Nothing has actually been sent back through time, but scientists from the University of Queensland, Australia, have used single particles of light (photons) to simulate quantum particles travelling through time. The model used actually showed that one photon can pass through a wormhole and then interact with its older self. Not long ago, the findings were published in the journal Nature Communications

To understand this experiment, you have to understand the concept of “closed timelike curves” (CTC). These are used to simulate powerful gravitational fields, like the ones produced by a spinning black hole, and could, theoretically (based on Einstein’s theory of general relativity), warp the fabric of existence so that spacetime bends back on itself – thus creating a CTC, almost like a path that could be used to travel back in time.

According to Scientific American, many physicists find CTCs “abhorrent, because any macroscopic object traveling through one would inevitably create paradoxes where cause and effect break down.” Others disagree with this assessment, however; in 1991, physicist David Deutsch showed that these paradoxes (created by CTCs) could be avoided at the quantum scale because of the weird behaviour of these fundamental particles that make up what we call matter.

It’s well known that at the quantum scale, these particles do not follow the rules that govern classical mechanics, but behave in strange and unexpected ways that really shouldn’t even be possible.

“It’s intriguing that you’ve got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away.” – University of Queensland physicist Tim Ralph (source)

The Experiment

Tim Ralph (quoted above) and his PhD student Martin Ringbauer simulated a Deutsch’s model of CTC, thus testing and confirming many aspects of the two-decades-old theory. Remember, this is just a mathematical simulation, but, as the researchers emphasized, their model is mathematically equivalent to a single photon traveling through a CTC. Again, nothing has actually been sent back through time though; to do that, scientists would have to find a real CTC, which has yet to happen as far as we know.

What’s known as the  ‘grandfather paradox’ is usually used to explain this concept a little more clearer; It’s a hypothetical scenario where someone uses a CTC to travel back through time to cause harm to their grandfather, thus preventing their later birth. Now imagine a particle going back in time to flip a switch on the particle-generating machine that created it – this is a possibility that these physicists say they have shown through their simulation.

A very relevant point made by Discover News:

“The maddening part of that problem is that the ability of particles to exist in two places at once is not a mere theoretical abstraction. It is a very real aspect of how the subatomic world works, and it has been experimentally confirmed many times over.” (source)

The New York Times states that:

“One of the supreme mysteries of nature… is the ability, according to the quantum mechanic laws that govern subatomic affairs, of a particle like an electron to exist in a murky state of possibility — to be anywhere, everywhere or nowhere at all — until clicked into substantiality by a laboratory detector or an eyeball.” (New York Times)

This means that one particle can exist in multiple states at one time. This is best demonstrated by the quantum double slit experiment. Recent experiments have also confirmed quantum entanglement, showing that space is really just a construct that gives the illusion of separation. One thing that suggests there is a high probably of time travel, in conjunction with the experiment mentioned in this article, is the fact that there are experiments showing that particles can actually be entangled through time.

This is illustrated by what is called the ‘delayed choice experiment.’

Like the quantum double slit experiment, the delayed choice/quantum eraser has been demonstrated and repeated time and time again. For example, physicists at The Australian National University (ANU) have successfully conducted John Wheeler’s delayed-choice thought experiment. Their findings were recently published in the journal Nature Physics. (source)

We recently published an article on the Delayed Choice experiment, you can read that here.

In 2007 (Science 315, 966, 2007), scientists in France shot photons into an apparatus and showed that their actions could retroactively change something which had already happened.

This particular experiment illustrates how what happens in the present can change what happened in the past. It also shows how time can go backwards, how cause and effect can be reversed, and how the future caused the past.

“If we attempt to attribute an objective meaning to the quantum state of a single system, curious paradoxes appear: quantum effects mimic not only instantaneous action-at-a-distance, but also, as seen here, influence of future actions on past events, even after these events have been irrevocably recorded.” – Asher Peres, pioneer in quantum information theory (source)(source)(source)

Although we do not have access to a CTC quite yet, there are good reasons to believe that this type of time travel is possible at the quantum mechanical level, and that is why I chose to mention these other experiments, to show that ‘time’ doesn’t even really exist as we think it does, and to show that findings within the realm of quantum physics are always inching a step closer to leaving the ‘theoretical realm.’

Why these same quantum mechanical laws have not been observed on the macroscopic level is yet to be understood, but physicists are working on the problem. For example, in 2012 physicists David Wineland and Serge Haroche received the Nobel Prize in physics for demonstrating how “quantum weirdness” could not only exist at the subatomic micro-world level, but also show itself in the macro-world. At one time, superposition was only thought to exist in the inaccessible quantum world, but not anymore. We know it’s possible, we just haven’t figured out how. We do, however, seem to be getting closer to finding out. (source) (source)

Perhaps one day, we will have determined the key to this puzzle and be able to observe large objects like cars, humans, apples, and oranges behave in the ways that matter does on a subatomic level, and perhaps one day we will find a wormhole, or a CTC in space, to conduct actual experiments that go beyond theory. That being said, a lot of what used to be considered theoretical in quantum physics is no longer theoretical, like quantum entanglement.