Time Exists. It Just Can’t Agree With Itself
I was relieved to learn recently that physicists have concluded time probably exists.
At my age, I thought the evidence was already fairly convincing. There is the face in the mirror, the growing list of doctors, and the remarkable speed with which another birthday arrives. Apparently, however, these observations do not satisfy the standards of theoretical physics.
Physicists want to know whether time is fundamental to the universe, something that emerges from other physical processes, or possibly a very convincing illusion. They have equations, theories, and experiments devoted to answering the question.
I have a calendar.
Our methods are different, but so far we have reached approximately the same conclusion.
My interest in space goes back to the 1960s, when I worked as a structural engineer on the Saturn program. My job concerned the hardware required to get us into space, but the program also encouraged me to wonder about what was waiting out there.
I began reading Carl Sagan and Frank Drake, and later Stephen Hawking and Neil deGrasse Tyson. I never developed their degree of fascination—or their ability to understand the mathematics—but I continued following the questions they were asking. During the last couple of years, I have added astrophysicist Ethan Siegel to that reading.
Perhaps that is why I am willing to follow physicists into a discussion about whether time exists. Once you begin wondering how the universe works, apparently there is no sensible place to stop.
Astrophysicists Live in the Past
Astrophysicists seem to look at the world differently from the rest of us. That may be an occupational hazard.
Most of us look out the window and assume we are seeing what is happening now. An astrophysicist looks into the sky and knows that almost everything being observed happened earlier.
Sunlight takes about eight minutes to reach Earth. We do not see the Sun as it is now. We see it as it was eight minutes ago.
The light from the nearest star beyond the Sun takes more than four years to reach us. The Andromeda galaxy appears as it was roughly 2.5 million years ago. When astronomers observe galaxies billions of light-years away, they are looking billions of years into the past.
Astronomy is really a form of archaeology conducted with light.
That creates an unusual profession. Astrophysicists develop theories about events that happened millions or billions of years ago, then search for ancient light, radiation, or other traces that might show whether those theories are correct.
A geologist can pick up a rock. A biologist can look through a microscope. An engineer can build something and see whether it falls down.
The astrophysicist may have to wait for a telescope to detect a faint signal that began traveling toward Earth before humans existed.
This may explain some of their quirkiness. When your normal working day includes black holes, curved spacetime, invisible matter, exploding stars, and events that occurred before the Earth was formed, your definition of “reasonable” probably changes.
Nobody Agrees on Now
The strangeness does not end with distant galaxies. According to Einstein, there is no single universal clock keeping time for everyone.
Time passes differently depending on how fast you are moving and how strongly gravity is affecting you. Two people can experience different amounts of time between the same events, and neither one is wrong.
That sounds like something invented to excuse being late for a meeting, but it has been tested.
In 1971, physicists Joseph Hafele and Richard Keating took atomic clocks aboard commercial airplanes and flew them around the world. Some of the clocks traveled east, in the same direction as Earth’s rotation. Others traveled west. Another clock remained on the ground.
The scientists were testing two of Einstein’s ideas simultaneously. Motion affects the passage of time, but so does gravity. Flying faster changes time in one direction. Flying higher, farther from Earth’s gravitational pull, changes it in another.
After the trips, the clocks no longer agreed.
The clock flown east had lost 59 nanoseconds compared with the clock left on the ground. The clock flown west had gained 273 nanoseconds. Those results were close to what Einstein’s theories predicted.
A nanosecond is one-billionth of a second. Most of us would not notice losing 59 of them. The average medical waiting room can take considerably more.
But physicists considered the experiment a success because the clocks disagreed in precisely the way the theories said they should.
Only a physicist could fly around the world, return with clocks that show different times, and announce that everything worked perfectly.
A Different Kind of Time Travel
Technically, the people aboard the airplanes also experienced slightly different amounts of time. Flying west made them a few billionths of a second older than the people who remained behind.
This is time travel, although it will not compete successfully with science fiction. Nobody disappeared into the future, met their younger self, or returned in time to buy Microsoft stock.
The travelers simply came home an almost immeasurably different age.
This is probably not enough to influence vacation plans. If flying west makes me several nanoseconds older, I will accept the risk. The time spent waiting at the airport will do considerably more damage.
Yet the experiment demonstrated something profound. Time is not merely a number displayed on a clock. It is connected to motion, gravity, and the structure of the universe. Clocks moving along different paths can experience genuinely different amounts of it.
Modern GPS systems must account for these effects. Satellites move rapidly and orbit where Earth’s gravity is weaker. Without adjustments based on relativity, their clocks would drift away from clocks on the ground, and GPS locations would become increasingly inaccurate.
Einstein’s strange view of time is therefore helping ordinary people find the nearest Costco or gas station.
Why Does Time Move Only Forward?
Physics becomes even stranger when it asks why time seems to move in only one direction.
Many basic laws of physics work just as well forward or backward. If a physicist watches a film of two billiard balls colliding, it may be difficult to determine whether the film is running normally or in reverse.
Everyday life is less cooperative. Eggs can be scrambled but do not unscramble themselves. A glass can fall from a table and shatter, but the pieces never leap back together and return the water to the glass. Hot coffee cools, but cold coffee does not spontaneously become hot.
And people get older.
Physicists connect much of this to entropy—the tendency of energy and matter to spread out and become less available to perform useful work. Entropy gives the universe what scientists call an “arrow of time.”
It points in the same direction as my knees.
The mystery is that scientists can measure time with extraordinary precision and describe how it changes with speed and gravity, yet they still debate why we experience it moving forward and whether time is a fundamental part of reality.
Time is measurable. It affects physical events. Clocks record it. GPS systems depend on it. That seems to establish that time is real.
But “real” and “fundamental” are not necessarily the same thing to a physicist.
The Strange View May Be the Useful One
Astrophysicists may seem strange to the rest of us because the reality they study has repeatedly turned out to be stranger than common sense.
They work with evidence that may have traveled for billions of years. They accept that two clocks can disagree and both be correct. They consider whether the present moment exists everywhere—or whether “now” depends on where you are and how you are moving.
That perspective has produced theories that sounded ridiculous and later became essential to technologies we use every day.
Most of us will continue living as though time is straightforward. We will make appointments, complain about waiting, celebrate birthdays, and wonder how another year disappeared so quickly.
Physicists will continue asking whether time is fundamental, emergent, relative, or something we still do not properly understand.
I am satisfied knowing that time exists.
I would just like someone to explain why the fifteen minutes before an appointment can last an hour while ten years of retirement disappear before lunch.
