Jetpacks, disintegrator rays, nuclear energy, Mars.

Decades ago, academics, futurists and government agencies cast their predictions of what would happen by the year 2020. Would submarines reach historic depths? Who would lead nations, and which ones would be global superpowers? Would Planet Earth even exist as we knew it?

"I shall not be surprised if on my 92nd birthday I am able to go for a ride in an antigravity car," mathematician and scientist D.G. Brennan wrote in 1968.

Some, like Brennan, were overly optimistic. Others were spot-on. Here's what happened, what didn't and what was just plain crazy.

1. Life expectancy will rise past 100

Futurist Ray Kurzweil predicted in 1999 that human life expectancy would rise to "over one hundred" by 2019.

"Computerized health monitors built into watches, jewelry, and clothing which diagnose both acute and chronic health conditions are widely used. In addition to diagnosis, these monitors provide a range of remedial recommendations and interventions," he wrote in "The Age of Spiritual Machines."

Nope: While Kurzweil may have accurately predicted health-related gadgets (such as fitness watches, BioScarves and EKG apps for your smartphone), he jumped the gun on life expectancy.

In 2019, the average life expectancy of the global population was 72.6 years, according to the United Nations. That average is slightly higher in the USA, at 78.6 years in 2017, according to a report in the Journal of the American Medical Association.

Though life expectancy in many industrialized nations continues to inch up, it has been going in the opposite direction in America. The United Nation predicts the average global life expectancy to rise to 77.1 years by 2050.

Life expectancy:What countries have the longest?

2. Computers will be invisible

Kurzweil had several other prophecies for the year 2019, including invisible computers.

"Computers are now largely invisible. They are embedded everywhere – in walls, tables, chairs, desks, clothing jewelry and bodies. People routinely use three-dimensional displays built into their glasses or contact lenses. ... This display technology projects images directly into the human retina."

Keyboards and cables would become rare, he said.

It's true: Computers are embedded everywhere these days. We have smart homes, smart tables, smart chairs, smart desks and more. Though we may not project images directly into our retinas, Google Glass comes pretty close.

(Per Kurzweil's prediction, scientists are developing smart contact lenses capable of monitoring the physiological information of the eye and tear fluid that could provide "real-time, noninvasive medical diagnostics." Several groups are testing smart lenses that would measure glucose levels in the tears of people with diabetes.)

3. Books will be dead

"Paper books and documents are rarely used or accessed. Most twentieth-century paper documents of interest have been scanned and are available through the wireless network," Kurzweil predicted.

Wrong: Though the net revenue of the U.S. book publishing industry has decreased since 2014, the industry still sold 675 million print books and brought in nearly $26 billion in 2018, according to the Association of American Publishers' annual report.

4. Your every move will be tracked

Kurzweil predicted that privacy would be a huge political and social issue and that "each individual’s practically every move (will be) stored in a database somewhere."

True, most say: Your TV watches you. Your smartphone follows you. Your web browser traces your digital trail. In an era when some populations worldwide live under 24/7 high-tech surveillance, most U.S. adults say they do not think it is possible to go through daily life without having data collected about them by companies or the government, according to a survey of U.S. adults by Pew Research Center.

More than 80% say the potential risks they face because of data collection by companies outweigh the benefits.

5. World's population will reach 8 billion

In 1994, the International Food Policy Research Institute projected the world population would increase by 2.5 billion to reach 8 billion by 2020. India, Pakistan, Bangladesh and the continent of Africa would add 1.5 billion people, the institute predicted.

Close, but no cigar: The world's population is 7.7 billion, according to a report in June from the United Nations. The report expects the population to grow by 2 billion in the next 30 years. Around 2027, India is projected to overtake China as the world’s most populous country, the report says.

6. China will be the world's largest economy

Speaking of China... in a 1997 article titled "The Long Boom," futurists Peter Schwartz and Peter Leyden said China would be on top.

"By 2020, the Chinese economy has grown to be the largest in the world. Though the U.S. economy is more technologically sophisticated, and its population more affluent, China and the United States are basically on a par," they said.

Close: In 2019, China still trailed the United States as the world's second-largest economy, by nominal GDP. Recent reports predict that China and India will overtake the United States by 2030.

7. We'll have self-driving cars

"Self driving cars are being experimented with in the late 1990s, with implementation on majors highways feasible during the first decade of the twenty-first century," Kurzweil wrote.

Kind of: Dozens of companies – including Tesla, Google spinoff Waymo and ride-sharing giants Uber and Lyft – are testing self-driving vehicles in select locations, such as Boston, Las Vegas, Phoenix and Columbus, Ohio.

Widespread use of a truly autonomous vehicle is not yet here: Experts say it might still be decades before we see a car that can drive anywhere it pleases.

Self-driving shuttle:Why a Rhode Island police officer pulled it over

In April, Tesla CEO Elon Musk said the company would have fully self-driving cars ready by the end of the year and a "robotaxi" version – one that could ferry passengers without anyone behind the wheel – ready for the streets next year. Tesla owners can "summon" their cars, which can drive themselves from parking spots to the curb, where the owner is standing.

What about flying cars? Porsche and Boeing partnered to create "premium urban air mobility vehicles," and Uber plans to launch a flying taxi service in 2023.

(Planes are landing themselves.)

8. It will be normal to retire at 70

In his 1994 book "The World in 2020," British commentator and editor Hamish McRae foresaw retirement ages reaching 67 to 70.

"The main motive for this in Europe is cost to the state. Governments feel that if retirement ages are not raised, the burden of paying pensions will be so high that working people will not be prepared to pay the tax levels necessary to fund them," McRae wrote.

No: In the USA, the average retirement age in 2016 was 65 for men and 63 for women, according to the Center for Retirement Research. That number has stayed relatively steady for men over the past few decades but has increased for women. For full Social Security benefits, the age is slowly climbing and depends on the year that you were born.

Some European countries set 67 as retirement age, the earliest age when citizens can start withdrawing pensions, and several plan to raise the age in the next few years, according to the Finnish Center for Pensions. For many Europeans, the topic is a matter of fierce debate.

9. Americans will vote electronically from home

As the millennial generation comes of age, they'll be able to vote electronically from home, Schwartz and Leyden predicted – possibly as soon as the presidential election of 2008.

Not yet: Amid fears of foreign interference in U.S. elections, lawmakers aren't about to let you vote in next year's presidential election on your iPhone. Proponents of online voting say it could improve turnout and prevent voter suppression at polling stations.

Startups are developing solutions for online voting, such as the Voatz mobile voting platform, which has used biometric/facial recognition in at least four public election pilots in the USA. Last year, West Virginia began using Voatz for absentee voting for military personnel stationed overseas. In one Utah county, citizens with a disability were able to vote electronically on their smartphones in a 2019 municipal general election.

10. China will be on a path to democracy

Schwartz and Leyden predicted that, despite taking "draconian measures" to avoid an internal crisis in the first decade of the new century, China "is generally acknowledged to be on a path toward more democratic politics – though not in the image of the West."

Not really: China faces scrutiny over human rights abuses against pro-democracy protesters in Hong Kong and against nearly 1 million Uighurs, a predominantly Muslim population, members of which have been arbitrarily detained and imprisoned in "reeducation" camps in China's Xinjiang region.

11. We'll have 'personal companions'

In his 1999 book, "Business @ the Speed of Thought," Bill Gates predicted personal devices that "connect and sync all your devices in a smart way, whether they are at home or in the office, and allow them to exchange data."

"The device will check your email or notifications, and present the information that you need. When you go to the store, you can tell it what recipes you want to prepare, and it will generate a list of ingredients that you need to pick up. It will inform all the devices that you use of your purchases and schedule, allowing them to automatically adjust to what you're doing," Gates wrote.

Hey, Alexa: Add milk to my shopping list.

Though Alexa isn't sorting through your emails (that we know of) and your smart thermostat isn't tracking your purchases, Gates isn't far off. Siri, Google Assistant, Amazon Alexa and an array of smart tech in the Internet of Things readily exchange data with your other devices and respond to commands.

12. Cars will be able to go months without refueling

Schwartz and Leyden predicted that by 2010, "hydrogen would be processed in refinery-like plants and loaded onto cars that can go thousands of miles – and many months – before refueling."

By 2020, they said, almost all new cars would be hybrid vehicles, mostly using hydrogen power.

What's the future of the auto industry? Hydrogen cars appear to give way to electric

Not yet: Toyota and Honda lead the hydrogen-powered car market, but it's an uphill battle against competitors peddling battery-powered electric vehicles. In 2018, 2,300 hydrogen fuel-cell vehicles were sold in the USA – less than 1% of the number of electric cars sold, according to InsideEVs, which covers electric vehicle news.

In other green transportation news: Last year, European railway manufacturer Alstom launched the world’s first hydrogen fuel cell train, and next year, London is likely to roll out double-decker hydrogen-powered buses.

13. Heart disease, depression will be world's top diseases

In 1996, the Harvard School of Public Health and the World Health Organization predicted that by 2020, the world's top two causes of the global burden of disease – a measurement of the number of healthy life years lost because of sickness, disability or early death – would be ischemic (coronary) heart disease and unipolar major (clinical) depression.

At the time, the leading causes were lower respiratory infections (such as pneumonia) and diarrheal diseases, the study said.

Close: In 2017, the most recent year that the data set was published, the five leading causes of the global burden of disease were neonatal disorders, ischaemic heart disease, stroke, lower respiratory infections and chronic obstructive pulmonary disease.

14. Global surface temperature will increase

(Climate predictions tend to have a longer range, but here's a snapshot of where 2020 stands.)

A report in 1995 by the Intergovernmental Panel on Climate Change predicted that the average global surface temperature could increase by about 3.6 degrees Fahrenheit by 2100.

The report predicted that sea level could increase by about 20 inches by the same year.

On track: With 80 years to go, both predictions appear possible. The global average temperature has risen a tad more than 1 degree Fahrenheit since the mid-90s, according to NOAA. Since 1992, the global sea level has risen a total of more than 3 inches, according to NOAA.

More:Were the predictions we made about climate change 20 years ago accurate? Here's a look

15. Humans will set foot on Mars

A report in 1996 by the Space Studies Board of the National Research Council said NASA would launch "possible human exploratory missions to the moon and Mars within the next quarter century," predicting that humans would land on Mars by 2018.

Schwartz and Leyden envisioned a similar scenario: "In 2020, humans arrive on Mars. ... The four astronauts touch down and beam their images back to the 11 billion people sharing in the moment. The expedition is a joint effort supported by virtually all nations on the planet, the culmination of a decade and a half of intense focus on a common goal."

Not quite: Though we haven't set foot on Mars, we've landed eight unmanned spacecraft on the planet's surface.

16. Boris Johnson would lead Brexit

In 1997, British news organization The Independent forecast that in 2020, Boris Johnson would become a member of the Cabinet of the United Kingdom, a decision-making body composed of the prime minister and a team of handpicked members of Parliament.

At the time, Johnson, 32, was an outspoken editor and columnist but had not held public office. "Not shy in clashing with party lines, Boris would 'renegotiate EU membership so Britain stands to Europe as Canada, not Texas, stands to the USA,' " the journalists wrote.

Pretty close: Have you heard of Brexit? Johnson became prime minister in July. He served in the Cabinet, starting in 2016 as foreign secretary under Theresa May. In December, Johnson led his Conservative Party to victory in a national election on the promise to "get Brexit done."

17. Antigravity belts will revolutionize warfare

Imagine a world where battles are fought a few feet above the ground, as soldiers hover in midair. In 1968, mathematician and scientist D.G. Brennan predicted that antigravity belts would "revolutionize the tactics of land warfare," writing that "even if the antigravity mechanism did not itself provide horizontal propulsion, relatively modest sources of thrust could easily be provided."

He suspected that by 2018, humans would have antigravity cars and jetpacks capable of operating for 30 minutes.

No: (Unless you're Luke Skywalker or Buzz Lightyear.)

18. Nuclear will replace natural gas

In 1968, Stanford University professor Charles Scarlott predicted that nuclear breeder reactors would make up the majority of U.S. energy production by 2018 as natural gas faded.

"Energy from water power, solar radiation, the wind, tides, or earth heat will not figure large in the totals. Power from nuclear power plants should be available in large amounts at low cost," Scarlott wrote.

Wrong: In 2018, fossil fuels – petroleum, natural gas and coal – accounted for about 79% of total U.S. primary energy production in 2018, according to the U.S. Energy Information Administration. About 12% was from renewable energy sources, and about 9% was from nuclear electric power.

For the electrical power sector, in particular, nuclear creates 19% of energy.

19. Americans will work 26 hours a week

In 1968, physicist Herman Kahn and futurist Anthony J. Weiner said that by 2020, Americans would work 1,370 hours a year (or 26 hours a week), instead of the 1,940 hours (37 hours a week) that was average at the time.

Unlikely: Though we work less than we did in 1968, the average American worked nearly 1,800 hours in 2018 (35 hours a week), according to the Organisation for Economic Co-operation and Development.

20. Nationalism will wane

In 1968, Ithiel de Sola Pool, a political science professor at the Massachusetts Institute of Technology, predicted that better communication, easier translation and greater understanding of the nature of human motivations would make it easier for people to connect across ethnic and national lines.

"By the year 2018 nationalism should be a waning force in the world," he wrote.

The opposite is true: Fueled by backlash against immigration, globalization and the political establishment, populist nationalism was a driving force behind Brexit, the election of Donald Trump and the rise of right-wing politicians in France, Austria, Italy, Hungary and Poland, among other countries, academics said.

"Everywhere one looks, in fact, one sees nationalism at work in today’s world," Stephen Walt, a professor of international relations at Harvard University, wrote in Foreign Policy Magazine.