Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!


Forgot your password?

Slashdot videos: Now with more Slashdot!

  • View

  • Discuss

  • Share

We've improved Slashdot's video section; now you can view our video interviews, product close-ups and site visits with all the usual Slashdot options to comment, share, etc. No more walled garden! It's a work in progress -- we hope you'll check it out (Learn more about the recent updates).


Comment: Re:Help me out here (Score 5, Informative) 88

by stevelinton (#49155623) Attached to: Mysterious Siberian Crater Is Just One of Many

I think the idea is this:
You have a large volume of clathrates underneath ice or frozen soil.
As things warm, they start to break down and a reservoir of methane gas builds up
at high pressure.
Eventually the pressure reaches the point where it can push aside or lift up or whatever the ice at its weakest point
and it finds a route to the surface.

Now you have a LOT of gas rushing through some kind of hole, a little bit like an oil well blowout and the gas flow erodes the sides of the hole and throws soil or ice into the air and generally starts to make a crater.

Furthermore the escape of all this gas lowers the pressure down where the clathrates are quite suddenly, so the breakdown accelerated greatly, providing still more gas to ruch up through the hole.

So not really an explosion, perhaps more like a blowout, but still fairly violent simply because of the amount of gas and the pressure.

At no point does it combust.

Comment: Re:Help me out here (Score 5, Informative) 88

by stevelinton (#49155269) Attached to: Mysterious Siberian Crater Is Just One of Many

You need to find out about methane clathrates. They are very roughly a chemical compound of methane and water which is solid and stable at low temperatures and moderately high pressures (as found under a few hundred meters of water or ice, for instance. When they get a bit too warm, or the pressure drops a bit they turn back into methane gas and water. One cubic meter of clathrate released almost 200 cubic meters of methane gas, which then has to go somewhere producing something like an explosion. At no point did the methane burn (it was nowhere near any free oxygen until it got to the surface, it was just a gas pressure explosion.

Comment: Re:Jump That Gun (Score 3, Informative) 102

by stevelinton (#49083301) Attached to: Supermassive Diet: Black Holes Bulk-Up On Dark Matter

Know is a tricky word, but there is plenty of evidence that most of the dark matter is not baryonic. The proportions of light elements formed at the end of the big bang gives a contstaint on the baryon density of the universe at the time, as do the ripples in the cosmic microwave background (which reveal the balance between radiation pressure and gravity in the early universe and tell us that most of the mass did not interact with photons at all). The bullet cluster is another piece of evidence. The stars in the colliding galaxies interacted with one another and with dust and merged into one bigger galaxy, but something, detectable by its gravitational lensing of galaxies beyond it, went straight through. It's hard to see how brown dwarves would have done that.

Comment: Re:Solution to Global Warming (Score 2) 75

As I recall the best choice for a low-mass sunshield is a grid of fine conductive wires about 100nm or so apart in both directions. This forms a Faraday cage at optical frequencies. There's a complicated tradeoff between what you make the wires out of, how much is reflected and how much absorbed and how fine you can make the wires without them melting. I'm not sure what the winner is for this application, but the area density of such a material can be less than that of a carbon monolayer, since it's mostly holes, just holes too small for light to get through.

You can probably keep it on station without rockets by opening and closing flaps in the sail to manage light and solar wind pressure, although the control processing might be pretty severe.

You can use similar techniques to terraform Venus and Mars -- for Mars you make the "shield" into a Fresnel lens that actually concentrates sunlight.

Comment: Re:14 billion years seems very short to me. (Score 1) 237

by stevelinton (#48921975) Attached to: Gamma-ray Bursts May Explain Fermi's Paradox

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.

Comment: Re:We Really Don't (Score 2) 153

by stevelinton (#48902923) Attached to: How Do We Know the Timeline of the Universe?

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.

Comment: Re:Boom. Boom. Boom. Another one bite's the dust.. (Score 1) 121

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.

Comment: Re:How galaxies are disappearing from our universe (Score 1) 174

by stevelinton (#48729399) Attached to: How Galaxies Are Disappearing From Our Universe

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.

Comment: Re:How galaxies are disappearing from our universe (Score 1) 174

by stevelinton (#48727723) Attached to: How Galaxies Are Disappearing From Our Universe

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.

Comment: Re:How galaxies are disappearing from our universe (Score 1) 174

by stevelinton (#48727649) Attached to: How Galaxies Are Disappearing From Our Universe

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.

Comment: Re:How galaxies are disappearing from our universe (Score 1) 174

by stevelinton (#48726965) Attached to: How Galaxies Are Disappearing From Our Universe

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.

Genius is ten percent inspiration and fifty percent capital gains.