It’s the diagram that compares physically existing boundaries and the boundaries of what humans can experience. On the left (y axis) you have the time in log scale. The “now” is the begining (left corner down) and the end is the begin of the observable universe. On the bottom (x acsis) you have the distances again in log scale: left the smallest possible right the farthest physically observable (now it’s estimated to be cca 46 billion light years).
The line starting from the lower left ending in the upper right says: the earlier something happened the farther it cam be and remain physically observable (due to the speed of light being finite).
What humans can directly experience is of course small compared to the “all time” and “all space” and it would be invisible without the logaritmic scales used.
My interpretation is that day-to-day human experience is focused on things that are a small subset (range) of all possible scales, and the same applies to time: we don't live a billion years, nor can we perceive a difference of 0.000001 seconds. We have no direct "sense" of really small things nor really big things, and have no sense of really old things and really frequent/fast things.
Not saying Carl Sagan came up with this particular diagram while high on weed, but it’s not altogether irrelevant to the conversation to mention that he privately smoked throughout his career, and credited cannabis for providing insights, documented in an essay he wrote anonymously as “Mr X” for a book on marijuana from the 1970s. [1]
Felt it worth mentioning on the day Congress took the small (yet meaningful) step of legalizing industrial hemp production in the US.
I believe what he is getting at is that there is a limit on certain space times that are accessible to a given observer. Imagine an alien race (or a point in space) that is 10M light years away. We want to obtain information about the current state of that point in space. Unfortunately, the next X years of that section of space (I don't know enough relativity to calculate X) is totally inaccessible to us. There is absolutely nothing we can do to obtain information about the current state of that point until X years has passed.
There are points of space that will NEVER be visible to us as the rate of cosmological expansion over sufficiently large distances outpaces the speed of light.
There's a galactic scam for you: sell beings real-estate that they can never use nor inspect. "A Xerzigian Prince offered me a great deal on some used galaxies..."
The time scale for human experience is around a second or so to 100 years, which makes sense for what we experience, but I also don't understand what past and future mean in the diagram, the vertical axis seems to mean time duration rather than a time in past or future...
Observations take time. The y axis shows the duration of possible observations. Durations shorter than the plank time or longer than the current age of the universe are inaccessible; as are durations into the future, since we can only make observations in the past.
