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Isotropy of the universe is the idea that the universe is the same in all directions, and is a founding principle of the laws of physics. However, that principle has been called into question by observations of galaxy cluster temperatures and luminosities.
https://scitechdaily.com/fundamental-principle-of-cosmology-cast-in-doubt-by-compelling-new-study/
"No matter where we look, the same rules apply everywhere in space: countless calculations of astrophysics are based on this basic principle. A recent study by the Universities of Bonn and Harvard, however, has thrown this principle into question. Should the measured values be confirmed, this would toss many assumptions about the properties of the universe overboard. The results are published in the journal Astronomy & Astrophysics, but are already available online."
The paper (open access):
https://www.aanda.org/articles/aa/abs/2020/04/aa36602-19/aa36602-19.html
Journal Reference (open access):
K. Migkas, G. Schellenberger, T. H. Reiprich, F. Pacaud, M. E. Ramos-Ceja and L. Lovisari. Probing cosmic isotropy with a new X-ray galaxy cluster sample through the L X – T scaling relation , 8 April 2020, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/201936602
(Score: 5, Interesting) by Anonymous Coward on Friday April 10 2020, @03:36PM (14 children)
"FTL" is already allowed, kind of. Basically when you accelerate towards the speed of light there's not any magical force that suddenly makes further acceleration, from your perspective, impossible. Instead the whole universe just gets really messed up. Distances begin to physically contract and time itself begins to accelerate for everything that's at rest (relative to you).
Because of this it's possible for a single human to easily travel billions of light years in a single lifetime. The catch is that when you slowdown - billions of years will actually have passed for everything that was at rest, relative to you.
So this [convertalot.com] site has a calculator for a 'relativistic starship.' If we could create a starship capable of accelerating at 1g for lengthy periods of times you can achieve unimaginable things. KIC 8462852 - that weird ass dimming sta is in Sygnus, about 1,470 light years away from us. Our relativistic starship could actually get there in about 14.2 years, which includes 7.1 years of accelerating at 1G and then 7.1 years of braking at 1G - to get there at rest.
But the exponential nature of this is the really cool thing. Andromeda is 2 million light years away. Our 1G relativistic starship could take us there in 28 years. That means you could, potentially leave as a 20 year old - and get back to Earth as a 76 year old - a round trip all easily within a human lifetime. But the catch is 4 million years would have passed on Earth by the time you got back. Hahah, in space travel of the future - 'where am I' would not be the question, but rather 'when am I'?
The really crazy thing is that all of this is really precipitated on nothing but building an engine that can accelerate at 1G for years at a time. A huge undertaking but one that somehow does not feel impossible. Yet if achieved it would completely reshape our entire species. Our notion of the most fundamental thing imaginable to most - time itself, would no longer have a clear meaning. You'd have the rich accelerate themselves to the future that, paradoxically, we in the world they leave behind would be building. Speaking of paradoxes there are also really crazy things possible here. Imagine you accelerate off towards Andromeda and somehow you find a habitable planet. You might end up landing on the planet only to find a sophisticated human colony founded hundreds of thousands of years ago. Turns out thousands of years after you left we developed a new technology to enable 2G travel, and the crew that left thousands of years after you did - got there hundreds of thousands of years before you. Fun stuff!
(Score: 4, Interesting) by Immerman on Friday April 10 2020, @05:29PM (11 children)
That's not remotely FTL though. Yes, if you could consume the entire energy output of the galaxy to acceleration, you could flit around the galaxy at will within a single human lifetime - even the entire output from a star would do a decent job - but most of the visible universe would still be theoretically unreachable. And of course, from your perspective you'd be radically accelerating the aging of the universe. Jaunting to the other side of the galaxy and back would age your homeworld by at least 200,000 years, and doing so at 1G would still take a couple decades from your perspective.
Not to mention the only way a sustained 1G acceleration is remotely plausible is with a reactionless drive, which would break one of the most fundamental laws of physics. Any sort of propellant or consumable power supply, no matter how efficient, invokes the tyranny of the rocket equation, and severely limits how long you can sustain acceleration. Even a ramjet powered by antimatter would have a hard time keeping up that acceleration for long (a fusion ramjet is a beautiful idea, but I've heard the math doesn't actually work out as a viable means of acceleration)
On the other hand there's not actually any theoretical prohibition on FTL - just FTL *through space*. Wormholes, warp drives, etc. are all theoretically possible - they just make time travel possible as well. And we have no theoretical reason to prohibit time travel.
(Score: 2) by takyon on Friday April 10 2020, @06:05PM (4 children)
AFAIK usage of a warp drive (Alcubierre drive [wikipedia.org]) would not show symptoms of time dilation. The apparent travel time would be equivalent to the time that passes for observers at the origin and destination.
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(Score: 2) by Immerman on Friday April 10 2020, @07:19PM (2 children)
Right - FTL doesn't have those problems. My point was thateven in AC's magical universe where indefinite 1G acceleration can be sustained, it's still not remotely FTL.
(Score: 2) by takyon on Friday April 10 2020, @07:44PM (1 child)
Well, you said:
"Wormholes, warp drives, etc. are all theoretically possible - they just make time travel possible as well."
I don't think an Alcubierre drive would enable meaningful time travel or backwards time travel (there is a section on the Wikipedia article discussing it). And zipping between Earth and Proxima Centauri without time dilation effects doesn't violate causality or anything.
Obviously, warp drives and wormholes may be impossible concepts. Even traveling at relativistic speeds is in doubt, since stray particles could destroy the ship. If we do create a form of FTL, it would be an exciting development, and could indicate that aliens have/are visiting Earth because it is just 2 EZ. Maybe the same could be true for reliable relativistic/0.999c travel.
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(Score: 3, Interesting) by Immerman on Friday April 10 2020, @10:26PM
>Zipping between Earth and Proxima Centauri without time dilation effects doesn't violate causality or anything.
Not obviously, no. But the speed of light is more properly termed the speed of causality, at which *every* force propagates. According to Relativity, the ability to travel even slightly faster than light, can be harnessed under carefully constructed circumstances to make a round trip that will see you arriving back at Earth before you left. It's worth reading about how it's done - it's nothing super exotic as I recall, just sending an FTL signal between three different reference frames so that the returns back to the origin before it left, and of course with an FTL ship, you are the signal.
(Score: 3, Insightful) by hendrikboom on Saturday April 11 2020, @04:23PM
The only "practical" FTL drives I've seen require large amounts of matter of negative mass.
Since FTL is equivalent to destroying causality I suspect there can be no negative-mass matter.
(Score: 0) by Anonymous Coward on Friday April 10 2020, @08:26PM (5 children)
You would not need much energy to maintain 1g acceleration. Your common rocket generates way more than 1G of thrust (or it wouldn't get off the ground), and can generally maintain this for a matter of minutes. Multiplying that energy on the order of thousands to millions is not much. I think you might be mixing up things like light powered propulsion - one of the fun fantasy explanations for fast radio bursts - where you could potentially generate thrust using an EM burst. With a powerful enough source, you could accelerate something to near relativistic rates without even bother with fuel on board the ship. FRB's come from a source that generates as much power in a fraction of a second as our sun will generate over thousands of years.
The real problem is what you hit on - the tyranny of the rocket equation. Your speed is determined by how much stuff you expel. So you need to carry a *lot* of stuff to have stuff to expel for years. Yet, the more stuff you carry - the more you you need to expel to get the same amount of acceleration. And then the bigger you get the bigger of engines you need to even be able to move at all, yet those bigger engines in turn add even more mass, and so on. In general these ideas would not be possible without a technological revolution. But I would not discount that possibility. Until we discovered nukes nobody would have ever thought something the size of a table could create enough power to obliterate a city. That idea is, intuitively, absurd - yet of course it's very real. It's interesting to consider how absurd everything is before it's discovered, and then becomes obvious. Imagine going back some time in the past and suggesting that a pinch of charcoal, sulfur, and saltpeter would be be able to generate enough power to put a small rock through a human skull from hundreds of meters away, with centimeter accuracy. Again, it's intuitively absurd, and again it's completely real.
Our universe, without doubt, has many more secrets awaiting their discovery. It's important to always stay grounded, but it's perhaps even more important to also never succumb to the arrogance of the present. Who knows when the next discovery awaits? Maybe thousands of years from now, maybe tomorrow.
(Score: 3, Interesting) by Immerman on Friday April 10 2020, @11:15PM (4 children)
>Multiplying that energy on the order of thousands to millions is not much.
True, in principle, with optimism. Sadly, thanks to the rocket equation that only increases your top speed by maybe 3-6x. The energy demands for a given amount of delta-V increase exponentially. Or alternately, the speed you can generate increases logarithmically with the amount of energy you use.
Just getting from surface to orbit involves over an order of magnitudes more propellant than payload (yeah, much is wasted fighting gravity, but as it turns out that doesn't actually matter much in the end): lets say mass of the entire stage 1 rocket (S) = 10x mass of payload(10P). To get double that delta V, you essentially need to boost an equivalent fully-fueled rocket to the speed that just your payload was gong the first time. And the same propellant-to-payload ratio applies for the bigger rocket, so mass of 2 stages (2x speed) = 10x Mass of 1 stage = 10^2x the mass of the payload. Going 3x as fast means adding another stage, so you're at 10^3x the payload mass. If you wanted to travel at near light speed if relativity weren't a thing - in practice close enough that time dilation starts to climb into maybe the single-digit multipliers, you need 27,000x as much delta-v, which means 10^27,000x as much propellant. Ion drives improve things immensely - but in the face of a 10^27k multiplier, even a few orders of magnitude improvement in delta-V per kg of propellant doesn't make things feasible. Our sun is only 2x10^30kg, The entire Milky Way galaxy, core and dark matter included, is only around 10^42kg. The entire visible universe only around 10^53kg.
(Score: 0) by Anonymous Coward on Saturday April 11 2020, @06:16AM (3 children)
Ah, you were going that route. Yes of course that would not work. It would be analogous to building a weapon of atomic scale destruction with gunpowder. I am speaking in terms of the most basic amount of energy needed to generate 1G of thrust for 'x tons' for one second * [however many years]. How we generate that energy within the realm of possibility is something for future generations to discover, yet again it does not 'seem' as though it ought be anything remotely like impossible.
(Score: 2) by Immerman on Saturday April 11 2020, @03:10PM (2 children)
You're absolutely right - and if I recall correctly in those terms it takes the energy of a few gallons of gas to get a person into low orbit.
Unfortunately that amount of energy is only really relevant if you're using a reactionless drive (or a space elevator). As soon as you use an engine that needs propellant ( = doesn't violate conservation of momentum as we currently understand it), then the energy required to accelerate the payload will become an infinitesimal fraction of the energy required to accelerate all the propellant required to accelerate the payload near the end of its journey.
Even with a reactionless drive you don't escape the rocket equation entirely, because you still need to take your energy with you, and the energy that you use to accelerate on the last day has mass that needs to be accelerated until then. Works out much better over short ranges at relatively low speed, but the closer you want to get to light speed, the uglier it gets.
On the other hand, there's not necessarily any reason to travel super close to light speed. Traveling 99% the speed of light instead of 90% will only get you to your destination 10% faster from their perspective, while requiring vastly more energy. You're really just doing it for the relativistic time dilation, and there may be far more energy-efficient ways to slow the passage of time.
Though hmm... I think the infeasibility of a ramscoop was determined using "reasonable assumptions". I wonder what the numbers would look like on a ramscoop ship with a hydrogen-propellant ion drive powered by a hydrogen mass-energy converter (small black holes?). There might be some wiggle room with sufficiently advanced technology.
It's still not FTL though. Crossing the galaxy and returning may only seem like it takes 12 years each way to you (I think that's what 1g gets you), but 200,000 years will have passed back home. Not only will every person you knew be gone, but so will every civilization - hundreds will likely rise and fall before you return, and the human species will likely have changed immensely, if we still exist at all. And for shorter trips between nearby stars? 1g won't get you going fast enough for dramatic time dilation effects - without relativity 1g takes about a year to get you to light speed, relativity means you're just getting into the range where time dilation starts becoming obvious. So two years of nearly-normal timeflow for each end of the trip, with additional years seeing increasing time dilation the closer they get to the middle of the trip.
(Score: 0) by Anonymous Coward on Saturday April 11 2020, @07:59PM (1 child)
The only thing I'd hit on here is that I think you're taking the present state of society and positing it onto a post-relativity world (for lack of a better term here).
After we can reach relativistic velocities, the entire nature of civilizations will change. In particular it won't be you out there alone traversing the stars at relativistic rates, there will be what will likely be an unimaginably large number of people doing the same. And it will be people from countless eons of time. I do think time will start to become much more akin to distance because of this. When you meet another traveler, one thing you'd likely want to know - if out of nothing other than curiosity - is when they are from.
The point of this is that it'd be safe to start to expect civilizations to begin to persist, more or less, indefinitely. One-off cataclysmic events would no longer matter as we'll be spread throughout the entire traversable universe at through eons of time. When you reach a post-relativistic society, the entire nature of existence would be so very different.
No, the really weird stuff would be things like evolution. While the exact time is in dispute, it's likely we only evolved the ability to speak with one another about 40,000 years ago. Learning to speak to putting a man on the moon in 40k years? What might we evolve towards in the future? And perhaps it won't even be evolution (in the colloquial sense), and instead you might find humanity has devolved into barely sentient blogs indulging without end in in some sort of ancient post-scarcity technology.
There's so much room for more reality based sci-fi because, as usual, reality is far stranger than any fiction we could ever imagine!
(Score: 2) by Immerman on Saturday April 11 2020, @10:43PM
I seriously doubt it. Yes, people may be traveling around - but only a tiny proportion of them, the energy costs per kg are just too high. And the opportunity costs even higher - if it takes you 1000 years to get from A to B, it's very unlikely your knowledge or wealth will be relevant when you arrive at B. You'll be a thousand-year-old relic with little of value to contribute to a society that's had over 30 generations worth of advancements since you left. What would someone from 1000 years ago have to offer modern society? Do you really think bank accounts would just sit around accumulating interest for that long? Or keep track of your balance at all? And how would social stability improve by a few people skipping across history? Stability is based on the people actually living life within society, not those who remove themselves from it for centuries at a time.
(Score: 0) by Anonymous Coward on Friday April 10 2020, @09:29PM
Well, you build your 1G starship, get up to ludicrous speed, and die from X- and gamma-ray exposure as you've blue-shifted up all of that 3K microwave background to lethal levels.
(Score: 0) by Anonymous Coward on Monday April 13 2020, @12:21PM
I like the line in the opening of Brian Greene's book "The Elegant Universe," where he says that Physicists don't actually have a good understanding of what time IS. They know how things change relative to it, they know how to measure it extremely well, but to define it... well they haven't done a great job. So, he says, for this discussion, we will define time as that which is measured by clocks.