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4 Scientific American, April 2019

LETTERS

[email protected]

TALES OF ENTANGLEMENT

“Spooky Action,” by Ronald Hanson and

Krister Shalm, discusses quantum entan-

glement, in which two particles exhibit a

“spooky” connection regardless of distance.

The authors do not explain why some-

thing as nonspooky as the following can’t

be going on: Suppose I hide a pair of gloves

in two different envelopes and send one

(without knowing which) to my friend on

Mars, with a note to open it on receipt. The

envelopes are now “entangled” because if

my friend finds a left glove, then I will find

a right glove, and vice versa—before a light

signal has had time to travel to Earth.

Gordon B. Hazen Professor emeritus

of industrial engineering and manage-

ment sciences, Northwestern University

The article made this old sci-fi fan’s imag-

ination run wild. Do we know that entan-

gled particles are “monogamous”? If an

electron can be entangled with one part-

ner, why not multiple partners simultane-

ously? Could entanglement be a relation-

ship among a large number of particles

independent of location? And if so, could

manipulating allow for truly instant mes-

saging across interstellar distances?

Bob Morrison Asheboro, N.C.

THE AUTHORS REPLY: Regarding Ha-
zen’s suggestion: Just as “correlation does
not imply causation,” it does not always
imply entanglement. The nonspooky cor-

relation of the two gloves is determined

the moment they are placed in their enve-

lopes and is an example of a “hidden vari-

able theory.” John Bell showed that any

such theory will not have correlations that

are as rich as those allowed by quantum

entanglement. In our experiments, once

our particles are sent to their distant loca-

tions, they are randomly measured in one

of two ways. Because the particles do not

know in advance how we are measuring

them, they cannot agree ahead of time how

to correlate their outcomes. It appears as if

measuring one particle randomly and in-

stantaneously influences its distant part-

ner, which is the spookiness that Albert

Einstein referred to.

In answer to Morrison: It is possible to

entangle many different particles with

one another, and this is an active area of

research—for instance, for building quan-

tum computers. But if two particles are

maximally entangled, there can be no en-

tanglement with any other particles at the

same time. In that sense, entanglement is

indeed monogamous, which ensures a lev-

el of privacy that is unmatched in classi-

cal physics and is at the heart of quantum

applications in secure communication.

Alas, faster-than-light communication

must remain science fiction. With entan-

glement, the outcome is random but corre-

lated. Let’s say you and a distant friend

share entangled electrons and have agreed

that if they are measured to be “up,” that

means “yes,” whereas “down” means “no.”

Your partner will get the same result as you,

so the electrons appear to have somehow

influenced one another faster than the

speed of light. But there is no way to force

your electron to be “up” to send a “yes” re-

sponse. When it is measured, the electron,

not you, will “choose” with a 50  percent

probability of whether it will be up or

down. It is no better than flipping a coin.

WISE TECHNOLOGIES

In “Sacred Groves,” Madhav Gadgil dis-

cusses the ecological benefits of areas of

primeval forest in India protected as the

homes of deities. Gadgil illustrates how

the sacred in traditional cultures can

transmit practical wisdom distilled from

bitter experience, which we need to help

save our planet. Such perspectives make

me less sanguine about the technologies

Scientific American often features. I ap-

preciate the lure of discovery, having had

a 40-year career in the physics of the

earth, but not everything we might create

ought to be created. All technologies must

be evaluated in the current reality that

anything that can be exploited for a profit

probably will, regardless of the dangers.

The greatest service to humanity at the

moment would be rendered not by imple-

menting technical innovations of uncer-

tain benefit but by gaining the emotional

maturity to appreciate and act on the kind

of wisdom portrayed in “Sacred Groves.”

Geoff Davies Retired senior fellow,

Australian National University

OCEAN CONSERVATION

In “The Last of the Ocean Wilderness” [Fo-

rum], Kendall Jones and James Watson

raise the point that we have depleted about

“90  percent of formerly important coastal

species” and that any conservation agree-

ments should set wilderness-retention tar-

gets. But their recommendation does not

go far enough. I think to be successful over

the short and the long term, we must also

spearhead public campaigns that identify

overexploited ocean species, explain what

classifies as pollution and how it can affect

ocean life, and make a list of recommend-

ed actions geared toward the average con-

sumer. And we should monitor overfish-

ing or illegal fishing by enforcing existing

legislation; using nontransferable unique

numbers to tag fishing vessels; encourag-

ing seafood traceability through documen-

tation; and monitoring vessels via inspec-

tion stations, drones and satellites.

Vasilios Vasilounis Brooklyn, N.Y.

December 2018

“All technologies

must be evaluated

in the current reality

that anything that

can be exploited

for a profit probably

will, regardless

of the dangers.”

geoff davies

australian national university