When the telephone rang, Jason was in the bathroom washing her hands. She ran out into the hall, shaking droplets from her fingers and snatched it out of its cradle.
“Hello?” she said, holding the phone with her shoulder as she ran the towel over her hands.
“We’ve got it, Jay. Come as quick as you can,” said a masculine voice on the other end of the line. She nearly dropped the phone. If he was referring to what she knew he was referring to, well…! This was a very significant phone call, not just for her, but for human history!

She dressed quickly and scrambled out the door, nearly forgetting to lock up. Old newspapers were blowing back and forth down the cobbled street. The sky was burning a brilliant orange as the sun set, turning to purple. If she had had the time she would have marveled at it. As it was, she twisted the key in the lock, checked her bag and double-checked it, and set off at a brisk stride down the road, the red-bricked terraces rising on either side of her.
At the checkpoint, a new officer she hadn’t met before asked her to stop for a routine check.
“Papers, miss,” he said, a wispy moustache the only visible feature on his face under his black official visor. She had always disliked the visors on the checkpoint guards. Made them look like something inhuman.
“Where’s Anderson? If you don’t mind me asking?” she said politely, fishing in her bag for her ident card. Anderson was a good-humoured middle-aged guard who never stopped her, she thought because he was a bit sweet on her. The lips beneath the wispy moustache twisted up into a sneer, revealing nicotine-stained teeth.
“’E’s gone, miss,” said the officer in a reedy voice, accepting her ident card and glancing at it before handing it back to her. “Early retirement. Move along.”
“Oh, dear. Well, would it be possible to get a telephone number, or an address to send a letter to him? I’d quite like to congratulate him,” said Jason.
“That’s classified,” said the officer, stomping his black combat boots and pulling his arms in; it was bitterly cold. “Move along.”
With some reluctance, Jason moved along. Not too much reluctance though. She always had hated those checkpoints, the barricades, the ID cards. Even as light as they customarily were around here.

The Jarosova Institute was outwardly a squat building which was colloquially known as the Bluebottle for its distinctive shape and the way the cheap blue-tinted streetlights reflected off its metal frame across the glass awning that gave it the appearance of blue-veined wings. At the door, a night shift guard that Jason didn’t recognise gave her the once-over and twisted the key in the lock.
“Evening, Miss Castarina,” he said. Of course, his blue-lit eye implant would have scanned her face at a hundred and forty points and cleared her against the company’s records whether he knew her or not.
“Evening…” Jason trailed off, trying to read his badge. The print was just a little on the small side for this distance though. “I’m sorry,” she said with a self-conscious laugh.
“Jaeger, Miss C.,” said the guard. “Colin Jaeger. You see why they only put the last name on the badge now,” he said with an easy chuckle.
“J.C. and C.J.” said Jason. “What a coincidence.” She breezed past Jaeger into the elevator at the back of the room and took it down to the third sub-level, where a false back in a supply closet opened onto a room filled with green light and steam. In the middle of the room stood her colleague, Niko Paine, a dark blob in the mist, with his back to her.
She cleared her throat politely. Paine’s face came into view out of the mist.
“Ah, Jason!” he said, and vanished into the green again. The way the light refracted through the water vapour in the air, it seemed to be coming from all sides. Then there was the sound of a heavy switch being thrown and a loud extractor fan whirred into life. The vapour billowed into the small box vents that dotted the ceiling, and Jason wondered how powerful a fan you would need to take the air with them. Someone was probably working on that on one of the deeper sub-levels.
“You’re probably wondering why I called you here,” said Paine, putting a test tube full of the vapour in a rack. It seemed to be lit from within by the same green light. “I won’t mince words with you: We’ve made contact.”
Jason’s jaw dropped. “Contact?”
“Only rudimentary at this stage, only very very basic. But yes, Jason. We’ve touched the other side of the mirror and come back to tell the tale.”
Jason thought about an old movie she’d seen once where there was a mirror-earth on the opposite side of the sun to the regular earth. That had been the preserve of silly science-fiction then. “So show me,” she said impatiently. “I walked here in the cold to see it.”

It looked like a regular old looking-glass, as Carroll might have put it, but when Jason approached it, there was no reflection on the other side of it. She looked at it in disbelief.
“Great for tricking people into thinking you’re a vampire,” said Niko. “But not much else. Look.” he went on, pointing to the opposite test tube in the mirror. It was glowing just like the green one, but a rich purple.
“They’ve got nicer colours,” said Jason. She touched the mirror gently. It felt like any other mirror on earth. “How did you crack the block transfer computation?” she asked.
“Oh, it would take too long to explain. I’ll send you the proof when I have it written down,” said Paine airily.
Jason rolled her eyes. Paine’s worst habit as a scientist – never recording his method beforehand – was down to the same brilliant mental capabilities that meant he rarely needed to. Just once, she thought, use a goddamn calculator like the rest of us.
“So what’s the next step?” she said. “Today a party trick, tomorrow world domination?”
Paine chuckled. “Scale the timeline up a little bit, Jay. The company doesn’t even know about this yet. Let alone have a utilisation strategy in place.”
Utilisation strategy! So they’ve gotten to you, too, thought Jason. She nodded. “I suppose I can see applications. Can you tune it to different causality structures?”
“Only ones where this mirror exists, in this exact spot. It sounds limiting, but it’s theoretically infinite,” said Paine. “Of course, the number of causality layers we can’t observe is also infinite…”
“Call it fifty-fifty, then.”
Paine grinned. “Now you’re getting it.” He reached into the guts of a large and important-looking machine and there was an electrical noise. The mirror image, without anything seeming to change, was suddenly different. The lab was empty, packed up in cardboard boxes.
“That’s spooky,” said Jason. “How does it do that?”
“Buggered if I know,” said Paine. “Best working hypothesis is whatever the transition between layers looks like, your brain can’t make sense of it. Like how whenever you look at your watch that first second always seems to last a little bit too long.”
Jason peered into the dusty, empty lab. Something was distinctly wrong about the scene, in a way she couldn’t quite put a finger on. “Have you ever seen yourself in it?” she said. “Or me?”
“Wouldn’t you like to know?” said Paine. “I have, as a matter of fact. Hold on, let me just-” he reached back into the machine “-I really need to rig up a switch for this thing. Or a big radio dial or something.”
The strange lurching not-change happened again, and suddenly, another Jason appeared on the other side, almost the mirror image of herself. Almost, but not quite.
“Eerie,” said Jason. “Can they see us?”
“In theory, no. That being said, they are in the same lab as us, doing the same research as us.”
Jason tapped the glass with a fingertip. The mirror-Jason did the same. The two fingertips met, a slim layer of glass all that separated them.
“Yow!” both of them cried, putting the fingertips in their mouths instinctually. “Static shock!”

Jaeger was asleep at the front desk when the elevator dinged and Jason made her way back onto the street. Nearly every window of the Bluebottle was dark, security having done their final check of the night on the above-ground level. It must have been later than she thought, because the checkpoint was locked down, the guard engrossed in a slim paperback with a painting of a woman holding a gun across her chest on the cover.
“Hey!” called Jason, knocking her palm on the glass. The guard looked up.
“Oh, Miss C!” said the middle-aged man. “Go on, then.” He nodded to her and pulled the lever that opened the gate.
“Anderson?” said Jason.
“That’s Sergeant Anderson to you, miss,”