Albert Einstein said that time was relative, but what does that even mean? Does time really exist? Here's everything you need to know:
What is time?
The definition of time is actually highly contested. The most agreed-upon scientific definition is that time is the progression of events from the past to the future. It is integral to the way humans perceive the world. Time moves only in one direction and does not move backward.
However, the way we quantify time is not consistent on all planes of existence. On Earth, time is measured in hours, minutes, and seconds. A second is equal to the time it takes a cesium frequency to oscillate 9,192,631,770 times, a unit that was derived after extensive scientific study. In reality, the way time passes is greatly affected by the strength of gravity. For example, people in the International Space Station age slightly slower than people on Earth.
Because time doesn't exist in a physical form, many scientists have questioned whether it even exists, as well as how it passes and why. In our day-to-day life, time plays a major role, but it gets more complicated when looking at the larger scheme of the universe.
How is time relative?
According to Einstein, "The past, present, and future are only illusions, even if stubborn ones." This is based on his Theory of Relativity which describes how gravity warps both space and time. This essentially means that the same event can occur at one time for one observer and at a different time for another based on their frame of reference.
This is known as "block time" or the idea that the past, present, and future all occur simultaneously. Basically, the faster an object or person is moving, the slower it experiences the passage of time. It also supports the theory of a four-dimensional universe, where time has its own coordinate system like space, implying that each moment in time has equal existence within the universe.
"Life is like a movie, and space-time is like the DVD," explains physicist Max Tegmark for Space.com. "There's nothing about the DVD itself that is changing in any way, even though there's all this drama unfolding in the movie." He continues on to say that human's perception of time moving from past to future is merely an illusion based on our brain state. Scientists are still trying to explain why humans chronologize time in this way.
A good example of how theoretically, the past and present can occur at the same time is how we on Earth see stars. The stars seen from Earth are light years away so we see the star in the condition it was in several years ago. The Sirius star is 8.6 light years away, so the star we see in the night sky is actually how it was 8.6 years ago. What we as humans experience in the present is actually something of the past.
One thing scientists do agree on is that time does indeed exist and has played an integral role in the universe. The passage of time itself is a topic still being studied by physicists all over the world. One difficulty when studying the movement of time is that there is no physical measurable aspect to it, so defining "now" is nearly impossible.
"The essence of relativity is that there is no absolute time, no absolute space. Everything is relative," remarks Andreas Albrecht, a theoretical cosmologist. "When you try to discuss time in the context of the universe, you need the simple idea that you isolate part of the universe and call it your clock."
What are conflicting viewpoints?
The biggest point of contention among scientists is the idea of the "future." According to the block theory, the past, present, and future all exist in the same plane, however, some scientists disagree with this.
Avshalom Elitzur, a physicist and philosopher told Quanta Magazine "I'm sick and tired of this block universe," in 2016 adding, "I don't think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it's not there." Fellow physicist Lee Smolin agreed, saying, "The future is not now real and there can be no definite facts of the matter about the future."
The source of disagreement comes from the linear way in which we perceive time and how seemingly irreversible changes get made, like how you can't unscramble an egg. Per The Atlantic, Swiss physicist Nicolas Gisin released a series of papers claiming that "time really passes and new information is created" using a mathematical language called "intuitionist mathematics." "The block universe, which implicitly assumes the existence of infinite information, must fall apart," he said.
Does this mean we could find a way to time travel?
If everything exists at once, could we potentially travel back in time? The common consensus is no.
For one, it would violate the second law of thermodynamics stating that entropy or randomness of the universe can only increase. Scientists agree that the past is considered a low entropy state, while the present and future are high entropy states. Some also believe that the energy required to build a potential time machine is more than exists in the universe.
Also in theory, if time travel existed we would have already been doing it, writes physicist Peter Watson in The Conversation. There is also the question of paradoxes like the "grandfather paradox" where you could hypothetically go back in time and kill your grandfather thereby preventing your birth, meaning you couldn't have time traveled.
Based on what we know, time seems to pass in one direction and we don't have a way of tampering with it.