Communications of the ACM

behind the pedestal, which limits the viewing angle to those looking toward the projection optics, and so cannot emulate the movies.

Daniel Smalley, an assistant profes-
sor of electrical and computer engineer-
ing at Brigham Young University, says,
“Like many in the holography field, I
felt that holograms would provide the
3D images of the future, but the annoy-
ing issue is you have to be looking in the
direction of the screen that generates
them. It’s counter to what you expect 3D
displays to do in the future.”
Builders of volumetric displays that
can be viewed from any angle face their
own challenge. “Fundamentally, you
have the problem that photons will
just keep traveling until they bounce
off something,” says V. Michael Bove,
principal research scientist and head
of the object-based media group at the
Massachusetts Institute of Technology.

Systems such as the VX1 built by Australian company Voxon Photonics use a fast-moving sheet to provide a reflective surface for photons. At a high-enough speed, the sheet will seem to disappear, but bright lights bounced off it will persist to the viewer; the result is the illusion of a slightly translucent 3D object floating in space. Bove says the need to move the sheet at high speed makes this an intrinsically noisy option, and one likely to suffer from mechanical wear.

Another option is to disperse particles
into the air and illuminate them. A team
led by John Howell, a professor of phys-
ics and optics based at the University of
Rochester, used cesium vapor to create
the voxels in their experimental volumet-
ric display; the cesium atoms glow where
the light from two steerable lasers cross.
Yet in these displays, moving parts and
poisonous particles need to be encapsu-
lated in a transparent dome or sphere.

“What’s of increased interest is not have a display in the table but to interact with it in a meaningful way. Volumetric displays do have this talk-ing-head-in-a-jar character that works against that. You have the sense that this imagery is bottled up,” Bove says.

Smalley also sees interaction as key, citing another Disney movie franchise,

Iron Man, as additional inspiration for
his move away from holographic tech-
nologies. In the first installment of the
movie series, protagonist Tony Stark
uses a 3D projector not just to visualize
the elements of his powered suit, but
also to create a virtual gauntlet around
his hand.

Smalley’s team overcame the need to encapsulate their display by trapping and moving a single dust-sized particle. The prototype uses an ultraviolet laser taken from a Blu-ray player to capture and move the piece of dust. A visible-light source tracks and illuminates it. Physicists have yet to develop a theory that fully explains the process of such photophoretic trapping, but it appears to rely on local heating from being struck by photons. Gas molecules hitting the hotter surface acquire more kinetic energy as they bounce off, pushing the particle away.

Says Smalley, “On average it doesn’t
work very well at all, but in the [statis-
tical] tails you see incredible behavior.
The particle just stays there. You can
even blow on it gently. We had one par-
ticle trapped in there for 15 hours. It
could have stayed for longer: we had to
switch the machine off.”
The particle’s composition seems
to be crucial. Smalley’s team settled
on black liquor—a by-product of the
paper-making process—after trying
numerous candidates. “I do not be-
lieve we can say this is definitively the

All volumetric
displays to date share
the same problem,
Smalley says.
“You don’t have
the self-occlusion
to make objects
that look realistic.”

The gold rush in cryptocurrencies has led cybercriminals to adopt new tactics.

Cybersecurity provider Symantec says the profitability of ransomware dropped in 2017 from an average $1,017 in 2016 to $522 per ransomware event. That’s why many cybercriminals have shifted to using coin miners, software designed to mine cryptocurrencies.

Infecting the computing devices of others in order to amass the processing power needed to mine cryptocurrencies is called cryptojacking. Symantec recently reported the detection of coin miners on endpoint computers had increased 8,500% in 2017.

The risk of being caught cryptojacking is minimal; it is difficult to trace because of the anonymity of cryptocurrencies. Also, cryptojacking scripts do not damage computers or data, and nothing is stolen (except processing power), so there is little incentive to follow up when an attack is discovered.

A common method of cryptojacking involves executing a JavaScript in a browser, stealing resources from the user’s CPU, which are pooled with resources from other cryptojacked devices to mine cryptocurrencies.

Browser-based cryptojacking
doesn’t require a download, starts
instantly, and works efficiently and
surreptitiously in the background;
usually, until the browser session
is closed. Sometimes hackers
will launch a stealth “pop-under”
window or a tiny one-pixel browser
to continue illicitly accessing a
device’s processing power.

Victims might be unaware they have been cryptojacked. The effects are mostly performance-related, and include lags in computers’ execution of commands, slower performance, and overheating.

Most antivirus software
and ad blockers can now detect
coin-mining software. Browser
extensions like No Coin or
minerBlock, and JavaScript blockers
like NoScript, can be installed to
defeat cryptojacking.

Legitimate uses of cryptojacking are beginning to appear online. For instance, digital media outlet Salon started a beta test early this year, using Coinhive to mine the open source cryptocurrency Monero as an alternative to online advertising as a revenue stream. If a visitor has an ad blocker turned on when visiting Salon.com, they might see a prompt to either disable the ad blocker or select a “suppress ads” option. The latter choice allows Salon to put readers’ unused computing power to use mining Monero while they are visiting the site.