Sunday, January 30, 2011


Last night my foot hurt. Let me restate this: last night, in bed, my right foot was extraordinarily painful. My first thought was diagnosis. The entire foot was very tender but nothing about the shape felt odd: no breakages, no swelling.

I reviewed the events of the evening. I had accompanied Clare to six o'clock Saturday Mass so she could watch Andy Murray in the Australian Open final Sunday morning. I must have stepped off a curb awkwardly coming back and not noticed at the time. But God, how it hurt this Saturday night!

My mind wandered to the Spanish practice of bastinado, the beating of prisoners' feet as a form of discreet torture, leaving no marks. I had reflected in the past that maybe this didn't sound so bad, but I was learning just how many nerve endings the foot actually has, all exquisitely tuned for pain.

When I was doing research into Artificial Intelligence in the 1980s, I had read Daniel Dennett on pain. He said that if you concentrated you could make the pain go away. Fine.

So I really thought about it and sensation gradually congealed as a red-hot wire through my foot. Dennet was right: as I focused my attention the pain transformed into mere data. But as he further observed, pain is ultimately so excruciatingly tedious that you can't stay focused forever.

I normally avoid pain-killers. Don't mask the pain, I say to myself, it's your body trying to tell you something. But all my body was communicating at that moment was that I wouldn't be getting any sleep tonight. I put the bedside lamp on - ten past one a.m. - and tried to stand. The pain was shocking and I couldn't put any weight on the foot. Nothing for it, I poked Clare: "You awake?"

She was now. Clare has a bedroom pharmacy of little white tablets: I pleaded for pain-killers. A few minutes later a barely conscious wife offered me two lozenge-shaped pills which I instantly swallowed. God knows what they were, maybe vitamin pills. Within half an hour I was away with the fairies.

This morning with only the slightest twinge I strolled to the paper shop, a twenty minute round trip. What is it with feet?


My three samples were duly posted off to the NHS yesterday for checking. I expect to get a letter telling me whether I have colon cancer or not around February 14th. How romantic is that?

Saturday, January 29, 2011

Thursday, January 27, 2011

Causality in Physics

I took some trouble with my own answer to the following question on Physics StackExchange. So I have copied the details here so it doesn't just get buried. However, all the answers at the question are worth a look.


"I have never seen a “causality operator” in physics. When people invoke the informal concept of causality aren’t they really talking about consistency (perhaps in a temporal context)?

For example, if you allow material object velocities > c in Special Relativity (SR) you will be able to prove that at a definite space-time location the physical state of an object is undefined (for example, a light might be shown to be both on and off). This merely shows that SR is formally inconsistent if the v <= c boundary condition is violated, doesn’t it; despite there being a narrative saying FTL travel violates causality?

Note: this is a spinoff from the question: The transactional interpretation of quantum mechanics."



As the originator of this question I have reviewed and learned something from all the answers posted so far. I would like to summarise my own views here.

Where did this query come from? From a question about the Transactional Interpretation of Quantum mechanics (TIQM), where said theory’s reliance upon “retrocausality” (‘causality’ backwards in time) was held to be a fatal defect. This kind of causality-argument is common in physics: we say that faster-than-light travel in SR is ruled out because it would violate causality.

Such causality arguments are conducted in what you might call the “metalanguage of physics”: technical English which supports and explains formal results. However, the arbiter in the end is the maths, so how do we interpret the notion of causality within the formalism?

Physical theories are defined by mathematical relationships between entities (observed and unobserved) usually expressed by equations (think Schrodinger, Dirac, the Lorentz transformation). If we say that event E1 “causes” event E2, several answers here suggest that the interpretation of causality in the formal theory is that: (i) if E1 is postulated to occur then the theory logically implies that E2 must occur as well; (ii) E2 is within or on the future light-cone of E1 (we say “cause precedes effect”).

However, it’s possible that condition (ii) is too stringent. While logical entailment is obviously an essential part of any formalised theory, our smuggling in of the word “future” is already an extra assumption. Our fundamental theories do not impose a specific past-future direction on the time dimension. This means that if you reverse the film, the events you see are still consistent with our fundamental theories.

Sometimes people use causality-like words in the physics metalanguage without conventional time-ordering condition (ii). For example, a possible Feynman diagram for electron-positron scattering has a narrative that an electron travelling backwards in time from the future encounters a (normal) electron, they exchange a virtual photon and continue on their way scattered. The 'cause' of the scattering event was the arrival of the future electron. Many textbooks mention this way of thinking but we don’t mind because the underlying theory gives consistent results which accord with observation. Perhaps TIQM is like this despite its narrative of retrocausation.

So my conclusion is that we have to be careful about arguments concerning a theory’s validity relying upon causation arguments couched in physics’ metalanguage. It’s not a slam-dunk. Sometimes if a theory violates conventional “cause precedes effect” causation it indicates a breakdown in the underlying mathematics, normally inconsistency. At other times a 'causation' argument is just a way of talking about the entailment of the theory in an innovative or whimsical way, and the theory is actually OK. Go look at the maths.

NOTE: there is a whole separate discussion about why, in natural language, we think so naturally in terms of cause and effect. It links to discussions about the arrow of time and why we do seem to be unhappy about running the film backwards as a valid picture of reality. That is a whole separate issue but still, I suspect, part of physics judging by the number of recent books on the subject.

Wednesday, January 26, 2011

Unable to walk (Cabot Circus)

Down to Bristol today to visit my mother and my brother, Adrian who's currently staying with her. We lunched at Cabot Circus, the new mall in the centre of town.

From the multi-storey car park there's a curious walk-way at level 3 across to the shopping concourse, pictured above. The angled walls and roof create the illusion that the floor is tilted and it's very difficult to walk straight.

As you can see from her expression (click on the picture to make it larger) my mother can't actually cross this bridge with her eyes open. As she stopped, wobbled and flailed for the handrail she whispered "Everyone thinks I'm drunk!" Adrian, also pictured, offers scant sympathy.

We came back at ground level and rejoined the car via the lift.

Tuesday, January 25, 2011

The King's Speech

Finally saw it this afternoon. I was prepared to be sceptical, after all the hype but it's a very fine experience with brilliant acting by Colin Firth as King George VI, Helena Bonham Carter as Queen Elizabeth and Geoffrey Rush as Lionel Logue (the Australian speech therapist).

It turns out that King George VI died of a coronary thrombosis, brought on by heavy smoking, in 1952 after only 15 years on the throne.

Monday, January 24, 2011

Ed vs. Ed

What does psychological type theory tell us about the future evolution in the relationship between Ed Miliband, Leader of the Labour Party and Ed Balls, his new Shadow Chancellor?

Ed Miliband is an interesting case: he's most likely INFJ although he might score a low E. An INFJ is driven by a strong set of personal values, gives and respects loyalty and comes across as empathic and warm (extraverted F). This orientation to the human, emotional side comes at the expense of analytic skills and an interest in the logic of policies.

Ed Balls is the very picture of an ENTJ. Extraverted, intellectual and judgemental with an underdeveloped extraverted feeling (F) function (his weakest function - see here). Ed is superlative at mapping out a grand strategy and marshalling forces to accomplish it - but without much regard for casualties along the way, collateral or otherwise.

As a male-female partnershipo ENTJ and INFJ could work although there would be plenty of arguing (too many Js). As a male-male subordinate-boss relationship the dynamics are unstable.

Ed can only operate through a strategy which makes sense to him. If people get in the way of its execution he will simply work to eliminate them. This requires him to "tame" Miliband and bind him to the strategy. At that point, for Balls, Miliband has a function - which is to go play nursemaid to the wounded egos left in the wake of Mr Balls' operations, while being a publicist/frontman for the touchy-feely media.

INFJs are hopeless at strategy and focus instead on harmony which is why Miliband's trades union sponsors elected him in the first place, to 'harmonise' with their particular interests and to be a public lobbyist for them. However, Ed Balls' "no cuts" strategy is even more closely aligned with the special interests of Labour's core constituencies (especially the public sector unions who pay the bills) so if Ed Miliband is looking for help in that direction he can forget it.

In this fascinating opera to come, Ed Balls is like an evil spirit who for his very existence has to take over and possess the soul of young Ed Miliband, making him his puppet. It will be fascinating and horrible to watch.

If David Miliband had become leader, he would have found Ed Balls an equally difficult character. David Miliband has the same personality type as Tony Blair (ENTP), while Ed Balls is an extraverted version of Gordon Brown (who is INTJ). So you would have had a reprise of the Blair-Brown poisonous relationship but with added public venom.

Saturday, January 22, 2011

The Black Swan (film)

We braved the freezing wind and zero temperature and walked to the Wells Film Centre this afternoon, orginally to see The King's Speech. But, Satuday and all that, it was sold out. So we opted instead for The Black Swan.

Now all the bleating from the ballet "community" about how this film bad-mouths behind-the-scenes goings-on in their art is ridiculous. The film is about an immature 28 year old ballerina Nina who is an OCD perfectionist but dances from the head, not from the heart. In her new top role as the Swan Queen (in Swan Lake) she is required to dance the part of the black swan, a seductive temptress, as well as the virginal princess white swan.

Nina can, of course, do white swans with her eyes shut, but letting her emotional side hang-out seems beyond her. Under a combination of the stress, the well-meaning efforts of her friends to help her loosen up and some slipped drugs, she starts to crack up.

The film starts puzzling, then it flips a few red herrings to mislead you it's going to follow the usual shop-worn cliches, then the tension ratchets up and it all gets totally surreal.

We enjoyed it.

* In reviews, some fuss has been made about explicit sex and drug use. Relax: it's not that explicit and no horses were frightened in the viewing of this film.

** We're down to see The King's Speech later in the week.

Thursday, January 20, 2011


We were only out for about half an hour this afternoon. Just popped down to the shops to buy some milk plus a bag of "insect feast" (sic) and a carton of fat-balls for the birds.

This was the sight which greeted us when we got back: the evidence of a crime against the avian community.

Once a sparrow, now through murderous intent transformed into one more piece of "goodness" for the garden. Clare buried it near the rose bush: it was still warm.

A trivial piece of detective work unmasks the perpetrator, his reputation as a bird-lover now transformed for ever.

More physics questions!

I posted two more, frankly populist questions on the Physics StackExchange site and got good answers. My reputation now stands at 42! (Clare asks wearily "Is it all men on this site?").

First question: "Is there a maximum possible acceleration?"

Second question: "What would happen if a large relativistic object hit the earth?"

Click on the links to see the full questions and the rather interesting answers.


Mundane news: our replacement DVD player arrived today, accompanied by The Brothers Karamazov.

Tuesday, January 18, 2011

At last the sun!

After days of living on a cold, damp version of Venus the clouds departed and the daystar returned. On the strength of it we took a walk.

After all the rain, the Bishop's Palace moat is full and fresh and covered in ducks.

We walked out into the country and caught the picture above of sheep being herded against the backdrop of Wells cathedral.

In other news, we bought an ultra-violet torch (4 AAA batteries) to search for signs of cat-marking on the carpet. A patch of staining was found last night after dark, with all the lights turned off, and Clare fixed it with sodium bicarb and white vinegar. Actually, there was no odour there in the first place but the UV caught it fluorescing and after that its fate was sealed. has invited me to be a reviewer and we're currently sorting out the arrangements.

The physics stack exchange continues to be as addictive as crack cocaine. I'm toying with making it my homepage (just kidding, google!). Lubos Motl gave a good answer to the question "What is Energy really?" linking it to time-translation invariance. By coincidence I have just been working through Emmy Noether's proof in "QFT Demystified".

Monday, January 17, 2011

Quantum Mechanics of Thermal Radiation

I previously posted on this topic but I was unhappy with my answer. So I submitted the following question to the Physics StackExchange ...

What are the quantum mechanisms behind the emission and absorption of thermal radiation at and below room temperature? If the relevant quantum state transitions are molecular (stretching, flexing and spin changes) how come the thermal spectrum is continuous? What about substances (such as noble gases) which don't form molecules, how do they emit or absorb thermal radiation? Is there a semi-classical mechanism (with the EM field treated classically) and also a deeper explanation using the full apparatus of QFT?

... and got some good answers here.

Saturday, January 15, 2011

Death Panels

The father was a doctor, the mother a nurse. They pleaded with their GP to give their three year old daughter Lana the flu vaccine. She wasn't in the "at risk" group so they were denied. She went ahead to contract the disease and tragically died anyway.

The father, Dr Zana Ameen angrily called on the Government to make the flu vaccine available to all children under the age of 5. How does £8 (the vaccine cost to the NHS) compare to the life of a child he reportedly asked.

I believe that something like five children under 5 have died of flu in the current outbreak so the chances of such death under the current vaccination regime must be less than one in a hundred thousand. So at least 100,000 vaccinations would be necessary to prevent one death, at a cost of around a million pounds.

Dr Zana Ameen also had a choice of popping around to Boots, where he could have bought a flu vaccine for £12.99 according to newspaper reports.

We have a centralised, top-down system of health care in this country where the experts (backed up by expert cost-benefit analysis) make the decisions as to who gets treatment and who doesn't. Of course, if you don't get treatment, you could die. In the US this approach to socialised medicine was tagged with the emotive phrase "death panels": not wholly wrong if it's your child who gets to die.

The alternative relies on individual choice and market mechanisms. Dr Zana Ameen could have simply exercised his choice and bought the vaccine for his daughter himself. This option has attracted moral opprobrium from Health Service professionals, with some calling for the market in vaccines to be made illegal. This is a difficult argument to make convincingly: in interviews senior bureaucrats have been reduced to that shoddy time-worn argument "we know best". Not too convincing when your daughter just died.

Here's an honest argument in defence of "death panels". Vaccines, like most things, are a scarce commodity. To distribute them you can either rely upon prioritisation or market mechanisms. In the former case the bureacrats decide (no doubt with the best of motives) who gets to receive potentially life-or-death treatments; in the latter case those with money do the deciding. The egalitarian case is for the former since all lives are equally worthwhile (really?*).

IMHO neither position seems tenable by itself. So we muddle through with the NHS a bulk buyer in the market running a free-at-the-point-of-use rationing system and the market also providing treatment options for those the NHS excludes but who can nevertheless afford to pay. I have to say that for a vaccine costing less than £20 pretty much everyone in the UK could afford to pay if they wanted it.

Since the NHS had plenty of opportunity to put in the order it wanted in advance, I think we should hear less from their ideologists about how private buyers are "taking treatments away from the deserving". You really do have a moral right to buy treatment for yourself in a legal and open market if that's how you want to spend your money, don't you?


Having recently turned 60, my screening letter for bowel cancer arrived in the post this morning. Based on perfectly-orthodox cost-benefit analysis, I completely subscribe to the logic of participating, even if there didn't happen to be a history of the disease in the family. The pack arrives in two weeks.

* There is a "gut-instinct" ethics question where you are invited to consider a burning house. If you rush in the front door and turn right you get to save your child; if you turn left you can save some number of unrelated children. Which way do you turn?

The preference for close kin over unrelated individuals is of course evolution 101. However, post-neolithic states with millions of inhabitants have to make generic policies (see "nepotism"). They therefore create an ideological concept of "citizen", all of which are to be considered equal. This conflicts with:

* kin preferences
* the obvious realities of inter-personal differences
* the differing utilities of people to the common good.

For attempts to buttress the essential homogeneity of "citizens", see "political correctness".

Wednesday, January 12, 2011

On memory loss and doggerel

Clare continues to worry about her alleged loss of memory.

"Remember that little poem about a bear?" I said,

"Fuzzy-Wuzzy was a bear
Fuzzy-Wuzzy had no hair

... .

Just try and complete it."

She thought for a moment then exclaimed triumphantly:

"Fuzzy-Wuzzy was a bear
Fuzzy-Wuzzy had no hair
He was a hairless bear!"

I looked at her with a sorrowful expression and said:

"Fuzzy-Wuzzy was a bear
Fuzzy-Wuzzy had no hair
Fuzzy-Wuzzy wasn't fuzzy
Was he?"

I am a little unsettled by this childhood poem which I had always taken to be about identity and loss. Something not quite right about it in these more enlightened times. I honestly think she was on to something though. What do you think of this modern rewrite?

Hairless-Bear was a bear
Hairless-Bear had no hair
Hairless-Bear wasn't hairy
Was he?

I think that hits the mark.

Anyway, our conversation continued.

"Do you remember last night in bed, when we were talking about the cat bringing in voles, although thank God none recently, and I recited a little poem about it to you. Do you remember that?

She thought about it, screwed up her face and then a radiant smile tentatively appeared and she said hopefully "Ickle wickle little vole ..?" then stopped in puzzlement.

I held the moment then softly uttered

"Rolly-Voley pudden' and pie
"Loose in the kitchen!" hear her cry
"Catch him and save him and don't let him die"
Rolly-Voley pudden' and pie."

Oh, the forbidden delights of deep kitsch!

Finally, something for the LGBT community which was a little favourite of my sister Elaine and myself when we were both "tiny people", unaware of its darker, more modern and thoroughly adult themes.

Lickle fly upon the wall
Him's got no clothes, no clothes at all
Him's got no little shimmy-shirt
Him's got no little pleated skirt
Him's got no mummy to wash him's hair
Him don't care
Him's bald.

God, so many issues there I lose count!

Tuesday, January 11, 2011

Burnham-on-Sea in January

Looking for a walk where we wouldn't sink up to our middles in mud, we decided on the forty minute drive to Burnham-on-Sea. There we were able to promenade on the front and thence onto the sea wall abutting the river Brue at the south end of town. The tide was on the turn and for once we saw the parked-up sailing boats actually afloat.

The mouth of the River Brue at Highbridge

The seafront at Burnham on Sea

Only the hardiest of couples were out and about so the scene, as you can see above, was largely deserted.

Sunlight on the water and on Wales

Somewhere, out there across the water, the sun was shining on Wales: here, the best that could be achieved was a certain dappling on the shallow water.

Beach-front properties

Beach-front architecture is largely uninspiring in English seaside resorts. The apartments pictured above, with their top-storey white wood panelling and glass-protected balconies might appear quite attractive: they are marred, however, by the pervasive use of grey brick walling, reminiscent of breeze blocks. Short of demolition, a campaign of bright pastel colouring seems to be their only hope.

Clare and Nigel in the chill wind off the sea

Finally, a self-portrait of your intrepid reporters. It was pretty keen out there!

Saturday, January 08, 2011

Finding the perfect-ish partner

In a previous post I talked about Internet dating. In fact there are many drawbacks to such a virtual strategy in finding the perfect (or merely adequate) partner. Compare the actual rather than virtual equivalent ...

In physical reality you zoom down to the disco, club or party where you are sure to encounter other people in the dating game (why else are they there?). You get a chance to physically check them out (a pretty good evolutionarily-honed filter) and if they pass muster you can engineer a quick encounter: a dance or a drink. Within ten minutes it's a rejection by one or both parties (go forth and iterate the process!) or you have the basis for developing some kind of relationship. If you feel nervous about it all, have a few drinks first.

In Internet dating, the process is much, much slower. It takes far longer even to get to physically meet. And then the basis for arranging a meeting leverages far less information, and that of dubious quality (what did you really write about yourself? Which photo did you use?). Finally there's a lot of stone-cold sober involved in getting to that first encounter ... it's got to be nerve-racking.

I sometimes think that the only people who can actually make Internet Dating work are those people who really have no need of it at all. The owners who run these profitable sites are like the proprietors of gyms: many pay their subscriptions but few get to call.

The alternative is match-making. In "actual reality" many people meet their partners through the good offices of friends who "know someone who would be a good match for you" or through working in an environment where soulmates are likely to hang out.

The Internet equivalent is the Internet Introduction Agency. You will meet with an advisor and based on their assessment of you, they will suggest suitable prospective dates and facilitate the arrangements. The nerves are, I suspect, mostly the initial interview and that's not a date at all.

Here's a list of Introduction Agencies for professionals with their charges. At the top-end you could pay more than £10,000 to avoid having to pair-off with any old riff-raff. If the global elite is too constricting a social base for you, rest-assured that alternatives are cheaper: Drawing Down the Moon will be familiar to Guardian readers over the years.

Recharging batteries

Not much posted here during the last week as I have been occupied with security accreditation work. The schedule eased off yesterday and I discovered that the cold had finally done for Clare's car battery as her vehicle wouldn't start. We strolled into town, dodging showers and I bought some jump leads from Halfords.

This bright and sunny, albeit cold, morning was the opportunity to try them out. Success! To give the car a fighting chance to recharge itself we took off for Priddy where we did a circular, puddle-strewn walk passing the tiny Priddy church (pictured below) and ending up at the charmingly ancient Queen Victoria pub for lunch.

The interior of Priddy church

The Queen Victoria pub, Priddy

Saturday, January 01, 2011

Quantum mechanics of thermal radiation

UPDATE: (Jan 17th 20100).

I posted the question below at the Physics StackExchange and received some excellent answers here. They add to my post below by factoring in something I had ignored: Raman scattering events.

How do objects emit thermal radiation?

This is an example of a simple query for which Google fails to provide a simple answer. You are inexorably drawn into endless websites which describe:

* Planck's Law
* Wien's displacement law
* The Stefan-Bolzmann law.

In the process you will learn a great deal about cavities, naive application of the equipartition theorem and its consequent ultraviolet catastrophe, electromagnetic standing waves, Planck's introduction of the quantisation of radiated energy. Here's a good overview. What you will NOT learn is how objects get to emit/absorb thermal radiation in the first place.

You will be particularly confused by some of the non-sequiturs in the articles Google turns up. You will read about the discrete frequencies emitted and absorbed by atoms as their electrons jump between orbitals at different energy levels. And you will wonder what on earth this has to do with the continuous blackbody spectrum. (At room temperature, nothing).

The truth of the matter is well explained in this powerpoint presentation. At ordinary temperatures on earth thermal radiation is in the infrared. This frequency band is associated with energy transitions where molecular links vary their stretch, bend and spin in different modes of oscillation (recall inter-molecular links are modelled by techniques such as LCAO, with more detail on coupling to the electromagnetic field here).

It turns out that molecules have a very large number of energy levels when doing these contortions and so the emission and absorption spectrum looks like a comb with an uneven height profile (see slide 33 and following). However, we have to factor in two other phenomena (as described in slide 32 in the context of the atmosphere):

"Doppler broadening: random translational motions of individual molecules in any gas leads to Doppler shift of absorption and emission wavelengths (important in upper atmosphere).

"Pressure broadening: collisions between molecules randomly disrupt natural transitions between energy states, so that absorption and emission occur at wavelengths that deviate from the natural line position (important in troposphere and lower stratosphere)

"Line broadening closes gaps between closely spaced absorption lines, so that the atmosphere becomes opaque over a continuous wavelength range."

Similar phenomena smear out the quantised lattice vibrations in insulating solids. In metals the free conduction electrons have a very fine-grained hierarchy of energy levels determined by the wave-numbers which fit into a macrosized metal object.

So there you are. You get a smooth blackbody spectrum of thermal radiation because of the extremely large number of finely-spaced energy levels involved in molecular/atomic/electron motions in the bulk (degrees of freedom as we say). These generate a wide, dense comb-like quantised structure of emission/absorption frequencies which is then smeared into a continuous spectrum by doppler and bulk kinetic effects.

Other keyphrase: Quantum mechanisms of thermal radiation.

Somerset walks

On Thursday Alex, Clare and myself walked across the tops to Wookey Hole where we lunched in the local Inn. I was perhaps the most adventurous, ordering a cherry beer called Mort Subite and a wild boar burger. My schoolboy latin told me that the beer was "sudden death" but my classical pronounciation was corrected by the waitress who said it the Belgian way.

Yesterday Clare and myself took in the Ham Wall wetland sanctuary where we saw the starlings coming in to roost again.

Today we took a stroll up to Beacon Batch, at 325 metres the highest point on the Mendips and thence down to Burrington Combe (pix below).

As regards this post, Dr Roy Simpson comments sardonically:

Hi Nigel,

There is a conundrum here where I am not sure what your stance is.

Having expressed the view that Aliens are presumed bad guys, and that inadvertent or deliberate broadcasts will give us away, and having drawn the conclusion that good defensive weapons (e.g exawatt lasers) might help defend the planet when the time comes (which could be any day ... any decade) --- who builds these weapons?

The problem is that contemporary weapons builders are defence companies perceived as building increasingly powerful weapons against fellow man, rather than as NASA/ESA/CERN-like international organisations building things for the good of fellow man.

Should one advocate a continuation of the Military-Industrial-Complex after all, but at the Global Scale? Is such an idea feasible from any political or ethical perspective? On the other hand could the existing members of the MIC actually like this idea because it seems to me (and I am sure others) that building these big expensive weapons seems increasingly out of place in the current Geopolitical scene. As you know the best military R&D for the UK right now is anything to do with detecting/neutralising anti-personnel mines - not "large kit" systems, perhaps even true for US. As you know often these procurements happen to "keep skills" rather than it being objectively necessary that such systems are all needed. So could Earth "keep skills" by designing-researching and building "Earth Defences" - admittedly regionally at first?

(PS I have just read about some ex-Black projects and recently learned that Stealth is the result of a mathematician -- a Soviet mathematician from the 1960s. Interesting substory here by itself.)


To which I reply.

"The Exawatt lasers were actually mentioned in the context of SETI signalling at power levels significantly in excess of the earth's reflected light. This is not a trivial engineering challenge if you're talking about sustained power output. Apparently the human race currently consumes as a whole around 15 TW.

On defensive measures there's a whole book published on how to militarily defeat aliens. About on a par, I imagine, with a discussion amongst Australian Aborigines about how to defeat the current US marine corps at first contact!

As regards undefendable-against alien weaponry, I particularly like steerable asteroids and relativistic kinetic weapons - if you're into loud bangs. More sophisticated weapons need not be anything much more potent than a tailored virus."