Directions: After students have had a chance to review the article “Stephen Hawking’s legacy will live on,” lead a classroom discussion based on the questions that follow. See the “Related articles” section to find supplemental articles for students.

PHYSICAL SCIENCES

Discussion questions:

1. What are quantum fluctuations?

Just as there is a Heisenberg uncertainty relation between momentum and position (a small amount of uncertainty, given by Planck’s constant, must always remain in one or the other or both), there is also a Heisenberg uncertainty relation between energy and time. There is always a small amount of uncertainty in energy and/or time. In other words, the shorter the lifetime of a particle, the greater the uncertainty of the energy of the particle. A quantum fluctuation is a temporary variation in the energy in empty space due to the uncertainty principle. This violation of energy conservation, even if only for a very short time, gives rise to the brief existence of pairs of particles and antiparticles (electrons and positrons, pairs of photons or other particle pairs) that spontaneously appear in existence from empty space. Almost as soon as these particle pairs exist, they annihilate each other and disappear. Such particle pairs are “virtual” — the particles and antiparticles do not officially have enough energy to exist indefinitely.

2. What is Hawking radiation?

Throughout all of space, even empty space, virtual particle-antiparticle pairs continually appear and then annihilate each other. If a pair appears near the event horizon of a black hole, one member may fall into the black hole leaving its partner to escape. The black hole emits the escaping particle or antiparticle as radiation is emitted and the mass of the black hole is reduced accordingly.

3. What is a mini black hole?

Most black holes are thought to be created by the collapse of large stars, resulting in black holes with large masses. Stephen Hawking predicted that the very dense state of the early universe immediately after the Big Bang might have created mini black holes with far smaller masses. Stellar black holes are far from Earth, but mini black holes might be floating around anywhere in the universe, possibly even within our solar system.

4. What are multiverses?

Multiverses are parallel universes that exist in addition to our universe. Some parallel universes might be very similar to ours — parallel universes could have branched off from our universe when an event offered alternate outcomes. Other parallel universes might be very different than ours — fundamental constants such as the speed of light and strength of gravity might be different, or the laws of physics could be entirely different. Depending on the constants and the laws of physics, some universes could be more likely to give rise to organized matter and ultimately intelligent life, and other universes could be less likely to do so. Perhaps the Big Bang was when our universe branched off from another universe, or perhaps it was even when two universes collided to produce our universe. Parallel universes have yet to be observed.

Extension prompts:

5. How might you experimentally observe and confirm the existence of Hawking radiation?

To produce Hawking radiation, the starting virtual particle pairs require a sufficiently strong gravitational field at the event horizon of a black hole to rip apart. The rate of particles ripping apart is relatively small. Therefore, observers would need to be relatively close, closer than is possible without getting pulled into the black hole, to observe Hawking radiation. Scientists do not know of any black holes in our solar system that can be directly observed. However, scientists may have overlooked a mini black hole somewhere in our solar system. A mini black hole might evaporate in a relatively large burst of Hawking radiation, so perhaps such large but rare events could be detected with gamma ray telescopes or other instruments that monitor deep space. We might also consider artificial methods of attempting to create a black hole, or we might consider means of sending probes beyond our solar system to visit black holes.

6. How can the production of particle-antiparticle pairs be observed?

The electric field near an atomic nucleus (especially one with high atomic number) can briefly separate virtual electron-positron pairs, although the pairs can still recombine and disappear. Yet if enough energy is provided, those pairs can become real and continue to exist. High-energy photons (such as gamma rays) passing near atomic nuclei generate real electron-positron pairs. This process, called pair production, has been observed and is one of the main methods by which gamma rays are absorbed by matter.

7. What is a gravitational singularity?

Space and time may be regarded as a single flexible sheet in four (or more) dimensions, called spacetime. Gravitational fields are the bending and stretching of spacetime, and gravitational waves are vibrations of spacetime. A singularity is a point at which the curvature of spacetime becomes infinite, or at which the gravitational field becomes infinitely strong. The center of a black hole is a singularity. The initial state of the universe prior to the Big Bang may have been a singularity.

BIOLOGICAL SCIENCES

Discussion questions:

1. Stephen Hawking was diagnosed with amytrophic lateral sclerosis (ALS) when he was 21. What is ALS?

ALS is a disease that destroys the motor neurons, or nerve cells that control muscles. Early symptoms include muscle stiffness, weakness, twitching, cramps and atrophy. As the disease progresses, the patient usually loses the ability to move, speak, swallow and eventually breathe. Patients typically die from respiratory failure within about four years of symptom onset. Some famous people with ALS include: Stephen Hawking (1942-2018), a cosmologist who lived a remarkable 55 years with the disease after his diagnosis in 1963, and Lou Gehrig (1903-1941), who played baseball for the New York Yankees in the 1920s and ‘30s. ALS is still frequently called Lou Gehrig’s disease.

Extension prompts:

2. What are some potential causes of ALS?

The causes of ALS are not well understood. Some ALS cases appear to be due to inherited genetic mutations, such as mutations in the superoxide dismutase (SOD) gene. This gene’s protein product normally helps to eliminate damaging oxygen radicals in neurons and other cells. More specifically, SOD is an enzyme (produced by the corresponding gene) that converts O^2- superoxide radicals into normal O₂ or H₂O₂ (then the catalase enzyme converts H₂O₂ into H₂O and O₂). The enzyme therefore gets rid of damaging superoxide radicals that are by-products of oxidative phosphorylation in cells’ energy-producing organelles called mitochondria. If some ALS patients have a mutation that makes SOD ineffective or less effective, patients’ cells would presumably have more superoxide radicals and more oxidative damage. Why that affects primarily motor neurons and not lots of other cell types is not currently clear. Other ALS cases appear to be linked to repetitive head injuries such as those sustained by some soldiers and athletes. Many ALS cases have no obvious cause.

As ALS progresses, motor neurons in the brain’s motor cortex (muscle control region), brain stem and spinal cord die. Those motor neurons tend to contain abnormal clumps of proteins, including ubiquitin (a protein that usually tags other proteins to be safely destroyed within a cell) and SOD. Just as what triggers the death of motor neurons in ALS remains unknown, it is also currently unknown why other neurons are relatively unaffected by the disease.

3. What are some current and potential treatments for ALS?

Riluzole is a small drug molecule that blocks some sodium ion channels, NMDA receptors and other motor neuron receptors associated with ALS. But the drug currently prolongs life by only a few months.

Potential future treatments may include improved drugs that bind to relevant receptors or ion channels on motor neurons, fight inflammation, reduce protein clumping and target some cells’ protein-destroying systems. Various gene therapy approaches could potentially affect any gene products known or suspected to be involved in ALS, including sodium ion channels, NMDA receptors and SOD.

ENGINEERING AND EXPERIMENTAL DESIGN

Discussion questions:

1. In what ways can engineering be used to assist ALS patients?

Motorized wheelchairs, robotic arms and other technology can assist ALS patients with mobility. As ALS progresses, patients need mechanical assistance with bodily functions ranging from breathing to swallowing. Such devices are currently external, but future versions could potentially be implanted. The most challenging problem is obtaining output signals from patients with advanced ALS, who might only be able to slightly move one facial muscle (like Stephen Hawking). If devices could reliably convert brainwaves to output signals, for example, ALS patients may be able to communicate more information more rapidly, which might improve patients’ quality of life. 

Extension prompts:

2. Stephen Hawking believed that it was important to establish human colonies in space. He expressed that human life on Earth could be eliminated by a virus, war, asteroid or some other catastrophe. How could humans establish colonies in space?

Humans have lived on the International Space Station and other space stations in low Earth orbit for short periods of time over the last 40 years or more. However, those stations must be supplied by frequent cargo rockets from Earth. And the stations’ lack of artificial gravity causes bone and muscle loss as well as other physiological changes that limit the duration of human visits. To be more viable, a space station would need to be large enough to sustain a significant population of people, rotate to generate artificial gravity and have a self-sustaining environment with enough plants and essential materials onboard to provide humans with resources to live, such as breathable air. Living on another planet or a moon would provide access to raw materials, much more room and at least some gravity, but would also likely require initial resupplying. The moon and Mars are the most suitable locations for human colonization.

3. What do you think is the likelihood that human life on Earth will be wiped out by an infectious disease, war, environmental change, an asteroid or other factors? Explain your reasoning.

Student answers will vary. Students can self-select different groups that argue for specific catastrophes that may impact humanity, or argue that no catastrophe will, and summarize their reasoning for their peers.

4. Do you agree with Stephen Hawking that there is a moral imperative for humankind to spread to other parts of space to avoid extinction? Would the universe be better off if humans remain on Earth? Or do you have other ideas?

This is another opportunity for students to self-select into groups based on their opinions and to explain the reasoning for such opinions to the rest of the class. Groups could be in favor of spreading humankind to space, protecting space from humankind or staying on Earth but protecting ourselves from extinction.