Fields, Technology and Society

From equations to everyday life

This morning, a weather forecast used satellite images. A bank transfer was time-stamped with a signal from a GPS satellite. Somewhere in Canada, a patient had a PET scan with a tracer made in a cyclotron that morning. All of these depend on the fields in this unit.

In the last lesson about technology, Field Technologies: Mass Spectrometers, Accelerators and MRI, you analysed how machines work (D1.1). This lesson asks a different question (D1.2): what are their effects on society and the environment, and are the benefits worth the costs? There is rarely one right answer, but there are better and worse arguments. Good arguments use evidence, consider everyone affected and compare alternatives.

GPS: gravity, timing and relativity

The US Global Positioning System has about 31 operational satellites (the count changes as satellites are launched and retired) in six orbital planes, about 20 200 km up, each circling Earth about twice a day (see Satellites and Circular Orbits). Every satellite carries atomic clocks and broadcasts its position and the exact time.

• Your receiver measures how long each signal took to arrive, t, and finds the distance d = ct (light travels about 30 cm in a nanosecond).
• Each distance puts you somewhere on a sphere around that satellite. Three spheres meet at one point (trilateration).
• A cheap receiver clock is not accurate enough, so it needs a fourth satellite to solve for its own clock error as well as its three coordinates.

Because light is so fast, tiny timing errors matter: an error of just 1 μs gives a position error of about 300 m.

Why GPS needs Einstein

Two effects from relativity change the rate of the satellite clocks:

• General relativity: clocks run faster where gravity is weaker. At 20 200 km, Earth's field is much weaker than at the surface (g = GM/r²), so the satellite clocks gain about 45 μs per day.
• Special relativity: moving clocks run slower. At about 3.9 km/s, the satellite clocks lose about 7 μs per day (see Special Relativity).

The net gain is about 38 μs per day. Engineers set the satellite clocks to tick slightly slow before launch, and receivers apply further small corrections. Without them, GPS positions would drift by kilometres each day. GPS is one of the most direct everyday tests of general relativity.

GPS in society

Benefits: navigation for drivers, pilots, ships and hikers; faster emergency response; precision agriculture (tractors that steer within centimetres, using less fertilizer and fuel); surveying; earthquake and ice-sheet monitoring. Less obvious, GPS time signals help synchronize cellphone networks, power grids and financial transactions.

Concerns:

• Dependence: many systems would struggle if GPS were jammed, spoofed (faked) or knocked out by a severe solar storm. Other systems exist (Europe's Galileo, Russia's GLONASS, China's BeiDou), and many receivers use several.
• Privacy: phones and apps can record where you go. Who stores that data, and who can see it?
• Military origins: GPS was built and is run by the US military. Countries that rely on it depend on another country's decisions.

Watching Earth: storm tracking, sea ice and surveillance

Weather satellites in geostationary orbit (about 35 800 km up) watch the same part of Earth all day, so forecasters can follow a hurricane or a line of thunderstorms in near real time. Satellites in low polar orbits pass over the whole planet and measure temperatures and moisture through the atmosphere. Better forecasts and earlier storm warnings save lives and money.

Canada's RADARSAT Constellation Mission (RCM), three satellites launched in 2019 about 600 km up, carries synthetic aperture radar. Radar sees through cloud and darkness, which matters in a northern country with long polar nights. RCM data help:

• the Canadian Ice Service map sea ice for ships and northern communities,
• track ships in Canada's waters, including vessels that switch off their location transponders (for example, to fish illegally),
• map floods and wildfire damage for emergency crews,
• monitor crops, forests and changes in Arctic ice.

The same ability that helps a search-and-rescue team can be used for surveillance. Satellite images can show military activity and protect borders, but they also raise questions: Who may photograph whose land? How sharp should images sold to the public be? How should images of people's homes be stored and shared? Laws such as Canada's Remote Sensing Space Systems Act license and limit Canadian commercial imaging satellites.

Space debris: a tragedy of the commons

Every satellite eventually stops working. Low orbits clean themselves slowly, as thin air drags objects down, but higher orbits can stay cluttered for centuries. Space agencies now track tens of thousands of objects larger than a softball, and estimate that there are more than a million pieces larger than 1 cm, too small to track but fast enough to destroy a satellite.

At orbital speeds (about 7 to 8 km/s, and closing speeds up to about 15 km/s in a head-on crossing), even a small fragment carries an enormous kinetic energy. In 1978, NASA scientist Donald Kessler warned that collisions could start a cascade: each collision makes more fragments, which make more collisions.

The number of active satellites has grown very quickly in the 2020s, mostly from large constellations of thousands of small internet satellites in low orbit. They bring fast internet to remote communities, including in the Canadian North, and lower latency than geostationary satellites. They also crowd low orbit, leave streaks across telescope images and release combustion products and metal particles into the upper atmosphere when launched and when they burn up on re-entry, effects scientists are still measuring. Responses include designing satellites to de-orbit within a few years after their missions end, tracking and collision-avoidance manoeuvres, darker satellite coatings and research into debris removal.

Accelerators, medicine and the environment

The expectation for this strand gives a second example: particle accelerators used for medical imaging.

• PET scans use positron-emitting isotopes such as fluorine-18 (half-life about 110 minutes), made in hospital or regional cyclotrons. Because the isotope decays so quickly, it must be made close to where it is used.
• Technetium-99m, the most widely used medical isotope, was long made mainly in nuclear reactors, including Canada's NRU reactor at Chalk River, which supplied a large share of the world's needs for decades until it stopped routine isotope production in 2016 (it closed in 2018). Canadian teams, including researchers at TRIUMF, developed ways to make it with medical cyclotrons instead, with no nuclear reactor or uranium targets needed and far less long-lived radioactive waste.
• Linear accelerators in cancer centres treat many thousands of Canadians every year.

Costs and concerns: large research accelerators use a lot of electricity (CERN's yearly use is comparable to that of a mid-sized city); building them takes concrete, steel and large budgets; superconducting magnets (in accelerators and MRI) need helium, which is non-renewable and escapes to space once released; and accelerators produce some radioactive materials that must be handled and stored safely. Supporters point to medical benefits, spin-offs (the World Wide Web was invented at CERN in 1989) and training for scientists and engineers.