is in principle possible to use it to actually reach a destination with speed greater than light without invoking time travel paradoxes. And of course, matter with negative energy of that kind does not exist as far as we know.

What about the idea of using space folding to instantaneously travel? Like the off mentioned hypothetical wormholes.

(October 20, 2015 at 8:52 am)Lemonvariable72 Wrote:

(October 20, 2015 at 5:09 am)Alex K Wrote: Concerning the Alcubierre warp bubble, I am still not at all convinced that it is in principle possible to use it to actually reach a destination with speed greater than light without invoking time travel paradoxes. And of course, matter with negative energy of that kind does not exist as far as we know.

What about the idea of using space folding to instantaneously travel? Like the off mentioned hypothetical wormholes.

You need to create a region of intense negative energy for that. It is not known how one would accomplish such a feat...