What is Time?
You know what time is, but have you ever tried to define it? Today, we're gonna talk about the physics of time and how we know it exists as part of our universe.
Time - it’s been marching forward for the last 14 billion years, it doesn’t care if you studied for your test or not, and it can move at different relative speeds depending on an observer's speed or proximity to mass. It never stands still and it never moves backwards.
This is the first installment in a series about time, the third part of PNT (positioning, navigation and timing)! We just released the GNSSDO - a precise clock that can give you precise time measurements under a nanosecond! Today, we're going to look at the physical reality of time. Stay tuned over the next few weeks to learn more about the ways human society has evolved around it and how we've gone about understanding and measuring it.
For today, we're talking physics. Headache guaranteed, existential crisis encouraged. Let’s go!
The Fourth Dimension
We sometimes think of ourselves living in a 3D world, but time is another dimension to our space we don't always remember. If you're meeting someone at a building in a city, you're meeting them at a street address and a floor. Those are your X, Y and Z dimensions. But when is the meeting? If you show up at 2pm for a 9am meeting, you'll miss it! Time is a part of the way we experience space (and thanks to Einstein, we know it's more closely intertwined than you can even imagine), hence the term spacetime.
A way to visualize the four dimensions of spacetime is by looking at a lightcone. The three-dimensional surface of all possible light rays arriving at and departing from a point in spacetime. The more you move through space, the less you move through time, and vice versa. Only things contained within your past light-cone can affect you today; only things contained within your future light-cone can be perceived by you in the future. Fun!
Looking Backwards
Looking into space, we effectively look back in time due to lightspeed not being infinite. For example, the closest supernova to Earth in the last decade was SN 2023ixf, which was discovered in May 2023. It was located in the Pinwheel Galaxy, about 21 million light-years away. This means that this star actually exploded 21 million years ago, it just took that much time for the light coming from it to reach us.
The Earth is about 8.3 light minutes from the sun, so if a sunspot appears on the sun, it'll take observers on Earth 8.3 minutes to see it. This means they're not seeing the sun as it is now, but how it was 8.3 minutes ago. However, we can only look back so far. Our limits to how far we can see are defined by the age of the universe.
According to the Big Bang theory, our universe began as a singularity approximately 13.8 billion years ago, which might make you wonder — what was before the Big Bang? Well, in the context of the Big Bang, time itself doesn’t stretch back infinitely but rather begins at that singular point. This is like asking what's north of the North Pole? There simply isn't a "before" in traditional terms, as time itself, along with space, matter, and energy, came into existence at that moment. (Unrelated, but the magnetic North Pole is moving at what I consider to be an alarming rate, but no one else seems concerned. Thoughts?).
As the universe expanded from this incredibly hot and dense state, time began to tick forward, marking the cooling and formation of fundamental structures like atoms, stars, and galaxies. This expansion is still ongoing (and accelerating), which means the universe is getting roomier by the minute.
The Big Bang is evidenced by the cosmic microwave background, a thermal energy that exists everywhere in the universe and is pretty much the limit on what we can see in all directions. The CMB fills the universe, and before cable TV you could even observe it on the static of your home television screen.
There's more evidence that time had a clear beginning in pretty mundane places - like the fact the night sky is dark. Because we are looking back in time, if time had always existed, the night sky would be entirely full of starlight. We know time started at an exact point because there are many stars in the sky whose light hasn't had time to reach us yet.
What is Relativity?
Things get even more fascinating when relativity enters the chat. Einstein (have you heard of him?) tossed the classical notion of time as a constant backdrop out the window and introduced us to a universe where time is a flexible dimension, deeply intertwined with space itself.
There are two types of relativity, special and general. If you’re attempting to learn the math behind them, I recommend starting with special relativity unless you’re a huge fan of tensor fields. Regardless, the mathematics is really fascinating behind both of them!
According to Einstein's theory of general relativity, gravity is the warping of spacetime by mass which then affects how time passes. Near a massive object, like a planet or a black hole, spacetime is significantly curved, and as a result, time itself is experienced more slowly. If you’ve seen Interstellar, you’ll remember that the closer the team was to the black hole, the slower time moved for them, to the extent that an hour on one planet they visited close to the black hole was equivalent to 7 years passing on Earth.
Special relativity introduces us to the effects of speed on time. As one approaches the speed of light, time slows down relative to observers at rest. This isn’t just theoretical — it’s an observable fact confirmed by experiments involving very fast-moving particles and precise clocks. If you were zipping around the Earth in a super-fast spacecraft, you would age slightly less than everyone chilling back on Earth. This mind-bending reality where speed and gravity can dictate the pace of time itself has big-time implications not just for theoretical physics, but for practical applications like GPS satellites, which must account for these effects to maintain accuracy.
In both these theories, time is not just a passive backdrop against which events occur; it is a dynamic and malleable dimension. But why is it called relativity? Because everything is relative!
The terms “faster” and “slower” don’t mean anything unless they’re showing the difference in duration from one thing to another. Time dilation occurs to events from the point of view of an observer, which means two people flying past each other in ultra-fast spaceships would see the same event differently.
Due to this, events you may traditionally view as happening simultaneously might not be simultaneous, as it depends on the frame of reference of the observer. If two distinct events happen at different points in space, it is impossible to say they occurred absolutely at the same time. Weird!
Will Time End?
Intuitively, we know that something that has a beginning always has an end, but it's weird to think about time itself running out one day. There are multiple theories on what will lead to the end of the universe (and therefore the end of the dimension we know of as time), and they all depend on a funky thing called dark energy. Dark energy is exactly what it sounds like: mysterious and powerful. It's what we call the mysterious energy driving the accelerating expansion of the universe.
Since this energy is so dark and mysterious, we can only theorize about what will happen to the universe, and there are a few theories.
Big Freeze (Heat Death)
Sometimes referred to as "entropic exhaustion," the Big Freeze is the idea that the universe will expand to the point that no free thermodynamic energy is left and everything in the entire universe has reached thermal equilibrium. This is the current prevailing theory due to the universal expansion scientists are observing. Bummer!
Big Crunch
The Big Crunch is the idea that the universe will throw an Uno reverse at the Big Bang and collapse into an infinitesimal point of matter and energy (maybe even bounce back into another Big Bang?). This would require gravity to defeat dark energy, so there isn't as much support for this theory as there is for the Big Freeze.
Big Rip
The Big Rip is exactly what it sounds like - in the case that universal expansion sped up even more than it is now, spacetime itself, the universe and everything in it could be torn apart. Yikes!
Are We Still Studying the Physics of Time?
Until we figure out quantum physics, absolutely! Currently, the big-picture version of spacetime we've talked about doesn't agree with the linear, unchanging version of time that exists on the quantum level. Scientists have been trying to reconcile their observations on the macro scale with their observations on the quantum scale for years now. As a space fan, I've decided quantum physics is none of my business, but if that's your kind of thing I suppose I respect it.
Until Next Time...
I hope I made good on my promise for a headache AND an existential crisis! To let us know your thoughts on time, reach out to us on socials.
Come back soon to find out how we went from using sundials to measure time to devices as precise as the GNSSDO, which can tell time under a billionth of a second!
