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Q: My patio is lit by the summer sun, which illuminates the floor and
adjacent walls. Last year I noticed that when a bird flew overhead, it cast
a shadow on the patio. The bird was flying at the correct height to just
clear the wall on my right. Its shadow moved across the patio floor at the
same speed as the bird. However, when the shadow reached the base of the
wall, it accelerated instantly, climb-ing the wall before reaching the top
and continuing along the flat roof, again at the same speed as the bird.

If the bird had been an object travelling at the speed of light, then
the shadow on the ground and the flat roof would have also been travelling
at the speed of light. But what of the period when the shadow was travelling
up the wall? Under this condition, the shadow would have had to travel faster
than light, which would present a direct challenge to Einstein. Is there
any other explanation?

* * *

A: Yes, the shadow does indeed move faster than light. No, this is
not a violation of the special theory of relativity, because you cannot
transmit information using a shadow. This is one of the key points of the
special theory. Information can be transmitted by light, but a shadow marks
the absence of any light so no information is transmitted faster than light.

Many other examples can be described to show geometric objects moving
faster than light. For instance, take two straight, crossed lines with an
extremely small angle between them and move the end of one line slowly towards
the other. The crossing point will move with increasing speed. If the angle
is small enough this speed will be faster than the speed of light. However,
again no information is transmitted and no material object which could be
used to transmit information travels faster than light.

Alex Niemeyer Munich

* * *

A: The question does seem to indicate the possibility of movement faster
than light (FTL). However, the illusion is generated by our viewpoint –
nothing actually moves faster than the speed of light (c).

We can observe a similar eff-ect in the ‘lighthouse’ paradox. Imagine
the beam of a lighthouse rotating at 1 revolution per minute. As distance
from the lighthouse increases, the track of the beam, when obser-ved striking
a vertical surface, would seem to be moving faster as it passed along that
surface. Calculations indicate that at a distance greater than approximately
3 million kilometres the beam would be rotating at FTL speed. Of course,
the beam’s track may be rotating thus, but the quanta making up the actual
radiated light energy moving away from the lighthouse continue to travel
at good old c.

To return to the bird, if the creature were to fly over the patio at
c, all that would happen is that the roof would have the sunlight cut off
from its surface in a time slightly less than it takes for light to reach
the patio, similar to walls of increasing distances reflecting the lighthouse
beam. This is the time taken for the light to travel the height of the wall.

John Smith Leicester

* * *

A: A bird ultra-speedy in flight Won’t give Doc Einstein a fright Its
shadow’s an absence Of light, not a presence And nothing is faster than
light

Doug Cross Honiton, Devon

Left in doubt

Q: As a left-handed person I was both amused and annoyed by your article
‘Sudden death for left-handers’ (Science, 12 March), which suggested that
left-handed people are at greater risk of accidental death. How can this
be? Surely a right-handed person has just as much chance of dying accidentally
as I do. Or is there some unknown factor involved?

* * *

A: When approaching obstacles, right-handed people will, in general,
circumvent them by going to the right, while left-handed people will go
to the left. If two same-handed people approach an obstacle from the opposite
direction they will walk safely around it without bumping into one another
en route. If two people of different handedness approach an obstacle from
the opposite direction, they will pass on the same side leading, potentially,
to a bump. Because most people are right-handed, it is left-handed people
that most frequently find themselves bumped in these situations. This is
a simple example, but taken to extreme and multiplied by a lifetime of bumps,
the result is a shorter life expectancy for left-handed people.

Hannah Ben-Zvi New York

* * *

A: We left-handers are at greater risk of accidental death because industrial
tools and machinery are designed for the right-handed. Left-handers are,
therefore, more likely to chop off parts of themselves in all manner of
mechanical devices.

An interesting example is the SA-80 assault rifle. When fired from the
left shoulder, it ejects spent cartridges, at great velocity, into the user’s
right eye.

Daniel Bristow Kew, Surrey

Stirring stuff

Q: What is the significance of James Bond’s famous phrase ‘shaken, not
stirred’? Is there really a difference in the taste of a shaken vodka martini,
as opposed to a stirred one? And if there is, why?

* * *

A: Supposedly, when a martini is ‘shaken, not stirred’ it ‘bruises’
the spirit in the martini. To seasoned martini drinkers this changes the
taste.

Padraic O’Neile Newcastle, Australia

* * *

A: Because a martini is to be drunk within seconds of preparation rather
than minutes, there is a difference. The tiny bubbles caused by the shaking
mean that a well-shaken martini is cloudy. This will also have an effect
on the texture of the drink – it is less oily than the stirred version –
hence the taste. The rumour that the spirit was bruised by the process is
nonsense as vodka does not have a vascular system.

Peter Brooks Bristol

* * *

A: James Bond may have appreciated the ‘softening and ripening’ effect
of partial oxidation of the aldehydes in vermouth – akin to letting red
wine breathe before drinking. In a refined and homogeneous substrate such
as vodka martini, a good shake can speed the process.

Alan Calverd Bishops Stortford, Hertfordshire

This week’s questions

Fishing well: A number of different species of fish are often found
in wells in Somalia. These wells are dug by hand, usually in limestone or
gypsum rocks. How did the fish get into these wells and how long have they
been there?

J. Beckett London

* * *

Roman knows: How did the Romans do basic arithmetic using their system
of numerals? Using our Arabic number system we find it easy to place two
numbers, one above the other, and add or subtract. How can this be done
using a number such as MCMXCIV (1994)?

Richard Cook Otago, New Zealand

* * *

Head cold: Whenever I eat something cold, such as ice cream, too quickly,
I get a headache which lasts for a few seconds. This doesn’t happen when
I eat something hot too quickly. Why?

Stephen Savage Oldham, Lancashire