What even is time? It’s what many wish they had more of the night before an exam. It’s what we use to document and lay out events from past to present to future.

When U of T physicist Hazem Daoud spoke to The Varsity, he described time as an arrow pointing towards higher entropy. Entropy is the phenomenon in which the world is always moving towards — a state of higher disorder, or chaos. If ice cream is left out, it will melt over time instead of solidifying. Similarly, a clean room will get dirty over time without one’s active effort to keep it clean. The arrow of time is irreversible in the current state of the world, as entropy is always increasing, never decreasing.

Yet, all that to say, nobody actually knows what time is. It has certain properties but not a clear definition. A particularly interesting and useful property of time is that it is not the same for everyone and everything.

How to slow down time

To compensate for the greater distance, time slows down for the moving object.

Before Albert Einstein published his theory of special relativity in 1905, everyone thought that space and time were independent. Everyone’s clock would be the same no matter where they were or how they were moving.

However, Einstein’s publication changed this paradigm. He proposed that the speed of light is the same for all objects because of a deeper conservation of space-time: the conceptual fabric of the universe.

The speed of light for a stationary object and an accelerating one would be the same. This can seem strange because this phenomenon isn’t true for other things. For instance, to a person standing beside a road, a car moving at 60 km/hr will look like it’s going at 60 km/hr. However, to someone in another car moving at 50 km/hr in the same direction, the first car will look like it’s moving at 10 km/hr. This property does not apply to light.

Imagine a light source that emits a beam of light on a stationary object, goes up, bounces against a mirror, and returns to the object. If this object was moving, the beam of light would have to travel a larger distance to come back to the object after bouncing off the mirror. Yet, the speed of light is the same for both a stationary and moving object. To compensate for the greater distance, time slows down for the moving object.

Another factor that slows down time is gravity. This idea was proposed by Einstein in 1915 when he published his theory of general relativity, showing that gravity causes a bend in space-time under mass. The closer you are to a strong gravitational field, the slower time moves. This is a mathematically calculated concept known as time dilation, or the slowing of time through one’s position. In application, someone living at a higher elevation will have a faster clock than someone at sea level. In fact, satellites’ clocks have to take into account the time dilation caused by their altitude when making measurements.

Time-resolved studies

Time-resolved studies look at the intersectional relationship between physics, chemistry, and biology. They study reactions in materials at a molecular scale, using measurements taken in time. The challenge with these studies is that the reactions happen on the scale of femtoseconds, which are one-millionth of one-billionth of a second long.

To view these reactions, the relevant sample of ions or molecules are treated with an ultra-short laser pulse that triggers a reaction. This sample is then probed with ultra-fast pulses of light or electrons to capture the motion of the reaction. The typical way to get a higher resolution of the reaction is to simply make the pulses shorter, similar to how a camera needs a faster shutter speed to capture fast-moving images.

However, in 2021, R.J. Dwayne Miller — professor in the Department of Chemistry at U of T — and Daoud published a paper in the Journal of Chemical Physics suggesting an alternate way of getting higher resolution, where they slow down the movement of the molecule or atom instead of increasing the speed of the laser pulses.

A way to slow down time based on Einstein’s theory of relativity is by moving faster. Therefore, the atom’s clock can be slowed down by accelerating it to high speeds. The same experiment using an accelerated sample can be probed with the same speed lasers, but because the atom itself would be experiencing time dilation, the reaction experienced by the particle is slowed down. A probe laser or beam of electrons can then capture a higher resolution without a faster ‘shutter speed.’

The higher the energy of the accelerated particles, the higher the possible resolution. For instance, the Large Hadron Collider at the European Organization for Nuclear Research can accelerate a hydronium molecule to an energy of 1.8 Tera electron Volts (TeV) (1 TeV = 1012 electron volts). Miller and Daoud suggest that the acceleration can slow down the molecule’s clock by a factor of 100, following Einstein’s equation of relativity.

Using this method of accelerating particles, better resolution can be achieved with current technology. However, this method has its own challenges as well. For example, in order for a particle to be accelerated, it needs to be electrically charged. An accelerator like the Large Hadron Collider can then use an electromagnetic field — which a charged particle is able to interact with — to accelerate the particle. Particles of smaller mass that can be easily accelerated are best suited to this method, as the energy required to accelerate a particle increases with increasing mass. As a result, large, uncharged molecules would be extremely difficult to study using this method.

Nevertheless, this method takes advantage of time dilation and gets better resolution from the existing technology. It uses one of the known ways that time can be slowed down based on Einstein’s theory of relativity. Because of this, advances in research that rely on a higher resolution of chemical processes don’t have to depend on advances in technology — which is extremely useful and therefore quite exciting.

Ridhi Balani was the Physics Correspondent for Volume 144.