FTL space travel
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Posted 10/17/14
Hello everyone, there is this annoying law in physics which says you can not go past the speed of light. This sucks, because it means that (if true) it will forever be difficult to explore the universe. The question I would like to discuss then is, "is FTL travel possible and if you think it is, how do you think it should/will be done?". Note that it does not matter if you are actually travelling physically at a speed greater than C, ie. an alcubierre drive, or some method of locally raising the speed of light, or other such 'trickery', is still seen as FTL travel.

So: do you think FTL travel will ever be possible? I hope a working alcubierre or other warp drive will be made someday. Even if not, I am pretty sure mankind will eventually find some way of travelling the galaxy, given all the other things like 'flying' which were also considered 'impossible' not too long ago. What does everyone else think? Will we be flying arond the galaxy? And what method of propulsion would we be using ?
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Posted 10/17/14
Excuse me if this sounds mad, I am not that well in the know of physics apart from meteorological stuff, but, if light can be altered by large scale gravity, would that mean that the SOL increases around black holes?
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Posted 10/17/14 , edited 10/17/14

Ten-blades wrote:

Excuse me if this sounds mad, I am not that well in the know of physics apart from meteorological stuff, but, if light can be altered by large scale gravity, would that mean that the SOL increases around black holes?


I am not 100% sure if I am telling you the truth here either, but I believe it does not. I believe it only bends the light of its trajectory and into the hole so to say, because the escape velocity for such an object is higher than the speed of light.

Such a question is logical tough, so it never sounds 'mad' ^_^
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Posted 10/17/14
This isn't answering question as much as clarifying a false assumption.

Yes, information cannot go faster than the speed of light (at least, insofar as we can tell); however, this does not prohibit us in principle from exploring the rest of the universe. This is because you as your "spaceship" goes really close to the speed of light, the distance between you and your destination shrinks (this is length contraction--another consequence of special relativity). In other words, you can travel any finite distance in any arbitrary amount of time while still going slower than the speed of light.

Also, note that while from your point of view on the spaceship, it will appears that the distance between you and your destination shrinks, to someone on earth (or rather, any place that is essentially not moving w.r.t. your destination) it will appear you're just taking an eternity and a half to get there. This is time dilation.

That said, be careful when applying length contraction and time dilation; they help introduce new concepts in the context of special relativity, but another effect (specifically, the one that prevents information from traveling faster than the speed of light) means that you need to summarize both effects in the so-called "Lorentz Transformation." This just changes the mathematics involved; it doesn't undermine the rule I've been purporting that you can travel any finite distance in any arbitrary amount of time. In either case, while it's allowed in principle, be aware that going near the speed of light requires an absolutely obscene amount of energy--deriving the energy to allow us to travel close to the speed of light is the main difficulty involved.
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Posted 10/17/14

Zoraprime wrote:

This isn't answering question as much as clarifying a false assumption.

Yes, information cannot go faster than the speed of light (at least, insofar as we can tell); however, this does not prohibit us in principle from exploring the rest of the universe. This is because you as your "spaceship" goes really close to the speed of light, the distance between you and your destination shrinks (this is length contraction--another consequence of special relativity). In other words, you can travel any finite distance in any arbitrary amount of time while still going slower than the speed of light.

Also, note that while from your point of view on the spaceship, it will appears that the distance between you and your destination shrinks, to someone on earth (or rather, any place that is essentially not moving w.r.t. your destination) it will appear you're just taking an eternity and a half to get there. This is time dilation.

That said, be careful when applying length contraction and time dilation; they help introduce new concepts in the context of special relativity, but another effect (specifically, the one that prevents information from traveling faster than the speed of light) means that you need to summarize both effects in the so-called "Lorentz Transformation." This just changes the mathematics involved; it doesn't undermine the rule I've been purporting that you can travel any finite distance in any arbitrary amount of time. In either case, while it's allowed in principle, be aware that going near the speed of light requires an absolutely obscene amount of energy--deriving the energy to allow us to travel close to the speed of light is the main difficulty involved.


Wait wait wait... you shrink the distance? So if I plot a course to a black hole and increase energy output to 100%, the black hole suddenly zips to planet Earth??? Or am I misunderstanding something here? I mean, how can you diminish the distance between two objects without them moving?

But also, if that is the case, then why is not being able to go FTL generally considered a problem? Why are people trying to find loopholes to be able to go FTL? Is there some other problem with the method of travel you described besides energy requirements which makes it more difficult?
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Posted 10/17/14 , edited 10/17/14

eragon2890 wrote:

Wait wait wait... you shrink the distance? So if I plot a course to a black hole and increase energy output to 100%, the black hole suddenly zips to planet Earth??? Or am I misunderstanding something here? I mean, how can you diminish the distance between two objects without them moving?


Let's avoid talking about black holes. That deals with general relativity and complicates the details--in special relativity (which is what we're talking about), we can only really talk about cases where gravity is very weak.

First of all, it doesn't make sense to talk about energy output as a percentage. Suffice to say, there is a relationship between your kinetic energy and how fast you're going (namely, E=(mc^2)*(1/sqrt(1-v^2/c^2)-1), so the appropriate metric is what percent of the speed of light you're going. You can figure out how fast you need to go (in principle) from special relativity.

Let's talk about the Alpha Centauri instead, which is about 4.4 lys away. What happens is as you approach the speed of light, how far away Alpha Centauri appears to be will change by how fast you're going; however this distance must always be greater than zero. You can make it appear 1mm away by moving at will probably amount to just a tiny amount below the speed of light, but in order for the Alpha Centauri to be the same place as earth would imply you are going the speed of light--that's impossible.



But also, if that is the case, then why is not being able to go FTL generally considered a problem?


Because it would violate causality. Also, you can't just go faster than light; rather, you cannot even go past the speed of light. See here for a reason why, although I'm not sure if this is the most reliable source.: http://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality


Why are people trying to find loopholes to be able to go FTL?


I assume by people you mean sci-fi authors. Authors want to create a universe, and time dilation (length contraction's big brother) makes narratives go to hell. This is because as you move faster, earth's clock will appear to tick faster--in fact, it might be so fast that by the time you reach your destination earth may have already been consumed by a helium-rich sun. Time dilation in general just makes the narrative very wonky as every single location now needs to be associated with its own timeline. In short, it's probably more trouble then it's worth.


Is there some other problem with the method of travel you described besides energy requirements which makes it more difficult?


To my knowledge, no, there isn't any other way. That said, if there is another way, it'd probably be answered by general relativity which I don't know too much about.
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Posted 10/18/14

Zoraprime wrote:


eragon2890 wrote:


Why are people trying to find loopholes to be able to go FTL?


I assume by people you mean sci-fi authors. Authors want to create a universe, and time dilation (length contraction's big brother) makes narratives go to hell. This is because as you move faster, earth's clock will appear to tick faster--in fact, it might be so fast that by the time you reach your destination earth may have already been consumed by a helium-rich sun. Time dilation in general just makes the narrative very wonky as every single location now needs to be associated with its own timeline. In short, it's probably more trouble then it's worth.


It is not just science fiction authors. Many scientists are pursuing the same thing.

Mostly because going STL does not make exploring the galaxy practical. The Voyager I probe is mankind's fastest moving object at a blistering 15 km/s. That is 15 kilometers per second, which may not sound that fast, but that makes it about 54,000 kilometers per hour. Oh yeah, definitely faster than a speeding bullet. But in terms of galactic distances, slower than a snail. Comets are generally moving faster. I mean, 15 km/s is about .00005% the speed of light which is about 300,000 km/s. 299,768 or something like that. Which means at 15 km/s it would take about 220,000 years to make it 4.4 light years to Alpha Centauri which is currently our closest stellar neighbor. But the voyage would take so long that by the time a probe going that slow got there, Alpha Centauri would no longer be our closest stellar neighbor and would be in a completely different part of the solar neighborhood.

Lots of science fiction authors deal with pure STL and time dilation. It is not that big of a bother to work with really. Not when you are talking distances under say 50-100 light years. With cryostasis and low impulse ion engines, it could be possible to get a ship perhaps as high as half the speed of light. Making a 50 light year journey only about 200 years round trip. If you are writing a science fiction novel about the people doing this, that sort of time frame is not really that hard to deal with. Although when you think about it this makes stellar travel not very practical from a profitability point of view. Taking 200 years to complete a trade mission and what you get had better be worth a lot more than 20 million per kilo of unobtanium.

The real killer though, is that if FTL travel is possible, then where are the aliens? It is the same question that plagues SETI. If there are intelligent aliens out there, where are their transmissions? We have been searching long enough we should have gotten something if they were as relatively common as planetary searches indicate planets are. Even if alien races quickly stop using radio signals in favor of something else, they probably would have started with it. We should receive those, at least if intelligent life were relatively common.

The concensus SETI is slowly reaching is that either a) we are the first relatively intelligent life to come about in this part of the galaxy or b) we are the last and the others have wiped themselves out. But if it were B, we still should have detected them unless most intelligent races kill themselves off before achieving radio.

Oddly this is good news for FTL because if there are no other aliens out there, then lack of aliens with FTL is not an issue. Which means FTL may in fact be possible. Finding alien life on other planets that are advanced but do not have FTL would tend to indicate that FTL is not possible or practical. It also could be idiosyncratic to that alien race, but it would still be an issue for the concept of FTL. So no alien races is a good thing for FTL in general, but at the cost of being a lonely universe.
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Posted 10/18/14 , edited 10/18/14
-First of all, I'm not sure how many scientists out there are considering FTL travel very seriously. The only thing I can remotely see that scientists are looking into is a mention that NASA is talking about a warp drive. Even then, warp driven isn't actual FTL travel; rather, it's just bending spacetime to shorten the distance between two objects so that you can move FTL, but the device isn't actually going faster than light. Rather, the velocity we're defining is how far two objects are in earth's frame divided by the amount of time it takes in the rocket frame, and even such as velocity in SR turns out to be greater than c; it's just that that velocity doesn't correspond to an actual observable velocity.

Edit: I should be a little more careful what I mean when I say "many scientists out there are [probably not] considering FLT travel very seriously." What I mean is that all research into physical phenomenon wherein the speed of light is of interest will still try to be consistent with relativity. If, somehow, we could actually go faster than light, then we would need to explain how that's consistent with special relativity and possibly general relativity (I say "possibly," because quantum mechanics and GR don't play nice with one another, and as such, contradictions between one or the other wouldn't be unprecedented). Whatever research appears to be FTL insterstellar travel is, more likely than not, an attempt to reach an apparent speed that is FTL.





-Second, I don't think a lack of aliens is particularly great proof. I'm not entirely sure how long we have been searching for extraterrestrial intelligence, but SETI seemed to start in 1960-ish. This means we can either only see radio waves that have passed through earth int he past 55 or so years, which may be a problem itself. Specifically, if a planet is within 55 light years, they will have had to to emit a radio wave within the last 55 years for us to see it (i.e., if their civilization died off "within" the over 55 years ago, their radiation is too far away). Past that, for every n lightyears we go out, that planet will have needed to emit their radio signal between 55+ n years ago and n years ago. It's not just a matter of making radio--it's also a matter of making radio and having used it recently.

Moreover, this would also assume that life is common enough that they can evolve into an intelligent being and that they decided to use analog electronics like the rest of us, which I think is a more fatal assumption. It's mostly a moot point, since we don't know how life rises from abiogenesis, so we can't comment exactly how common life is.

Lastly, and the biggest issue I see is that we're assuming that if the alien civilization packed up and left, they would had to use FTL travel. Why not just use a slower-than-light speed and exploit length contraction or some similar phenomenon? And if so, why would they suddenly decide to pack up?

Overall, this argument seems to make a jump from "we can't find aliens" to "they may have had to use FTL travel to leave us." It seems like it's making a lot of implicit assumptions.
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Posted 10/22/14
You will need to bend light so that it takes longer to reach it's destination in order to travel faster than it.
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Posted 10/22/14

Sir_jamesalot wrote:

You will need to bend light so that it takes longer to reach it's destination in order to travel faster than it.


When we mean FTL travel.. we don't mean literally out-speeding a beam light... We're talking about achieving speed faster than lightspeed.
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Posted 10/22/14

shuyi000 wrote:


Sir_jamesalot wrote:

You will need to bend light so that it takes longer to reach it's destination in order to travel faster than it.


When we mean FTL travel.. we don't mean literally out-speeding a beam light... We're talking about achieving speed faster than lightspeed.


For that you will need more fuel than is available in the universe.
The best you can do is the light rocket which travels at the same speed of light.
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