Imagine the following scenario: You have two pendulum clocks capable of accuracies only known until now to atomic clocks. Both of the pendulum clocks are synchronized.

You go to a city, at sea level, and leave the first clock there. Then, you decide to climb Mount Fuji with the second clock. Common sense says that both clocks are still synchronized. Why wouldn't be?

But you'd be surprised to discover that's not the case. Without going into the technical details, the rules of general relativity imply a very non-intuitive effect called time dilation.

Earth, like any mass, has a gravitational field that gets weaker as you get further from the Earth's centre. Because the pace of time itself changes due to gravitational effects, clocks will run slower. So, a tick of our second pendulum clock, up in the mountains, will take longer to complete than a tick from our first clock in the city.

## The implications of time dilation

Time dilation breaks our intuition about time and how we understand it. This not only raises deep philosophical questions about the nature of time itself but also:

• Explains why an astronaut travelling at near-light speed would age slower relative to people on Earth (the "twin paradox").
• Is crucial for GPS systems, as time runs faster for satellites compared to receivers on Earth due to lesser gravity, and this difference must be accounted for to maintain accuracy.
• Implies that black holes, with extremely strong gravity, could cause time to slow to virtually a standstill from the perspective of a faraway observer.

If things weren't complicated enough already, thinking about the nature of time will only open up more questions:

• Is time absolute or relative? If two clocks can tick at different rates, that suggests time might not be the same for everyone, everywhere.
• If a year can pass differently for an astronaut travelling at near-light speed, what does that mean for our concept of ageing and experience?
• If extreme gravity can slow time down, could it be possible to manipulate gravity to control time?
• If time can stretch and shrink, what does that say about the nature of the future and past? Are they as fixed as we assume?

As a science fiction reader, two of these questions appeal to my brain the most:

### If extreme gravity can slow time down, could it be possible to manipulate gravity to control time?

Manipulating time would essentially involve manipulating the strength of gravity. Theoretically, if we could develop technology to create a gravitational field strong enough, we could create time dilation effects.

In a nutshell: to control time, you'd need to control gravity. Because in the weird and wonderful world of relativity, it's gravity that influences the flow of time.

### If time can stretch and shrink, what does that say about the nature of the future and past? Are they as fixed as we assume?

Our everyday experiences suggest time is linear; there's past, present, and future in order. But Einstein's relativity upends this notion. If time dilation can occur, it suggests time isn't strictly uniform.

If time can stretch and compress, then maybe our linear concept of past-present-future isn't entirely accurate. Our 'now' could be happening alongside multiple versions of the past and future, all relative to different observers.

In a vast ocean of time, you're in a boat, and the boat's speed and direction dictate your time flow. Another boat might cross your path, moving differently, hence experiencing time differently.

Now, a final question came up from this journey:

## How do we know that gravity influences time and is not time what influences gravity?

Einstein's theory of general relativity describes gravity not as a force, but as a warping of space-time by mass and energy.

However, if we follow the perspective of quantum mechanics, one could indeed argue that the properties of time could influence gravity. These two frameworks - general relativity and quantum mechanics - have yet to be reconciled in a satisfying way. The answer could be awaiting us in the realm of quantum gravity, string theory, or some yet undiscovered framework.

## Quantum gravity

Quantum gravity is a theoretical framework that attempts to reconcile the principles of quantum mechanics, which explains the behaviour of the very small (like particles), with those of general relativity, which explains the large scale, like galaxies or even the universe as a whole.

Currently, these two models are akin to different languages - exceptionally efficient in their own realms, but they don't agree when it comes to extreme corners of the universe, like black holes or the moment of the Big Bang.

Quantum gravity is that sought-after universal language, a theory that could successfully and coherently describe all fundamental forces of nature. However, Quantum gravity is still highly theoretical and abstract, existing more in equations than in tangible, testable predictions. Up until now, we haven't found a way to directly observe or test its predictions, making it remain in the realm of theoretical physics.

However, science is dogged and persistent. Many ideas that were purely theoretical have eventually found their evidence and moved into the realm of fact. So, who knows? Perhaps quantum gravity will have its day.