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. votingmachine
    votingmachine says:

    I am puzzled by the removal of posts. I posted something speculative, and said it was speculative. But it is not CONTRADICTED by current literature. It is not supported or contradicted … it was speculation. I hardly see the point of a comment board that only allows comments that have already been substantiated in the literature. To remove retrocausality I see two possible explanations:The path thru the first grate is not limited, but is actually both allowed paths. That is fairly standard. The second filter then collapses the possibility to the allowed path, acting as observation, even though the "detection" of that collapse happens at a later time. That allows for a causality in an ordinary forward flow of time. The path thru the prior filter was not "caused", but chosen. I believe that is the most accepted interpretation, although it puts the second filter into an observer sort of role.The other is to eliminate the time between the events, and postulate a frame of reference where the two filters are crossed at the same time. That removes retrocausality because there was no before and after. We have no evidence that a frame of reference exists with no time flow. "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.”"You can get around the objection by saying:the future "CHOSE" the past. Then it isn't causality, but observation and collapse of possibilities.there is no time between the events. They are simultaneous, and therefore there is no "past" between the two. The first is probably correct. I still think the second needs to be considered.Or there is no time flow and causality as we expect them.

  2. votingmachine
    votingmachine says:

    @Krell. Thanks. I read the link. I still think that it says what I said. Specifically it says:"there is no meaningful way to discuss how anything looks "from a photon's perspective" in special (or, for that matter, general) relativity. What we can say is that the spacetime distance* of the path travelled by a photon (through vacuum) is zero, but that the actual distance (and hence amount of time) the photon travels depends on the frame of reference."I agree. There is no meaningful way to speak of the time flow. We can say that the "spacetime distance" of a photon path is zero.It is a common conception to speak of the "time flow" rather than the "spacetime distance". I still see nothing wrong with phrasing it in terms of the time flow of the photon although I agree with the formal "spacetime distance=zero". I am not sure why it is a "mis"conception. But if we were to consider the "frame of reference" of the photon, it has a path that has a spacetime distance of zero. Which implies that it does not have any spacetime distance between filter 1 and filter 2. That before and after that we talk of, which create a causality issue, are not a before and after, in that unusual "frame of reference". Two simultaneous things may of course influence each other. I'm not able to dismiss causality. It is embedded too deeply in EVERY model I see. If we get to a result that says time went backwards and causality was reversed, with a later event causing a prior event, then I think it is only reasonable to speculate on explanations that change our interpretation to return to causality.

  3. dayalanand roy
    dayalanand roy says:

    Highly grateful having such an illuminating reply.Being a non-physicist, it is hard for me to conceptualize about the said QM experiment, but I have tried many a times. After reading the explanations given by Stephen Hawking in his 'the Grand Design', I tried to understand it in following way- Suppose we fire a bullet from a gun. While leaving the gun, the bullet will affect the gun ( as a jerk in the opposite direction)- a causality, but after leaving it, we may not think that it will again affect the gun (no retrocausality). But let us suppose that the bullet is tied with an almost invisible  long thread the other end of which is tied with the gun, then some time after leaving the gun, when the bullet travels the length of the thread, the bullet will again affect the gun that may appear to us as a retrocausality.I know that this situation does not simulate with the double slit experiment, but I have just tried to transform the bullet (a particle like thing) -by tying it with a thread- into a wave like thing and see the effect. The explanations of the results of double slit experiment also depend somewhat on the wave- particle duality. Apologies in advance, if my way of thinking is too wild.RegardsDayalanand

  4. dayalanand roy
    dayalanand roy says:

    Dear colleaguesOne more comment. It has been said that entropy is the process that requires 'arrow of time'. However, this requirement leads to many unanswerable qestions about time as it gives a sense of 'time as an entity' and then leads some people to say that time does not exist.In my view, it is more sensible to say that it is the entropy and the second law of thermodynamics and the irreversible processes that give us the feeling of the so called 'time's arrow'.Thanks and regards

  5. Fooality
    Fooality says:

    I just want to respond to say how much I love this kind of writing. The weirdness physicists encounter is a huge part of why I love this forum, even though I'm not a physicist. Its just amazing to read about. My impression (as a computer guy) is that the universe is explicable because the universe is in a sense, compressible. For instance, you could accumulate gigabytes of data on the locations on the path of a planet, or you could generate that same data from the comparatively small amount of math which predicts the path of that planet. In fact, because its compressible, if a physicist has enough information about a planet at one moment of time, she can expand the whole path, past and future, out of it. In this sense it collapses the path down from a set of previous possibilities. The reason this is possible is that the system is describable using less information that the universe actually manifests. Its compressible, thus is explicable. If it were not compressible, no point in the future or past would be predictable from any other point, just as an incompressible string is completely random.I haven't read the experiments linked to, but I've read about retro-causality in delayed choice quantum erasure, where the choice a photon makes down the road dictates whether the interference pattern will be observed earlier. You can choose to view that in terms of the choice retro-causing the pattern, or the pattern predetermining the choice, or you can see in terms of compressibility: The which-path information for the whole system, spread out over space and time, is described by only 1 bit, the seemingly spread out nature of it in space and time is the illusion, just as the planet is described by more simple equations. The actual reality is the underlying compressed form. Anyway, that's my two cents on making any sense of it. Thanks again for sharing!

  6. eltodesukane
    eltodesukane says:

    “Does the particle exist at all before it is measured?”
    — Just like in a 2D Pokemon video game I used to play.
    When you get to a certain point on the map, you can see Nurse Joy looking at you from the hospital’s window.
    Was Nurse Joy already at the window earlier? How could you tell?
    There is no way to tell while playing the game, as the window is only visible when you get there.
    Does the window even exist before you get there?
    Does Nurse Joy even exist before you can see her?

  7. bhobba
    bhobba says:

    I haven’t seen an analysis of the experiment mentioned in the paper, but it looks like a variation of the good old delayed choice experiment. Before going any further I think a careful analysis of that experiment needs to be part of the debate:

    Like a lot of things in QM where people talk about sometimes its a particle and sometimes its a wave, the reality when you look at it carefully, is its got nothing to do with that outmoded idea. Its never really a paticle or a wave – its always just quantum stuff. Whats quantum stuff:

    QM IMHO, as I have posted many times, is simply the most reasonable probability model that allows continuous transformations between pure states. The real interpretive issue is not the formalism – we understand it and why its like that pretty well – it’s a model of observations – observations are the primitive of the theory. This is what causes all the angst and worry. Everyday experience is the world is more than just observation. You see it all the time in discussions about QM – the deep need we seem to have that there is something behind those observations. I believe if you simply accept this one fact much fog and angst about it disappears.

    What’s going on with these delayed choice type experiments – IMHO its in simple cases decoherence can be unscrambled, as has been discussed a number of times here on physics forums.


  8. bhobba
    bhobba says:

    [QUOTE=”jim hardy, post: 5142535, member: 327872″]I have no doubt it’s someplace at every instant, just we cant measure where..[/QUOTE]

    As I said, that’s the problem that’s particularly hard to shake.

    Once you do, progress is swift in understanding QM.


  9. bhobba
    bhobba says:

    [QUOTE=”votingmachine, post: 5142909, member: 559673″]No time elapses at the speed of light.[/QUOTE]

    That’s a common misconception.

    SR is the theory about transformations between inertial frames. No inertial frame can travel at the speed of light so the theory says nothing about time at the speed of light.

    Even plugging c into the equations you end up with the dreaded 1/0 which is undefined.


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