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You don't understand galactic scales; Relativity is a bitch thread by Kiyotsugu Hirayama - Thu, 04 May 2017 18:10:19 EST ID:unNII3om No.56931 Ignore Report Quick Reply
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So imagine you have two cannons firing their cannon balls away from each other, and their muzzle velocity is the speed of light.

You'd think the relative speed of one cannon ball to the other would be 2x the speed of light, right?

Nope. The equation for relative speed between two objects looks like this:


Where v1 and v2 are the speeds of the two cannon balls and c is the speed of light.

So as the velocity of the cannon balls approaches the speed of light, their own speed doesn't matter and the limit of light speed is dominant. Relative speed of 2x speed of light is thus impossible.

Or in other words: wat.

You guys got some other relativistic mind-blowers?
Georges-Henri Lemaitre - Thu, 04 May 2017 18:21:39 EST ID:7jcVAyVz No.56932 Ignore Report Quick Reply
Here's something I've always wondered about the relationship between relativity and hypothetical tachyonic matter (so obviously this is a completely speculative thought experiment): if a mass were travelling faster than the speed of light (while still being in 'normal space' i.e. not in some kind of hyper-space or pocket universe or whatever) its time dialation relative to a non-relativistic speed observer would obviously increase the closer to c it got. If the dialation continued to increase for speeds beyond c, it would be travelling even further into the 'future' relative to the stationary observer.

So my thought is; if this mass were say a probe, and also had tachyonic sensors and communication equipment, effectively it would be moving into the future relative to the stationary observer, and thus would be able to communicate future happenings to the stationary observer. Which would obviously open the door to the possibility of temporal paradoxes. Which leads to my bigger picture thought; is FTL impossible for the same reason time-travel is, that they lead to violations of causality? Following this trail, is it possible that there is some deeper interrelationship between time, causality, and relativity that this reveals?
Kiyotsugu Hirayama - Thu, 04 May 2017 18:35:05 EST ID:unNII3om No.56933 Ignore Report Quick Reply

>Which leads to my bigger picture thought; is FTL impossible for the same reason time-travel is, that they lead to violations of causality?

Interesting point. This even applies to non FTL stuff like worm-holes. Just place one end near some huge mass, or just accelerate it enough then bring it back, and the two ends would be time-dilated relative to each other. Travel through the wormhole, and you'd travel through time.
Joseph von Fraunhofer - Sat, 06 May 2017 14:50:32 EST ID:7jcVAyVz No.56934 Ignore Report Quick Reply
Indeed! It's very unfortunate that that seems to be the case with wormholes as well, although we will have to see. I don't think Hawking's 'radiation-grandfather-suicide' explanation for why wormholes don't work is entirely air-tight, but I won't bother going off on that tangent.

It's not a brain-twister, but on the subject of relativity in general, it's really going to suck when we colonize space if we can't figure out even some method of FTL communication, because, for example, the internet can only work on any given planet, maybe within a single solar system, because of relativistic effects. *Culture* itself may require a single system to work, because of the combination of the relativistic effects of travel with the huge timescales involved.

Consider a species that sends one colony ship to the next system over, and one to the other side of the galaxy, with the goal of working toward one another. Because the close-colonizing systems would be able to share back and forth, they would both have some small degree of homogeneity, and also advance more quickly (because of networking of knowledge and sharing of resources.)

But the other group would arrive at their destination, from their perspective, much much sooner (than the other side would get to them, not to their own original destination.) By the time the two sides met, the far travelers would seem like the ancient primordial civilization to the near colonizers, whereas they would seem to be unfathomably alien and strange to the far travelers. Maybe life just isn't meant to be able to scale to the problems relativity brings about?
Joseph Lockyer - Fri, 12 May 2017 18:29:26 EST ID:unNII3om No.56936 Ignore Report Quick Reply

If true FTL is impossible, I'd expect a galactic "civilization" to spread in stages. The outer reaches being composed of descendants of the first interstellar waves of colonizers, then as you progress towards the origin then technology gets significantly more advanced as the ones with more 'time' back home catches up with the first-comers.

This hypothetical grading of civilization development continuously moves outwards. Maybe we'd even see luddite societies constantly trying to out-pace the encroachment of high-tech civilizations?

But at any rate, as far as we know, any system will be isolated from the next by years or decades of travel. One of the youth sci-fi book series in my countries deals with the effects of exactly this, where the exploring main characters spend decades in chryo, and try to deal with the new system cultures they encounter and the fact that all of their families and friends are long dead.
James Elliott - Sat, 13 May 2017 16:05:47 EST ID:7jcVAyVz No.56938 Ignore Report Quick Reply
That's pretty cool, what book series is that?
Walter Adams - Sat, 13 May 2017 20:13:30 EST ID:unNII3om No.56939 Ignore Report Quick Reply
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The Chronicles of the Starship Alexandria. That's the series' name at least.

Though I doubt you can get it outside Norway. Don't think you can get it here either tbh, it's an old 80's-90's thing.
Subramanyan Chandrasekhar - Sun, 14 May 2017 14:27:04 EST ID:7jcVAyVz No.56940 Ignore Report Quick Reply
Cool, thanks. I was hoping the internet would have some .pdfs, but it looks like you're right, this guy's stuff is pretty hard to track down. It's a shame the internet doesn't care as much about preserving the literature of quality authors not writing in English than it cares about holding onto a million sources for the same trashy English crap.
James Christy - Fri, 19 May 2017 04:02:03 EST ID:d98JL0hU No.56942 Ignore Report Quick Reply
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Well, as long as we're on things that'll blow your mind - causal violations are not a problem in relativity. They are in fact expected and on multiple levels. For instance, two people at different reference points, witnessing events unfold in a third, may disagree about their order. There's quite a few other bits of relativity that create causal paradoxes, some of which are plugged up by quantum physics, but it introduces its own:


There's a limit to the rate of causality, that makes the speed of light, but causality itself is not necessarily inviolable. It just hurts the brain to think of it.

It's also worth noting, that in the cannonball example of OP, the two cannon balls also effectively cease to exist to one another as no information can be passed between them. Worth noting because that's going to be the same fate of every red shifted galaxy in the universe, relative to us, in some hundred billion years (while the handful of blue shifted ones with merge with us). Evidence of the CMB will be gone as well, so for future observers, there will be no evidence of the creation of the universe nor that there were ever any galaxies other than their own.

Which makes you wonder what we're missing already - given that so many galaxies (some estimate 90% of them) have already passed beyond the observable limit.
Anders Angstrom - Fri, 19 May 2017 15:30:33 EST ID:7jcVAyVz No.56943 Ignore Report Quick Reply
Awesome paper! Skimming through it now. I certainly am open to the idea that violations of causality are possible; it never made sense to me that the universe would perform some kind of check to keep itself consistent, when it's already so goddamned insane. What then are your thoughts about Hawking's wormhole-radiation-grandfather-suicide claim that a specific causal violation that was loop like would necessarily destroy itself (and maybe everything) with it's own radiation?

>> there will be no evidence of the creation of the universe nor that there were ever any galaxies other than their own.
Sad to think about. Still, hasn't it been said that some very scientifically diligent civilization might be able to deduce all that information about the formation of the universe by studying hyper-velocity stars? (Obviously they would have to take that interpretation on faith, since it could never be confirmed, and would probably always be seen as a theory 'the alleged other galaxies.')

>>Which makes you wonder what we're missing already
Well, if it's the case that spacetime is infinite in extent (which, we have no reason to assume it's not, and the heterogeneity principle suggests it is) then there is a hubble volume somewhere out there in the universe if you travelled long enough that contains every different possible configuration of spacetime for that given volume (in every direction you would care to point into in the sky, as bizarre as it seems, thanks to the unique properties of infinity and probability.)

Which means that, although an infinitesimally small percentage of all the other things that have passed beyond our light horizon -- which would logically be mostly composed of every possible configuration of matter for our physical universe -- there would also be an endless number of Us-es, hubble volumes that, just thanks to statistics, are exactly like our own, with other entities exactly like us, just by chance, some further along, and some further behind. We have missed our own destinies, slipped beyond our light horizon (or, more accurately, probably so far away to begin with their light would never even reach our starting position within the lifetime of the universe, nevermind the redshift.)
Irwin Shapiro - Fri, 19 May 2017 23:22:57 EST ID:yzfSDg8q No.56944 Ignore Report Quick Reply
> it's really going to suck when we colonize space

>By the time the two sides met, the far travelers would seem like the ancient primordial civilization to the near colonizers, whereas they would seem to be unfathomably alien and strange to the far travelers. Maybe life just isn't meant to be able to scale to the problems relativity brings about?

I wouldn't worry about it, that's just the natural way of things. Since the far travelers could colonize billions of times the amount of space, they would be the ones spreading across the universe. The close travelers might seem like the primitive culture then, for not being able to grasp the technology. Maybe they could avoid their own destruction long enough to detect the first signals of the far travelers... Can you imagine being in their shoes? Reaching out of their tiny bubble at some point in the distant future and waking up to a universe that's suddenly becoming colonized at the speed of light? They would have no hopes of catching up to the far travelers, unless they develop the technology to travel even faster and contain them somehow. Then they would become the far travelers. Life would get along fine either way, because the universe got colonized.
William Lassell - Fri, 23 Jun 2017 17:18:20 EST ID:iClpwVzv No.56959 Ignore Report Quick Reply
Well, this one's related to the OP.
Even if you have tow frames of reference tat are moving relative to each other, the sped of light (any light) is equal in both frames of reference.
In other words, here on Earth, light moves in all directions at the same speed, even though we're moving around the sun at 108,000 km/h.
But light also moves in all directions at the same speed in a frame of reference having no motion relative to the sun.
This seems really weird, but as Einstein pointed out: the alternative is that one frame of reference is magically better than all the other frames of reference.
Carl Seyfert - Sun, 25 Jun 2017 15:00:13 EST ID:oJOQRPZS No.56963 Ignore Report Quick Reply
I don't need no Einstein to point out that the only one true frame of reference is mine.
Henry Draper - Thu, 29 Jun 2017 01:41:44 EST ID:kjgELPni No.56969 Ignore Report Quick Reply
Then why hadn't anyone figured it out until Einstein came along? The Theory of Relativity was one of the guiding influences on the conceptual development of social and cultural relativity, which you are (humorously) referencing. Don't pretend that you would have invented the concepts of perspective and subjectivity, which took millennia for humanity to uncover, all on your own.
Cecelia Payne-Gaposchkin - Fri, 30 Jun 2017 02:40:33 EST ID:a3KY2hIa No.56970 Ignore Report Quick Reply
Multiple scientists, most notably Poincaré and Lorentz, worked in the last decades of the 19th century on the mathematical framework that would eventually be used in the theory of Special Relativity. Before Einstein, everyone just assumed that both the aether and a real "universal time" existed, and suggesting they didn't was a counterintuitive, radical development. Einstein also showed how the previously derived mathematics followed from his simple axioms.
Robert Dicke - Mon, 03 Jul 2017 08:44:19 EST ID:VlPllY39 No.56972 Ignore Report Quick Reply
why is the speed of light 299,792,458m/s? why can't it be faster? what if it is faster somewhere the gravity has a different kind of an effect?
Edward Pickering - Thu, 06 Jul 2017 16:09:41 EST ID:unNII3om No.56974 Ignore Report Quick Reply

If I understand the theory correctly, the speed of light is what it is because that's the constant "speed limit" of the universe. Light itself can travel slower if it's in a medium where it can interact with matter, which is why you get refraction in water for example, but once in a vacuum it travels as fast as it can.
Joseph Taylor Jr. - Sun, 09 Jul 2017 11:58:05 EST ID:unNII3om No.56976 Ignore Report Quick Reply

To explain a bit further: Light is mass-less, and from this we can 'assume' that its velocity in a vacuum should be infinite. This of course is impossible, so its speed is exactly the maximum speed possible. The reason why this limit exists and why it's a constant and not relative itself is because it's deeply tied into the general laws of physics, which we assume at least are universal everywhere.
Maximilian Wolf - Sat, 29 Jul 2017 13:55:31 EST ID:+G8ef2Iy No.56989 Ignore Report Quick Reply
Light isn't really devoid of mass, it simply has no rest mass. Using the concept of mass you are is a bit of an outdated/antiquated concept. As it relates most specifically to this situation, the difference between a particle or an object's mass and rest mass is that it takes into account what we learned about mass and energy through the lens of Special Relativity. Things that are accelerating (acceleration is an important distinction to make here because in space it's not really possible to establish fixed frames of reference and whatnot) have more mass than things that are not accelerating. Until the acceleration approaches velocities very near the speed of light, however, the difference in mass is pretty much infinitesimal and therefore pretty much negligible.

The guy that responded to you is right, light does travel at velocities less than what the speed of light is measured at in a true vacuum. However, most of known the observable universe where otherwise nothing appears to exist is actually a false vacuum. A true vacuum is characterized by the lowest possible energy state, but a false vacuum just happens to be the local minimum (keeping in mind local can encompass some pretty vast sectors of space). If I remember correctly, in areas that there appears to be nothing and you'd expect it to be vacuum, there are actually photons left over from the Big Bang (hence being able to measure the Cosmic Microwave Background Radiation).

As for why the speed limit couldn't be higher... You just have to think about it for a minute. All measurements, whether they're of speed/velocity, distance, or what have you, are relative to other measurements. That's why we establish a basic unit that we can more or less view as arbitrary that we can use to orient ourselves and make sense out of the measurements we make. The speed of light just is what it is. What numbers and measurements we assign we to it are entirely for our benefit and as long as you're decent at math it's pretty easy to manipulate things so that the numbers could be higher or whatever.

But, what you meant more specifically is why the speed of light not able to be faster, not why our numbers representing it can't be higher (although I believe to some extent the impetus for your asking was caused by seeing it represented as a numerical value). It's kind of like asking why couldn't someone else have been born instead of you or why didn't the universe have slightly different laws which may have made our existence a total impossibility. These things are just observable facts and we've hit the limit of what science can do to help us understand things. Science only helps us figure out how, not really why.

Truthfully though we could represent the speed of light however we wanted, all we would have to really keep in mind is the fact that we can actually represent it and get meaningful results from mathetmatical equations we formulate around it is that it's finite. Which, that just makes sense really--it essentially acts as a speed limit for the universe and you can't very well have something be infinite if it has limits. All that's really important regaarding it is that it is what it is and the the velocities everything else travels at is all relative to that. It's essentially the universe's established standard unit by which everything else works in relation to.
William Lassell - Wed, 02 Aug 2017 17:06:58 EST ID:uSMmLmww No.56992 Ignore Report Quick Reply
i am going to think about your words, max. ed's and joseph's too, thanks fellas.
Harlow Shapley - Wed, 02 Aug 2017 20:26:06 EST ID:d6k0JkjC No.56993 Ignore Report Quick Reply
TLDR: Because light is a self-propagating electro-magnetic wave per Maxel's Equations.

William Hartmann - Mon, 11 Sep 2017 13:15:57 EST ID:kRyBQtrI No.57015 Ignore Report Quick Reply
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But you're helpful otherwise.

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