54 | New Scientist | 7 December 2019
Shrinking gravity
The first law of thermodynamics
is the conservation of energy.
The first law of geology is that
rocks fall downhill. Falling rocks
gain energy. The energy must
come from gravity. So why doesn’t
gravity get less every day?Oliver Knott (age 12)
Olney, Buckinghamshire, UK
When rocks are taken uphill,
they gain gravitational potential
energy from whatever brought
them up. When they fall down,
this becomes kinetic energy,
which is released as heat or used
to break the rocks when they
reach the bottom. The energy
doesn’t come from gravity,
it comes from whatever brought
the rocks up. So gravity doesn’t
diminish over time.Robert Willis
Nanaimo, British Columbia,
Canada
Rocks build up potential energy,
usually over eons, as the ground is
pushed up by tectonic movements.
This transfers some of a tectonic
plate’s kinetic energy into potential
energy in the now-higher rocks.
At some point, probably due to
erosion, they fall back down,
quickly converting their potential
energy back to kinetic energy.
This is then transferred back to
the underlying tectonic plate as
the rock hits the ground. The
plate stores the “new” potential
energy until the next upheaval,
when it starts again.Spencer Weart
Hastings-on-Hudson, New York, US
To convince yourself gravity
doesn’t diminish over time, try
this. Take something heavy in
your hand and move it up and
down with your eyes closed,
while imagining it is attached
to the floor by a spring that you
are stretching and relaxing. The
“spring” is the gravitational field.
When you raise the rock, you put
some energy into the field. When
you lower it, the field gives back
the energy. This doesn’t changethe strength of the spring that
pulls on other rocks or on you.Mark J. Bridger
Oxford, UK
The question bears on the
accelerating expansion of the
cosmos caused by dark energy.
Because things are moving
apart, the gravitational potential
energy in the universe should be
increasing. Yet kinetic energy is
also increasing, seeming to break
the overall energy conservation
principle. An alternative is that the
accelerating expansion is itself a
kind of “falling”, a motion caused
by gravity. That might suggest the
existence of a greater gravitational
universe outside our own.Super seers
I have heard that it is possible
for some people to see ultraviolet
light. Is this true, and if so, how is
it possible?Richard Swifte
Darmstadt, Germany
The human retina is sensitive
to the ultraviolet (UV) spectrum
down to about 300 nanometres,but the lens of the eye filters it out.
This adaptation perhaps arose to
protect the retina from the more
damaging UV. It also avoids the
increased blurry effect of having
too wide a spectral range, since
different wavelengths focus at
different distances from the lens.
Artificial lenses are designed to
block UV. But people born without
a lens, or who have a lens removed
and not replaced, sometimes
report seeing ultraviolet as a
whitish-violet light. One example
is the Impressionist painter
Claude Monet, who developed
bad cataracts in later life and
eventually had his left eye’s lens
removed. His subsequent works
heavily feature bluish colours,
often thought to be the result
of him seeing UV.Brian Horton
West Launceston,
Tasmania, Australia
Normal colour vision rangesfrom wavelengths of around
380 nanometres (violet) to
750 nanometres (red). Most people
can’t easily see light shorter than
380 nanometres because the lens
of the eye absorbs it. If the lens is
missing or removed, often due to
cataracts, light below the violet
range isn’t blocked and can be
detected down to around
310 nanometres. Without the
lens to focus light, these people
are far-sighted and need corrective
lenses to focus at short distances.
Insects can see ultraviolet
light, and some other animals
have vision in this range too.Bob Butler
Llangoed, Anglesey, UK
Some years ago, after being
admitted to hospital with
sepsis, I developed uveitis, an
eye inflammation that could
have caused permanent loss of
vision. The lens of my right eye
was removed and replaced with
an artificial one. The new lens
meant I could see better through
this eye than I ever had before.
On leaving hospital, I decided
I deserved a pint of bitter. Standing
at the bar of my local pub, I noticed
that their device for detecting
counterfeit banknotes was
emitting very bright bluish light.
I mentioned this to the barman,
who looked at me with a very
quizzical expression but made no
comment. I then realised that he
couldn’t see the light: it was visible
through my right eye alone.
It seems that the natural
lens in the eye has a filtering
effect as a protection against
ultraviolet light. I owe the staff
of the emergency eye clinic my
thanks not only for saving my
eyesight, but also for my ability
to see UV light. ❚This week’s new question
Gambler’s fallacy When using an online casino, how can I tell
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stacked against me, or by an AI that knows how frequently
and by how much I, personally, have to be baited to maximise
the casino’s revenue? Dave Neale, Truro, Cornwall, UKWant to send us a question or answer?
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