Nearby, Recent Interplanetary Collision Inferred 88
The Bad Astronomer writes about a new discovery by the Spitzer Space Telescope, which detected signs of an interplanetary smashup only 100 light-years from here, and only a few thousand years ago. There's a NASA-produced animation of the collision between a Mercury-sized planet and a moon-sized impactor. The collision's aftermath was detected by the presence of what are essentially glass shards in orbit around the star. Here's NASA's writeup.
Re:Neat video, but not very accurate (Score:5, Informative)
Re:A question for interstellar arms dealers... (Score:3, Informative)
There are probably more efficient ways of wiping out life than pouring on the order of 10^30 joules into accelerating a gigantic impactor.
Re:And nothing of value got lost? (Score:3, Informative)
Having an FTL drive doesn't mean it's a time machine. The actual method of travel is important here. It's impractical to go at relativistic speeds that are a considerable fraction of the speed of light, and it's pretty darn impossible to accelerate even beyond 99% of c. Theoretically, going faster than c could mean going back in time, but there is simply no way to accelerate normal matter in this fashion.
It's very likely any FTL drive technology would have to employ other means, like bending spacetime so the external distance traveled is way bigger than the subjective distance for the spacecraft in question. This could be done with a wormhole-like mechanism for example. Whether or not time flows differently for the travelers (relative to the galactic frame of reference) depends entirely on the details of this technology that we do not yet have access to.
For example, if I arrive at Alpha Centauri in two minutes from now, and come back to Earth in two more minutes Earth time, that doesn't necessarily mean I have traveled back in time 8 years. It just means there is no way I could have done that as a lump of ordinary matter traversing the entire distance through normal space.
Re:Hmm... sample of 1... (Score:3, Informative)
Yeah it kinda does. (Score:3, Informative)
Having an FTL drive doesn't mean it's a time machine. The actual method of travel is important here.
Yes actually FTL does mean you have a time machine, and the method of travel doesn't really matter. It's not like a Back to the Future time where you can arbitrarily go backwards and forwards as far as you want, it's limited to past-only and by how far and fast you can actually travel and how fast your non-superluminal spaceships can travel. But from some observer's reference frame you will have traveled back in time and broken causality by arriving at your destination before you left, simply by moving faster than c relative to them.
And if you incorporate a second FTL journey, it's actually possible to arrive at your starting point before you left according to all reference frames.
Here's an explanation [wikipedia.org]. There's a nice explanation with graphs and everything . [theculture.org]
Note that it does not depend on Lorentz Transformation of the super-luminal traveler/communication. The mechanism isn't important. That observers in normal, relativistic reference frames see you traveling faster than c is what is important. If you can do that, you can go back in time.
Whether or not time flows differently for the travelers (relative to the galactic frame of reference) depends entirely on the details of this technology that we do not yet have access to.
Time may pass differently for the travelers relative to some reference frame, but remember there are no privileged reference frames in Relativity. You can break causality if you go FTL relative to any reference frame, and if you aren't traveling FTL with respect to any reference frame, then you can't really be said to be traveling FTL can you?
Re:Yeah it kinda does. (Score:3, Informative)
Oops, my second link to the page with nice graphs [theculture.org] was hidden in a period. The explanation on that page uses instantaneous communication as its example for clarity and simplicity, but all you really need to do is break out of the light cone and you can potentially break causality with time travel.
Absolutely does. (Score:3, Informative)
If that were true, quantum entanglement would break causality.
Not it wouldn't, because nothing is traveling faster than c in entanglement, not even information. In fact, it is exactly for the reason I'm describing that demonstrates why quantum entanglement can't be used to send information. Nor can any effect resulting from the entanglement being collapsed on the "other end" be distinguishable from it collapsing on your end. There is no possibility of breaking causality.
However macroscopic-you traveling FTL most definitely involves transferring mass and information and thus causality can be broken.
Actually, what we perceive as causality is a symptom, not the cause. Hence it's an illegal assumption that time travel would _have_ to occur every time local effects shift (for lack of a better word) between two points in space faster than it would take a photon to traverse.
Actually, causality is one of the basic assumptions of Relativity. That's how Einstein ended up arriving at the conclusion that nothing can travel faster than light. He assumed causality was inviolate, and he assumed that c was constant for all observers. It was the latter notion that led to the idea that different observers could see things happening at different times. And this led to the notion that if one could travel faster than c, some observer would see effect happen before cause, violating causality.
And I didn't say time travel necessarily had to happen in any particular instance of FTL. I said FTL necessarily allows for time travel. Which, if you look, every reference agrees with.
I think most people here, including possibly myself, already know about relativistic effects, there is no need to preach. The whole discussion was _not_ based on the idea of actually accelerating any quantity of matter to causality-breaking, faster-than-light speeds to begin with. There is simply no propulsion system that can do that. What we may be able to build, however, is a system that achieves the same effect by bending spacetime in a very neat way. This is what SciFi nerds call FTL. It's a theoretical system with theoretical properties. But _if_ it works, it's not a time machine simply because it teleports matter between two points "faster" than it would take a ray of light to do so.
Yes I understand that and I thought I was pretty clear in stating that I was not referring to an object accelerated past c. Any method of travel which appears to be super-luminal to any observer breaks causality. The poster was suggesting a method where some observer would agree that you traveled FTL, yet because you didn't "really" go FTL you get around Einstein's conclusion. That simply isn't true.
Energy-wise, accelerating anything to c is impossible. Causality-wise, going faster than c by any method is impossible. I'm a Sci-Fi nerd, and hey it's neat to think about warp driving letting you get FTL without actually having to accelerate to make it seem semi-plausible. Nevertheless, even though it doesn't work that way in Star Trek, warp drive allows time travel.
In order for it to really work, we need more than just warp drive. We also need to violate Relativity. Either causality is not inviolate, and time travel is really possible, or some other assumption of Relativity is broken. That's always possible, sure, but in a Relativistic universe, FTL == Time Travel. Seriously, look it up.
Re:Absolutely does. (Score:3, Informative)
However, I think this statement goes a little far. FTL travel almost certainly implies time travel, but relativity only makes a preferred frame seemingly unnecessary given current observations. It doesn't rule out a preferred frame altogether.
Relativity is predicated on the assumption that there is no preferred reference frame. It is because of that assumption that many of the laws of Relativity are required to ensure that it is the case. If there was a preferred reference frame such that the laws of physics only had to apply to it, but causality could be broken elsewhere, then the Theory of Relativity would be very different.
Also, violating causality is a good reason to be suspicious of a phenomenon, but I don't think it deserves the "impossible" label.
It's impossible in the Relativistic Universe. It is always possible that Relativity is wrong, and that its assumptions are wrong. Hell, Newton was wrong about his basic and seemingly safe assumption that time was the same for all observers (and I hear he was even smart enough to recognize he was making that assumption and write it down). Maybe causality can be broken. The implications for physics would be profound.
Things like Warp Drives are ways to get around the limitations of Relativity without it having to be "wrong" in the hypothetical universe. But it doesn't work.
Grandfather paradoxes are (IMHO) the only reason to doubt the possibility of breaking causality, and the many worlds interpretation of quantum mechanics eliminates these paradoxes.
I'm going to wait until we unify QM and GR before I agree with that. It isn't clear to me at all that having a loop in causality in your spacetime graph would necessarily mean you have a separate outcome from some quantum waveform collapse such that you are in a different universe at the "end" of the loop vs the "beginning". There is nothing in Relativity (obviously) that would require that to be the case, and there wouldn't be a "end" or "beginning" either.
I'm also going to wait until we have some reason to actually prefer the many world interpretation over others before I agree with that. Just because it would be convenient for solving time travel paradoxes in a universe where FTL travel is possible doesn't mean its actually true. Seems more likely (as in agrees with current best theory) that time travel (and thus FTL) is simply impossible.