Musings Of A Physicist

“There was no ‘before’ the beginning of our universe because once upon a time, there was no time.” ~ John D. Barrow.

In the beginning, almost fourteen billion years ago, everything in the universe—space, matter, and energy—was packed into a tiny area smaller than one trillionth the size of a microprocessor.

It was so hot that the natural forces that shape our universe were all together. We don’t fully know how this began, but this tiny universe rapidly expanded in an event called the Big Bang.

This event marked the start of “time” on a cosmic level.

Cosmic time is the time we refer to when discussing the Big Bang and how the universe expands uniformly. This means we assume the universe has the same density everywhere at each moment.

The ideas of time and space are so connected in our everyday and scientific discussions that it’s tough to separate them and give a clear definition.

Going back to the philosopher Aristotle (around 350 B.C.), thinkers have tried to understand what time really means through various theories. Philosopher Leibniz believed that time should be understood alongside matter and movement, claiming that space and time, while different, are linked together. Many scientists argue that to understand time, we need a clock.

Some scientists think time can be fully described with a four-dimensional mathematical structure, called a manifold. But even though we can express time mathematically, it’s wrong to say it’s just a mathematical idea. A few physicists suggested that if we see time as a container related to space, and apply Einstein’s field equations to it, we might find a physical basis for measuring time, regardless of matter. However, the mathematical structure alone cannot define time without physical scales. So, without some material content to give it a scale, time has no physical basis.

It’s hard to define time simply using concepts that are separate from time. Plus, finding a meaning for time through something real is challenging since we can’t easily point to it.

In everyday language and in physics, we often say things like “this event took time” or “this happened a while ago.” These phrases are usually clear because we can relate them to physical processes that act like clocks. For example, how should we understand “3 years” in a sentence like “3 years passed with no physical process”?

We use ‘clock’ to mean a “physical process that acts as a clock.”

Usually, a clock is a system that undergoes a physical process with a counter that tracks time. Interestingly, a change in the counter (like the hands of a grandfather clock moving) is itself a physical process acting as a clock.

Even though natural laws refine our understanding of time, they don’t give a simple definition because many concepts in those laws still depend on time.

Physicists say time should be defined to make mechanical equations as straightforward as possible. This means we focus on which time parameter works best for the situation we’re looking at.

Since natural laws are closely linked to how we see time, we can say that “time has a clear use” when that use is based on a physical process.

Time also depends on the chosen reference frame within the limited coordinate transformations in special relativity. Thus, to form a universal idea of cosmic time, we need to define a specific coordinate frame. Cosmic time is measured by clocks at rest compared to the expanding universe, starting from the moment of the Big Bang.

Having physical tools like rods and clocks, which provide scales for length and time, is vital in General Relativity to give these concepts a solid foundation.

Two physicists talked about “in-built” rods and clocks in space-time, even without matter. They imagined a clock made of two mirrors moving parallel, reflecting light. However, this approach failed because light’s behavior doesn’t set a unique scale for specific times in the universe, especially during early moments when a physical process can’t be identified.

So, what kinds of clocks could we use in cosmology?

One option is the cosmic scale factor ‘R(t),’ which describes how the universe’s size changes over time. While the equations are complex, they can be solved using the Friedmann equations with certain conditions involving the universe’s energy and geometry.

Other types of clocks could include temperature, black body radiation, the decay of unstable particles, or atomic clocks, each having specific measurement properties.

After exploring time and how to measure it fairly, we can briefly discuss the direction in which time “flows” as we perceive it.

Entropy is one of the few physical quantities that gives time a specific direction. According to the second law of thermodynamics, as time moves forward, the entropy (or disorder) of an isolated system can only increase. This means measuring entropy helps us tell the past from the future.

We all have a sense of how entropy works. For example, it’s easy to notice the difference between watching a video play forward versus backward. A video of a fire melting ice makes sense in one direction but seems impossible in reverse.

Interestingly, for most physical laws, including the second law of thermodynamics, the laws remain valid whether time runs forward or backward. When a physical law works the same way going forward or backward in time, it shows T-symmetry. Because of the second law, entropy stops large processes from showing T-symmetry, which only becomes obvious at a larger scale.

On a microscopic level, we can’t make conclusions since the laws imply T-symmetry.

Thus, the direction of time, or the idea that time only moves forward, is reasonably called the “thermodynamical arrow of time.”

Clausius put it simply:

“The energy of the universe is constant. The entropy of the universe tends to a maximum.”

The “increase in disorder or entropy is what distinguishes the past from the future, giving direction to time.”

While entropy provides a direction to time, time exists even without it.

Since ancient times, people have wondered about time travel—going back to the past or forward to the future. This may seem like fantasy to many, but it’s not entirely out of reach.

One intriguing point of Einstein’s Relativity is that time is relative to the observer. An object moving at high speeds experiences time more slowly, meaning it moves into the future. Other effects include the shrinking of the object’s size and the increase in its mass. As it approaches light speed, more energy converts to its mass, needing even more energy to keep moving. Thus, the speed of light seems to be a natural limit.

This very restriction could make time travel possible, hypothetically.

However, we would need an infinite energy source to reach near-light speeds, which isn’t possible with our current technology.

Another way to see the future is to be near a massive gravitational field, like a black hole. We would age slower than those on Earth because time moves slower in strong gravitational fields, meaning time dilation occurs. So, in a way, we might time-travel to the future.

But what about going to the past?

Currently, per our understanding of physics, we can’t physically travel to the past, mainly due to the rules of entropy. However, we can still observe the past of many distant objects in the universe, given that their light takes time to reach us. For example, light from the Sun takes eight minutes to reach us, so we see it as it was eight minutes ago. Similarly, we can observe galaxies and stars as they were millions or billions of years ago. This method is crucial in astronomy and cosmology for studying the universe’s birth and evolution.

There are countless theories on this topic and many ways to rethink the concept of ‘time’ itself.