A look at 'Looper's' potential for real world time travel

A new movie - "Looper" explores closed time loops as a time travel mechanism.
You can read more about it in the following article by CNN. The discussion revolves around some of the topics we have already covered in the blog post on various forms of possible time travel.

Time Travel - Various methods of Time Travel

We are always travelling in time to a future at a near constant rate. This is the most trivial form of time travel. It is currently unknown if the laws of physics allow time travel to the past. Backward time travel also introduces a whole set of problems and paradoxes.But we will come to that later, now we will look at various possibilities of time travel which are theoretically allowed by the current day physics.   

• Wormholes - A wormhole is a hypothetical feature of the space time curvature. According to the general relativity gravity curves the space time. There are theoretical solutions of general relativity which contain wormholes. A structure in which space time curves onto itself looking something like the following picture. In such a structure we can travel through the hole as a shortcut in space time. No such structure has been observationally detected yet. The possibility of engineering such a structure for time travel is quite far fetched. 

• Primordial Black Holes - If we cannot create a wormhole the other possibility is that there may already exist some really old black holes. These black holes formed due to collapse of extremely dense matter in the early stages of the universe. Some of these black holes could be traversable due to their geometry and form candidates for time travel.  Even though they are considered as a candidate for dark matter none has been detected observationally yet.

• Cosmic Strings - These are large 1-D topological defects associated with early universe.The  formation of cosmic strings is somewhat analogous to the imperfections that form between crystal grains in solidifying liquids, or the cracks that form when water freezes into ice. They are also predicted by some string theories. Cosmic strings can also exist with negative tension and hence negative mass. If this is wrapped around a wormhole it can be used for time travel.

• The Special Theory of Relativity - Time dilation is a form of time travel permitted by Einstein's special (and general) theory of relativity. Time elapsed in two frames of reference can be different and this can be perceived as a forward time travel by one of the observer. The following gif shows the difference between time experienced by green and red clock. Time experienced by a moving body is slower than experienced by an inertial frame at rest. This effect is observed and accounted for in satellites used for GPS.

• Negative Energy, Mass and Flux - Exotic forms of matter like negative pressure, energy etc can be in theory manipulated to create possibilities for time travel. Study of dark matter and energy is one of the hot topics in current research in physics.

• Tachyons - These are hypothetical faster than light particles. If they exist they may be used to send signals which can travel in time. There have been some faster than light neutrino anomalies detected in experiments but they turned out to be due to some errors in the setup and calculation.

• Time Reversal - This phenomenon is not really related to time travel but an interesting possibility. Most laws of physics are invariant under time reversal transformation except maybe the second law of thermodynamics. We are used to a universe where the time increases in the direction in which entropy increases. But the question is if would you experience time backwards if entropy was always decreasing in the universe ?

A Song by Kumar Vishwas

मैं भाव सूचि उन भावों की,

जो बिके सदा ही बिन तोले | 

तनहाही हूँ हर उस ख़त की,

जो पढ़ा गया है बिन खोले |

हर आसूं को हर पत्थर तक,

पहुँचाने की लाचार हूक  |

मैं सहज अर्थ उन शब्दों का,

जो सुने गए हैं बिन बोले |

जो कभी नहीं बरसा खुल कर,

हर उस बादल का पानी हूँ |

लव कुश की पीर बिना गयी,

सीता की राम कहानी हूँ |

जिनके सपनों के ताज महल,

बनने से पहले टूट गए |

जिन हाथों मैं दो हाथ कभी,

आने से पहले झूट गए |

धरती पर जिनके खोने,

और पाने की अजब कहानी है |

किस्मत की देवी मान गयी,

पर प्रणय देवता रूठ गए |

मैं मैली चादर वाले उस,

कबीरा की अमृत वाणी हूँ |

लव कुश की पीर बिना गयी,

सीता की राम कहानी हूँ |

कुछ कहते हैं मैं सीखा हूँ,

अपने झक्मो को खुद सी कर |

कुछ जान गए मैं हसंता हूँ,

भीतर भीतर आसूं पी कर |

कुछ कहते हैं मैं हूँ,

विरोध से उपजी एक खुदार विजय |

कुछ कहते हैं मैं रचता हूँ,

खुद मैं मर कर खुद मैं जी कर |

लेकिन मैं हर चतुराई की,

सोची समझी नादानी हूँ |

लव कुश की पीर बिना गयी,

सीता की राम कहानी हूँ |

Time Travel - Arrow of Time

The term "Arrow of Time" was first coined by Arthur Eddington in 1927. It is used to describe the "one way direction" in which we perceive the passage of time. Time is a rather curious dimension in that we only see it increasing in one direction. The laws of physics at the microscopic level stay the same under something called the "T-Symmetry". T-Symmetry states that the laws of physics stay invariant under the time reversal transformation. A notable exception to this is the second law of thermodynamics. Entropy of the universe increases with passage of time. Or in other words we perceive time in the direction of increase of entropy. Since the beginning of universe started at big bang the entropy of universe has been increasing. The early universe had a low level of entropy as all the mass was concentrated at one point. With the expansion of universe this mass gets distributed across the space. The following figure shows a graph plot of Entropy v/s Time. 

Thus we see that the arrow of time points to a state of increasing entropy (or expanding space). The direction of the arrow in a way gives a default form of Time Travel. We are all traveling in the future at all times. Even though the universe had a single big bang singularity in the past, we have several black hole singularities in our observable universe. We know now that information loss can occur when a black hole evaporates. As such the entropy at the edge of a black hole may actually increase. What does it mean for the direction of Time ? We will explore some of these ideas in the next post on possibilities for Time Travel.

For now we update our previous picture of the pear shape of time with a direction as follows. 

Not only does Time have a space but also it has a direction. Possibilities of Time Travel should take into account our physical understanding of Time. The shape and arrow of Time limit the forms of Time Travel we may imagine.