“Have you ever hear of the Delayed Choice Quantum Eraser experiment?” asked Doctor Franklin Lum, sitting down at the coffee shop table across from me.
That was it. No preamble, no “Hi, hello, how’s it going Wendi?” Just a question about physics.
Not that I was surprised mind you, Frank was a brilliant scientist and quantum mechanics savant, but his people skills were a little… lacking.
“Hi Frank,” I said pointedly. “Oh, I’m fine, how are you?”
My sarcasm wasn’t lost on him, and he blinked and looked a little sheepish.
“I did it again didn’t I?” he said. “I’m sorry Wendi, I just get so excited, and this one is… “
I rolled my eyes and he stumbled to a halt.
“Er - How is, um… everything?”
I sighed. It wasn’t worth making the poor guy uncomfortable, so I relented. “Nevermind Frank, go ahead tell me about this Quantum Choosey… thing. Do you want a coffee?”
He relaxed visibly. “No. Delayed Choice Quantum Eraser… Um… have you heard about the Two Slit experiment?”
I nodded. That was basic oddball quantum weirdness at it’s core. “Yeah,” I said. “You fire a photon at a pair of slits. When you don’t know which slit it goes through it acts like a wave, but when you do it acts like a particle.”
He nodded. “Most physicists chalk the whole thing up to some kind of oddity with how you measure the photon and which slit it goes through.”
I nodded.
“So with the DCQE they use Entangled Particle Pairs. They disentangle after going through the slit and one of the particles goes left, and the other goes right. But the result is the same. If you measure, both pairs show up as particles, if not - waves. With me so far?”
I nodded again.
“Okay, so some bright people figured out a way to tell which slit the particle comes through by measuring the delay from when the one on the left is measured versus the one on the right. They don’t detect anything until AFTER it’s already gone through the slit.”
I blinked. “So they don’t look at the slit to see if a particle went through?”
He shook his head, grinning. “Nope. They look at the delay. Slightly longer equals left slit. Slightly shorter, right slit.”
I nodded. I could see where he was going with this. “But… okay so you know which slit it goes through. That means it always acts like a particle, right?”
His grin widened. “Yeah. So what they did was set up a random chance - 50/50 - that the data would kinda… self destruct on the right side. Guess what they found?”
I grinned. “When the data self destructed on the right-hand particle…” I said slowly.
He nodded eagerly.
“The left-hand particle acted like a wave?”
“YES!” He practically shouted. “It was so accurate that they could determine the results of the right-hand particles by looking at the left-hand results! They knew if they saw a wave pattern the results on the right were lost!”
“The effect came BEFORE the cause?”
“Exactly!” he said. “The wave pattern showed up before the data was lost. The only element that was different?”
“That we knew the results of the other tests. We knew which slit the particles went through.”
“Right!”
I nodded. It was pretty cool, but not very practical. “Okay, so why are you so excited?” I asked him.
He was bouncing in his seat now like a kid who needed to pee, he was so excited.
“Okay. So, we know how to see effect preceding cause right? What if we tweaked the circumstances?”
I shrugged. “How?”
“Okay so if someone sees the result it makes a particle, if the result is never known it shows a wave. What if we make a machine that does the test multiple times and the result is… Oh let’s say marked on a piece of paper or something. Just an L for left slit, or R for right slit, right?”
I nodded.
“Okay so you have the papers all arrayed in a line, but you only look at say 3 out of 10. The rest are burned. What happens?”
I thought I could see what he was getting at. “If the papers are burned and no one ever sees them the results are lost. They should be wave patterns.”
“And the ones you looked at?” he prompted.
“They should be particles.”
“And what if you wait five minutes… a week… a YEAR… to look at the results?”
I started to get excited too, because I could see what he was driving at. “You would have the results… the wave vs particle pattern, but you would have it RIGHT NOW.”
He grinned again and asked, “Meaning…”
I looked at him my eyes wide, and I said in a quiet voice, “Meaning you could get messages from the future.”
***
“Okay, I have some concerns,” I said.
It was two weeks later, and we were in his lab at MIT, putting the finishing touches on the machine. The design was a little more elegant than he had originally proposed. Basically it would automatically run the Delayed Choice experiment, but keep all the results on a hard drive. In order to create the message we would type the message into the computer, which would be converted into binary. Then the computer would bring up a grid of results, which would display L for left slit, or R for right slit, but only for those we needed in order to create the message we wanted to send. The rest of the results would be obliterated from the hard drive by overwriting them 64 times to ensure that nobody would ever know those results, so that the message could not be corrupted. For simplicity we decided that we would limit the first set of messages to 50 characters so that we wouldn’t have to run the Delayed Choice Double Slit experiment too many times.
But I was starting to wonder if any of this was a good idea.
Frank looked up from where he was coding the program to run the experiment.
“Suppose we get a message but we go back and look anyway? At the results?”
He looked puzzled for a moment, thinking about it, then just shrugged and said, “Then we won’t get a message. If we don’t do this right, it simply won’t work.”
I sighed and tried again. “Okay, then what if we get a message, but then refuse to ever go back and type the message in?”
He stared at me nonplussed. “Uh… then someone else will type the message in. Otherwise we wouldn’t get a message at all.”
“Well then what if we get a message, but then we try to type in something else completely? I mean, what about freaking FREE WILL Frank!?”
He frowned at me like I was crazy.
“Look, you have free will. Everyone does. But time is linear. Whatever happens happens. It has nothing to do with free will or anything else. Time moves the way time moves, and that’s that.” He looked thoughtful for a moment, then stood up and walked over to me. “Look I get what you’re saying. Suppose you get a message that you don’t like, and you don’t want to have to go back and send that message later. But then why would you even bother sending that message in the first place? If you get a message from your future self it will be something that you think you need to know now. You WILL want to send that message, because you WILL have sent it. You wouldn’t send it if you didn’t want to, so don’t worry.”
“Okay… I guess that makes sense…”
He smiled.
“But what about paradox,” I said suddenly.
He frowned and his head dropped.
“What if I get a message that says ‘Don’t Drink the Water, it’s poison’ and I don’t drink the water? I wouldn’t have known it was poisoned if I hadn’t told myself, so how did I know?”
He sighed. “There’s no such thing as paradox, it can’t happen.”
“How do you know? What if we rip a hole in the space-time continuum?”
He rolled his eyes. “You watch too many movies.”
I shrugged. “Still it could happen.”
“No, it can’t. This isn’t time travel Wendi, you can’t go back and kill your own grandfather.”
“But what if the message comes back and says, ‘Quick, kill yourself or the world will end,” and you do? Then who sent the message.”
He looked at me. “Obviously someone else. Someone with a sick, twisted sense of humor.”
“But wh-”
“No, no, no,” he said gently. “Just stop. Everything will be fine. If anything were to go wrong, we simply wouldn’t get a message at all. Nothing would come through, and we’d have to scrap the experiment and start over. In fact the fact that we scrapped it would probably be why nothing came through.” He giggled at that. “Temporal mechanics is weird.”
He looked at me again. “Are you going to be okay?” he asked.
I looked at him again. He seemed so confident, and it was very reassuring. I nodded.
He smiled. “Good. Because I ran the experiment about two minutes before you started panicking. It should be just about done now, and we’ll be able to see the message any minute. Of course we have no idea who it’s from, so why don’t we just say that you get to be the first one to send a message. In that case, it should be from you. Sound good?”
My heart started beating fast in my chest, as he went over to the computer and brought up the message and looked at it. He frowned.
My heart started beating fast in my chest, as he went over to the computer and brought up the message and looked at it. He frowned.
“We got something, but I don’t get it. It’s dated a week from now, and it’s just a string of random numbers. Then at the end it says, ‘Call Mom.’”
I frowned. “What if I don’t?” I asked. “Will that screw everything up?”
He shrugged. “My guess is that you either sent this message because you missed an opportunity and you want to try to change it - a futile gesture of course - or because you already did, and you knew that you would want to. If it were me, I would call, but either way, whatever happens now has already happened from the point of view of the person who sent the message.”
I made up my mind in an instant. Of course I would call my Mom. My mind ran through all the scenarios that might prompt me to send such a message. Would she die in the next week? Would I find out she has cancer and this would be my last chance to say goodbye? Was she about to be in a car accident?
I pulled out my cell phone and tapped her name in my call list.
By the third ring I was practically pulling my hair out, when I heard her voice.
“I’ll be right back, do you want me to get you anything?” Her voice was distant as if she were holding the phone away from her face, speaking to someone else. “Okay, one minute. Hello?” she said into the phone.
“Hi, mom. How’s it going? Is everything okay?”
“Fine sweety, why? What’s going on? Are YOU okay? It’s not really like you to call in the middle of the day like this, is something wrong?”
I was starting to feel relieved, but remembered a lot could happen in the space of a week.
“Uh, no mom, I’m fine,” I said. “Look, what are you doing right now, are you busy?”
“Well, no, not really. Your dad and me just stopped at a gas station on the way home from the grocery store. I was going in to pick up some drinks and maybe some lottery tickets for the drawing next week. Is everything okay sweety?”
I closed my eyes and laughed. Frank looked at me with one eyebrow raised.
“Everything is just great Mom. Hey, if you’re going to get a lottery ticket, pick one up for me too would you? I’ve got some numbers to try, and I’ll split with you if we win.”
“Aw, that’s sweet. Sure I can do that sweety. Can you actually imagine if we won? What are the odds?”
Messages from the Future by R M Swanson is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work at http://billyuno.blogspot.com/2015/09/have-you-ever-hear-of-delayed-choice.html
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