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Gamma-ray Bursts May Explain Fermi's Paradox

stevelinton Re:14 billion years seems very short to me. (235 comments)

I seem to recall third, but the stars that make and scatter medium-weight elements are big bright short-lived ones, so the first generation might only have taken 10 million years. There is some uncertainty about where the heavier elements (gold, uranium, etc.) come from. It is possible they are produced by a much rarer process.

2 days ago
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Gamma-ray Bursts May Explain Fermi's Paradox

stevelinton Re: Not really. (235 comments)

You don't get relatavistic velocities with fission or fusion propulsion at reasonable mass ratios. You need antimatter for that.

2 days ago
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How Do We Know the Timeline of the Universe?

stevelinton Re:We Really Don't (152 comments)

You talk about ""science" --- the one with hypothesis, testing, reproduction of results". These things do kind of apply to cosmology. Hypothese are about things like the statistical distribution of galaxy sizes and redshifts, or the exact spectrum of the cosmic microwave background or the proportions of elements in the oldest stars or ... The speculators are working out these prediction so that the observational astronomers can test them with their next set of instruments. Or in some of the other areas, about what we will see in the LHC when we reproduce on a very small scale certain conditions.

Reproduction of results is harder, because we only have one universe, but people only become convinced of an explanation when there are multiple chains of evidence supporting it. So dark matter is supported by galaxy rotation, features of the cosmic microwave background spectra, gravitational lensing AND siumulations of galaxy distribution.

3 days ago
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Astronomers Record Mystery Radio Signals From 5.5 Billion Light Years Away

stevelinton Re:Boom. Boom. Boom. Another one bite's the dust.. (121 comments)

creating a spark that lasts seconds and outputs more energy than the sun has in the past million years.

Actually it lasts only about a millisecond, but the 1 MYears of solar output part is right. It's about the mass of the moon converted to RF energy in
1 ms.

about two weeks ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

Constant approaching velocity is special relativity again, and again the velocities don't add the way you expect.If the planets in your example are approaching at 2/3 c they each see the other approaching at 12/13 c and they will very definitely and messily interact. Each exists for the other.

In this case acceleration makes no essential difference though. In either planets frame of reference there is an event horizon behind it (in GR acceleration and gravity are equivalent) but none in front of it, so they can see each other and interact freely.

about three weeks ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

If you're dealing with constant velocities, you are in the territory of special relativity. In this world there are no event horizons and every object can interact with every other. If two galaxies are each receding in opposite directions from a third central one at 2/3 c they will each see the other receding at 12/13 c according to https://en.wikipedia.org/wiki/... (section 2). Velocities do not add up the way you think they do and when they get to a decent proportion of light speed it starts to matter. This has been experimentally checked using moving atomic clocks. Thus they can keep on exchanging messages, although the messages will be quite redshifted when they arrive and take longer and longer to make the journey.

However, the original article deals with accelerating motions, since that is what the universe seems to be doing. This is crucial.

One way of seeing what happens is to imagine two galaxies accelerating away from one another. Assume there are clocks freely falling in both galaxies.
Define a function f so that a signal sent from one galaxy at lightspeed (could be photons, gravitons, neutrinos, doesn't matter) at time t on the local clock arrives at the other galaxy at time f(t) on its local clock. It's not hard (for anyone with a degree in astrophysics) to work out exactly what function f is. It turns out that there is critical time T such that as t approaches t from below, f(t) approaches positive infinity. In other words the last few signals emitted by one galaxy as it's clock ticks towards T are spread out across the whole of the rest of time when they finally catch the other galaxy and no signal emitted at or after time T can ever arrive. The critical time T depends on the current separation, velocity and acceleration of the galaxies in a fairly straightforward way. After local time T nothing you do can affect the other galaxy. After its time T you can never find out what happened to it.

about a month ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

Could do, although there is no evidence of such an effect up to now. The laws of physics could also just change tomorrow for no particular reason, in thi sarea, or in some much more down-to-earth one, like whether the proton is stable. We can never know.

The article is essentially in the business of explaining the consequences of the laws as we currently conjecture them to be (which fit what we can observe pretty well). It can't make any stronger claim to be "correct" than that, but, apart from refining "pretty well" to "very well" nor can any physical theory.

about a month ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

To get very far away from us they started receding from us at a higher speed than objects that are closer. However, nobody can point to where an object "disappeared" - it's all conjecture unsupported by experiment or direct observation. Who knows, maybe when the fabric of the universe gets too thin, the repulsive force becomes an attractive force. We simply don't know enough yet.

Of course. Anything could happen, but there is a remarkably consistent, and mathematically simple, if somewhat unintuitive picture emerging of how the universe has evolved on the largest scales. The picture in general (dark matter, dark energy, etc,) is consistent with a number of independent sets of data, for example supernova surveys and detailed analysis of the cosmic microwave background. The article is trying to explain the consequences of this picture.

What we can see are galaxies at very high redshifts and evidence for accelerating expansion. If the dark energy explanation for the expansion is right, then lighjt emitted from those galaxies (which we can see) a few billion years after the light we see them by now, will never reach us. Of course some unknown thing could intervene to prevent this happening, but we see no sign of such a thing yet.

about a month ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.

Objects don't have to be gravitationally bound to influence each other. A rogue plantoid passing through our system isn't gravitationally bound to it, but our gravity still can modify its path.

You're right, but you've misunderstood my point. If A, B and C are all "far" apart then all the distances are increasing at an accelerating velocity and the situation is as I described it. The last paragraph deals with the special case where B and C are close enough that they are not accelerating apart. In this case B and C will remain in contact forever, and so A will lose touch with both of them at the same time.

about a month ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

Doesn't work. If you try and relay light (or any other message) along the line from the distant galaxy to us, what happens is that it reaches each relay station just as the relay station loses contact with us. It never arrives.

about a month ago
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How Galaxies Are Disappearing From Our Universe

stevelinton Re:How galaxies are disappearing from our universe (174 comments)

Can there still be interaction between the galaxy that just disappeared, and a galaxy mid-way between us? Yes.

Can there still be interaction between the middling galaxy and us? Yes..

Both true, but these interactions don't combine. Suppose you have three galaxies in a line A--- B---C and A and C are just leaving causal contact.
Suppose a light-speed message is sent from A towards B and C. B will indeed receive it, and be able to reply to it (maybe) but that will happen just as B and C leave causal contact (the universe having carried on expanding), so that if that message is forwarded towards C it will still not arrive. The photons in the forwarded message cannot overtake those in the original message that are still flying from B towards C.

If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.

about a month ago
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What Language Will the World Speak In 2115?

stevelinton Everyone can speak their own language (578 comments)

Real-time audio translation is just taking off. By 2115 everyone should be able to speak and hear others speak in whatever language they like, including perhaps one that they and their personal AI made up as they were growing up.

about a month ago
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2014: Hottest Year On Record

stevelinton Re: noooo (560 comments)

As I said in my first comment, I'm just curious why some data is used, and other data is ignored.

As to that last comment...in general, if there's adjustment happening to something like research data, it has to be disclosed in findings reports. Eg, "I adjusted the weight readings by 10.5 grams, because I forgot to tare my scale with the crucible on it". I don't think that all unadjusted data is better. I think that if data is adjusted, the reasons for doing so and the method should also be fully disclosed.

If you go back to the original papers you will (by and large) find all the disclosure you want. The problem is that that mans reading and understanding tens of thousands of pages of complex mathematical arguments to see how each aspect of the data analysis was arrived, at, validated, etc. You are (probably) reading summaries of summaries of summaries of review articles of the papers. Complete disclosure there would make the documents thousands of times longer and completely unreadable.

about a month ago
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2014: Hottest Year On Record

stevelinton Re: noooo (560 comments)

On current trends solar and wind are set to hit that goal within a decade or so. There are some interest engineering problems around storage/demand management and power transmission, but the trend lines look quite good. Especially if you enforce even reasonable local environmental standards on mining and burning coal.

about a month ago
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2014: Hottest Year On Record

stevelinton Re:noooo (560 comments)

Just about all European players have or do reprocess -- France at Cap de la Hague, the UK at Sellafield. It turns out to be
remarkably messy, difficult and expensive, and very prone to radiation leaks of one kind or another. It was only really economic when there was a military market for the plutonium at basically "any price".

The problems are not fundamental, they are all engineering, but there were lots of them, and they never really stopped. You're working with something that has a horribly mixed chemical composition, and was designed (as a fuel element) to be tough enough to survive inside a reactor for a few years. You have to dissove everything in loads of hot concentrated nitric acid just to get started, so now you've got industriial quantities of hot radioactive acid laced with a not exactly known mixture of salts, plus insoluble sludge of one kind or another gunging everything up. And you can't ever go into the plant to unjam a conveyer, clear a stuck valve or clean a filter.

about a month ago
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Pope Francis To Issue Encyclical On Global Warming

stevelinton Re:Doesn't matter (341 comments)

A modern wind turbine in typical European conditions generates enough energy to "repay" the costs of building and installing it in about six months. http://www.theguardian.com/env...

Backup can mostly be other renewable sources (solar, hydro, biomass) demand management and storage (pumped water at the moment). For the rare but real occasions when none of this covers the need, cheap gas turbines designed for a low duty cycles seem like the best option.

about a month ago
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SKA Telescope To Offer Neighbors Cheap Broadband

stevelinton Re:Fail (63 comments)

The telescope when finished (2025) will need more total bandwidth between its antennae than the entire remainder of the internet is projected to need at that time.

It will be dedicated fibre, about 50 000 km of it.

What we're talking about here is connecting the (very few) isolated farms and villages within one or two hundred miles of an antenna. With a population that
distributed it isn't a last mile issue it's a last hundred miles issue.

about 2 months ago
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SKA Telescope To Offer Neighbors Cheap Broadband

stevelinton Re:It's not long-term cheaper to trench? (63 comments)

The population is VERY spread out -- that's why they put the telescopes here in first place. Think rural Nevada, then take 90% of the people away.

about 2 months ago

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