Carrock

5 hours ago, avicenna said:

If the voltage across a long wire is constant, is the current uniform throughout the wire length.  

In a 50 Hz ac voltage across a long wire, at a certain moment how does current vary along x, the wire length. 
 

There's been a bit of conflation in some replies between A.C. and D.C..  D.C. is conserved but A.C. isn't.

So: V.H.F. A.C. whose effects are more obvious than 50Hz.

You could have 100 meters of 50 ohm coaxial cable with 10db loss per 100 meters. Connecting A.C. power at say 100MHz 100V to the (resistive) cable gives I =V/R = 2 amps input (200W). Connect a 50 ohm resistive load to the far end and you'll get ~ 31.6V at 0.632A i.e. 20W output. If you use a 200 meter cable you'll get 10V at 0.2A i.e. 2W. The input is still 200W into 50 ohms.

The current drops exponentially along the cable. The main power losses are I^R losses in the conductor, dielectric (insulator) heating and radiation from the cable. There is no A.C. current conservation; some of it charges and discharges the dielectric and current is also involved in creating magnetic and electromagnetic fields.

One way of dealing with reactance is to consider the effect of a load impedance mismatch.

e.g. terminate the cable with 25 instead of 50 ohms. This will cause a power reflection back into the cable to compensate for trying to connect a 50 ohm cable to a 25 ohm load. If 20W output then 20 *(50 -25)/(50+25)W i.e. 6.7W is reflected back into the cable and after attenuation 0.67W reaches the source.

You'll get standing waves on the cable; every half wavelength (About 1.5m) you'll get maximum voltage and minimum current; between these nodes you get a minimum voltage, maximum current node. A.C. current can be created and destroyed without breaking conservation laws. Some energy is stored in various fields and doesn't reach the load; sometimes it's called imaginary power(it can be treated as 90 deg or sqrt(-1) out of phase with 'real' power) or reactive power(capacitors and inductors have reactance).

This is sort of real; there are meters which measure forward and reflected power in coax cables...

I didn't want to oversimplify too much; this post ended up much longer than I intended.

On 4/19/2024 at 2:25 AM, npts2020 said:

I would quibble slightly with this, in that there is also the matter of surviving long enough to actually reproduce, although I think high reproduction rates definitely are a major positive contributor to a species survival in most cases.

If a predator's reproductive success is greater than that of its prey, high reproduction rates definitely are a major negative contributor to a species survival in most cases.

Predator numbers would closely track prey numbers as they increase; in bad times for prey reproduction, prey and then predator numbers would crash as prey was eaten by faster breeding predators. Eventually, perhaps after a few repeats, the predator and/or the prey would become extinct.

In practice, e.g. introducing feral cats on a small island, most of the prey species are driven to extinction but the predator often scrapes a living in an impoverished environment.

11 hours ago, Ken Fabian said:

@Carrock

Seems to me we would try for Near Earth Objects (NEO's) by preference. Atiras asteroids are inside Earth's orbit around the sun without crossing it, Atens are inside but do cross it, Apollos are outside and cross it and Amors are outside and don't. They seem likely to have recurring "windows" where lowest delta-v will be possible. I wouldn't start with the Asteroid Belt.

I've suggested C-type (carbonaceous) asteroids by choice, for the water content (for reaction mass) however in general those are more likely to be found further out in the solar system and the Near Earth ones more likely to be S-type (stony). All probably contain nickel-iron but whether they contain carbonaceous or others suitable for extracting fuel/reaction mass is not clear. But the fact that carbonaceous meteorites are common suggest that carbonaceous asteroids could amongst those NEO"s. And if going further afield then there are Mars' moons, which appear to be carbonaceous. Any potential target would deserve some survey sampling.

Unless Atiras asteroids cross Venus' orbit no flyby assist is possible. It would be necessary to change the payload's orbit such that its aphelion is at earth's distance from the sun. This would generally require a lot more than 10mph or even 100mph delta v. Similarly for Amor asteroids which don't cross Mars' orbit the payload needs to be decelerated for an earth distance perihelion. There is only one convenient planet to aim for and if the asteroid's orbital period is similar to earth's you may have to wait years for a lowish energy/fast transit window.

Quote

From Wikipedia

The Aten asteroid with the smallest known perihelion is also the one with the highest known eccentricity: (137924) 2000 BD19 has an orbit with an eccentricity of 0.895, which takes it from a perihelion of 0.092 AU, well within Mercury's orbit, to an aphelion of 1.66 AU, which is greater than the semi-major axis of Mars (1.53 AU).

That Aten asteroid does look quite promising...

Within reason you want an asteroid from which as many planets as possible can be reached occasionally by a low delta v burn. I suspect, without (years of) calculation that asteroids from which Mars, Jupiter and Saturn can sometimes be reached with low delta v would be good. Several asteroids in different orbits could effectively spread out the windows.

The point of planetary flyby is to donate to or abstract from the flyby planet's velocity and momentum with a very small initial delta v rather than obtain it all from burning fuel.

You would only need a tiny amount of fuel (tons of payload per gallon of fuel) which I expect would be present in adequate quantities in most asteroids.

b.t.w. I'm still very dubious about asteroid mining....

1 hour ago, exchemist said:

OK, Interesting. Roughly how many of the requisite planetary alignments would there be per year, or per decade? 

For something like the Voyager grand tour, less than one a century...

For a slow journey, all you need is the delta v to get to the first flyby and for course corrections; any asteroid chosen with this in mind would have frequent low delta v options, I'd guess one every year or two. Planetary alignments, possible journeys and required delta v would likely be all worked out long before any mining.

There would be a balance between the cost of rocket fuel etc to minimise journey time and the cost of mined resources unavailable during the journey.

NASA manages these flybys regularly and, it seems, generally chooses to keep time to ultimate target at less than ten years.

On 3/18/2024 at 4:57 PM, exchemist said:

My understanding of this subject is that the killer, economically, is the huge cost of the change of momentum required to bring extracted minerals back to Earth. These asteroids are on a very different orbit from that of the Earth and momentum change (rocket power) is very expensive, per kilo of payload.

and similar comments...

There are a lot of issues with mining asteroids but I don't think this one is significant.

All the delta v you need is sufficient to arrange a first planetary flyby, with slowing and deflection towards another flyby planet, basically the reverse of the many flybys used to get spacecraft from earth to e.g. Jupiter. The minerals would also need entry protection to survive entry at somewhat more than earth's escape velocity.

e.g. a package which would miss a planetary flyby by ten million miles in 10 years' time would only need a delta v around 100mph.

On 1/23/2024 at 6:10 PM, Peterkin said:

That's not the Mensa test I was given, sometime back in the bronze age. That took an hour and half, went on and on. One of my fellow inductees scored 152. She was a lively, interesting person, while the membership we encountered was mostly trendy young couples with whom we had nothing in common. We dropped out pretty soon - no loss.

As Strange mentioned a long time ago, confirmed by some of my friends, the Mensa lower membership limit is 148 and Mensa pass scores range from 149 to 152 - at least from 5 samples (6 including Peterkin's example).

Explains the fact that an an improbably large number of apparently intelligent people think a good use of their time is to hang out with people who, like them, are good at 'passing' IQ tests.

On 1/22/2024 at 6:23 PM, Otto Kretschmer said:

Since IQ tests have the celling of 160, are IQs higher than that possible or not?

The test has to be optimised for an IQ range. The highest IQ for an individual test corresponds to getting all the answers correct.

On 1/16/2024 at 5:42 AM, sethoflagos said:

Revisiting Kirchoff's Law briefly, there are some useful bits of information to be had from Electromagnetic Reciprocity.

This caught my eye:

Quote

Forms of the reciprocity theorems are used in many electromagnetic applications, such as analyzing electrical networks and antenna systems.^[1] For example, reciprocity implies that antennas work equally well as transmitters or receivers, and specifically that an antenna's radiation and receiving patterns are identical.

 

As you've mentioned reciprocity a few times in this thread, just a clarification that reciprocity is not always valid.

From Electromagnetic Reciprocity

Quote

One case in which ε is not a symmetric matrix is for magneto-optic materials, in which case the usual statement of Lorentz reciprocity does not hold (see below for a generalization, however).

This is  commonly used in radar, when the transmitter and receiver are both permanently connected and are effectively part of the aerial; output power to the rest of the aerial is from the tx, while input power is to the rx, isolating tx and rx.

Using the ionosphere for radio communications similarly sometimes produces non reciprocal paths.

4 hours ago, Paulsrocket said:

Not everyone can get the TS-SCI level clearance to be in the radio room, or to be captain just as not everyone can do what my son does for Lockheed Martin.  My son went thru rigorous testing and passed, did you?

A win-win question from you.

It's in the public domain that Britain's Official Secrets Act is pretty draconian and signing it is for life, not just Christmas.

So if I answer 'no' I'm lying or telling the truth.

If I answer 'yes' I'm lying or breaking the Official Secrets Act.

A few people who had very high level clearance:

Burgess, Philby, MacLean, Blunt, Cairncross, Fuchs, Gold, Greenglass, Hall etc.

In case there is any truth in what you've said, this is my last post on this thread.

Quote

All Navy sub communications are encrypted

Except when your son is carrying clear messages to the captain or other crew.

As someone who chose not to start WW3 by faking messages, I'm surprised he didn't end up as chief of naval operations.

1 hour ago, Carrock said:

Even worse is the idea that there are people in the U.S. military whose job includes having unnecessary access to highly secret information. 😧

Silly me, I didn't for a moment believe that was true.

1 hour ago, Paulsrocket said:

How about you?  Got cool photos from ICEX at Santas workshop at the North Pole too.  

Like you, I'll go with vicarious achievement.

Best I can come up with is my dad was on various merchant ships escorting U.S. - Britain convoys in WW2. Only got his feet wet once when the wheelhouse windows were smashed by a nearby shell.

BTW none of my close family, including my father, were in the military, so you 'win' on that.

Quote

It's not the captains job to decrypt all messages, it's the radio operators job, though there may well be certain messages for the captains eyes only, and if so my son would notify him.

Quote

Benjamin Franklin: "Three may keep a secret, if two of them are dead."

Quote

...whose job was to decrypt messages meaning that he would know what the ship's new orders were before the ship's captain.

So the unimportant things like ship's orders are fine for the radio operator to know.

Next post

Quote

The radio operator is not allowed to leave the radio room when on duty, so he does not carry messages, the captain comes to him if need be.

Is the radio operator trusted not to make his own private copy? If not, whoever searches him when he leaves would need the same clearance as he might see a decrypted message.

Much better of course for the radio operator to memorize the messages.

Quote

if you find yourself in a hole, stop digging.

20 hours ago, Paulsrocket said:

Before working as a software engineer at Lockheed Martin my son was a radio operator on a nuclear sub, whose job was to decrypt messages meaning that he would know what the ship's new orders were before the ship's captain.

That's interesting. I know a few people who were radio operators on commercial ships decades ago and none of them were able to decrypt secure messages not addressed to them.

If, very improbably, the captain gave decryption information to your son, he deserved to be court-martialed and his security clearance revoked.

Even worse is the idea that there are people in the U.S. military whose job includes having unnecessary access to highly secret information. 😧

13 hours ago, StringJunky said:

Is that principally by  social engineering methods?

I don't know but probably just some saddo annoying me. It's the first time anyone's ever made even a token effort to hack me AFAIK.

Updating my password on my various devices was tedious but probably very low security risk.

On 1/7/2024 at 4:40 PM, StringJunky said:

To be a subject of Pegasus software, for what it costs to implement, one would need to be a very high value target. I'm under no worries about Pegasus.

Nor me.

However, hacking my email enough (more if competent) for my ISP to change my password and inconvenience me was a lot simpler/cheaper.

It's rather sad that mentioning public domain work by Frank Miller (1882) and Hedy Lamarr (1942 patent) is considered worth this response.

There's an arms race between security and hacking in encryption and I hadn't really thought about the internet, computers and O.T.C.

The securely encoded data in O.T.C. is actually encode once, send, copy or read many. Only the copies of the O.T.C. key must be held securely.

So send an encrypted file via internet to another computer as described by Sensei. DOS attacks etc don't deplete O.T.C. as the same encrypted data can be resent. For best security copy the file to a computer which never has internet access. Plug in a memory stick with a copy of the one time key used by the originator. Run a simple program which uses the one time key to decrypt the file. Don't save the decrypted data , unplug the stick and switch off the computer when finished.

The O.T.C. doesn't need to be significantly larger than the total of all files sent.

44 minutes ago, Sensei said:

That's why people invented checksums, hash functions, digital signatures.. and a pretty simple project for a first-year student with just socket(), listen(), accept(), connect(), send(), recv(), close(), fopen(), fread(), possibly select() (for async), which has just raw data, will grow to a pretty complex.. just to detect malicious data in our stream..

In the worst-case scenario, if you send an (unused yet) offset to the secret key to the other party (to regain any encrypted communication after introduction of any malicious data in the stream), MITM could use DoS/DDoS to cause the secret key to be depleted on the machines.. 64 GB / 100 Mbps (12.5 MB/s) = ~ 5243 seconds..

O.T.C. isn't particularly suited for sending vast amounts of data or for the internet as I hinted at in my preference for cheap and crude over sophisticated and expensive. Fairly secure encryption is pretty much necessary for the internet.

To avoid rather than solve these problems, if phones with their metadata are also unsuitable, one of many alternatives: frequency hopping radio with one time code to set the frequencies.

This is very hard to jam and a fairly secure version is widely used by the military and other clandestine communicators. One time code isn't normally practical for large numbers of users. Probably civilians couldn't get a license as governments like to eavsdrop.

3 minutes ago, Sensei said:

We have two phones or two computers. They open a connection from one to the other over a potentially hostile environment, the Internet. One uses socket send(), the other uses recv(), and then vice versa. One byte sent, one byte received, so the sender's counter is incremented accordingly. The same thing happens to the counter on the receiving machine. MITM intercepts the connection and injects the byte(s) in the middle of the stream, resulting in them being at different offsets in the secret key..

 

For some forgotten reason I originally assumed you meant random data from the sender rather than malicious interference.

Malicious interference can disrupt any communication; the important point with O.T.C. is never ever  reuse it for any reason.

6 hours ago, Sensei said:

..sending random data to the recipient would result in desynchronization. They would be in different bytes of the key..

 

I don't see why anything but noise would cause desynchronization.

To (re)synchronise if frequently needed, use a predefined part of the key (different each time) to send e.g. ZZZZZZZZZZafdc74cf to indicate the next byte used for message encryption. Fairly straightforward software could be used to find which part of the key decodes as ZZZZZZZZZZ.

1 hour ago, Sensei said:

The keyword here is "single-use", which is virtually impossible. Sooner or later you will end up sending the same key again.

Goods e.g. a key can be sent by trusted courier which has been possible for a few thousand years. This code, like any other, relies on physical security.

Any finite length one time code code will, if generated often enough, eventually repeat by random chance, in this case (64GB) after at least many billion uses, but that doesn't assist decoding.

7 hours ago, Sensei said:

 

The key must be generated in some way. Discovering the key generation algorithm increases the chance of decoding the signal.

 

Instead of brute force WiFi password hacking, you can check the device manufacturer and see how they created the password creation algorithm..

 

A recommended way here is to use a noisy reverse biased diode in avalanche mode and an analogue to digital converter to generate the code. With some care, the slight preferential bias for some digits can be made far too small to be useful for decoding. I don't think knowing the algorithm would help.

6 hours ago, Sensei said:

If someone has an iOS older than 14.8, Pegasus can instantly remotely hack it, as long as they have Internet access:

........

If someone has an iOS older than 16.6.1, Pegasus can instantly remotely hack it, as long as they have Internet access:

.....

 

The important issue here is to realize that crude and cheap is much better than sophisticated and expensive. An ancient phone with no internet access and only text and voice can't be hacked without physical access, assuming the phone designer was honest and competent. Some software would be needed for de- and encryption. If the phone was stolen, all its one time code would be compromised but not other phones with different one time code.

7 hours ago, Sensei said:

Desynchronization forced by a third party or transmission errors will require resending the data.

If you want to annoy a spook monitoring your comms, send this plain text message first.

'To make sure you get the full list of our secret agents in Britain, I'll send it 10 times.

Naturally you use different parts of the one time code for each message and actually send it 1000 times.

One time code is inconvenient but has its advantages.

I recall many years ago hearing people droning out interminable lists of numbers on short wave radio; my first experience of one time code.

Slightly off topic, but defeating quantum decryption is quite simple (human stupidity excepted) and the technique has been known since at least 1882.

From Wikipedia

In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is larger than or equal to the size of the message being sent.

A couple of phones, each with say 64GB identical one time code storage and no ability to change the software without physical access should be fine for a few years of communication.

For obvious reasons, adverts for increasingly powerful phones and encryption rarely mention this.

From later in the Truth Table wiki

Quote

Note that this [truth] table does not describe the logic operations necessary to implement this operation, rather it simply specifies the function of inputs to output values.

So, I think, this wiki is stating that you can have a truth table with or without the additional steps required to implement it, which seems rather obvious.

Complementary to Studiot, I can't see any way to implement a nontrivial truth table without at least logic gates.

The Wiki refers to Logic Gate  for practical implementation...
 

Wikipedia is certainly oversimplifying. Like, say, a photon, a virtual photon is calculated or observed to exist from when it is created to when it is destroyed, which for massless virtual particles has no upper time limit.

The second sentence is is even more misleading. Oppositely electrically charged particles experience a net attraction by exchanging virtual photons; this attraction does not happen with real photons but the arguably very different characteristics of virtual and real photons in this instance does not shorten the virtual photons' lifetime.

15 hours ago, mistermack said:

One other question that occurred to me is how virtual gravitons comply with the description of virtual particles being transient and short-lived.

What description? Photons, for example, are stable and therefor virtual photons have infinite range.

On 10/20/2022 at 3:35 PM, hypervalent_iodine said:

Correct. 

They are listed in the staff directory as an admin and the mods and admin all know who he is, but that doesn’t always mean we can get in touch easily. Honestly, it wouldn’t be SFN if we didn’t let the domain lapse every few years. 

[morbid]

Can anyone confirm that the domain owner is immortal, or at least that their will won't take a couple of years to clear probate?

[/morbid]

After I recently discovered this serial has been made publicly available I immediately watched it again....

"Young Dracula," is a tragicomic fantasy melodrama for children and adults, spanning six years, dealing with the battle between evil and good from the viewpoint of Count Dracula's children: Ingrid, 14, and Vlad, 12, who know they will soon transform into evil, bloodsucking vampires, or cease to exist.

The dialogue, plot and character development are IMO better than most adult tv drama.

It's now available on the BBC IPlayer and also free on UTube

12 hours ago, toucana said:

A report by NATS (National Air Traffic Services) says that a catastrophic failure of the UK air traffic control system on the August 28 Bank Holiday this year was a caused by a “one in 15 million’ software failure event.

From the report, a single software failure in calculating a flight path was designed to cause a fatal exception i.e. crash the whole system rather than e.g. generate a 'NOT VALID FLIGHTPATH -MANUAL INTERVENTION REQUIRED' warning for controllers. The backup system must have been designed solely for hardware failure since as soon as it was enabled it experienced the same software failure and crashed.

This particular problem has been fixed so crashing the system was always an unnecessary requirement in handling this safety critical information.

The claim that this failure, after 15 million successful flight plans, is a  ''“one in 15 million" software failure event' implies that there are no more unintended fatal exceptions in the software.

Really?

Fortunately the whole system is now getting a very necessary upgrade.

I can't see how to do this without at least implicit equations...

12 hours ago, Scienc said:

Guys, why in physics does the work done by the system have the POSITIVE sign, but in chemistry does the work done by the system have the NEGATIVE sign?

Gets worse than that. Sometimes in the same system work done in e.g. charging a capacitor is regarded as positive, while work done discharging a capacitor is, er, positive.

You can then say (e.g. from https://www.allaboutcircuits.com/textbook/alternating-current/chpt-6/q-and-bandwidth-resonant-circuit/ )

Quote

More formally, Q is the ratio of power stored to power dissipated in the circuit reactance and resistance, respectively:

Q = Pstored/Pdissipated = I2X/I2R Q = X/R where: X = Capacitive or Inductive reactance at resonance R = Series resistance. 

You can actually buy a power buildup cavity using the same general idea.

From Wiki

Quote

The quality factor of atomic clocks, superconducting RF cavities used in accelerators, and some high-Q lasers can reach as high as 10^11[3] and higher.^[4]

It seems like you could put in 10mW and get 1 MW out. Apparently the concept of imaginary power or reactive power (which is not available for continuous  work) is not used here; 'stored power' is just power.

It's rather like saying torque is energy because the units are dimensionally the same.

There's a simple definition of Q to avoid this confusion*

Q(omega)= omega*(maximum energy stored/power loss)

*My confusion anyway - there must be some rationale to the "stored power is not energy" concept.