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Comment: Re:We already got Blender? (Score 5, Informative) 198 198

by hyperfine transition (#49326007) Attached to: Pixar Releases Free Version of RenderMan

And back in the 1990's we had BMRT (a free renderman clone); until they came and paid/threatened the guy to stop making the free clone available.

Sorta. Larry Gritz, the author of BMRT, went to work for Pixar and then left to start his own company, Exluna, whose main product was a Renderman competitor called Entropy. Unfortunately Pixar's lawyers jumped on Exluna and Exluna was vaporised. BMRT and Entropy were no longer available after this. Larry Gritz went to work for Nvidia after that on a GPU-accelerated renderer, I think.

Comment: Re:Of course NIST would say that! (Score 1) 166 166

They are the official timekeepers for the US, along with the U.S. Naval Observatory (which also operates the timescale that GPS satellite clocks are steered to) but they are not timekeepers for the world.

The international standard for time is UTC, a 'paper' clock which is the average of atomic clocks from all around the world.

Comment: Re:You're doing it wrong. (Score 1) 166 166

Standard OS clocks only tick at about 100hz, so you're always out by an average of 5ms anyways.

Nope. Although the system interrupt is only between a few hundred Hz and a kHz, other, faster counters are used to interpolate between these ticks. So on Linux, eg the Time Stamp Counter in the CPU can be used to improve the timestamp resolution to a microsecond, or even nanoseconds, with the nanokernel patch (which is standard in the BSDs, I think).

Comment: Re:I have two problems with this article. (Score 1) 287 287

by hyperfine transition (#49253833) Attached to: NTP's Fate Hinges On "Father Time"

In my experience of operating a network of geographically-dispersed stratum 1 NTP servers, there is frequently asymmetry at the 1 to 2 s level and occasionally worse. An NTP implementation like ntpd filters out these outliers but the simple protocol you are suggesting would not.

PTP cannot account for network asymmetries any more than NTP can. It can only guarantee symmetric paths when all the hardware between two endpoints is PTP-capable, meaning that each boundary device has to implement a PTP clock.

In the end, it seems silly not to synchronise device clocks to a universal reference . There are many local applications which require a timestamp that must be compared with a time stamp on another device at some time down the line.

Comment: Re:Where is the center? (Score 1) 174 174

by hyperfine transition (#48729369) Attached to: How Galaxies Are Disappearing From Our Universe

I just read a SF story that used an idea like this - that analogous to the time dilation experienced as you fell into a 'normal' black hole, you would see spatial dilation as you fell into a black hole 'in time'. the idea that the universe is inside a black hole has been bandied about but I can't find a reference for the 'temporal black hole' idea ( I feel sure that the story I read was based on a scientific paper).

Comment: Some more plasma physics is needed in their model (Score 4, Informative) 179 179

The paper is a one pager of introductory plasma physics. It isn't a serious calculation and it wasn't meant to be. Anyway ...

Their model is as follows. A plasma will reflect all electromagnetic radiation below a certain frequency, determined by its density. The plasma exerts a pressure like a gas and they then assume confinement of the plasma with a magnetic field, balancing the plasma pressure with the 'pressure' that a magnetic field exerts on charged particles. They then say that we can make magnetic fields in the range up to 100 T and working back, estimate the plasma frequency, which turns out to be in the UV. So great, you can deflect lasers into the UV with a modest confining field.

You need to look at some of the other numbers though.
First, what sort of plasma density do you need to reflect UV ? The answer is something like 10^28 per cubic m. This is enormous - fusion plasmas are about a million times less dense). It's getting close to solid state density eg if a solid has atoms 0.2 nm apart this is 10^29 atoms per cubic m. That is not going to be easy .... The other problem is that at such a high density, the collision frequency is very high so that a magnetic field is not very effective at producing confinement. Probably useless in fact.

The other thing to look at is the required plasma temperature. They assume a temperature of 1000 K, Unfortunately, the density of a plasma at 1000 K at thermal equilibrium is extremely low unless the background pressure is huge. So it has to be a lot hotter, in particular, comparable with the ionization energy which is roughly 100 000 K. And really, we need a fully ionized plasma because the magnetic field is not going to confine the neutral gas that we are using to make the plasma so that means we need a 100 000 K plasma. This means that the required magnetic field goes up by a factor of 10.

Would somebody else like to estimate how much power you need to dump into the plasma ?

Comment: Re:And how do you start it? (Score 1) 127 127

by hyperfine transition (#46664853) Attached to: New US Atomic Clock Goes Live

Typically, there is no single clock that is "the reference". The international time standard, UTC, is an average of about 400 or so atomic clocks from all around the world. There are a number of caesium fountains currently contributing to UTC. Even in a single laboratory, where there a number of clocks, these clocks will usually be averaged and one clock then adjusted to keep to this average. There are various practical reasons for this. If you have a number of similar clocks, the average will have a more stable frequency than a single clock. Some clocks are better at very short averaging times, others better for the long term so you can make an average that takes this into account. State-of-the-art clocks seldom work continuously so you need something else as a flywheel between operating times.

The main use for very good clocks is as frequency references, or to measure the time interval between two events (in which case any offset cancels out) rather than time of day references. Sub-nanosecond synchronisation of distributed systems is only used in some very specialised scientific applications like Very Long Baseline Interferometric telescopes.

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