Retrocausality: How Fundamental is the Arrow of Time?

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In this Insight, I’ll discuss a recent experiment ([1]  [2] [3]) by some Australian physicists that has brought a thought experiment into the real world [4], and in doing so has lended some ammunition to a most unlikely idea:

Retrocausality: A reversal of the concept of causality, allowing effects to occur before the events that cause them.

The Arrow of Time

light cone

Figure 1: Events occur at points in spacetime. At each point Q there is a light
cone consisting of two parts. The future light cone is the three-dimensional
surface generated by the light rays emerging from Q. The past light cone is similarly defined by the light rays converging on Q. (The labels ‘future’ and ‘past’ are conventions at this point in the discussion. In Section IV we will define them in the cosmological context.) Points inside the light cone of Q are timelike separated from it; points outside the light cone are spacelike separated. Points inside the future light cone of Q are in its future; points inside the past light
cone are in its past. Points outside the light cone are neither.

The arrow of time” simply refers to the way that time passes. Most people take the arrow of time for granted, because we observe it as having a direction: forward. That’s the way we perceive it, the way we interact with it. Water does not spontaneously jump back into a glass, buildings do not materialize from rubble, etc… These things all serve to give time a definite sense of direction. When you do things, they have consequences that come afterwards. For scientists, the idea that all effects have a preceding cause is referred to as causality.

Causality is a pretty deeply-rooted idea. There is an awful lot of evidence that the arrow of time has a single direction. It’s one of the assumptions made in Special Relativity [5], because without causality there is the ability to move faster than light, and an effect can be observed before its cause (see Figure 1). It’s also required by the Second Law of Thermodynamics [6], whereby the perpetual increase of entropy in closed systems requires a single direction for time. There is even evidence [7] that the arrow of time arises naturally out of a universe that has gravity.

My personal feeling is that the arrow of time is fundamental and shouldn’t be thrown around lightly. Without an absolute arrow of time, we end up with a universe where time travel is possible, but events are mostly deterministic, resulting in free will being only an illusion. Depending on your views of the world, a lack of free will can be disturbing

The Experiment

The experimental setup is quite clever, as described in [3]:

“…They fired the atoms at two grate-like forms created by lasers, although the effect was similar to a solid grate.

However, the second grate was only put in place after the atom had passed through the first one. And the second grate wasn’t applied each time, only randomly, to see how the particles reacted differently.

What they found was that, when there were two grates in place, the atom passed through it on many paths in a wave form, but, when the second grate was removed, it behaved like a particle and took only one path through.

So, what form it would take after passing through the first grate depended on whether the second grate was put in place afterward. Therefore, whether it continued as a particle or changed into a wave wasn’t decided until a future event had already taken place.

Time went backwards. Cause and effect appear to be reversed. The future caused the past. The arrow of time seemed to work in reverse.”

This amazing result caused one of the scientists to say, “It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it.”  Though that seems a bit sensational to me, it really is what’s on the table!

The Strangeness of Wave-Particle Duality

photo of light as both a particle and a wave

The first ever photo of light as both a particle and a wave.

A key sentence in the description of the experiment is this one:

Therefore, whether it continued as a particle or changed into a wave wasn’t decided until a future event had already taken place.

This is key, because it says that retrocausality is only implied by the experiment if this choice is actually occurring! In other words, if you operate under the assumption that the atom “decides” to behave as a particle, or as a wave, depending on the physical situation, then the information involved in making that choice would seem to violate causality.

But why do elementary particles sometimes act as particles, and other times act as waves [8]? Are they really just both all the time? For that matter, what does it mean to exist as both? Does the particle exist at all before it is measured? These are all questions that we don’t yet have solid answers to!

It’s timely that this experiment was reported about the same time that the first ever picture of light as both a wave and a particle was released. This helps bring several conflicts in physics into the public eye, which is a great opportunity for discussion.


Here are a few interpretations of this experiment:

  1. Causality is being violated. The atom is choosing how to behave midway through the experiment based on future information about whether the second grate will be added.
  2. Causality is not being violated. The atom could not have decided to “be a particle” or “be a wave” before the actual measurement was made at the end of the experiment.
  3. Causality is not being violated. Particles never “decide” to be a particle or a wave, they are always both, all the time. This experiment just displays the inadequacies of measurement.

What do you think? Do you have a different interpretation? Is the arrow of time fundamental, or can it be tampered with?


References and Additional Reading

  1. Experiment Confirms Quantum Theory Weirdness (
  2. Experiment Confirms Quantum Theory Weirdness (
  3. Scientists show future events decide what happens in the past (
  4. Wheeler’s Delayed Choice Experiment (Wikipedia)
  5. Special relativity: Causality and the prohibition of motion faster than light (Wikipedia)
  6. Entropy’s Arrow of Time (Wikipedia)
  7. How Gravity Explains Why Time Never Runs Backward (
  8. Wave-Particle Duality (Wikipedia)
  9. The Physics of ‘Now’ James B. Hartle (
  10. The first ever photograph of light as a particle and a wave (
26 replies
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  1. Nugatory
    Nugatory says:

    A number of posts in this thread have been removed, as they were driving the discussion off-topic.

    I’d like to remind the participants about the Physics Forums rules about personal theories. They are not allowed unless supported by peer-reviewed publications or other scientifically accepted sources – and the books that pump out misleading half-truths like “time slows down as the speed of light is approached” aren’t that).

  2. kreil
    kreil says:

    [USER=559673]@votingmachine[/USER] : Thanks for reading, and welcome to PF! Your interpretation of the time dilation equations of Special Relativity is, as bhobba pointed out, a common misconception. See [URL=’’]this well written response[/URL] for a good explanation.

    [USER=465472]@dayalanand roy[/USER] Thanks for reading, Dayalanand. Let me respond to your questions in order:

    1. This experiment is indeed another type of delayed choice experiment. You can read more about it here: [URL]’s_delayed_choice_experiment[/URL]

    2. I chose irreversible examples to imply that entropy is always increasing (the second law of thermodynamics). Entropy is one of the only quantities that requires a specific direction for time. Reversible thermodynamic processes are not particularly interesting to this end since they are performed at equilibrium and keep entropy constant. In fact, it is this conservation of entropy that allows them to be time-reversible! If the entropy had increased, the process would be irreversible, hence the arrow of time. Irreversible processes are primarily what occurs in nature.

    3. For a full explanation of why entropy implies an arrow of time, read this: [URL][/URL]

    4. On this point we can agree somewhat. I don’t think time travel is in the cards, either.

    5. Spacetime is the fusion of three dimensions of space and one dimension of time into a single continuum. I don’t think calling it the “fourth dimension of space” is technically correct. While spacetime was a major leap forward in our understanding of time, note that this experiment was dealing with quantum mechanics instead of relativity, and the two theories are famously incompatible. Our explanations of relativistic time do not account for many of the weird things in QM.
    When you get right down to it, I think you’re correct that dimensions do not “exist” in a physical sense. They are descriptions we have applied to nature for the purposes of prediction, and they do a great job at that.

  3. kreil
    kreil says:

    [USER=331]@HallsofIvy[/USER] [QUOTE=”HallsofIvy, post: 5142928, member: 331″](there is, at one point, “particle or atom”, but I presume that is a typo).[/QUOTE]

    Thanks for pointing out the error, I fixed it.

  4. bhobba
    bhobba says:

    [QUOTE=”votingmachine, post: 5144675, member: 559673″]Time DOES slow down as the speed of light is approached. Where is there any evidence of the opposite?[/QUOTE]

    Its misleading. Proper time, for example, remains the same. Suppose you have two inertial frames moving close to the speed of light relative to each other. Each has a clock placed at the origin. Each will read the same time on their clocks, but see the other clock as running slower. Which is correct? Which runs slower?

    What’s really going on is its like rotating a rod to fit through a door. The rod length remains the same – but its relation to other coordinate systems is different. In fact formally length contraction and time ‘slowing down’ can be viewed as hyperbolic rotation.

    But really this issue should be discussed on the relativity sub-forum – not here.


  5. atyy
    atyy says:

    Does this experiment violate any Bell inequality?

    I can’t find the article, but it seems there is only one particle involved, so it seems unlikely that a Bell inequality is violated. If no Bell inequality is violated, then we are not forced to abandon classical relativistic causality, so it seems that causality should still be intact.

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