The prank, and she was now certain it was a prank, had crossed the line. Leaving a filthy coffee mug in the middle of the floor could've been an accident. The potted plant a week later had to be deliberate, and seemed to represent some kind of escalation. Today's little display, without doubt, was supposed to be the punchline, but involving a live animal was just wrong. At least it didn't seem to have been harmed.
       So, who would do this, and what was the joke? Heather was sure she was the target, as the person who always came into the lab first in the morning. The setup was absurd enough; she had to admit that a fully inflated one-person life raft next to the computer racks was impossible to ignore. But putting a cat in a cage and setting it on board the raft was going too far.
       "Good morning, Heather . . . uh, what's that?"
       "Morning, Jianxing. Looks like a life raft with a caged cat in it to me."
       Jianxing was another early bird, and also, Heather thought, a leading suspect. Best to play it cool.
       "I can see that. Is it yours?" he asked.
       "It just appeared here overnight. Like the coffee mug and the plant. I wonder who did it." Might as well give him an opening to claim credit.
       "Not me. I'm allergic to cats. That one's kind of cute, though."
       The cat, a striped tabby, looked at them and meowed.
       Heather walked over to the life raft and saw a jar tied to the side of the cage, with a note folded up inside. She opened it and read:
       "If found, please contact David Stein, Physics Department, Columbia University, New York, NY." The David Stein, she wondered? She remembered that the now-famous physicist had started his career at Columbia, so it made some sense.
       "Oh, I get it. You want me to call the Institute for Advanced Study in Princeton, ask for a Nobel laureate, and tell him his cat showed up in his old lab, right? Make a complete fool of myself to a famous scientist?"
       "I told you I didn't do this. I don't know who did," said Jianxing. After a pause, he added, "But yeah, that's probably the setup. What are you going to do?"
       Heather thought he looked upset enough to be telling the truth. "I don't know. The first thing is to deflate the raft and find a more appropriate place to keep this poor animal."
       The cat meowed again and rubbed its face against the side of the cage.
       "It looks friendly, at least," said Jianxing.
       It was friendly. She had to sneak it into her apartment building inside her jacket so the superintendent wouldn't see, and then spend a chunk of her meager graduate student stipend on cat supplies at the pet store just down Broadway, but with a snuggly ball of fur curled up in her lap purring, her initial anger was fading.
       There was still the note, though. She unfolded it again. What were the odds this was really David Stein's cat? It was a stretch, even for the most dedicated pranksters she knew, but it wasn't impossible. Someone who would put a cat in a cage for a practical joke just might be enough of a jerk to steal a pet.
       She picked up the phone and called directory assistance for the number of the Institute. Then she dialed it, and after explaining herself three times to the bemused secretary, managed to leave a message for Professor Stein. He would get it, realize it was either a joke or an error, and then, she hoped, forget her name. With the cat settled in, she went back to the lab and continued with her day.
       That evening, she returned to the apartment to find a cat happy to see her, and a blinking light on the answering machine. She sighed. Her mom, no doubt. She loved her mother and used to enjoy their conversations, but ever since realizing that her showtunes-humming son wasn't going to give her grandchildren, she'd locked her maternal lasers onto her daughter. Heather would have to call back for the inevitable passive-aggressive exchange that would start with "Are you dating anyone?" and end with "I just don't want you to have regrets when you're older."
       She pushed the message button, opened the refrigerator to see about dinner, then spun around and stared slack-jawed at the answering machine. She replayed the message twice: once to confirm it was real, and once to copy down the home phone number he'd left. Whoever had pulled this off, she had to hand it to them; this was one epic prank.
       Fingers trembling, she dialed the number.
       "Hello, Dr. Stein? This is Heather McLaren."
       "Oh, hi. I got a message that you found my cat."
       "Yes, that's right. I'm a physics graduate student here at Columbia, and someone left it in the lab where I work," she said, omitting the cage and life raft.
       "What does the cat look like?"
       "It's a striped tabby, with green eyes and a white nose and belly. A neutered male, I think. There was no tag or collar."
       The line was silent.
       "Dr. Stein, look, I'm sure this is a stupid practical joke, and I don't know who stole your cat for it, but I assure you that when I find out, I'll give them an earful."
       "Was there anything with the cat?"
       She took a deep breath and told him about the cage, the military surplus-looking life raft, and the note. "The cat seems to be fine, and I have him set up with food and litter here in my apartment," she added.
       "Has anything else strange happened in the lab recently? Maybe other things showing up?"
       How did he guess that? she wondered. Could he be in on this? "There was a coffee cup a few weeks ago, and a potted plant about a week after that," she said.
       Another awkward pause.
       "Dr. Stein, I don't have a car, and I don't know whether cats are allowed on the train, but I can be here whenever you want to come pick him up. Tomorrow, if you like, or whenever. I can take care of him until you come."
       "Thank you for calling. I think you're right, someone's putting you on. I'll come up there tomorrow, and no harm done. His name's Schrödinger. Stupid joke, I know, what can I say?"
       "Okay, sure. I'll be here." She gave him her address and they hung up.

       David Stein, who Time magazine had called 'The Man Who Understands Everything,' sat back down at his kitchen table and stared at his half-eaten meal of Chinese takeout. Every few minutes, he glanced up at the telephone on the wall, then at the calendar, and then back at the food. He didn't understand.
       "It's 1995," he said to the calendar, "where the hell did he go for 30 years?"
       A vague memory surfaced. He ran to his study, pulled down a book, and flipped to a set of tables. Yes, 30 years. That couldn't be a coincidence. But how did it work? And did that mean what he hoped it did?
       Dinner forgotten, he pulled an old research paper from his filing cabinet, sat at his desk, and started scribbling equations on a pad of paper. Several hours and two more pads later, he'd reached an impasse. He had a promising theory, but confirming it would require a computer model. Collaborators. Time on a computing cluster. Things that would draw attention.
       Old memories came back. Security clearances. Sitting with Ada in a windowless office, both of them talking excitedly about their findings. Pitching a grant proposal. Deadlines. A placard on a desk, quoting Benjamin Franklin: "Three may keep a secret, if two of them are dead."
       His chest tightened.
       "Shit," he said to the walls. "I have to make sure that poor girl doesn't get pulled into this."

       Heather's apartment intercom buzzed just after eight in the morning.
       "Heather? It's David Stein."
       "Hi, Dr. Stein. I didn't expect you so soon. Give me a few minutes, and I can bring Schrödinger down."
       "No, no, leave him there. I'll be at the diner on the corner. Come join me whenever you're ready and I'll buy you breakfast."
       "Okay, thanks. I'll be right there."
       She stood in front of her closet, holding up different pieces from her small wardrobe while looking in the mirror, oscillating between jeans and a sweatshirt, her usual lab attire, overdressed and trying too hard, which would be a disaster, and he's waiting at the diner, just go already, dammit. Upgrade the sweatshirt to a sweater, tie this unruly red hair in a ponytail, and it would have to be good enough, she decided.
       He was easy to spot in a booth near the door: receding gray hair and bifocals, but with the same bright eyes and inquisitive expression she'd seen on magazine covers and textbook sidebars. "Dr. Stein. Hi, I'm Heather."
       "Please, call me David. We're colleagues," he said.
       "Oh, okay," she said, as her mind screamed Holy crap, David Stein just called me a colleague!
       They ordered, and then he peppered her with questions about her work. He seemed very interested in her computer science background and her current project in astrophysics. As they walked back toward her apartment afterward, she was embarrassed to realize she'd just had breakfast with one of the most famous people in her field, and had spent the whole time talking about her piddly little thesis.
       "Let's walk a little further, to the park. It's nice weather and I haven't seen the neighborhood in ages," he said.
       "Sure," she replied, glad that there was still a chance to stop making a fool of herself.
       As they entered Riverside Park, his pace slowed.
       "Be careful chasing rabbits," he said.
       "Excuse me?"
       "Do you ever get drawn into a problem and forget about everything else? Look up from your work and realize your colleagues have all left the lab, the sun's gone down, and your uneaten lunch is still in the bag?"
       "Yes. That's a good thing, isn't it?"
       "It can be. But people like us have to be wary of it, too. We dive into a rabbit hole, and it becomes our whole world. The best work comes out of times like that, but it can also blind us to things that are much more important."
       She knew she would be replaying that 'people like us' in her mind for years. "What's more important than science?" she asked.
       "That's exactly the problem," he said, with sudden sharpness. Then he stopped, looked straight into her eyes, and said, "Never, ever forget that science serves life, not the other way around."
       "Okay," she said, not knowing how else to respond. She'd been riding a wave of starstruck bliss all morning, and now she was tumbling in the surf and trying to catch her breath.
       They resumed walking, silent for a few minutes. When he spoke again, his casual tone had returned.
       "By the way, I'm going to be traveling for a few weeks. I was planning to hire a pet-sitter, but you'd be doing me a huge favor if you could just hang onto Schrödinger and take care of him until I get back. I'll pay you."
       "Oh, okay. I don't mind at all, no need to pay me."
       "I insist. I still remember what it was like to live on a graduate student stipend, and it's no trouble for me," he said, with a slight smile.
       After that, he reverted to small talk: how things had changed and not changed in the neighborhood. Back at her building, she invited him up to visit his cat.
       "No, that's fine. I trust you're taking good care of him. I'll call you when I get back and pick him up then."
       They said goodbye, and he strode down the street toward a parking garage.
       Back in her apartment, Heather sat down and stared at Schrödinger, sound asleep on a sunny windowsill.
       "What. The. Hell?" she said aloud.
       The cat opened one eye a fraction of an inch, then closed it again.
       "Colleagues." "People like us." That was just buttering her up for the punchline: deprioritize your work and focus on "life." She knew what that meant. Find a nice guy, start a family, drop out of your career for a few decades. It was like talking to a tall, balding version of her mother.
       The thought hung in her mind for a long moment. Did Mom set this up? The woman was capable of anything, and she had motive and opportunity. Still, David Stein? How would she even get in touch with him? And why would he go along with it? He'd often used his fame to advocate for greater gender parity in science. "We can't solve the big problems by leaving half our brains behind," he'd been quoted as saying.
       Maybe she was misinterpreting his warning. And she still couldn't make sense of him driving two hours to pick up his cat, then leaving without even looking at it. What the hell was going on?
       She had to figure it out.

       David kept his composure all the way to his car, and all the way out of the city, before pulling off the Turnpike at a rest stop named after Thomas Edison. Then he parked the car and burst into tears. After a few minutes, he regained control of himself.
       Taking slow, deep breaths, he replayed the conversation in his head. He'd realized a few minutes into breakfast that he had to do something drastic. She was smart, with the math chops and computing background to figure this out. And if his theory was right, that would get her into serious trouble. But she was also starstruck enough that his warning seemed to hit home, even if she couldn't understand the context.
       Do as I say, not as I do, he thought.
       Rabbit holes. He and Ada went down one together, but only one of them came back out. They were certain they'd figured out the problem after the plant, and the Agency had set such tight deadlines on the project that they'd rushed to the "mammalian life form" milepost with the only one handy. Then they made the same mistake again, thinking they finally understood. They could only see one way it could have gone wrong, so they accounted for that and pushed ahead.
       Ada insisted that she should be the one. She was sure it would bolster her long-running effort to persuade NASA to take female astronauts, and he knew she wouldn't back down. They took every precaution. But they forgot what mattered most.
       Thirty years of regret. Now he knew where to find her, and there was only one way to get there. He went inside the rest stop, found a payphone, and made a call.

       In a nondescript building in a suburb of Washington, DC, Jim Turner sat at his desk in a windowless office. The only ornament on the desk was a placard with a quote from Benjamin Franklin. Wearing one of his usual dark suits, he could have passed for an unassuming middle manager in some dull corporate job, the kind of person nobody would notice. His most trusted subordinate, a young African American woman with close-cropped hair, sat across from him.
       "Our sole priority is keeping this project secret," said Jim.
       "We also need to ensure the safety of the individual in question, of course," said Nia.
       "You read the file, right?"
       "Yes."
       "So you know what this thing is."
       "Not all of the physics of it, but yes, I get the idea."
       "And what impact do you think it would have on the nation's most important interests if it got out?" he asked, leaning forward and beginning to scowl.
       "I assure you, I understand the importance of secrecy here. I was just saying there are other considerations as well."
       "No, there aren't."
       "I beg your pardon?" she asked, her mouth hanging open for a second afterward.
       "You heard me. This must stay buried, no matter what. The Director has authorized us to take all necessary actions to that end," he said, then added, "and I do mean all necessary actions."
       "You can't really mean what I think you're saying."
       "I mean exactly what I said. And I expect you to follow orders, soldier. Is that clear?"
       "Yes, sir," she said.

       Heather spent the weekend in the library, poring over an archive of David Stein's early work. While it was a bit outside her specialty, she was able to follow along. Indeed, many of the findings had long ago entered standard textbooks. It was interesting history, but it wasn't getting her any closer to understanding the man behind the work.
       Then some equations in a 1964 paper caught her eye. After staring at them for a few minutes, she grabbed her scratch pad and checked something. It didn't make sense.
       The original math was fine as far as it went, but if it was correct, it implied a way to move mass through space at the speed of light. Teleportation. . She knew that if the equations pointed to something like that, there had to be a howling blunder somewhere. The great David Stein must have made a wrong turn. Setting aside the research papers, she leafed through some of the other documents, filed in reverse chronological order. Digging back through time, she saw course syllabi, a blurb about him from a university promotional pamphlet, and some news clippings about his wife's mysterious disappearance in 1965. Ada Stein was a physics professor at Barnard, Columbia's women's college. She left her apartment with several suitcases, told the doorman she was going to visit a friend in Nevada, then vanished. The police found no leads, and the case went cold. Did she just want out of the marriage? Why disappear instead of filing for divorce?
       Heather flipped to the next item. It was a feature in the student newspaper a few months earlier, profiling the academic power couple. The article had a photo of them, sitting in their apartment, comfortable and smiling. No sign of marital discord, just two young geniuses, full of potential, relaxing at home.
       She looked closer at the photo. A chill ran up her spine. Then she chuckled at herself. Of course, that wasn't Schrödinger in Ada's lap; cats don't live that long. And, of course, someone who likes tabbies would own another one now. The weird math error was clearly messing with her head, and it was time to get out of the library.
       After a few deep breaths, she made photocopies of the research paper and the newspaper story, then returned the originals to the librarian and went home. In the morning, she would run some simulations on the lab computing cluster and find the error she knew must be in the 1964 paper. Correcting a mistake by David Stein, even a minor one, would be enough for a little research letter to the journal.
       One convenient aspect of working with computer models is that your labmates can't tell what you're doing just by glancing over your shoulder. On Monday, Heather was at her lab desk as usual, coding a simulation. Nothing strange about that. Instead of modeling the formation of black holes, though, she was exploring something far outside the lab's usual orbit. She was trying to prove to herself that she hadn't lost her mind.
       It wasn't working.
       Her colleagues were long gone when she noticed the light outside. Dawn. She went back to her apartment, showered and changed, then returned to the lab again at her usual time. The same pattern continued for a few days before she was forced to sleep. After that, she took an alarm clock to work to remind her to go home and rest for a few hours each night.
       The problem wasn't that her model failed. The problem was that it worked, and it shouldn't. It was modeling impossibilities. There had to be an error in the original equations, and if she couldn't find it, was she really David Stein's colleague, his equal?
       She made excuses to her advisor about the lack of progress on her thesis project, and he offered sympathy and suggestions for the problems she'd made up. It hurt a little to lie to him, but she couldn't let this go. When she returned to her apartment late every night, there was Schrödinger, all snuggles and purrs. He showed up in her dreams, too, batting electrons around in their orbits and chasing gamma rays across space-time.
       What pulled her back, after more than two weeks, was a voice in her head. "Be careful chasing rabbits," it said.
       Of course. He'd lost himself in this exact problem, hadn't he? This was what he meant. Maybe it's what he was doing when his wife left him.
       She deleted all of the code and dropped the photocopies and all of her other notes into the recycling bin. It had all been one big dead end, and other than learning the useless trivia that caesium-137 has a half-life just over 30 years, she'd learned nothing. Time to get back to what mattered.
       Giving up was a huge relief. Her thesis project got back on track; she started sleeping much better, and neither an arcane error in an ancient paper nor David Stein being a cryptic weirdo was her problem anymore. Schrödinger was delightful, and if the great genius never came back for him, that was fine with her.
       She was lounging on her couch with the cat and a book one evening when someone knocked at the door. The peephole revealed a well-dressed African American woman about her age, and she opened the door.
       "Hi, sorry to bother you. I'm Aaliyah. I'm a third-year law student, and I live just down the hall."
       "Hi, Aaliyah. I'm Heather."
       "It's great to meet you. Um, this is kind of weird, but can I just look out your window? I think there's a guy following me around, and I want to see if he's out front."
       "Sure," said Heather, opening the door further and letting Aaliyah in. As she turned back to close the door again, she felt a sharp sting on her shoulder and spun around. "Ow! What was that?"
       "Sorry, it was the only way. . ." Aaliyah started, but then Heather's world went gray.

       When she awoke, she knew from the rushing white noise and small round windows that she was on an airplane. She'd never been on a private jet before, but this was no airliner. The cabin was small, with pairs of comfortable seats facing each other. Aaliyah sat across from her. On the other side of the aisle was a man she'd never seen before, wearing a dark suit. He looked like a stereotypical businessman, so much so that she got the disturbing sense that he was in disguise, trying to pass as something nonthreatening. She was equal parts reassured and terrified to see David sitting across from him. Somehow, he was involved with these people who'd drugged and abducted her?
       The mystery man spoke first. "Ah, you're awake. I'm Jim Turner. I work for the government. I believe you've met my associate, Nia Jackson, and of course, you know David."
       "Oh, so it's Nia now?" Heather shot a look of betrayal at her seatmate. Nia shrugged an apology.
       "Heather, I'm so sorry I dragged you into this. I tried to tell them you don't know anything about it," said David.
       All the code she'd deleted and papers she'd thrown out leapt back into Heather's mind, and her eyes widened. "Holy crap, it's real, isn't it? You and Ada built it, and it worked, just not the way you expected," she said. Everyone else on the plane stared at her, mouths agape.
       Turner broke the silence with a chuckle. "Doesn't know anything, huh?"
       "Wait, what is it you think is going on?" asked David.
       She told him about digging through his archive, the 1964 paper, her calculations and models. "You didn't have the computing power to model it back then, but I did, and it kept coming out the same. You were trying to build a teleporter, weren't you? But you wanted to move things through three dimensions, and thought they were just going farther than you intended. My model showed it could only move them one way, in the fourth dimension, by one half-life of the isotope you used to calibrate it. The objects stayed in the same place, but skipped 30 years into the future."
       "You worked all that out from the paper?"
       "Well, there was one other clue," she admitted. "In the archive, there was a picture of you and your wife in your apartment in 1965. She had a cat in her lap."
       David's face went from puzzlement to amazement, then twisted into a smile. "You and Ada would have gotten along very well," he said.
       "I'm guessing we're headed to see her now, aren't we?"
       "We're on our way to a secure location where we expect something to happen. It remains to be seen exactly what," said Turner.
       There was so much Heather wanted to talk to David about, but not in front of these evasive spies. Instead, she sat quietly, her mind buzzing with both excitement and fear. She wondered exactly where they were going and what the government planned to do with this technology. Humming behind those thoughts, though, was a tremendous sense of accomplishment. She'd been right. She had figured out something that had stumped David Stein.
       The jet landed at an enormous air base surrounded by desert, and then a military convoy transported them across trackless miles of sand and sagebrush. They got out, and Turner dismissed the convoy. Then the four of them stood amid vast emptiness, their long shadows stretching across the desert as dusk fell.
       Turner and David were staring at a spot about 50 meters to the south, so Nia and Heather stared at it too. Then, just before it got dark, the spot changed. There was no 'whoosh,' no flash of light, just a sudden switch from nothing to something. Heather realized that if she'd been present when the cup, plant, or cat had arrived in her lab, she would have checked herself into a mental hospital. Now, though, after studying the equations and models for weeks, she felt a profound sense of awe. Theory manifested as reality.
       The life raft was bigger than the one in the lab, but with the same military surplus look. Inside it sat someone wearing a survival suit and a helmet. The figure stood, picked up a metal suitcase, turned around, and froze. Then it dropped the suitcase and removed its helmet. Long, black hair fell down its shoulders. She looked just like the picture in the newspaper article.
       "David?" shouted the figure.
       "Ada!" he yelled back. Tears ran down his cheeks.
       "Stop!" said Turner. He was pointing a pistol at David.
       "What the hell?" said David.
       "Nobody move. Ada, if that is who you are, approach slowly and keep your hands where I can see them."
       "You know goddamn well that's who she is, you moron! Who the hell else would it be?" said David.
       Heather looked at Nia, who also held a gun now. Hers was pointed downward, but that was no more reassuring than a coiled rattlesnake with a closed mouth; she was a lethal force ready to strike any target in a blink. It was all happening in slow motion, Heather's thoughts rushing ahead. These goons don't care about people, just their damned secrets. We're going to die, and there's nothing we can do about it.
       Or was there? The life raft. The survival suit. David and Ada had thought the device worked in three dimensions, and that it was just sending things farther away than they'd expected. They'd equipped Ada to survive anywhere. Wouldn't they have equipped her to come back?
       "Wait," said Heather. They all looked at her. "That suitcase contains a spare teleporter, doesn't it?"
       "Yes," said David.
       "Then you and I should go secure the caesium in it," she said, trying to give him a meaningful look. He returned a puzzled stare.
       She tried again. "I mean, we should go over there and explain the situation to her, before it starts a chain reaction," she said. Finally, something dawned on him.
       "Right, of course," he said.
       "What are you two talking about?" asked Turner.
       "The device uses a radioisotope, caesium-137, and after the jump, the spare one she carried with her could undergo a decay event at any instant," said Heather.
       "She's right, isotopic decay is a stochastic process, and if it happens now, well, I can't answer for the consequences," said David. "She and I should go help Ada secure it immediately."
       It was the perfect kind of bullshit: true, but not the whole truth. Radioactive decay is random and unpredictable, but the minuscule amount of caesium-137 in the device posed no immediate danger to anyone.
       "I don't trust you two. Nia, go over there with Heather and watch her secure the device. If you even see a hint she's up to something, stop her," said Turner. "You stay with me, David."
       David nodded at Heather, and she and Nia walked slowly toward Ada. Heather tried to focus on her breathing, but her knees felt weak. She had hoped to get the scientists separated from the spies, but Turner's paranoia had prevented that. Now, if she tried to explain her plan to Ada, Nia would shoot them both.
       When they had gotten about halfway to Ada, Nia spoke just loud enough for Heather to hear. "You know he plans to kill all three of you, right?"
       "Yeah, I figured that out. But why are you telling me?"
       "Because I'm not a murderer. If you have some plan for knocking him out or even just stunning him for a second, all I need is an opening. I have your back."
       The woman who tranquilized and kidnapped me has my back, thought Heather. Great. Still, it seemed like the only chance they had.
       They reached Ada. "Hi, I'm Heather. I'm a physicist and a colleague of your husband. The year is 1995. You traveled through time instead of space. I know this is a lot to take in, but we need to act carefully and fast."
       "I'm Nia. I think you know my boss over there, Jim."
       "I sure do," said Ada, frowning toward the men in the distance. She seemed upset, but not surprised, that Turner was pointing a gun at her husband.
       "Right. This project is still top secret, and we're the only five people in the world who know about it. Jim thinks that's too many," said Nia.
       "I think we can resolve this without anyone getting hurt," said Heather. She gestured toward the suitcase. "Can you adjust that to target a point about 50 meters away?"
       Ada looked at Heather, then at Nia, then nodded. "I see. Okay." She opened the case, faced its upturned lid toward the two men, and knelt down with Heather in front of it. Nia stood behind them. They spoke in whispers as the device started to hum.
       "Nia, everything okay over there?" Turner shouted.
       "Yes, sir, they're just about done," she said, then added under her breath, "and so are you, you sonofabitch."
       Ada flipped a switch, and then only three people in the world knew about the project.

       One instant, there was darkness and desert, and three figures crouching over a suitcase in the distance. The next instant, an entire squad of military police tackled Jim. "Welcome to 2025, Mr. Turner. You're under arrest," said their sergeant.
       David turned away from the commotion and saw two familiar-looking women standing in front of him. For a moment, he was puzzled by the streaks of gray in their hair and the faint lines on their faces that hadn't been there a moment ago. Then a wave of relief and awe washed over him. "Holy crap, it worked," he said.
       "Yes, it did. Good to see you again, David," said Heather. "Nia sends her regards. She wanted to be here, but was stuck in a cabinet meeting in DC."
       "You have a lot of catching up to do, dear," said Ada, smiling as a tear ran down her cheek. She drew him close, and they held each other, and time stretched out before them.