There are no existing time machines capable of sending humans forward or backward in time. The closest we have come to time travel is using particle accelerators to cause subatomic particles to experience time dilation (i.e., forward time travel). There is a significant amount of time dilation data available. Particle accelerators succeed in achieving time dilation by accelerating subatomic particles close to the speed of light. Unfortunately, though, backward time travel has no similar body of experimental data. The major problems with creating backward time travel appear to fall into three categories:

  1. Backward time travel appears to require negative energy, based on arguments made by American theoretical physicist Kip Thorne and British theoretical physicist/cosmologist Stephen Hawking. Many in the scientific community acknowledge that negative energy likely exists, and point to the Casimir effect, discussed previously, as an example in nature. However, today’s science is unable to harness negative energy in any meaningful way to make a time machine.
  2. Many in the scientific community, like physicists Dr. Olum and Dr. Everett, believe the amount of energy required to twist space sufficiently for spacetime manipulation and enable Dr. Mallett’s time machine to work is enormous. Conceptually, we may be talking about the amount of energy provided by a star, similar to our own sun. Harnessing this level of energy is far beyond today’s science. Science’s best efforts to study high-energy physics has to date been confined to particle accelerators, such as the Large Hadron Collider. There is no experimental evidence that Dr. Mallett has succeeded in manipulating spacetime.
  3. Many in the scientific community are concerned with causality violations, especially regarding backward time travel. However, as we learned in the section titled “Twisting the arrow of time,” there can also be causality violations regarding forward time travel. The causality violations are generally termed “time travel paradoxes,” which we will discuss in detail in the next chapter.

Having made the above points, I think it is important to point out that some physicists believe subatomic antimatter particles travel in the opposite direction in time (i.e., backward in time) versus their matter counterparts. For example, some physicists assert that positrons, the antimatter equivalent of electrons, travel backward in time, while electrons travel forward in time. In solid-state physics, if we consider a current flowing in a semiconductor, electrons in a semiconductor move as a current in one direction, while the “holes” (i.e., the position the electron occupied in the semiconductor, which becomes vacant when the electron moves as a current) move in the opposite direction. Physicists differ on whether the “holes” represent positrons (i.e., actual physical antimatter particles). I mention this for completeness. There is no scientific consensus that antimatter travels backward in time.

Where does this leave us? I think this question deserves a complete answer. Stay tuned for part 2.

Source: How to Time Travel (2013), Louis A. Del Monte