Einstein’s Theory of Relativity says that time travel is perfectly possible — if you’re going forward. Finding a way to travel backwards requires breaking the speed of light, which so far seems impossible. But now, strange-but-true phenomena such as quantum nonlocality, where particles instantly teleport across vast distances, may give us a way to make the dream of traveling back and forth through time a reality. Step into a time machine and rewrite history, bring loved ones back to life, control our destinies. But if we succeed, what are the consequences of such freedom? Will we get trapped in a plethora of paradoxes and multiple universes that will destroy the fabric of the universe?
Could We Travel Back in Time, Thanks to Entanglement Physics?
By Susan Nasr, HowStuffWorks.com
Einstein said that nothing travels faster than the speed of light, but when physicists look at how entangled particles behave, they get stuck in a mirage in which that tenet appears not to be true.
Physicists don’t fully understand entanglement, beyond it being a relationship between particles. If you want to know what entanglement looks like, pull up a chair to an experiment that has produced it. Researchers at the University of Nice-Sophia Antipolis and the University of Geneva shone a laser made of photons, the basic units of light, into the crystal. When the laser’s photons hit irregularities in the crystal, single photons sometimes split into two. These daughter photons were related to one another, and to their parent photon, in how much energy they had.
You can think of the parent photon as being like a train, and the crystal like many bumps. When the train hit the bumps, it broke into two chains of cars with related directions and speeds. These daughter photons weren’t just related, but entangled. Particles are entangled if they’re related in one property but random in the rest.
Entangled particles don’t have to be related in energy. They might be ions whose spins are always opposite, ions that always move in opposite directions, or an ion that always spins in a certain way when a photon moves in a certain way. Since the property is always one of particles — photons, electrons, neutrons and the like — and quantum mechanics are the rules that govern particles, the state is called quantum entanglement.
Whatever links these particles, the link holds over any distance. Another group at the University of Geneva entangled photons used the crystal setup just mentioned and sent the photons 18 kilometers (11 miles) apart and showed they were still entangled.
So far, entanglement sounds like a force. After all, electrons repel, whether they’re millimeters or miles apart, but that repulsion weakens the farther apart the electrons are. Entangled particles have related properties, no matter the distance, and that’s just one way in which entanglement acts unlike the forces we know.
A Challenge to Special Relativity?
What links entangled particles then? By one idea, information travels between them. Physicists have found, though, that if information — say, a wave — did travel between entangled particles, it would have to move faster than the speed of light. That’s a problem because Einstein’s special relativity, an undisputed theory of physics, says nothing can travel faster than the speed of light. Was Einstein wrong?
In an opposing idea, called nonlocality, no information, no particles, no anything travels between entangled particles. There’s no need. The particles are strangely related in a way nature knows but our physics hasn’t yet defined.
Physicists don’t yet know which, if either, of these ideas is accurate. Clearly, the possibility that special relativity needs corrections causes some physicists distress.
For fun, let’s suppose that entangled particles achieve their state by sending information between one another faster than the speed of light. We need a messenger to carry the information. The candidate? The tachyon, a hypothetical particle not known to exist that travels faster than the speed of light. Now that we’ve broken special relativity’s rules and made tachyons exist, let’s run an experiment.
Pretend all the clocks in the world are broken. To tell time, we have only point A and point B. We say time has moved forward when light, leaving A, reaches B. When light leaves A, it’s ‘now.’ When light reaches B, it’s ‘later.’ Now, we release two things: a photon (light) and a tachyon from A. No surprise: The tachyon will reach B first. When the tachyon reaches B, what time is it? It’s before now. What happened? Did the tachyon move backward in time?
It’s weird, but those are the rules in a world where tachyons exist. So we can see how the idea of information traveling faster than light, if that’s indeed how entangled particles achieve their relationship, would stir up physics. But so far, this looks unlikely.
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