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(PhysOrg.com) -- Among the many intriguing concepts in Einsteins relativity theories is the idea of closed timelike curves (CTCs), which are paths in spacetime that return to their starting points. As such, CTCs offer the possibility of traveling back in time. But, as many science fiction films have addressed, time travel is full of potential paradoxes. Perhaps the most notable of these is the grandfather paradox, in which a time traveler goes back in time and kills her grandfather, preventing her own birth.
In a new study, a team of researchers has proposed a new theory of CTCs that can resolve the grandfather paradox, and they also perform an experiment showing how such a scheme works. The researchers, led by Seth Lloyd from MIT, along with scientists from Piazza dei Cavalieri in Pisa, Italy; the Tokyo Institute of Technology; and the University of Toronto, have published their study in a recent issue of Physical Review Letters. The concepts in the study are similar to an earlier study by some of the same authors that was posted at arXiv.org last year.
In the new theory, CTCs are required to behave like ideal quantum channels of the sort involved in teleportation. In this theory, self-consistent CTCs (those that dont result in paradoxes) are postselected, and are called P-CTCs. As the scientists explain, this theory differs from the widely accepted quantum theory of CTCs proposed by physicist David Deutsch, in which a time traveler maintains self-consistency by traveling back into a different past than the one she remembers. In the P-CTC formulation, time travelers must travel to the past they remember.
Although postselecting CTCs may seem complicated, it can actually be investigated experimentally in laboratory simulations. By sending a living qubit (i.e., a bit in the state 1) a few billionths of a second back in time to try to kill its former self (i.e., flip to the state 0), the scientists show that only photons that dont kill themselves can make the journey.
To demonstrate, the scientists stored two qubits in a single photon, one of which represents the forward-traveling qubit, and one of which represents the backward-traveling qubit. The backward-traveling qubit can teleport through a quantum channel (CTC) only if the CTC ends by projecting the two entangled qubits into the same state.
After the qubits are entangled, their states are measured by two probe qubits. Next, a quantum gun is fired at the forward-traveling qubit, which, depending on the guns angle, may or may not rotate the qubits polarization. The qubits states are measured again to find out if the gun has flipped the forward-traveling qubits polarization or not. If both qubits are in the same state (00 or 11), then the gun has not flipped the polarization and the photon survives. If the qubits states are not equal (01 or 10), then the photon has killed its past self. The qubits states were always equal, showing that a qubit cannot kill its former self.
In a new study, a team of researchers has proposed a new theory of CTCs that can resolve the grandfather paradox, and they also perform an experiment showing how such a scheme works. The researchers, led by Seth Lloyd from MIT, along with scientists from Piazza dei Cavalieri in Pisa, Italy; the Tokyo Institute of Technology; and the University of Toronto, have published their study in a recent issue of Physical Review Letters. The concepts in the study are similar to an earlier study by some of the same authors that was posted at arXiv.org last year.
In the new theory, CTCs are required to behave like ideal quantum channels of the sort involved in teleportation. In this theory, self-consistent CTCs (those that dont result in paradoxes) are postselected, and are called P-CTCs. As the scientists explain, this theory differs from the widely accepted quantum theory of CTCs proposed by physicist David Deutsch, in which a time traveler maintains self-consistency by traveling back into a different past than the one she remembers. In the P-CTC formulation, time travelers must travel to the past they remember.
Although postselecting CTCs may seem complicated, it can actually be investigated experimentally in laboratory simulations. By sending a living qubit (i.e., a bit in the state 1) a few billionths of a second back in time to try to kill its former self (i.e., flip to the state 0), the scientists show that only photons that dont kill themselves can make the journey.
To demonstrate, the scientists stored two qubits in a single photon, one of which represents the forward-traveling qubit, and one of which represents the backward-traveling qubit. The backward-traveling qubit can teleport through a quantum channel (CTC) only if the CTC ends by projecting the two entangled qubits into the same state.
After the qubits are entangled, their states are measured by two probe qubits. Next, a quantum gun is fired at the forward-traveling qubit, which, depending on the guns angle, may or may not rotate the qubits polarization. The qubits states are measured again to find out if the gun has flipped the forward-traveling qubits polarization or not. If both qubits are in the same state (00 or 11), then the gun has not flipped the polarization and the photon survives. If the qubits states are not equal (01 or 10), then the photon has killed its past self. The qubits states were always equal, showing that a qubit cannot kill its former self.
