Does the Past still exists?

The concept of time is one of the greatest mysteries in both our existence and the field of physics. We perceive time as a continuous flow, with a distinct moment called "now." As you watch this video, half an hour has passed since you were engaged in something else. Whatever you did in that time cannot be changed, and what you will do in the next half hour is entirely up to you. However, the understanding of time in physics differs significantly from our perception. Albert Einstein, the renowned physicist, was the one who unraveled this disparity. It seems he had quite a comprehensive understanding of the subject.

What did Einstein teach us about the past, present, and future? That is precisely what we will delve into today. The topic we are discussing today is explored in greater detail in my upcoming book, "Existential Physics," set to be published in August. For more information about the book, please visit existentialphysics.com. Typically, we think of time as a uniform concept that applies to everyone and everything. If one second passes for me, it also passes for you and the clouds above. This notion makes time a universal parameter, representing the passage of time and our shared understanding of "now."

However, Herman Minkowski was the first to question this perspective. He observed that Maxwell's equations of electrodynamics made more sense if time was treated as a dimension rather than a parameter. Similar to how a ball remains unchanged when you rotate one direction of space into another, Maxwell's equations exhibit the same consistency when you rotate one direction of space into time. Minkowski proposed the idea of combining space and time into a four-dimensional spacetime, allowing for the rotation of space into time, just as we can rotate two directions of space into each other. This concept naturally explains the symmetry found in Maxwell's equations.

It is not dependent on electric and magnetic fields but rather stems from the fundamental properties of space and time themselves. While I cannot visually depict a four-dimensional object like a flower, I can represent it with two straight lines one for time and the other for space. Being referred to as an observer entails the responsibility of observing various aspects. Surprisingly, there is a significant amount of debate on this topic within scientific literature. Let's bypass this intriguing debate and adopt the term "observer" in the same manner as Einstein did, referring to it as a coordinate system. This coordinate system is utilized by theoretical observers, whether they be dolphins or any other entity. While this may not align precisely with the FBI's definition of an observer, if it was acceptable for Einstein, then it is acceptable for us as well. Einstein's assumption essentially implies that any coordinate system is equally valid for describing physical reality. These four assumptions may seem innocuous initially, but they have far-reaching implications. Let's delve into the first and third assumptions. The finite speed of light and the principle that nothing can exceed the speed of light are fundamental concepts. However, a physicist might argue that you cannot be certain because the light emitted from the screen has not yet reached you. How can you be certain that your phone is still there? One way to address this question is to acknowledge that you may not know for certain if anything exists at this moment aside from your own thoughts. Descartes famously stated, "I think, therefore I am," encapsulating this idea. While this perspective is not incorrect, it does not align with how most individuals commonly use the term "now."

We frequently discuss events occurring in the present without considering the time it takes for light to travel. Why can't we simply agree on a shared concept of "now" and proceed accordingly? Observers believe that the events that occur now are usually too small to notice in everyday life. It is only when relative velocities approach the speed of light that these events become noticeable. Therefore, we typically do not perceive them. If you and I were to discuss who knocked on the door at this moment, we would understand each other without any confusion, even if we were moving at nearly the speed of light. However, referring to the concept of "now" would become very perplexing. This is already quite mind-boggling, but it becomes even more complex. There are two space-time diagrams and consider any events that are not causally connected. Diagrammatically, this is represented by a line connecting the two events with an angle less than 45 degrees to the horizontal. The previous example with mirrors demonstrates that for any two such events, there is always an observer for whom those events occur simultaneously. You just need to imagine the mirrors flying through the events and the observer flying directly in the middle, adjusting their velocity until the photons hit both events at the same time. So, any two causally disconnected events happen simultaneously for some observer.

Now, consider any two events that are causally connected, such as eating too much cheese for dinner and feeling terrible the next morning. Find an event that is not causally connected to either of these events, for instance, a supernova exploding in a distant galaxy. There will always be observers for whom the supernova and your cheese dinner occur simultaneously, as well as observers for whom the supernova and your morning after occur simultaneously.

Let's bring all of this together. If you are comfortable with the idea that something exists now that is not here, then according to Einstein's fourth assumption, this must be true for all observers. Therefore, if you believe that certain events are happening now, and all other observers claim that the events happening at the same time are different, then it is possible for all of these events to exist simultaneously.