John A. Wheeler - The Nature of Time
Nautil.us has done it again! In this article they paint a beautiful picture of a tortured physicist searching for meaning through it all! Wheeler was constantly asking himself the real big questions, the kinds that only those with an accomplished career can tackle.
He was obsessed with figuring out the nature of time and its role in the universe. Haunted by the death of his brother and thoughts of his own mortality, he was always thinking about the flow of time, and whether it is a real phenomenon or not! The article described his famous “delayed choice” experimental paradigm, trying to illuminate the role of the observer in actually realizing the universe itself!
The article does a great job contextualizing Wheelers work, with his own history, the history of QM and World War II. Not only did he give the name to “black holes” and “wormholes”, but he revolutionized how we think about the flow of time. When he and Dewitt tried to write down a “wave equation” describing the evolution of the entire universe, they found themselves unable to account for the flow of time, for how could the “clock” tick outside of the universe itself?
He sought an answer in another mystery: quantum-measurement.
when an observer makes a measurement, he doesn’t measure something that already exists in the world. Instead, his measurement somehow brings that very thing into existence.
The article does a great job describing the famous double-slit experiment showing this strange phenomenon. When someone actually tries to measure the slit the photon travels through, the wave behavior disappears.
By choosing which property of a system to measure, we determine the state of the system. If we don’t ask which path the photon takes, it takes both. Our asking creates the path.
To address this, he proposed the famous delayed choice experiment! Here Ill copy from the article since it does a great job explaining it:
Wheeler had realized that it would be possible to arrange the usual double slit experiment in such a way that the observer can decide whether he wants to see stripes or blobs—that is, he can create a bit of reality—after the photon has already passed through the screen. At the last possible second, he can choose to remove the photographic plate, revealing two small telescopes: one pointed at the left slit, the other at the right. The telescopes can tell which slit the photon has passed through. But if the observer leaves the plate in place, the interference pattern forms. The observer’s delayed choice determines whether the photon has taken one path or two after it has presumably already done one or the other.
It gets really crazy when Wheeler suggests “scaling that up”:
Imagine light traveling toward Earth from a quasar a billion light years away. A massive galaxy sits between the quasar and the Earth, diverting the light’s path with its gravitational field like a lens. The light bends around the galaxy, skirting either left or right with equal probability and, for the sake of the thought experiment, arrives on Earth a single photon at a time. Again we are faced with a similar choice: We can center a photographic plate at the light’s arrival spot, where an interference pattern will gradually emerge, or we can point our telescope to the left or right of the galaxy to see which path the light took.
The crazy part:
Our choice determines which of two mutually exclusive histories the photon lived. We determine its route (or routes) start to finish, right now—despite the fact that it began its journey a billion years ago.
WHAT?! Thats absolutely mind blowing! Even crazier is that this experiment was actually done using mirrors on the moon (the first of which was set down by Neil Armstrong in 1969), and the results where just as he predicted!
Time as we once knew it does not exist; past does not come indelibly before future.
And so we’re left with the conclusion that quantum measurement allows observers in the present to create the past. But how?
As usual, I can’t recommend the article enough. Please give it a read!
- Original Article [link]
- FQXi “Physics of Information” Conference [link]
- Wikipedia page [link]
- Google Search for “It From Bit” [link]
Note: This post (and most of the ones to follow) is a combination of paraphrasing, copy/pasting, and my own thoughts/questions.
P.S. Interestingly enough and somewhat unsurprising, I just learned that his student was Richard Feynman! It makes perfect sense now when I think about it.