A Question about Universal Expansion

[Gunboat Diplomat]

Basically, Gunboat is saying that any form of FTL travel will lead to paradoxes and other are asking "why is that a problem?". I realise that everyone understands that a paradoxical situation is a bad thing but what you are asking is "why does one paradoxical situation mean all other FTL travel situations are impossible?"

Actually, I think only 5810Singer thought that this might not be a problem. As far as I can tell, everyone else sees the construction of paradoxes as a fatal error...

Our current understanding of physics theoretically allows for the creation and stabilisation of wormholes that can connect two distant points allowing one to travel from point A to point B through the wormhole faster than light could travel from point A to point B outside the wormhole. Now, I don't understand why by Gunboat says that FLT travel necessarily allows time travel. My lack of understanding doesn't matter for this example because the ends of wormholes can be placed in different points of time as well as different points of space, theoretically allowing time travel. Allowing time travel leads to the possibility of creating paradoxes which is what Gunboat has been explaining. So, why is this a problem? It is a problem because this means that the best application of our mathematical understanding of current physics says that paradoxes can occur, but they can't, that's the point. Therefore, there is something wrong with the initial physical assumptions or the mathematics that allowed wormholes and, presumably, other FTL travel methods to exist in the first place. The most likely explanation is that FLT travel is always impossible, even if we don't understand why just yet.

Phew, I hope I'm right about all that. Feel free to correct me, physics is not my strong suit.

Your physics may not be strong but your reasoning is sound! Yes, like proof by contradiction, the construction of a paradox shows that the current understanding is logically flawed. Therefore, there exists some assumption in the current understanding which is false. In the case of faster than light travel, relativity and causality, it's most likely faster than light travel that's the false assumption. Otherwise, in order to show that faster than light travel is valid, someone has to demonstrate how relativity, causality or some other implicit assumption is false...

Incidentally, Aught3, you don't understand how faster than light travel causes paradox? What did you think of my assassins and diplomats gedanken?

 

[Aught3]

Incidentally, Aught3, you don't understand how faster than light travel causes paradox? What did you think of my assassins and diplomats gedanken?

I understand how time travel can cause paradoxes and I understand that the same types of FTL travel techniques can be used to time travel but they are still separate issues in my head - I think I may be in the same situation that Pulsar is/was earlier. As to the example I'll have a think about it tomorrow morning and let you know how I get on.

 

[scalyblue]

Incidentally, Aught3, you don't understand how faster than light travel causes paradox? What did you think of my assassins and diplomats gedanken?

I understand how time travel can cause paradoxes and I understand that the same types of FTL travel techniques can be used to time travel but they are still separate issues in my head - I think I may be in the same situation that Pulsar is/was earlier. As to the example I'll have a think about it tomorrow morning and let you know how I get on.

Remember that you can't travel through space without travelling through time as well.

 

[Gunboat Diplomat]

I understand how time travel can cause paradoxes and I understand that the same types of FTL travel techniques can be used to time travel but they are still separate issues in my head - I think I may be in the same situation that Pulsar is/was earlier. As to the example I'll have a think about it tomorrow morning and let you know how I get on.

There are so many things about this statement that I don't understand...

First, what do you mean by "the same types of FTL travel techniques?" What types are those? Did you notice that in the assassins and diplomats example, there was no mention of the mechanism of the magic walkie-talkies? That's because the method is irrelevant. They could be using warp fields, wormholes or voodoo for all it matters...

Second, how on God's green Earth can the two be "separate issues" in your head? If you can travel faster than light then you can travel back in time. If you can travel back in time then you can construct paradoxes. How are they "separate issues?"

Finally, if you're "in the same situation as Pulsar" then you're in denial 'cause that's where he seems to be. So far, his only response to me has been that there's no localized superluminal motion even though all it takes to violate causality is a macroscopic change in position faster than light...

I hope you make some headway with understanding the assassins and diplomats example...

 

[Aught3]

Yes, thank you I made a lot of headway with the assasin example. I actually found it to be very clear but I did have to draw myself a couple of diagrams first. I now see that all FTL techniques would be able to produce this paradoxical situation. I was also thinking about the Alcubierre drive and I realised that if you hopped in zoomed around for a bit and came back to the same place you would have travelled backwards in time. I guess the reason why time travel and space travel are two separate issues in my head is that I've never had any formal training in relativity and for me it's something I can wrap my head around for short bursts only. I realise that I'm not thinking about these issues in quite the right way but I'm having a go

 

[Pulsar]

Sorry for the long delay, life is getting in the way, and this thread is taking more time than I'm willing to invest...

Finally, if you're "in the same situation as Pulsar" then you're in denial 'cause that's where he seems to be.

LOL, thanks for those kind words. I'm not in denial, your assassins-example is very nice, and I tend to agree with you (how about that! ). One thing though: your thought experiment depends explicitly on special relativity alone. Of course, there's no dispute there: FTL violates causality in the framework of special relativity. But by distorting spacetime, general relativity pops up. As you said, there was no mention of the mechanism of the magic walkie-talkies, but this mechanism is important, as simple special relativity is no longer sufficient.

If the assassins and diplomats no longer move in a Minkovski space, then each has his own, local inertial frame, so the postulates of special relativity no longer hold globally. The question then becomes: would the distortion caused by the warp drive be of such magnitude that the paradox can be avoided? I have no idea, I guess it depends on the details of the metric involved.

I've searched the web for more info on the Alcubierre drive and causality, but I haven't found a peer-reviewed paper that settles the issue once and for all. It certainly goes beyond my own knowledge of general relativity, so as it stands, your argument is convincing.

To be clear, I'm absolutely no advocate of this Alcubierre drive: I think it's totally insane. Even if causality isn't an issue, it involves negative energy densities, and even worse, tachyons. This might be resolved in further research (C. van den Broeck made some improvements), but my gut feeling says that Nature will not allow these bubbles...

 

[scalyblue]

Wouldn't an Alcubierre drive require some rediculous-huge amount of energy, like a contained neutron star?

 

[5810Singer]

Wouldn't an Alcubierre drive require some rediculous-huge amount of energy, like a contained neutron star?

Isn't that the main problem with all the really cool "sci-fi" tech,....energy production?

And then once you've produced such an insane amount of power how do you control it?

 

[Gunboat Diplomat]

Sorry for the long delay, life is getting in the way, and this thread is taking more time than I'm willing to invest...

No worries. Life trumps all...

Finally, if you're "in the same situation as Pulsar" then you're in denial 'cause that's where he seems to be.

LOL, thanks for those kind words. I'm not in denial, your assassins-example is very nice, and I tend to agree with you (how about that! ). One thing though: your thought experiment depends explicitly on special relativity alone. Of course, there's no dispute there: FTL violates causality in the framework of special relativity. But by distorting spacetime, general relativity pops up. As you said, there was no mention of the mechanism of the magic walkie-talkies, but this mechanism is important, as simple special relativity is no longer sufficient.

I'm sorry if that was harsh. I was annoyed at how your responses ignored key points in my posts, such as the results of relativity being independent of method or the semantics of movement. Hence my claim of denial...

If the assassins and diplomats no longer move in a Minkovski space, then each has his own, local inertial frame, so the postulates of special relativity no longer hold globally. The question then becomes: would the distortion caused by the warp drive be of such magnitude that the paradox can be avoided? I have no idea, I guess it depends on the details of the metric involved.

The postulates of special relativity always hold globally...

Firstly, even if the two theories were opposed, as long as your drive restores space-time back to what it was before, special relativity will apply again and you'll have violated causality...

Secondly, the two theories are not opposed but are instead complimentary. General relativity is an extension of special relativity with the addition of the equivalence principle, where all methods of acceleration are indistinguishable from each other. If you're moving within a gravity well, you will experience time dilation from both your movement and the curvature of space-time. They compliment each other...

Besides, doesn't this Alcubierre drive warp space-time locally? How is that supposed to fix the global issues of special relativity and causality?

I've searched the web for more info on the Alcubierre drive and causality, but I haven't found a peer-reviewed paper that settles the issue once and for all. It certainly goes beyond my own knowledge of general relativity, so as it stands, your argument is convincing.

This is what I've been saying: no one is addressing this issue. In order for faster than light travel to be possible, special relativity must be wrong... but then why is it so right? You see our problem?

To be clear, I'm absolutely no advocate of this Alcubierre drive: I think it's totally insane. Even if causality isn't an issue, it involves negative energy densities, and even worse, tachyons. This might be resolved in further research (C. van den Broeck made some improvements), but my gut feeling says that Nature will not allow these bubbles...

To be clear, I'm not an opponent of faster than light travel. I very much want it to be true but I can't believe it without an explanation as to how it can be possible while there is so much evidence to show that it is not.

Not coincidentally, I'm also an atheist...


Finally, so what if it uses a lot of energy? That's just an engineering problem!

 

[Aught3]

Okay, time to revive this thread.

What about multiverse theory? Can you avoid the problem with causality if you travel back in time to a separate universe?

 

[obsidianavenger]

assuming infinite universes its already happened hasn't it?

 

[aeroeng314]

Okay, time to revive this thread.

What about multiverse theory? Can you avoid the problem with causality if you travel back in time to a separate universe?

Provided it's a one-way trip. But that raises the question of, if it's possible to get to that universe from this one, why would it not also be possible to get to this universe from that one? What would make this universe more special than the others?

 

[borrofburi]

Okay, time to revive this thread.

Yet another revival time...

Pulsar never responded to GD's post... I'm curious what he thinks.

 

[SagansHeroes]

I was reading something the other day that has more relevance to the original topic I guess.

http://www.newscientist.com/article/mg20827825.200-big-bounce-cosmos-makes-inflation-a-sure-thing.html

 

[Pulsar]

Okay, time to revive this thread.

Yet another revival time...

Pulsar never responded to GD's post... I'm curious what he thinks.

Well, GD had convincing arguments. If an Alcubierre drive is a local, temporary distortion, and the space-time between source and observer before and after the FTL transport hasn't fundamentally changed, then yes, it does seem to lead to causality problems. I don't know enough about the properties of the proposed device to argue whether such time paradoxes can be avoided, so I've given in. Kudos to GD.

 

[borrofburi]

Pulsar never responded to GD's post... I'm curious what he thinks.

Well, GD had convincing arguments. If an Alcubierre drive is a local, temporary distortion, and the space-time between source and observer before and after the FTL transport hasn't fundamentally changed, then yes, it does seem to lead to causality problems. I don't know enough about the properties of the proposed device to argue whether such time paradoxes can be avoided, so I've given in. Kudos to GD.[/quote]

That's cool... I'm just in an argument about the idea that there's any reason to give credence to the idea of aliens on earth, and one of my objects is that any travel that allows you to get from point A to point B (including wormholes and Alcubierre drives can cause paradoxes, or at the very least is indistinguishable from proposing that the magical blue leprechauns are knocking over my neighbor's garden gnomes.

But when I returned to this thread to find the assassin-diplomat-train illustration, I noticed that there was a sort of "outstanding objection", and I wasn't certain if you had been convinced or not.

Thanks for responding.