A/N: Okay, this one actually has a little physics in it - but just a very little, and I probably got it wrong.

Felicity Smoak didn't like Werner Heisenberg. She'd never actually met the man – he'd died well before she was born – but she held him responsible for single-handedly ruining what could have been a perfect college grade point average. Heisenberg was one of a group of men who founded a branch of physics known as quantum mechanics. The group included no women because Felicity was fairly certain that women were far too practical to come up with such irrational, unprovable theories. She relented a little in her opinion of quantum mechanics when she learned that quantum tunneling enabled the development of computer chips. Computers were, after all, beautiful things. However, she steadfastly maintained her grudge against Heisenberg. Heisenberg and his stupid Uncertainty Principle.

Felicity never understood why a Computer Science major had to study physics anyway. There were so many other, more relevant courses. Coding, scripting languages, operating systems, encryption – plenty of material to fill up a four year degree. But, no, MIT had to insist on the sophomore physics class. Some of it was fine because, frankly, classical physics was pretty easy. Potential energy, conservation of momentum, force equals mass times acceleration – it was all stuff you could touch and feel, very intuitive. But Heisenberg's quantum mechanics was the complete opposite. It defied common sense – especially that fucking Uncertainty Principle. As hard as she studied, she couldn't seem to get the concept to click. And so, thanks to Heisenberg, her chance for a perfect GPA was trashed in her second year of college because she got her one and only 'B' in a transcript full of 'A's.

Cooper, her college boyfriend, thought she was silly to care so much. Grades were irrelevant, he said, just another means of oppression imposed by the wicked, nameless System. But while Felicity usually agreed with Cooper and his hacktivist agenda, she couldn't bring herself not to care about her GPA. For starters, she'd received a generous scholarship from MIT and wanted to show the school she was worthy of that investment. But mostly, it was a milestone to strive for. Coop could sport his 'C' average if he wanted, but Felicity saw nothing wrong with reaching for perfection. Of course, years later when she found out that Cooper was a complete ass (and after comparing him to Oliver Queen, really not all that great in bed either), his opinions on grades and pretty much everything else ceased to matter.

As best as Felicity understood it, Heisenberg's Uncertainty Principal said that if two things were related to each other, you couldn't be completely certain about both of them. If you knew one of them perfectly, then you had to have some question about the other. The example they used in physics class was position and velocity. If you knew position, then you couldn't precisely know velocity - and vice versa. To Felicity, that seemed stupid. If she were driving a car, the GPS would tell her exactly where she was and the speedometer would give her the velocity. Problem solved. But Heisenberg disagreed. Of course, he was mainly thinking about subatomic particles and not a 2004 Honda Civic, but why should the two things behave any differently? Felicity did her best to ignore the absurdity of the Uncertainty Principle and just use the textbook equations in her exam, but despite her best efforts all she could achieve was the 'B.' Perfection ruined.

Flash forward eight years later, and that tiny flaw in her scholastic record was all but forgotten. She was residing in Star City as the CEO of multi-billion dollar technology company, and living with a man who – while certainly not perfect – was perfect for her. Oliver Queen was smart, brave and completely and unutterably sexy. And he loved her every bit as much as she loved him; she had no doubt about that. Things might get scary at times, but overall, life was good – so much better than she'd ever imagined it could be. She never once thought about Heisenberg and his silly Uncertainty Principle.

That is, until their return from Central City after Vandal Savage.

She knew something had happened on that trip to put Oliver off his game, just a tiny bit. He'd been preoccupied and Felicity was sure that he and Barry had had words over whatever it was. Oliver had originally promised to tell her, but when they got back to Star City he'd dismissed it, saying only "it doesn't matter, it's over." It had worried her at the time and she meant to follow up, but then they got engaged, and she had to recover from the shooting, and she and Curtis had deadlines at Palmer, and her mom was dating Captain Lance, and life had moved on. Plus, Oliver was more devoted than ever. He'd always been a physically affectionate person, but now when they were home he just about draped himself over her in his efforts to stay close. She felt warm, protected and loved. It was wonderful. But every now and then, at the oddest moments, her gut would tell her that something wasn't quite right. She'd look at Oliver and she could tell that he'd gone somewhere else in his head, if only for a few seconds. And that bothered her.

It was bothering her late one night as she lay in bed. She hadn't been able to fall asleep, despite the two peel-me-off-the ceiling orgasms Oliver had given her. He'd dropped off almost immediately but, satisfied as she was, she couldn't seem to do the same. Instead she stared into the darkness, listening to his slow, steady breathing. Normally it was one of the most soothing sounds in the world, better than a sleeping pill. But tonight it wasn't working - she couldn't shake a feeling of fear.

And that's when she remembered Heisenberg's Uncertainty Principle. Almost a decade after physics class, the silly thing just popped into her head as she tried not to toss and turn, worrying about whatever it was that Oliver wouldn't tell her. Heisenberg had said that if two things were related, you couldn't be completely certain about both of them. If you knew one of them perfectly, then you had to have some question about the other.

Love and Trust…two things that were related. Felicity rolled onto her side facing away from Oliver and thought about love and trust. If you knew one of them perfectly, then you had to have some question about the other. Felicity was completely certain that she loved Oliver and that he loved her. She would stake her life on it.

So where did that leave trust?

She gazed at her engagement ring, twinkling dimly on the bedside table. She would never say that she didn't trust Oliver. She knew, for example, that he would always do the honorable thing. She knew that he would sacrifice himself for the people he loved. And she was confident that his days of serial dating were behind him. But was her trust absolute? She thought about the second lair that he'd kept from Diggle and herself for almost two years. She thought about the plan he'd cooked up with Malcolm Merlyn to defeat Ra's al Ghul – a plan he hadn't shared with his two closest friends. And she thought about his "it doesn't matter" statement when they returned from Central City.

And she concluded that, no, her trust wasn't absolute. Or, more importantly, maybe his wasn't.

Two things related to each other – love and trust. If you knew one of them perfectly, then you had to have some question about the other. Damn Heisenberg and his Uncertainty Principle. She sighed.

"Felicity?" Oliver's voice was low and a little groggy. She hadn't realized that he'd awoken.

"Yes?"

"I can feel you thinking. What's got you up at this time of the night?"

She decided to answer him honestly. "Heisenberg's Uncertainty Principle."

"Excuse me?"

"Heisenberg's Uncertainty Principle. See, Heisenberg was a 20th century physicist who said…"

"I know who Heisenberg is, Felicity."

"You do?" That was a surprise. After dropping out of four colleges she didn't think Oliver had ever come close to a physics class, let alone quantum mechanics. She hoped this wasn't like cooking, where he had easily mastered something that completely eluded her. "How do you know about Heisenberg?"

He slid closer to her in their bed, spooning against her and wrapping one arm around her waist. "I read The Elegant Universe, at least four times. Shado had a box of books on the island. When we weren't running for our lives or trying to find food, I used to read. We didn't have ESPN, I needed some way to occupy myself."

Really - Oliver read on the island? Felicity had always assumed that Oliver and Shado had found more athletic ways to while away the hours. She rolled onto her back. "Oh, wow, Oliver. That's…cool. The Elegant Universe. It's one of my favorite books. You know the author - Brian Greene? He's kind of sexy for a physicist. The girls at MIT used go to New York once in a while just to hear him lecture. Cooper thought it was stupid but I think he…"

"Felicity!" Oliver sounded much more awake.

"What?"

"You care to tell me why you're thinking about Heisenberg in the middle of the night?"

Oh boy. Not really. She took a deep breath, "Well…"

And suddenly he was on top of her, his hard body contacting everywhere, his lips close to her ear. "You know what? You can tell me in the morning. Right now, I can make you forget both Heisenberg and Brian Greene." His breath was warm and ticklish on her neck, and she felt a tingle all the way down to her toes. Her breath hitched as Oliver lowered his lips to hers. And the Uncertainty Principle was once again forgotten.

Unfortunately for Felicity, spending all of her time in bed with Oliver was not an option. So when he was not within kissing distance, she continued to worry about the thing that he didn't seem to want to tell her. Some days were worse than others, but it was never completely out of her mind. The frustrating part was that she knew it was within her power to find out. She had the skills and technology. She could pull his phone records to see who he called, she could hack his cell's location services to discover where he went, and she could probably even plant some of the nanotech she and Curtis were working on to record his activities. All of which felt sneaky and wrong. She held off doing anything for several weeks.

But eventually she just couldn't take it anymore. She'd tried numerous times to get him to talk, even plying him with a few extra glasses of wine over dinner, and she was still none the wiser. She thought briefly about withholding sex until he came clean, but her body was never going to cooperate with that plan. Typically it required only the right, smoldering look from him for things to go damp. So, instead, she put her considerable skills to use. She had to know.

She figured out quickly that he was going to Central City a couple of times each month. His visits usually coincided with days that she had to work late – board meetings or business trips. The address in Central City was puzzling. She initially assumed that Oliver was going there to do something with Barry and the crew from S.T.A.R. labs – something he didn't want her or Digg to know about. Maybe there was a new meta-human, she thought, or some experimental technology that Caitlin wanted him to test. But he never actually went to the lab, and when she Googled the street view of the address, it looked like he was visiting a residential neighborhood. It didn't completely rule out the meta-human theory, but it made it a lot less likely.

She stewed on that for a couple of more weeks before deciding to check the address out for herself. Oliver was going to be locked up with his campaign manager for most of the day and her meeting schedule was light. With the corporate jet she could be in Central City and back in an afternoon, putting her worries to rest once and for all. She had a last minute attack of nerves, debating whether she really wanted to act like a suspicious girlfriend, then told her assistant that she had errands to run and to hold her calls. At lunchtime she grabbed her purse and left.

The flight took less than an hour and she quickly found herself in a rental car, headed toward the outskirts of Central City. It had been a while since she'd driven herself anywhere – between limos and bodyguards she was almost always the passenger these days – and she enjoyed the freedom of handling her own vehicle on a sunny, spring afternoon. As she neared the address, she noticed that it was a nice neighborhood. The houses were modest but well-kept and the yards were tidy. She arrived around 3:00 and parked on the street, a short distance away from the small, brown two-story home.

And her courage deserted her.

She wasn't sure whether to laugh or cry. It certainly was ironic. After wrestling with her suspicions for months she'd finally come this far – fallen this far might be a better word – only to lose her nerve. Hell, she'd already done most of the ugly stuff; used her tech to spy on Oliver, lied to her assistant, and taken the corporate jet for a purely personal matter. All she had to do now was walk another 50 yards and knock on a damn door to get answers. And she couldn't seem to do it. She sat in the car, her hand reaching several times for the door handle, then stopping. Part of it was fear of what she might find, but most of it, she realized, was not wanting to be this person…a person who checked up on someone she loved. Love and Trust. If you knew one of them perfectly then you had to have some question about the other.

Felicity sighed and started the car back up.

She began to pull away from the curb, then hesitated as a school bus drove slowly past her. The bus stopped in front of the house and a boy got off. He was slim, with sandy hair and blue eyes – maybe 8 or 9 years old. The door to the house opened and a woman stepped out to greet the boy, a large smile on her face. She was pretty, with dark hair and a petite build, and she looked too young to have a child that age. Felicity wondered briefly if she were the nanny, but her loving expression clearly said that she was the boy's mother.

And this was the house that Oliver was visiting. He was going to see a young woman, and a boy with sandy hair and blue eyes.

Felicity felt sick.

Of all the bad scenarios she'd imagined – and she'd imagined some pretty bad ones – Oliver seeing another woman had never been on the list. She'd thought perhaps he was planning to take on Damien Darhk by himself or was cooking up some scheme with Harrison Wells that he knew she and Digg wouldn't approve of. He'd pulled so many self-sacrificing stunts in the past that all of her thoughts had been along that vein – Arrow-related, not personal. Because she had been certain – absolutely certain – that Oliver loved her the same way she loved him, with every fiber of his heart and soul. Watching the woman and boy walk back into the house, she tried to tell herself that she was only seeing a piece of the puzzle. There was probably a good explanation; maybe this woman had a husband to whom Oliver owed a favor, maybe this was some kind of big-brother program. But seeing the boy's features, features that reminded her so much of Oliver, she knew that wasn't the case. This was a family…and it appeared to be Oliver's family.

Love and Trust. If two things were related to each other, you couldn't be completely certain about both of them. What would Heisenberg say if you couldn't be certain about either one?

She wasn't sure how she did it, but she managed to turn the car and navigate her way back to the airport. She had to pull over twice to throw up on the side of the road, leaving a sour, spoiled taste in her mouth. When her flight crew worriedly asked her if she felt alright, she told them she had eaten something bad but would be fine for the flight home.

Fine, of course, being a completely stupid word and home not being much better. When the plane touched down in Star City she realized that, for the first time since they'd started living together, she didn't want to return to the home she and Oliver shared. He was going to look at her face and know in five seconds that something was very wrong, and she had no idea what she was supposed to say . I traced your movements for the last couple of months and I saw your other family in Central City - congratulations, it's a boy?

She thought briefly about calling John Diggle, but that seemed totally unfair. John had had a front row seat to all of her drama with Oliver over the years, and he didn't deserve having to play relationship counselor to the most serious issue yet. Besides, it felt a little like asking him to take sides, and neither side was very attractive. There was her spying versus Oliver's cheating – tough to tell who was on the side of the angels.

In the end she went back to her office at Palmer. She had spare clothes and a toothbrush, and it wouldn't be the first time she'd pulled an all-nighter on the job. She texted Oliver to tell him she'd be working late and not to wait up. Then she changed into a comfy pair of leggings and an oversized sweatshirt, and pulled her hair out of its ponytail. If by some miracle she managed to feel drowsy, she could catch a few winks on the couch. Trying not to be a complete liar, she sat in front of her computer and pulled up the specs for their latest project. In her state of mind they made almost no sense, but she forced herself to concentrate, to think about something other than a sandy-haired, blue-eyed boy.

Two hours later the same specs were on her computer screen and she had accomplished almost nothing. It was past dinner time - nearly 9:00 - but she had no appetite, despite having lost her breakfast by the side of the road.

"Felicity?"

She looked up from her computer to see Oliver at the door, a bag of take-out from the local Thai restaurant in his hands. He caught sight of her face, dropped the bag, and rushed over to her chair.

"Felicity, what happened?"

She shook her head and refused to look at him, casting her eyes down on her keyboard. It was hard to imagine explaining her trip to Central City, and even harder to see how any rationale Oliver might offer could make this better. She gestured limply at the specs on the monitor, hoping that he would accept her frazzled appearance as an extreme case of work stress.

But Oliver, of course, wasn't buying it. He turned her chair away from her computer to face him, and his index finger went under her chin to tilt her face upward until their eyes met.

"Felicity, please tell me."

He was gazing worriedly into her eyes and there was nothing else to do except tell him. She could feel herself starting to tremble, trepidation taking hold of her body. "I saw them, Oliver."

"Saw them? Saw who? Darhk's ghosts?"

"No. The woman and boy. The…family…you've been going to see in Central City. I saw them today."

His eyes widened and he stepped back. The hand under her chin dropped to his side. "How did you know…" He paused and his face darkened. "You tracked me."

She nodded, the trembling growing stronger. "I did. I had to…I had to know what was going on and you weren't going to tell me."

He was silent. She watched a range of emotions cross his face – panic, guilt – before he finally settled on anger. The hand at his side clenched and unclenched. "No, you didn't have to, Felicity," he said sharply. "You could have left it alone. You could have trusted me. After all we've been through, you could have given me the benefit of the doubt." His voice was indignant.

To her surprise, instead of feeling defensive, she felt her own answering spark of anger. She reminded herself that she was not the only one who had done wrong here. In a steadier voice she said, "And you could have trusted me, Oliver. You could have told me about them - given me the benefit of the doubt." Her trembling stopped.

They stared at each other, neither giving an inch. He shook his head heatedly and opened his mouth, but then suddenly paused. A little of the tautness left his body. "I couldn't, Felicity," he said more calmly. "I made a promise that I wouldn't."

"A promise to who?"

"To Samantha – the woman you saw."

Oh God, this was exactly what she had been afraid of. Her gut wrenched, the anger from a few seconds ago slipping away. He'd made a promise to another woman – a pretty, young woman. Despite her best effort to rally her ire – which she now knew was justified - she felt the sharp sting of tears as she asked, "Who is she, that you'd make that kind of promise, Oliver?" She paused, dreading the answer to the next question. "Do you love her?"

His face softened. "Of course not, Felicity. I met her when we were practically kids, well before the island. We had a brief hook-up and William, the boy you saw, is the result. He's my son." When she said nothing he continued, "I found out about him when we went to Central City to fight Vandal Savage. Barry and I were in Jitters, and I saw the two of them. I had to…I had to know."

"And the promise?"

"Samantha didn't want the world to know that William is my son. She was afraid that once the paparazzi got ahold of the story he wouldn't have a chance for a normal upbringing. She's probably right."

"You could have told me, Oliver. I'm hardly 'the world.'" There was an edge to her voice.

He sighed, "I know." He stared at his shoes as he added, "It was a condition of seeing him…not letting anyone know about William. I was so...excited and...scared when I found out about him that I was ready to agree to anything in order to be a part of his life." He returned his gaze to her face and his anger also seemed to have disappeared. "I know I should have trusted you…I should have told you right away. But somehow, the longer things went on, the harder it was to say anything."

She didn't know how to answer that and so they were silent, Felicity still sitting in her seat with Oliver standing in front of her. Oliver's explanation about William and Samantha was not what she was expecting. It wasn't good, but somehow it wasn't as awful as it could have been. He had a son, but he was a son conceived almost a decade ago, when Felicity was still in high school. Oliver didn't love Samantha, he had made an agreement in desperation. Oliver hadn't trusted her, but then she hadn't trusted him either. Love and Trust. If you knew one of them perfectly then you had to have some question about the other.

"Do you love me, Oliver? No questions, no doubts?"

He reached out to stroke her cheek with the backs of his fingers. "I do, Felicity. No doubts whatsoever."

"And I feel the same about you."

The room felt a little brighter. The issue was by no means resolved, but she had a feeling that they would figure it out. She sat back in her chair and thought about the last few months – about Love and Trust and Heisenberg's Uncertainty Principle. Sometimes you couldn't know everything with absolute certainty…and maybe that was okay. She wondered if she took that physics exam now if she'd get an 'A.'

"Felicity?"

"Hmm?"

"Please tell me what you're thinking."

She looked up into his worried blue eyes. "Do you remember a few months ago, when I couldn't sleep?"

He frowned, "I think so. You were thinking about some physicist."

"Heisenberg, I was thinking about Heisenberg."

"Right. And you're thinking about him again now?"

And so she told him. She told him about the Uncertainty Principle and Love and Trust. If two things were related you couldn't be absolutely certain about both of them.

"I think," she said slowly, "if I can only be certain about one of the things, I'd rather it be love."

He leaned down to kiss her forehead. "I still think, with some work, maybe we can be certain about both of them. Prove the Uncertainty Principle wrong."

She smiled.

Take that, Heisenberg!