How long is a moment? Travelling at the speed of light – whatever that is; The music of hell? It has to be the bagpipes

How long is a moment?

A moment is force times the perpendicular distance from the fulcrum to the line of action of the force.

Brian Doyle, Crewe

The epidemiologist and poet Miroslav Holub developed an attractive theory in his collection of essays The Dimension of the Present Moment (1990). Based on some psychological evidence and a knowledge of poetry, he argued that the human consciousness lives in a present that is a few seconds long – about the time it takes to read an averagely long line of verse. All those blank verse soliloquies really were on to something.

Mike Bradshaw, Ormskirk, Lancs

At last, a question to which a precise answer can be given! In the middle ages, an hour was considered to be made up of 40 minutes, with each minute divided into 40 moments. Disregarding the elastic notions of time then prevailing, and allowing 3,600 modern seconds to the hour, that gives us 90 seconds to each minute. These minutes would contain 40 moments of precisely 2.25 seconds each.

The word "moment" is used shockingly casually these days. Personally, if anyone says to me, "I'll be with you in half a mo'", I get impatient one and one-eighth of a second later. Yes, I did used to be a schoolteacher.

Alan Rooks, Blaby, Leicester

Time itself being quantised the smallest possible duration in terms of being able to detect something happening before or after something else is the Planck time, 10 to the power of 43 seconds give or take a (very small) bit. But don't blink or you'll miss billions of them.

Ken Sheridan, Stafford

If you are an estate agent, about 20 minutes (as in "moments from Hyde Park"). Otherwise, when time stands still. "The more the personality disappears in the twilight of mood, so much the more is the individual in the moment" – Søren Kierkegaard, Either/Or.

J Castles, London W2

It is as long as you enjoy it.

Mike Hall, Northampton

Why does light travel at the speed it does?

In the answers to this question (N&Q, 2 February) there is an underlying assumption that outer space is a vacuum, which isn't strictly true.

Within galaxies, such as our own Milky Way, the interstellar space, besides being full of stars, planets, asteroids, comets and meteorites, is also pervaded with the "interstellar medium", which is about 99% gas (hydrogen with a little helium) and 1% dust (not the stuff you find under your bed), and fills the "space" between bodies in galaxies and even extends somewhat into inter-galactic space.

However, there is no evidence of a pervading "intergalactic medium" in any way analogous to the interstellar medium, so the space between galaxies is much closer to pure vacuum — closer even, than man-made vacuums on Earth.

Given this variation in vacuum quality between intergalactic and interstellar space, light must travel faster between galaxies than it does within them. I don't know if anyone has calculated the difference, but I doubt that you could live on it.

Phil Coughlin, London E1

If heaven has angels playing harps, what is hell's official musical instrument? (Will not accept bagpipes as an answer.)

The human voice, which musical instruments (including bagpipes) have been successfully designed to improve upon.

Peter Hepworth, Wolverhampton

Sorry, but none other than Scotland's national poet, Robert Burns, in his poem Tam o' Shanter, actually portrays the devil playing the bagpipes. On his way home from a heavy session in the pub, Tam sees a coven of witches dancing wildly in the churchyard at Alloway. The music is provided by the devil himself:

There sat auld Nick, in shape o' beast . . .

He screwed the pipes and gart them skirl,

Till roof and rafters a' did dirl.

It is a very funny poem. I recommend reading it with a Scottish friend (for translation purposes) and several single malts.

Dave McAllister, Preston

Why do self-adhesive postage stamps still have perforations?

The perforated edges look nice as well as making it easier to obsessively position three or more stamps so they are spaced equidistantly, and horizontally and vertically aligned.

Sheila Kirby, Esbjerg V, Denmark

Any answers

Why do atoms need to be so small?

Ruth Yudkin, Bristol

Why do budget airlines have such dreadful colour schemes?

Alain Head, London EC1

What was in the arches of inner-city railway bridges before they filled up with lock-up garages, bars and restaurants?

Simon Hugo, London SE17

Send questions and answers to nq@theguardian.com. Please include name, address and phone number.

• This article was amended on 23 February 2011. The original did not make it plain that one of the replies referred to 10 to the power of 43.