Astronomy – October 2019

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Considering all this evidence together and recogniz-

ing that the behavior of Earth’s dynamo is difficult to

predict, we cannot confidently say that our planet’s

magnetic field will soon reverse. We also know from

paleomagnetism that reversals don’t happen periodically.

They occur rather randomly in time — about once every

half-million years or so (on average) — and, typically, a

reversal takes a few thousand years to be completed.

Reversals haven’t had any apparent effect on the

g loba l env i ron ment , t he y don’t a f fec t t he mot ion of t he

continents, and they are not affected by the Sun.

If humankind survives into the distant future and

finds itself amid a reversal, the impact on compass-based

nav igat ion w i l l l i kely be ac com mod ated by tech nolog ic a l

progress. Beautiful displays of aurorae, usually seen only

at high latitudes at night and during magnetic storms,

might be visible at lower latitudes. Unfortunately, I don't

think I’ll live long enough to enjoy such a spectacle.

Jeffrey J. Love

Research Geophysicist, U.S. Geological Survey, Denver

QI

HOW DOES GRAVITY AFFECT

PHOTONS (THAT IS, BEND LIGHT)

IF PHOTONS HAVE NO MASS?
Robert Arrington
Butner, North Carolina

AI

While it is true that photons have no mass, it is

also true that we see light bend around sources

w it h h ig h ma s s due to g r av it y. T h i s i s not bec au s e t he ma s s

pulls on the photons directly, but instead because the mass

warps the space-time through which the photons travel.

Imagine a bowling ball on a mattress. The ball is a

massive object — say, the Sun — and the mattress rep-

resents space-time, in which it sits. (Of course, space-

time is four-dimensional, but it’s a bit harder to imagine

that!) When you place the bowling ball on the mattress,

it deforms the surface. If a grid were drawn on the mat-

tress, you would see the grid deform, so the straight lines

of the boxes were no longer straight. The same is true for

a star sitting in space-time — the star deforms space-

time around it, causing it to curve toward the star.

Now imagine a marble; this represents a photon. If

you roll the marble in a straight line on the mattress and

it comes too close to the bowling ball, the marble will

curve because the mattress it’s traveling on dips and

curves around the bowling ball. This is what happens to

light traveling through space: When it comes too close

to a massive object, it encounters warped space-time and

curves not because it’s being pulled by gravity, but

because t he space-t i me it ’s t ravel i ng t hroug h is cu r ved,

so its “straight” path becomes a curved, bent one.

Alison Klesman

Associate Editor

QI

DO BLACK HOLES AND

DARK MATTER INTERACT

GRAVITATIONALLY? DOES DARK MATTER

FALL INTO BLACK HOLES?

James Smith

San Jose, California

AI

Da rk mat ter doe s i nter ac t g r av it at iona l ly w it h

normal matter and, yes, that includes black

holes. So dark matter can absolutely fall into black holes.

But there’s a catch. Matter, both dark and normal,

can orbit a black hole without falling in, provided it’s

beyond the event horizon, the black hole’s point of no

return. But as normal matter clumps together and inter-

acts with other (normal) matter, friction causes that

matter to lose energy through heat. This causes the

matter to slow down and lose angular momentum,

which it needs to stay in orbit. If the matter loses enough

angular momentum and slows down enough, it will fall

into the black hole.

Dark matter is different. By nature, dark matter

rarely interacts either with other dark matter or with

normal matter, and therefore it isn’t likely to experience

friction or lose angular momentum. Because it can

maintain its angular momentum, most dark matter

orbiting a black hole tends to stay in orbit around the

black hole without falling in.

Alison Klesman

Associate Editor

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All objects with mass

warp space-time

around them; the

more massive an

object, the more

pronounced the

warping it causes.

When photons travel

through the region

near a massive object

that has caused

significant warping,

they follow curved

paths because the

space-time through

which they are

moving is curved.

ASTRONOMY: ROEN KELLY

CURVED

SPACE-TIME,

CURVED

PATHS