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A physics professor told me once that pretty much all of current cosmology rules out FTL, but then he said he doesn’t 100% dismiss it due to how weird things in

by api 4y ago

A physics professor told me once that pretty much all of current cosmology rules out FTL, but then he said he doesn’t 100% dismiss it due to how weird things in QM keep turning out to be. He said that if FTL were to turn out to be possible it would mean the universe is yet another order of magnitude stranger than we thought due to all the insanely weird implications it would have. One speculation he threw out was that due to causality issues if you flew FTL you might actually be going to parallel universes without realizing it until you noticed hundreds of years later that your light speed radio emissions never made it home. They wouldn’t arrive because you sent them from another universe. Would also solve the Fermi paradox via a combinatorial explosion of universe X FTL trajectory combinations. Your warp ships never meet anyone else because that would violate causality. One warp ship arrival per universe only. Edit: thought of an even weirder Fermi paradox answer based on the above. Since intelligence eventually figures out FTL, only one intelligence exists per slice of the multiverse and they will never meet along any FTL hyper-trajectory. Causality protection conjecture gone wild. Also wouldn’t it suck if your multiverse trip was impossible in reverse? Go to the Centauri system, shoot some cool selfies, and come back to a barren lifeless Venus-like Earth. Every warp drops you in a universe where you are the only sentient life.

midasuni 4y ago
Imagine a wormhole which dumped you from New York to LA instantly. How would that provide time travel? Ok you beat your message by a few milliseconds, HFTs love it, but that’s now different to the ping routing via Sydney. how do you get the message back to the location it left before it left? If an FTl spaceship went to a different universe then presumably a ship in orbit of Earth aiming at mars would simply vanish and never appear in mars orbit, telescopes on earth wouldn’t see it 8, 16, 30 minutes or even years later
- jleahy 4y ago
  What if the other end of the wormhole is travelling close to the speed of light? What if one end of the wormhole is accelerating?
- SkittyDog 4y ago
  So just to start with... Every legitimate physicist accepts that time travel and other major causality paradoxes are an inevitable result of ANY mechanism that provides a meaningful version of FTL travel or communication. This is provable from the math of special relativity. You need some algebra to show it, but the proof isn't super difficult if you already understand the math of special relativity. • https://physics.stackexchange.com/questions/574395/why-would-ftl-imply-time-travel https://physics.stackexchange.com/questions/574395/why-would... • https://selfawarepatterns.com/2019/02/03/why-faster-than-light-travel-is-inevitably-also-time-travel/ https://selfawarepatterns.com/2019/02/03/why-faster-than-lig... There are a lot of YT videos that discusses it, too. But none of this is likely to convince a skeptic if you don't already have a mathematical understanding of SR. So if you're serious, you should really start by learning that.
  - scatters 4y ago
    But our universe doesn't obey special relativity, since it has gravity; you have to use general relativity. FLRW has a preferred velocity at all points, right? Couldn't you use that to construct a global time coordinate and require FTL to experience that time coordinate as increasing? (This would be dependent on the metric; I realize that GR has solutions that have closed timelike curves inherently.)
    - SkittyDog 4y ago
      I don't know where you got the idea that our universe doesn't obey special relativity? That's an odd thing to have learned... SR is a subset of general relatively. So SR can be used perfectly well in situations that obey the constraints under which it was designed to be used. As for your second paragraph, I'm not qualified to answer your question. GR is an extremely rigorous subject that requires a very high level of math to understand. But something about your tone makes me wonder if you might be engaged in some kind of motivated reasoning... That you desire the viability of FTL travel because it's exciting or romantic? If you think that describes your feelings on the subject, then the answer is almost certainly going to be "No".
      - 4y ago
- AnotherGoodName 4y ago
  It comes down to special relativity not allowing an absolute frame of reference. Without any absolute frame of reference there is no instantaneous. You may think of instantaneous as being 90 degrees away on a spacetime diagram. But someone else's frame of reference will observe your instantaneous at a different angle than 90 degrees on their own spacetime diagram, perhaps they'll even see it as going backwards in time. Their frame of reference is valid too. So if they sent a message back at a 90 degree angle on their own spacetime diagram using the same mechanism they'd be sending a message to us back in time in our frame of reference. It gets really funky when you do this with 3 different observers all observing each other communicating backwards in time to the point where they can cycle a message between the three of them that goes further and further back in time. You'd have to first throw away frames of reference to make such travel possible but that's the entire basis of relativity and good luck finding a better explanation as to why light always appears to go at the same speed for any observer in the universe.
  - Arwill 4y ago
    The geometric description of the universe is a high-level description of the universe, but its just a description, its not what fundamentally is. The large-scale universe emerges from the low-level behaviours of its smallest parts, the particles and the vacuum. Causality is not a product of the universe, but the other way around.
grishka 4y ago
> pretty much all of current cosmology rules out FTL Except current cosmology is full of strange things that feel to me like they are only there as placeholders for our gaps in the understanding of the universe. The expansion of space only exists because we need something to explain redshift. Dark matter only exists because we need something to make the galaxy rotation speed match our formulas. I do have a feeling that some of these ideas may turn out wildly wrong in the future and we'll be looking at "the universe is expanding" the same way we look at "combustion releases phlogiston out of whatever you're burning" today.
- plutonorm 4y ago
  I completely agree and often get told I'm an idiot for it. But some of it just doesn't ring true for me.
  - whatshisface 4y ago
    If you're not convinced, you should look in to the evidence and the deductions that lead to what is now presented as fact. Very few popular presentations cover the reasons behind the wacky stuff scientists say, which is to blame for you evaluating that stuff on the basis of whether it rings true - because you've been given no other way to evaluate it.
- andrewflnr 4y ago
  Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence commonly cited, including but not limited to galaxy rotation curves. Honestly skepticism of dark matter is understandable, but what the heck else are you expecting to pop up to explain redshift, a phenomenon that happens always and only when an object is moving away from you? What's the mystery there? You might be confusing universe expansion with dark energy (which IMO might well disappear, being vulnerable to improvements in the cosmic distance ladder), or cosmic inflation (apparently well substantiated but quite ad hoc, but if so you should really learn your terminology before you try to make predictions about future discoveries.
Tade0 4y ago
That last paragraph would make an interesting setting for a book.
- toastedwedge 4y ago
  Or a scifi short on Dust
bitcharmer 4y ago
I always struggled with Fermi paradox. We've been floating through insanely vast universe for laughably short time and to me its just bizarre to assert that if we couldn't find evidence of other civilizations so far, that for sure means we're the only intelligent species there is. As much as I appreciate Fermi's work, this is something I can't at all agree with.
- ClumsyPilot 4y ago
  1 - we think that a highly advanced alien civilisation would build massive startships that move at increadible speeds, Radio beacons that signal to alien civilisations, dyson spheres, etc. These things would be massive, easy to detect, and stick out like a sore thumb. 2 - with our instruments, we can observe vast swathes of space, many galaxies, quite precisely. So it appears that no highly advanced alien civilisation is in our galaxy or the nearby galaxies 3 - Even at our technology level, If we wanted to signal out into the void on purpose, we could signal quite far - thousands of lightyears, and if we wanted to target a particular star we could signal precisely with a large laser and that signal would be detectable across most of the galaxy. So it stands to reason that there is no civilisation similar to ours in the near couple hundred lightyears and that there is no advanced civilisation, the way we envision them, in our or nearby galaxies. It could be that everyone is hiding, or our idea of advanced civilisation is wrong. Or anytthing else really. That;s hwy it's a paradox/
- alasdair_ 4y ago
  One obvious answer is that any civilization that is advanced enough to be at the dyson sphere level probably has no problem hiding their existence from humanity if they want to.
  - astrange 4y ago
    That’s quite the coordination problem and it doesn’t seem like you can solve that by being an advanced civilization. (Inventing cars didn’t stop us from littering.)
    - 4y ago
nootropicat 4y ago
Ftl means there's one global reference frame most likely the cmbr one. The reason relativity predicts weird effects like time travel is because it doesn't allow ftl as an assumption, so the only way to hack ftl into it is negative time. It's baffling how many people seriously argue about consequences of something that completely disproves the theory itself while still using it to generate predictions.
mcswell 4y ago
"Every warp drops you in a universe where you are the only sentient life." Well that explains the Fermi Paradox.

by api 4y ago

midasuni 4y ago

Imagine a wormhole which dumped you from New York to LA instantly. How would that provide time travel? Ok you beat your message by a few milliseconds, HFTs love it, but that’s now different to the ping routing via Sydney. how do you get the message back to the location it left before it left? If an FTl spaceship went to a different universe then presumably a ship in orbit of Earth aiming at mars would simply vanish and never appear in mars orbit, telescopes on earth wouldn’t see it 8, 16, 30 minutes or even years later

jleahy 4y ago
What if the other end of the wormhole is travelling close to the speed of light? What if one end of the wormhole is accelerating?
SkittyDog 4y ago
So just to start with... Every legitimate physicist accepts that time travel and other major causality paradoxes are an inevitable result of ANY mechanism that provides a meaningful version of FTL travel or communication. This is provable from the math of special relativity. You need some algebra to show it, but the proof isn't super difficult if you already understand the math of special relativity. • https://physics.stackexchange.com/questions/574395/why-would-ftl-imply-time-travel https://physics.stackexchange.com/questions/574395/why-would... • https://selfawarepatterns.com/2019/02/03/why-faster-than-light-travel-is-inevitably-also-time-travel/ https://selfawarepatterns.com/2019/02/03/why-faster-than-lig... There are a lot of YT videos that discusses it, too. But none of this is likely to convince a skeptic if you don't already have a mathematical understanding of SR. So if you're serious, you should really start by learning that.
- scatters 4y ago
  But our universe doesn't obey special relativity, since it has gravity; you have to use general relativity. FLRW has a preferred velocity at all points, right? Couldn't you use that to construct a global time coordinate and require FTL to experience that time coordinate as increasing? (This would be dependent on the metric; I realize that GR has solutions that have closed timelike curves inherently.)
  - SkittyDog 4y ago
    I don't know where you got the idea that our universe doesn't obey special relativity? That's an odd thing to have learned... SR is a subset of general relatively. So SR can be used perfectly well in situations that obey the constraints under which it was designed to be used. As for your second paragraph, I'm not qualified to answer your question. GR is an extremely rigorous subject that requires a very high level of math to understand. But something about your tone makes me wonder if you might be engaged in some kind of motivated reasoning... That you desire the viability of FTL travel because it's exciting or romantic? If you think that describes your feelings on the subject, then the answer is almost certainly going to be "No".
    - 4y ago
AnotherGoodName 4y ago
It comes down to special relativity not allowing an absolute frame of reference. Without any absolute frame of reference there is no instantaneous. You may think of instantaneous as being 90 degrees away on a spacetime diagram. But someone else's frame of reference will observe your instantaneous at a different angle than 90 degrees on their own spacetime diagram, perhaps they'll even see it as going backwards in time. Their frame of reference is valid too. So if they sent a message back at a 90 degree angle on their own spacetime diagram using the same mechanism they'd be sending a message to us back in time in our frame of reference. It gets really funky when you do this with 3 different observers all observing each other communicating backwards in time to the point where they can cycle a message between the three of them that goes further and further back in time. You'd have to first throw away frames of reference to make such travel possible but that's the entire basis of relativity and good luck finding a better explanation as to why light always appears to go at the same speed for any observer in the universe.
- Arwill 4y ago
  The geometric description of the universe is a high-level description of the universe, but its just a description, its not what fundamentally is. The large-scale universe emerges from the low-level behaviours of its smallest parts, the particles and the vacuum. Causality is not a product of the universe, but the other way around.

grishka 4y ago

> pretty much all of current cosmology rules out FTL Except current cosmology is full of strange things that feel to me like they are only there as placeholders for our gaps in the understanding of the universe. The expansion of space only exists because we need something to explain redshift. Dark matter only exists because we need something to make the galaxy rotation speed match our formulas. I do have a feeling that some of these ideas may turn out wildly wrong in the future and we'll be looking at "the universe is expanding" the same way we look at "combustion releases phlogiston out of whatever you're burning" today.

plutonorm 4y ago
I completely agree and often get told I'm an idiot for it. But some of it just doesn't ring true for me.
- whatshisface 4y ago
  If you're not convinced, you should look in to the evidence and the deductions that lead to what is now presented as fact. Very few popular presentations cover the reasons behind the wacky stuff scientists say, which is to blame for you evaluating that stuff on the basis of whether it rings true - because you've been given no other way to evaluate it.
andrewflnr 4y ago
Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence commonly cited, including but not limited to galaxy rotation curves. Honestly skepticism of dark matter is understandable, but what the heck else are you expecting to pop up to explain redshift, a phenomenon that happens always and only when an object is moving away from you? What's the mystery there? You might be confusing universe expansion with dark energy (which IMO might well disappear, being vulnerable to improvements in the cosmic distance ladder), or cosmic inflation (apparently well substantiated but quite ad hoc, but if so you should really learn your terminology before you try to make predictions about future discoveries.

Tade0 4y ago

That last paragraph would make an interesting setting for a book.

toastedwedge 4y ago
Or a scifi short on Dust

bitcharmer 4y ago

I always struggled with Fermi paradox. We've been floating through insanely vast universe for laughably short time and to me its just bizarre to assert that if we couldn't find evidence of other civilizations so far, that for sure means we're the only intelligent species there is. As much as I appreciate Fermi's work, this is something I can't at all agree with.

ClumsyPilot 4y ago
1 - we think that a highly advanced alien civilisation would build massive startships that move at increadible speeds, Radio beacons that signal to alien civilisations, dyson spheres, etc. These things would be massive, easy to detect, and stick out like a sore thumb. 2 - with our instruments, we can observe vast swathes of space, many galaxies, quite precisely. So it appears that no highly advanced alien civilisation is in our galaxy or the nearby galaxies 3 - Even at our technology level, If we wanted to signal out into the void on purpose, we could signal quite far - thousands of lightyears, and if we wanted to target a particular star we could signal precisely with a large laser and that signal would be detectable across most of the galaxy. So it stands to reason that there is no civilisation similar to ours in the near couple hundred lightyears and that there is no advanced civilisation, the way we envision them, in our or nearby galaxies. It could be that everyone is hiding, or our idea of advanced civilisation is wrong. Or anytthing else really. That;s hwy it's a paradox/
alasdair_ 4y ago
One obvious answer is that any civilization that is advanced enough to be at the dyson sphere level probably has no problem hiding their existence from humanity if they want to.
- astrange 4y ago
  That’s quite the coordination problem and it doesn’t seem like you can solve that by being an advanced civilization. (Inventing cars didn’t stop us from littering.)
  - 4y ago

nootropicat 4y ago

Ftl means there's one global reference frame most likely the cmbr one. The reason relativity predicts weird effects like time travel is because it doesn't allow ftl as an assumption, so the only way to hack ftl into it is negative time. It's baffling how many people seriously argue about consequences of something that completely disproves the theory itself while still using it to generate predictions.

mcswell 4y ago

"Every warp drops you in a universe where you are the only sentient life." Well that explains the Fermi Paradox.