WHEN IN DOUBT FREEZE the signs said, sometimes out loud.
"Sentient Security Systems authorized to use lethal force"

  A gray cube of texturized carbon, the bunker looked out of place in the park-like setting. The afternoon sunshine played over walls so smooth they resembled early computer graphics. Eagles soared before the hazy mountains. Three perfectly circular clouds floated lazily above each other.

  Guests approached along the garden paths like living extensions of the Net. They had waited years for a chance to learn something completely unpredictable. That happened maybe once a lifetime in this post-historical era, though they could expect to change identities every decade.

  Reporters and spies staked out the entrances to the Torus Acceleration Center nestled in the Rocky Mountains. For a few hours, the world's physicists would outrank its celebrities, many of whom had tried to get invitations. The control room was filled with the experts who had made this experiment possible. One Netrepreneur had disguised himself as Cheng Xiao, the eccentric Taiwanese cosmologist, but he was unmasked when his comments made too much sense.

  A hush fell as Mike Denning, head of the Unified Field Group, entered the control room, temporarily rearranged into an auditorium.
Almost sixty, the bald engineer looked gaunt from his radical life extension diet. He was testing new methods to survive another half century. His skin had a slight purplish tint.

  Bradd Belder had arrived an hour early, waiting for the lecture to begin. He had a penetrating, almost haunting stare.
He had also lost weight recently, and had been questioned when the security cameras hadn't recognized him.
He pushed his vScreen up his nose. When he sank into his chair, he became hard to notice.

  Mike took his position on the podium. Behind him, the curved screen lit up like the dawn. He tested the hidden acoustics.

  "Thanks for showing up," he told the swarming cameras. "Our experiment is almost ready. We're about to witness history, and should be proud to be a part of it. For the benefit of our casual viewers, I will recap the Torus project."

  Here we go, Bradd thought. The speech was unavoidable, but he wanted to get the experiment over with.

  Mike cleared his throat. "We started making rapid progress after the main string equations were unexpectedly solved back in 2019. Superstrings are the basic components of everything, the smallest things that can exist. People are made of them, and stars, and laser beams, even gravity waves. It doesn't matter whether strings really exist, as long as our predictions match observations. As you know, Burt Garilliak's extraordinary final work about the Multiverse was tragically lost when he immolated himself. I believe the truth is even stranger than he could imagine . . . How to put it?" he wondered theatrically. "Every string is a byte in the software of the universe."

  Much too simple, Bradd thought, but the crowd was captivated.

  "A string can vibrate an almost infinite number of ways," Mike was saying. "Each type of vibration is a different particle. Strings exchange harmonic modes through temporary dimensions. There's an unlimited number of forces, but we can only detect the four weakest ones. The higher forces cause tiny disruptions in our calculations. Reality is the result of many interacting universes deriving from a single equation."

  "The theory makes some unexpected predictions. One of the strangest is the Torus. The Unified Field Group has been testing it for two years now. As you know, our original mandate was to build a scanner capable of finding hidden nuclear bombs. You'll recall we saved Tokyo with minutes to spare. We have since gone far beyond that."

  The screen filled with equations like particularly obnoxious graffiti. When Mike turned to look, he briefly looked ancient.
"String theory is extremely strange," he said. "We can't renormalize the brane volumes. A gravitational field contains a certain amount of energy which, according to relativity, causes more gravity, and so on. We can only make sense of these paradoxes through transfinite math the human brain can't handle for long, yet the mental imagery is unbearably compelling. Anyone who thinks too long along these lines risks madness."
The first-stage symptoms were a badge of honor among hardcore nerds.

  "We had reached the limits of the knowable. The neural restoration drug NHV-4676 was developed just in time. Originally a dementia drug, it temporarily creates new neurons in proportion to existing brain structures, without forming new connections between them. The drug causes a trancelike state of intense focus. The new neurons dissolve afterwards, except in Garilliak's case of course. You forget your most profound 'insights', so there are few lingering effects. Then we let the computers take over." Mike paused. "There may be some slight delusions or fantastical thoughts," he mused, "but they never gave me any trouble. Mr. Belder, sitting in front of me, pioneered the use of NHV-4676 as our human guinea pig."

  Bradd acknowledged the applause with a small nod.

  Mike gestured at the screen. An endless pipe hung in deep dark water, anchored to the seafloor by steel cables. A giant squid passed in the background. "Here we see the floating CYCLOPE linear accelerator, built in the Pacific by India and China. Designed to test string theory predictions, it has fulfilled its purpose."
  "In early 2026, it created and trapped the Torus particle in an ultra-strong magnetic field. Stabilized by its internal rotation, the object could be any size, but its intersection with our dimensions is microscopic. It appears as a speck of dust, but is really a four-dimensional doughnut. Any particles trapped inside the Torus will go round and round forever. From the particle's viewpoint, the Torus appears like an endless tunnel."
  "Those who understood particle accelerators immediately grasped the Torus's importance. It was suddenly possible to accelerate particles arbitrarily close to the speed of light, since there were no centripetal effects to slow them down. For over two years, we have been accelerating a single electron inside the Torus. Recently, it has attained Planck-level energies." Mike looked down the middle of the room. "We can't explain what we're seeing yet, but we may have an answer tonight. One thing I can promise with certainty: there is no danger. The energy throughput is limited to 1.499 milliwatt. Our facility has a fully certified, UN-approved, Level Five safety plan."

  The screen showed the Torus itself, a blue oval flickering like an alien emergency blinker. Slowed down a million times, the video had been taken with Bradd's own detector, a one-meter black box next to the confinement magnet three kilometers from this room. It gave him an icy sensation.
  Like Mike said, no reason to worry.

  "As you know," Mike continued, "this object no longer bears any resemblance to the original Torus. It repels normal matter, which also keeps it centered. Unfortunately, there have been some irresponsible rumors. They even claim we found a doorway to Hell, which is quite untrue. However, by accelerating the Torus with an entangled particle inside, we might be able to create a basic time machine. That would take a few millennia though."

  A stern middle-aged woman joined Mike on stage. They conversed quietly.

  Mike returned to the lectern and said: "Dr. Dubroff has studied the first nonillion digits of Pi, and she knows pattern anomalies. The Torus response has changed in the past week, and is definitely no longer random. This has persuaded us to perform our upcoming experiment."

  He surveyed the packed room, seeming to make eye contact with everyone. "It appears our Torus is no longer confined to our own universe. We're about to peek through the keyhole without opening the door. Did anyone remember to bring popcorn?" There was laughter and disarray. The room hushed.

  The screen showed another close-up of the Torus, manipulated by huge magnets. Everyone leaned forward.
The flashing became irregular and then stopped. Bradd had trouble breathing. Would his detector, one of four around the Torus, work properly?

  They stared down an endless tunnel half a millimeter wide. Reflecting the scanning laser's blue-green glow, the walls were made not of empty space, but nothing: the universe simply ended there. Particles trying to leave the tube were dragged back by their own quantum ghosts before they could blink out of existence. The tunnel converged to a vanishing point ahead.

  Suddenly, it widened into a disk made of pure static.

  "Remember," Mike said breathlessly, "this image is generated by the Keyhole Scanner. No matter remains in the tunnel." A thin line ran down the middle, the accelerated electron's path, a Kerr black hole stabilized at unprecedented cost.

  The disk was a perfectly smooth mirror, reflecting the scanning beam back to its source and over the curving tube wall. Then the laser was no longer being reflected. Another, very similar beam shone out of the tunnel.

  "Symmetry breaking..." Mike said. "They broke through first."

  "The new beam has standard scanning frequencies," a technician sitting behind Bradd reported, "Irregular modulation."

  "Looks like a Himax format," a second engineer declared. "Sixty hertz per channel, hundred megabits per second. We're trying to convert it."

  This simple task took the longest fifteen minutes of Bradd's life. Suddenly, a monotonous, gender-neutral voice boomed from the hidden acoustics. There were shrieks, and a senator fainted in the front row.

  "Our Union much observed your various transmissions," the voice said without pausing between sentences. "Your contiguities are of a kind not previously absorbed."

  A journalist asked Bradd, "is this what I think it is?"

  Transfixed, Bradd could barely nod. "They may have picked up some of our Wifi through the Torus. Should have buried it deeper."
He learned something about himself: being under cosmic-level stress was not unpleasant, just paralyzing. He was clutching his pad so tightly his fingers were turning white, but couldn't release the pressure.

  "New problem!" the first technician said, wiping his forehead with his tie. "Something is hacking our Triple-S."

  Mike asked, "An infiltration?"

  "Standard Achilles trapdoor, Trojan-Dunkirk variant," the technician responded. "Himax is a closed format, so the data was processed offsite through our shared network."

  "Then these . . . beings could have already accessed our controls."

  "In a limited way maybe."

  "To maximize efficiency an amalgamation of our spheres must occur," the voice was saying. "Emissaries will now accomplish the alteration."

  "Don't necessarily like the sound of that," a reporter mumbled, tapping notes.

  "Shut it down!" Mike bellowed as alarms went off.

  Before anyone had time to think, something approached through the wormhole at hyperspeed. The necessary precautions had been implemented years earlier. Four redundant klystron detonators went off simultaneously, cutting power to the Torus. In a few picoseconds, the circling electron destabilized, and the tunnel was pinched off. The main screen flashed once, and was replaced by shocking blackness.

  "Status report," Mike said as the ground moved beneath him.

  "Shutdown successful," the technician said. "Not one nucleus got through." There was a tremendous rumble all around.

  "Why have we lost contact with the Torus chamber?"

  Bradd stood up. "The Torus self-annihilated as predicted," he said. "Many secondary neutrinos were detected. The chamber has been destroyed, and the area outside this bunker has been seriously damaged. No reported casualties outside the valley containment zone. My calculations predicted this outcome."

  "The Torus released all its energy at once," a technician added. "Years of hard work wiped out in a moment."

  Mike clutched the lectern, but that only made him tremble harder. A water bottle rolled off the podium. As if from a distance he heard the reporters' shouts. Scrolling text on the surrounding screens itched in his eyes.

  ~~~

  Bradd listened to the sound of his breathing as he entered the dark Torus chamber with Mike Denning at his side. They wore orange spacesuits. One whiff of the fumes in this room could kill them. Designed to absorb toxins and isotopes, UN Law prevented the Center from venting these gasses. They would have to be chemically reabsorbed.

  As intended, the Torus disintegration had released most of its energy in the form of seismic waves. Half a kilometer under a hilltop, the chamber had actually been enlarged by the Torus oscillations, and was now surrounded by a cooling sphere of compressed rock. The floor was filled with a deep layer of fallen rubble. In the center was a mass of warped iron, all that remained of the giant magnets. Bradd's sensitive equipment had been pulverized, even though it had absorbed only a tiny fraction of the energy release.
They were looking for any remaining evidence of alien intelligence, a high-risk mission requiring human interaction. Remotes and robots didn't function well in the strong fields still pervading this chamber.
If necessary, the exploration team could also function as a First Contact Committee.

  Mike led the way. "I can't believe this," he said. "Who were they?"

  Bradd didn't reply.

  "That was a brilliant detector you invented," Mike said, pointing at the location of Bradd's black box. The rubble tilted in several directions around them.

  Always surprising himself, Bradd spoke the truth at last. After six years, it felt almost perfunctory.
"I didn't come up with the concept of the graviton amplification detector," he said. "Burt Garilliak invented it six years ago. I stole his idea. He didn't really commit suicide, either. I bombarded his stored anti-carbon sheet with a neutron beam from half a kilometer away while he was testing it."
"You could say the guilt was killing me," he added.

  Mike stopped next to the strangely undamaged central magnet.

  "Why are you admitting this now?" Mike asked.
Apparently, they were no longer in contact with the control room. His microphone didn't work. In the background, he could hear his own voice and Bradd's giving a detailed report about a piece of debris. Bradd had planned for everything.

  "I had to destroy the Torus to prevent the truth from coming out." They had to speak louder so they could hear each other through their suits.

  "You did all this?" Mike exclaimed. "Mind if I ask for details?"

  "You still don't understand. Good, I hope to keep it a secret. We created a gateway to a world almost identical to ours, a tunnel to a parallel Earth, only mirrored. They even have a duplicate Center on the other side, behind their own Torus. After a few moments of contact, the two universes became slightly different, and the corridor opened for two-way traffic."

  Mike thought hard while Bradd spoke, trying to anticipate the results of years of scheming.
"Have you ever studied the standard black hole solutions? Because of symmetry rules, every black hole must have a 'far side', a four-dimensional funnel leading to points unknown. Where does it go? A different universe or perhaps into the past? No. The simplest solution is that it leads to an identical universe on the other side. Separated by the black hole horizon, the two halves are indistinguishable. If they could ever be connected, they wouldn't stay identical, however. I've thought a lot about this. Even a narrow, low-bandwidth gateway to a parallel Earth would transform both Earths. We couldn't trade matter or energy with our other halves, but we could cooperate on software, design work, entertainment. Each side would specialize on different tasks."

  "What are you talking about," Mike protested. "That voice threatened to assimilate us."

  "A simple special effect," Bradd explained. "I installed an additional laser projector in my black box, and a collimated positron gun: my Torus alien." He pointed to the ruins around them. "Mine was never used. The identical copy on the other side was activated first, since the symmetry break started there. The selection was purely random. They had no Torus shutdown on their end, only a sudden, unexplained cut-off."

  "You still haven't given me your motive," Mike said.

  "You really think we'd be satisfied with just one gateway?" Bradd asked. "We'd open links with more and more parallel universes, and each of them would do the same. The number of linked Earths would explode into an endless web. Soon, some other version of myself in some other universe would admit to Burt's murder, and every version of us would then be punished. Even if by some miracle no one talked, the rate of progress would accelerate so much I would become obsolete. That fate does not appeal to me."

  Mike stared. "Wait, you have an alibi! When Burt killed himself, you were taking part in the first clinical trials of NHV-4676. There's no way you could have murdered him. When you're under the influence, you can't move a muscle."

  Somehow, Mike figured it out.
"You stole your ideas from me!" he exclaimed. "You were there in the recovery room three weeks later, after my first session with the drug. You took those notes no one could decipher, and knew I wouldn't remember."

  The confrontation had come not a moment too soon.
Bradd said, "If... if I could change the past, I would. I can't allow the truth to come out. I wanted to confess to something, hoping for some sympathy here, some understanding instead of useless rage, before..."

  "What? Before killing me? You have the motive," Mike said slowly, "but you seem to have forgotten about the means and the opportunity."
He stared at his opponent's mirrored faceplate. There was no way Bradd could have smuggled a weapon into the chamber.

  Mike glanced at Bradd's backpack, connected to his own. The oxygen and power umbilical remained attached, but the network cable was unplugged, so no one had overheard their conversation. He'd probably also hacked the cameras.
Apparently, Bradd was a genius at illusions, not physics.

  "Plug it back in," he said, stepping closer.

  So it ends, Bradd thought.
He was wrong.

  Even now, the Torus hadn't vanished completely. Trapped within the central magnet's residual field, it still formed a tiny bridge between the two universes, less than a proton in width.
The chamber had been electrically grounded and neutralized just before they had entered. Using Mike's forgotten equations, Bradd had calculated that would cause the Torus to completely decay almost two minutes ago.

  It happened when Mike put his hand on Bradd's shoulder.

  The Torus could only release its remaining energy in the form of gravitational waves. Everything in the room began to vibrate, the edges seeming to blur. The waves didn't kill Mike, powerful as they were. It was the tidal force. Bradd had planned for Mike to be standing next to the core magnet at the critical moment, himself a few meters away, but Mike had frustratingly refused to be guided.
Still, the uneven force worked even at twelve meters, leaving no external marks for the autopsy to find. Mike simply fell on his face. The official cause of death would be cardiac arrhythmia and cerebral hemorrhage.
Though he had seemed vigorous, Mike's calorie-restricted diet and fringe life extension treatments had weakened him.
Bradd had spent many hours training for this moment, riding the exercise centrifuge at the Life-Fitness center. That alone saved him.

  The perfect alibi, was his last thought. The murder weapon came from another universe . . .

  ~~~

  He woke in intensive care, a pleasant, sympathetic environment. Despite his unexpected internal injuries (gravity micro-tides were much stronger than expected; there had to be a paper in that), he felt at ease. No one would suspect a thing. He was the victim here.
He had solved the problem of his guilt. Justice would be served. There was an identical copy of himself on that other Earth. Since the evidence hadn't been destroyed there, they would have detected his tampering with their Torus detector, and that other Bradd would have already confessed to everything.
When they found out about his plan to eliminate Mike, they would probably execute him under that other Colorado's "zero strike" law.

  Yet still he felt the urge to confess.
But the only person he could have trusted to keep his secret was himself. And he had effectively closed that door, hadn't he?