How the Quantum Eraser Rewrites the Past | Space Time | PBS Digital Studios

Video Statistics and Information

Video
Captions Word Cloud
Captions
[MUSIC PLAYING] This episode is sponsored by Audible. Can reality be adjusted after events have occurred? That's the unsettling implication of the delayed choice quantum eraser experiment. We recently talked about the weird results of the single particle double slit experiment. They imply some startling things about the nature of reality. Today, I want to talk about an additional, possibly even stranger, version of this experiment, whose results force us to reconsider the nature of causality itself. Now this episode isn't going to make a lot of sense unless you've seen the first one. So go ahead and watch it, if you haven't already. I'll wait right here. OK. So the single particle double slit experiment suggests that things may not exist as well-defined, even real particles, in that strange interim between creation and detection. There's a fuzzy space in which we don't know the particle's location or path. The Copenhagen interpretation would tell us that in this space, a particle is only its wave function, a distribution of possible properties. It's a probability wave that does all the usual wave-like stuff like making interference patterns, until something happens to collapse it. At that point, the Copenhagen interpretation tells us that a true transition happens between wave and particle. But is that right? And if so, what really causes this transition? Does observation of the particle's location force the universe into settling down and deciding which particular reality we happen to be in? To head off any wild metaphysical giddiness, I want to say right now that there's absolutely no reason to believe that observation by a physicist is better at collapsing wave functions then observation by an electronic detector. Or a houseplant, for that matter. We'll talk about what observation really means at a later point. But it's still pretty interesting to see what happens if we try to observe the wave function at different points in the double slit experiment. The great mystery of the experiment is that very particle-like things appear to traverse both slits simultaneously, like you might expect of a wave. Physicists love a good mystery, and so have tried very, very hard to peek to see which slit these particles actually travel through before they produce the famous interference pattern. Turns out the universe is on to us. Any experiment that determines unambiguously which slit the particle traverses destroys the interference pattern. Instead, particles land in simple clumps, one for each slit, as though they were traveling as simple particles the whole time. This is even true if you place detectors on the far side of the slits after the wave particle thing should have already been interfering with itself, just like the wave function is collapsing retroactively, as if the universe is saying, OK, guys. The human figured out which way you went. No more funny stuff. Better pretend like you are particles that whole time. But there's a huge problem with this interpretation. It's impossible to make these measurements without messing up the wave. The interference pattern happens because the waves emerging from each slit are what we call coherent, which is a fancy way of saying that the relationship between the wave form is emerging from the two slits. So the locations of peaks and valleys is predictable and stays consistent as the waves move forward. This coherence is what allows the waves to produce the interference pattern in the first place. But when you place some device in the path of either wave, you mess with this coherence, and so affect the pattern that reaches the screen. By the way, the double slit experiment where you try to determine which slit is traversed is called a "Which Way" experiment. And if the test is done on the far side of the slits, it's called a "Delayed Choice" experiment. Physicists hate being outsmarted by the universe. So they've come up with clever ways to measure which way the particle traveled while still preserving coherence. I'm gonna talk about the most famous, performed in 1999. This experiment made use of a very special type of crystal that absorbs an incoming photon, and creates two new photons, each with half the energy of the original. These new photons are twins of each other. In fact, they're an entangled pair. So fundamentally connected in strange ways that we'll come back to. Place this crystal in front of the double slit to make coherent entangled pairs of any photons passing through. Send one of each pair off to the screen to produce our interference pattern. And use the other to figure out which slit the original photon passed through. Let's focus on detectors A and B here. Detector A lights up if the original photon passed through slit A. And detector B lights up for slit B. If we run this for a bunch of photons, we see that whenever detectors A or B light up, we get a simple pile of photons here at the screen. No interference pattern at all. As though any knowledge of which way the original photon traveled stops it from acting like a wave during its passage through the slits. And crazier, this experiment was set up so that photons reach A or B after their twins reach the screen. So a photon lands on the screen according to the pattern defined by its wave function. And then later, its untangled partner reaches detector A or B, and somehow retroactively influences the previous landing position. It's like the second photon is saying, whoa, whoa, whoa. Someone figured out which slit I came through. You better look like you came through that one, too. This is amazing. And given that we only interact with one of the entangled pair, surely that means we aren't messing with the other. So we aren't ruining the interference pattern with decoherence. Could it get any weirder? This is quantum mechanics. So, yeah. This extra stuff is the quantum eraser. Its job is to destroy any information about the path of the photons. These devices are beam splitters. Just half-silvered mirrors. They work by allowing 50% of the photons through, while reflecting the other 50%. Now you have a new possible outcome. Instead of being reflected to detectors A or B, half of the photons end up in detectors C or D. But this clever arrangement ensures that if C or D light up, we have no idea which slit that photon came from. If we only look at the photons whose twins end up at detector C or D, we do see an interference pattern. It looks like the simple act of scrambling the "Which Way" information retroactively sends the message, OK. Chill. The observer lost the info of which slit we went through. It's safe to have gone through both again. Are we forced to double down on the interpretation that observation of the path causes the collapse of the wave function, and that the wave function can collapse all the way back to wherever our new knowledge extends to in the past? Some sort of retroactive reality cascade? This is a pretty wild story. For that reason it makes sense to be cautious, even of the Copenhagen interpretation. Part of the appeal of the Copenhagen interpretation is that it avoids any physical interaction that moves faster than light. See, when a spread out wave function resolves itself into a set of known properties, say, the location of a particle on the double slit screen, somehow the entire wave function knows to do this-- to collapse at the same instant. But if these wave functions are physical, as the Copenhagen Interpretation would tell us, then there is no real instantaneous physical interaction. By contrast, a physical interpretation of the wave function, like the De Broglie-Bohm Pilot Wave Theory, requires an underlying physicality, a set of defined properties that evolves with the wave function. So-called hidden variables. That's a bit uncomfortable, because to explain experimental results, those physical properties need to act and change instantly at any distance. They need to have what we call non-locality. Now the delayed choice quantum eraser double slit experiment doesn't tell us whether the wave function is physical or not. But it tells us that the Copenhagen, or any other metaphysical interpretation of the wave function, is no less well, crazy-sounding than a physical interpretation. In fact, the solution may lie in this fascinating phenomenon of quantum entanglement. As we'll see in the future, entangled particles really are able to influence each other instantaneously. And we'll see that their non-locality doesn't violate causality. So perhaps they can even affect coherence and decoherence retroactively and physically without making a causal mess. Perhaps this thing we call observation is just entanglement between the observer and the experiment. Perhaps the evolving tapestry of entanglement in all its impossible complexity is what really defines reality in this space time. This episode of "Space Time" is supported by Audible.com. Right now, Audible is offering "Space Time" viewers a 30-day trial period. Check out audible.com/spacetime to access their audio programs and titles. I recently listened to "The Big Picture" by Sean Carroll. It's rare to find a brilliant theoretical physicist who's willing and able to discuss the philosophical and human implications of the science without the nonsense mysticism. Go to audible.com/spacetime. And make sure you use that link so they know we sent you, and to get a membership trial. Last week, Neal Stephenson came on the show to help us figure out how to save humanity from the end of the world. You guys had your own ideas. Mircea Kitsune asks, what if a black hole was headed towards the Earth? Could we stop it? This is a great "end of world" scenario. And the short answer is, no. Long answer-- depends on the type of black hole and how close it gets. The only black holes that we know for sure are buzzing around our galaxy are stellar remnant black holes. And they're at least several times the mass of the sun. Something like that passing through the solar system, even if nowhere near the Earth, would probably disrupt planetary orbits and either rearrange or scatter our planetary system. Even if it passed by the outer reaches of the solar system, it would perturb the Oort cloud and send swarms of comets plummeting inwards. However, there's a theoretical type of black hole, so-called primordial black holes, which may have formed in the first instance after the Big Bang. These may have masses similar to planets rather than stars, if they exist. And one of these passing through the solar system would only be dangerous if it was moving slowly enough, and if it came very close to the Earth. In fact, impact by a primordial black hole was one of the hypotheses that Seveneves scientists proposed for the moon's inexplicable destruction. Tsjoencinema would like to know what would exactly happen if a gamma ray burst hit the earth. I'm glad you asked. By happy coincidence, the wonderful channel, Kurzgesagt, recently published a whole episode on gamma ray bursts. I'll let them answer this one for you. Check the link. A few of you pointed out that I neglected to mention certain known ends of the world that are coming in the distant future. And yeah. Sure, Earth is certainly doomed. The increasing temperature of the sun will cause all of Earth's oceans to evaporate in a billion years, plus or minus, depending on the model. But that's the sort of predictable event that hopefully a super advanced post-humanity can deal with with a bit of basic geoengineering. Increasing atmospheric reflectivity for example, which is something we could do in the very near future, if we wanted to. As for the death of the sun and Andromeda's collision with the Milky Way, OK. Let's deal with one end of the world at a time, people. I also asked you to submit your ideas for extinction-proofing humanity. The prize-- a place on the ark. Here are the winners. In the event of End of World, you guys should show up at your local spaceport. Your names will be on the door list. Error 404, Hoder Not Found had a great idea in which you collect asteroids and put them in orbit around the Earth, ready to sling at any oncoming impactors before they reach us. This is actually a really nice one. The rocks don't even have to be all that big, as long as you spot the incoming rock on one of its nearer passes before the impact pass, you could play a little cosmic billiards to knock it just off course. TimacTR suggests we upload our minds into deep underground computers. Go full virtual? I'm down with that. Although who's to say we didn't already do that? Caleb Tandberg wants to build underwater cities instead of underground cities to protect against gamma ray bursts. I really love this idea. You don't even need much water at all to fully protect us from the gamma ray burst or the subsequent UV. So just below the surface is great. All surface life is laid waste. But hey, The Deathless Face of the Unborn Mind has an extinction proofing idea. Reinvent our monetary economic systems to produce efficient stable and healthy societies that actually have the energy and collective intelligence to progress technologically. OK. Well, if you're going to be sensible about it. But can we please still have the underwater cities? [MUSIC PLAYING]
Info
Channel: PBS Space Time
Views: 1,864,015
Rating: 4.8967757 out of 5
Keywords: pbs, space time, spacetime, quantum mechanics, quantum physics, astrophysics, quantum, quantum eraser, double slit, experiment, photons, restrocausality, causality, time travel, copenhagen interpretation, which-way, delayed choice, interference pattern, wavefunction, physicality, particle, wave, beam splitter
Id: 8ORLN_KwAgs
Channel Id: undefined
Length: 14min 40sec (880 seconds)
Published: Wed Aug 10 2016
Reddit Comments

I really liked this

👍︎︎ 4 👤︎︎ u/Mr_Genji 📅︎︎ Feb 10 2017 🗫︎ replies

your link is wierd

👍︎︎ 1 👤︎︎ u/__sharp 📅︎︎ Feb 10 2017 🗫︎ replies
👍︎︎ 1 👤︎︎ u/niceguy191 📅︎︎ Feb 10 2017 🗫︎ replies

What the fuuuck, that blew my mind.

👍︎︎ 1 👤︎︎ u/thestickystickman 📅︎︎ Feb 10 2017 🗫︎ replies
Related Videos
Note
Please note that this website is currently a work in progress! Lots of interesting data and statistics to come.