4 ms·

Briefly put: the speed of light. It takes a long, long time to get anywhere, because light moves at a relative crawl compared to the distances between stars, s

Briefly put: the speed of light. It takes a long, long time to get anywhere, because light moves at a relative crawl compared to the distances between stars, so it would be a long and energy-intensive process to visit another planet, especially given how hard it would be to detect a civilization there before you departed. The Earth didn't emit much of a signal on radio waves until about 1900, so even if the aliens were only sixty light years away and could move at the speed of light, they'd have had to launch just as soon as they got some signals. Ah, but FTL, you say. But nothing. FTL implies Closed Timelike Curves: time travel, or the informational equivalent, with all of the logical paradoxes that can entail. I'd sooner believe in breaking the conservation of mass-energy than beating the speed of light.

cgriswald 6y ago
I think your sibling poster has it right: lack of evidence. All the speed of light problems can be ameliorated. The entire galaxy could be visited (if not colonized) in a relatively short amount of time by something like a Von Neumann probe at even 0.01c. The 'aliens' needn't be biological.
heyitsguay 6y ago
Agreed but to be fair - mass-energy is not conserved across intergalactic scales.
subsubzero 6y ago
The speed of light is a constant but we are still very behind in terms of understanding the universe and physics, we still do not have a unified theory which bridges general relativity to quantum mechanics. We know next to nothing about dark matter(haven't found any yet), also in terms of energy production we have not achieved - room temperature superconducting and fusion reactors. We can barely get rockets outside of earths orbits, let alone to our closest celestial neighbor the moon. That being said, I think given our current technological shortcomings we are in no position to rule out 100% possibility of life from another world being here.
- at_a_remove 6y ago
  In literally every protracted discussion I've had about FTL since before I got my physics degree, someone has brought up the "science doesn't know everything yet" angle. Here's the thing: science is never done, it will never know for finally and for certain. And yet two of my objections will remain unaddressed: 1) FTL implies CTCs, that means paradoxes. How are those going to be resolved? Because the Cosmic Censor stuff never quite makes sense -- exactly what mechanism is going to forbid you from opening the message that says, "Do not send this message back in time?" 2) Again allowing for FTL ... where are the self-replicating machines spreading across the cosmos? Pop to another star system, pop to another galaxy, or even a local cluster. They should have already been there. The universe is vast. Most species will realize that this is a terrible ass idea, but there's always a lunatic in a large enough crowd, and it'll be a pretty large crowd. Nobody ever deals with these points. People want FTL to be true because, well, Star Trek (or Star Wars, or Farscape, take your pick). It's a bit dull, plodding along at some small fraction of c, but the universe doesn't have to be exciting for us. And I'm not going to bet otherwise.
  - ikrenji 6y ago
    1, without having a mechanism for FTL how can we say for certain that it implies CTCs? maybe relativity is an approximation of a more complete theory which allows FTLs without CTCs... 2, didn't the universe itself expand faster than light during inflation? what does relativity have to say about that? if we can't use relativity to explain inflation, why do we so comfortably use it to forbid FTL?
    - at_a_remove 6y ago
      1) Oh, it turns out that the mechanism for getting CTCs out of FTL is completely irrelevant. Construct your light cones, see the results. Warp drive, wormholes, whatever, they all produce CTCs. You can look into it if you don't believe me. 2) None of the matter in the universe was expanding, the spacetime was. You're not the first person to think of this. Probably the first question that popped into anyone's mind when variants of the concept were being kicked around. Anyway, the expansion doesn't generate CTCs because there's no information and/or matter to circle around, it's just the stretch between these. Now, you'll probably ask about -- well, why can't we shrink spacetime selectively to get around all of that? Ah, that requires some exotic matter, matter with negative mass-energy. No, anti-matter does not have negative mass-energy (first thing people reach for). In fact, we've never found anything with negative mass-energy and that concept itself raises a ton of fun paradoxes which I could get into. And you still get CTCs. There's probably a large tree of arguments out there on the Internet somewhere which addresses all of these common objections.

cgriswald 6y ago

I think your sibling poster has it right: lack of evidence. All the speed of light problems can be ameliorated. The entire galaxy could be visited (if not colonized) in a relatively short amount of time by something like a Von Neumann probe at even 0.01c. The 'aliens' needn't be biological.

heyitsguay 6y ago

Agreed but to be fair - mass-energy is not conserved across intergalactic scales.

subsubzero 6y ago

The speed of light is a constant but we are still very behind in terms of understanding the universe and physics, we still do not have a unified theory which bridges general relativity to quantum mechanics. We know next to nothing about dark matter(haven't found any yet), also in terms of energy production we have not achieved - room temperature superconducting and fusion reactors. We can barely get rockets outside of earths orbits, let alone to our closest celestial neighbor the moon. That being said, I think given our current technological shortcomings we are in no position to rule out 100% possibility of life from another world being here.

at_a_remove 6y ago
In literally every protracted discussion I've had about FTL since before I got my physics degree, someone has brought up the "science doesn't know everything yet" angle. Here's the thing: science is never done, it will never know for finally and for certain. And yet two of my objections will remain unaddressed: 1) FTL implies CTCs, that means paradoxes. How are those going to be resolved? Because the Cosmic Censor stuff never quite makes sense -- exactly what mechanism is going to forbid you from opening the message that says, "Do not send this message back in time?" 2) Again allowing for FTL ... where are the self-replicating machines spreading across the cosmos? Pop to another star system, pop to another galaxy, or even a local cluster. They should have already been there. The universe is vast. Most species will realize that this is a terrible ass idea, but there's always a lunatic in a large enough crowd, and it'll be a pretty large crowd. Nobody ever deals with these points. People want FTL to be true because, well, Star Trek (or Star Wars, or Farscape, take your pick). It's a bit dull, plodding along at some small fraction of c, but the universe doesn't have to be exciting for us. And I'm not going to bet otherwise.
- ikrenji 6y ago
  1, without having a mechanism for FTL how can we say for certain that it implies CTCs? maybe relativity is an approximation of a more complete theory which allows FTLs without CTCs... 2, didn't the universe itself expand faster than light during inflation? what does relativity have to say about that? if we can't use relativity to explain inflation, why do we so comfortably use it to forbid FTL?
  - at_a_remove 6y ago
    1) Oh, it turns out that the mechanism for getting CTCs out of FTL is completely irrelevant. Construct your light cones, see the results. Warp drive, wormholes, whatever, they all produce CTCs. You can look into it if you don't believe me. 2) None of the matter in the universe was expanding, the spacetime was. You're not the first person to think of this. Probably the first question that popped into anyone's mind when variants of the concept were being kicked around. Anyway, the expansion doesn't generate CTCs because there's no information and/or matter to circle around, it's just the stretch between these. Now, you'll probably ask about -- well, why can't we shrink spacetime selectively to get around all of that? Ah, that requires some exotic matter, matter with negative mass-energy. No, anti-matter does not have negative mass-energy (first thing people reach for). In fact, we've never found anything with negative mass-energy and that concept itself raises a ton of fun paradoxes which I could get into. And you still get CTCs. There's probably a large tree of arguments out there on the Internet somewhere which addresses all of these common objections.