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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: 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.