00:00Your past was written just now. That is not philosophy. That is laboratory physics.
00:06In 2007, scientists ran a bizarre experiment and the results silenced the entire physics community.
00:12The glance you gave the night sky a moment ago was rewriting the fate of a photon that left its
00:16star 131 years ago. Not influencing the future. Rewriting the past. You think you live in the now.
00:24But the now is a sensation your brain invented. And for that photon, leaving and being seen
00:29happened at the same instant. 131 years of travel. For the light itself, that journey took zero time.
00:36So here is the question. If the past can be rewritten by the present, and the present does not exist,
00:42then who is writing your life? This insane idea came from an even more insane man.
00:47His name was John Archibald Wheeler. An American physicist. He invented the word black hole.
00:54But the most unsettling thing about Wheeler was not black holes.
00:58It was a thought experiment he proposed in 1979. The delayed choice experiment. The conclusion is
01:04terrifyingly simple. The observation you make right now can decide what already happened in the past.
01:10That sounds like science fiction. It is not. It is a physics fact, confirmed in a laboratory over and over.
01:17Let us start with an experiment you may have heard of. The double slit experiment.
01:22Fire a beam of light at a wall with two narrow slits. The light passes through both. And on the
01:28screen
01:29behind, it forms a pattern of alternating bright and dark bands. That tells us light is a wave.
01:34Two waves interfere, and that pattern appears. But here is where it gets strange.
01:40Fire the photons one at a time, letting them pass through the double slit individually.
01:44The interference pattern still appears. What does that mean? A single photon somehow went through
01:50both slits at once and interfered with itself. And it gets worse. Put a detector beside the slits to
01:57see which path the photon actually took, and the interference pattern vanishes instantly.
02:01The photon behaves as if it knows you are watching. It stops being a wave and becomes a particle,
02:06taking exactly one slit. This is the strangest thing about quantum mechanics. Observation itself
02:13changes the behavior of what is observed. But Wheeler did not think that was crazy enough.
02:18He asked a sharper question. What if we wait until the photon has already passed the slits,
02:23and only then decide whether to observe it? In 1979, at a symposium at Princeton marking the
02:29centenary of Einstein's birth, Wheeler proposed a delayed choice experiment. Here is the design.
02:35Send a photon through a half-silvered mirror. It has a 50% chance of being reflected and 50%
02:40of
02:40passing through. The two paths eventually meet at a point. If you place a detector there,
02:46you can tell which path the photon took. It behaves as a particle. But if you place a second half
02:52-silvered
02:52mirror at that point, the two paths interfere, and you see interference fringes. It behaves as a wave.
02:58Here is the key. You can wait until the photon has already passed the first mirror,
03:03already chosen its path, and only then decide whether to insert the second mirror.
03:07Common sense says the photon decided long ago whether it took one path or both.
03:12But the result is this. No matter when you make the decision, the outcome matches your choice
03:17perfectly. Choose to measure the path, and the photon took one path. Choose to measure interference,
03:23and it took both. And that choice was made after the photon was already on its way.
03:28In other words, your choice now determined the photon's behavior in the past.
03:32In 1984, physicists at the University of Maryland actually ran the experiment.
03:37The result matched Wheeler's prediction exactly. A delayed choice by the observer really does reach
03:43back and affect the photon's earlier behavior. In 2017, French scientists rebuilt it with far more
03:49precise equipment. The conclusion never changed. Your choice now can rewrite the past.
03:54But Wheeler was not satisfied with laboratory distances. He scaled the experiment up to the
04:00size of the universe. Imagine a quasar 10 billion light years from Earth. Its photons are already flying
04:07toward us. Between the quasar and Earth sits a massive galaxy. Its gravity bends the light like a lens.
04:14This is called gravitational lensing. Because of the lensing, the quasar's light splits into two paths,
04:19bends around the galaxy, and meets again at Earth. Is that not a double-slit experiment on a cosmic scale?
04:26The two paths may be millions of light years apart. And the photon's journey may have lasted 10 billion
04:31years. But here is the question. If you now choose to measure which path the photon came from,
04:37it behaves as a particle, taking one path. If you choose to let the two paths interfere,
04:42it behaves as a wave, taking both. And when you make that choice, the photon has already been traveling
04:48through space for 10 billion years. Which means a decision you make tonight is rewriting the fate of
04:53a photon that left its quasar 10 billion years ago. And that is not the end of it. Wheeler went
04:59further,
04:59with an idea that chills the spine. Perhaps the past of the entire universe is not fixed at all.
05:04The universe is like a feedback loop. Observation now is constantly shaping, defining, even creating
05:11the past. We are not merely observers of the universe. We are participants in it. Perhaps
05:17even its builders. Wheeler called this the participatory universe. He said the universe
05:22was not fully determined at the moment of the Big Bang 13.8 billion years ago. It becomes real,
05:28little by little, through our ongoing observation. Those photons that have been traveling since the Big
05:33Bang, their history and their fate are only settled today, by our looking. And before being observed,
05:39they exist in a strange superposition, all possibilities at once. This is the core weirdness
05:44of quantum mechanics. Before you open the box, the cat is in a superposition of dead and alive.
05:50And before you observe it, the past of the universe is in a superposition of countless possibilities.
05:55But there is one more thing you may not have considered. For a photon, time does not exist at all.
06:01According to Einstein's relativity, when you travel at the speed of light, time stops completely.
06:06What does that mean? A photon leaves a quasar 10 billion light years away and arrives at your eye.
06:12In its own experience, those two events happen at the same moment. Departure and arrival are one
06:18instant. 10 billion years of travel. For the photon, the elapsed time is zero. So Wheeler said that a
06:25photon's history can only be written from its future, because for the photon, past and future do not exist.
06:31Does that mean time is just a human habit? Physicists do think so. In the equations of
06:37physics, there is no such thing as now. Only past and future. The now is something our brains fabricate
06:43to make sense of the world. In 2020, the Nobel Prize in Physics went to three scientists who used
06:49entangled quantum experiments to prove that there is no objective reality independent of the observer.
06:54Different observers may see different facts. And every one of those facts is real. That
06:59conclusion cuts straight through our basic assumptions about reality. So here are the
07:03questions. If the past can be rewritten by the present, if the present does not exist,
07:08if different observers see different realities, then what exactly is the world in front of us?
07:12Are our memories real? Has our life already happened, or is it still being written?
07:18His answer was this. Perhaps the universe needs observers. Without observers, there is no observed
07:24reality. The universe created us. It may be that we, in turn, create the universe. That was Wheeler's
07:32craziest claim. He told us that we are not insignificant specks in a vast cosmos.
07:37We are the necessary condition for the cosmos to exist. Every time you look up at the stars,
07:42you are taking part in the creation of the universe. Every time you make a choice,
07:47you are rewriting some corner of history. After all, as Wheeler himself said,
07:51this idea is crazy enough that it might just be right. And the moment you finish watching this video
07:56may, somewhere unimaginably far away, be rewriting the fate of a single photon that has been traveling
08:01for 10 billion years.
