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Comment by throwaway7645 | Hacker News Reader

Didn't read the article, but it's always fun following these lines of thought. We have no evidence of life outside earth, but darn it...we can't be the only ones here...I mean we can, but that just doesn't sit well with me. I wonder if almost any civilization would end up using lightsails powered by beams or if many skip it in favor of some other technology.

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I've always wondered if we're already observing evidence of extraterrestrial civilizations, we just think it's some natural phenomenon. All of astronomy is based on observation, if we mistake alien activity for a natural process it could completely screw up our understanding of what's going on. For instance, maybe Quasars are artificial super-structures, created as a power source, or a beacon to signal other civilizations (similar to the signals SETI broadcast) or something else.

NOTE: I don't know nearly enough about astronomy to speculate which phenomena could be potentially artificial, but I do think it is a distinct possibility.

Still unlikely, see the Fermi Paradox and The Great Filter

http://waitbutwhy.com/2014/05/fermi-paradox.html

Favorite of mine. The author did a tremendous job.

It will probably be something similar to this [1] that allows humans/aliens to travel to distant worlds. As per my limited understanding, nothing can move faster than light in space-time, but it seems that it may be possible for a space-time wave to move faster than light. We would just ride that wave.

[1] https://en.wikipedia.org/wiki/Alcubierre_drive

That requires the existence of exotic matter, something that really isn't known to exist at all, and it'd be pretty surprising if it did. Pretty much all FTL schemes require some exotic thing that's unlikely to actually be feasible in our universe.

The bending of space is possible though. We could figure out how to bend space in a different way. Think about it, alchemists were trying to figure out how to turn lead into gold. We now know how to transmute an element into gold, although not economically. Science is till pretty young. I wouldn't rule out something similar to the Alcubierre Drive for space travel.

Remember that any such device allows you to travel backwards in time. Does it still seem so plausible to you when you realize that 'c' isn't some kind of cosmic speed limit, but the speed of causality in the universe?

Other than the fact that it seems mind-boggling, is there any reason to believe that time travel is forbidden by the laws of physics, or that causality is an absolute?

You've heard of the progress of entropy and the "thermodynamic arrow of time" right? Time travel into the past, violating causality... it breaks the math used to formulate the most basic physics at a very fundamental level.

Does that mean that it's impossible, or that our current model of physics simply doesn't account for causality violations?

We got along okay with Newton until Einstein came along and upgraded the model.

Definitely the latter, there's no way to say it's impossible, just incredibly unlikely. It's easy to imagine a universe in which it is possible, and that universe could be consistent with the math. The big "but" however is that our universe has none of the features we'd expect from one with the necessary curvature or exotic matter/energy.

Does that make sense? I mean, if we discover that there are multiple universes, or that Block Universe is true.... who knows? Even then however, I suspect we'd run into something like Novikov Self-Consistency Principle, or we'd have noticed time travel by now.

Time travel isn't forbidden. At least, there are solutions to the Einstein field equations that allow for the existence of wormholes between two points in space-time. But that doesn't mean there are wormholes in our universe, or that wormholes could be created (that last part is a little tricky, it's not easy to talk about causality when dealing with time travel). There could well be other laws of physics that prevent time travel, but general relativity seems to allow it.

It's a solution that assumes things like a perfectly uniform sphere of gas collapsing from infinity (Schwarzschild geometry), without any angular momentum. Even then, the ERB isn't traversable. A lot of the talk around wormholes and time travel suffers from map-territory issues.

For example, if you look at the maximally extended Schwarzschild geometry (the Kruskal-Szekeres extension) you see two asymptotically flat regions of spacetime connected by a "wormhole". It's probably just an artifact of the model though, and not anything physical.

A lot of the, "Math allows" stuff probably falls into that category, which is why when it comes to relativistic masses and velocities, you can't ignore quantum effects either.

Personally, I like to believe that NP hard problems can be solved efficiently in our universe. If time travel is possible, PSPACE can be solved efficiently. That is a pretty good argument for me that time travel is not possible.

http://www.scottaaronson.com/papers/ctc.pdf

That's an interesting perspective I've never considered, coming at it from the physics end of things. I appreciate what amounts to a much more grounded and logical take on why time travel makes so little sense, and is so hard to consider seriously.

>>the speed of causality in the universe?

I'm not sure it matters as long as space-time in one section is independent from another section. Maybe? Of course, I don't really know what I'm talking about since I'm not a Physicist. On a related note: As a kid, when I was still an innocent soul, I tried to build a perpetual motion machine and transmit high rate video-without first encoding it- through a phone line. It wasn't until I learned about conservation of energy and bandwidth that I understood the error of my ways. Point is, my ignorance is probably taking me astray.

I don't think you're that far astray, but you might be missing one key factor. There is a concept in physics called the spacetime interval, which can imply one of three kinds of separation: Timelike (events which can be causally connected, i.e. events which are within each other's light-cones), Null (the worldlines followed by photons), and Spacelike (events which are causally disconnected). For an easy example of "Spacelike", anything within an event horizon has a spacelike separation from anything beyond that horizon (an extreme example).

However

What if you could connect those regions with this "warp drive"? You could create a causal link between regions that would otherwise be causally disconnected. You could carry information from the past to the future, or visa versa. You could essentially ignore the very fabric of spacetime.

Now... maybe that's possible, but I doubt that it's possible in the context of warping gravity. If M-theory is correct in some form, or MWI is correct (basically any multiverse theory) then maybe you could do it. After all, if the universe is a lower dimensional structure embedded in a higher one, you could imagine traversing those spacelike separations in the same way that you could do things in 3D that someone bound to 2D could not. That however, depends on so many "if's" that it's basically magic.

I like to think that black hole propulsion is the way to go

https://arxiv.org/abs/0908.1803

Fusion candles.

https://www.schlockmercenary.com/2003-08-03

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