Here's how theoretical physicist Sabine Hossenfelder answered one of Life's Biggest Questions in her book, Existential Physics -- which I wrote about a few days ago.
This is the brief answer to "Does the past still exist?" (At the end of each chapter, Hossenfelder gives a one paragraph summary of her take on the Big Question tackled in the chapter.)
According to the currently established laws of nature, the future, the present, and the past all exist in the same way. That's because, regardless of what you mean by exist, there is nothing in these laws that distinguishes one moment of time from any other. The past, therefore, exists in just the same way as the present. While the situation is not entirely settled, it seems that the laws of nature preserve information entirely, so all the details that make up you and the story of your grandmother's life are immortal.
Her answer is in accord with Einstein's conception of the space-time continuum, as laid out in his special theory of relativity.
A hundred years' worth of observation have confirmed that time has the properties Einstein conjectured at the beginning of the twentieth century. According to Einstein, time is a dimension, and it joins with the three dimensions of space to one common entity: a four-dimensional space-time.
...You can advance this argument for any two events anywhere in the universe at any time and arrive at the same conclusion: the physics of Einstein's special relativity does not allow us to constrain existence to merely a moment that we call "now." Once you agree that anything exists now elsewhere, even though you see it only later, you are forced to accept that everything in the universe exists now.
This perplexing consequence of special relativity has been dubbed the block universe by physicists. In this block universe, the future, present, and past exist in the same way; it's just that we do not experience them the same way. And if all times exist similarly, then all our past selves -- and grandparents -- are alive the same way our present selves are.
They are all there, in our four-dimensional space-time, have always been there, and will always be there. To sum it up in the words of the British comedian John Lloyd, "Time is a bit like a landscape. Just because you're not in New York doesn't mean it's not there."
More than a century has passed since Einstein put forward his theories of special and general relativity. But here we are today, still struggling to understand what it really means. It sounds crazy, but the idea that the past and future exist in the same way as the present is compatible with all we currently know.
In the course of discussing how the measurement in quantum mechanics is an exception to time-reversibility (the evaporation of black holes is the other exception). Hossenfelder speaks about the role, or more accurately non-role, of consciousness in quantum measurement.
This was music to my scientifically-minded ears, since New Age types are notorious for exaggerating the role consciousness plays in creating reality through measurements of quantum phenomena.
"But what is a measurement?" you may ask. Yes, good question. This certainly bothered physicists a lot in the early days of quantum mechanics. By now this question has, luckily, largely been answered.
A measurement is any interaction that is sufficiently strong or frequent to destroy the quantum behavior of a system. Only what it takes to destroy quantum behavior can be (and, for many examples, has been) calculated.
Most important, these calculations show that a measurement in quantum mechanics does not require a conscious observer. In fact, it doesn't even require a measurement apparatus. Even tiny interactions with air molecules or light can destroy quantum effects so that we have to update the wave function.
...This is also why you shouldn't listen to anyone who claims that quantum leaps allow you to think your way out of illness or that you can improve your life by drawing energy from quantum fluctuations and so on. This isn't just off-the-mainstream science; it's incompatible with evidence. Under normal circumstances, quantum effects don't play a role beyond the size of molecules.