Physics and Philosophy

Over at the other blog, my coverage of William Lane Craig’s cosmological arguments has attracted the attention of a commenter whose URL is He has at least one comment on each of my posts starting with the one I published three weeks ago, and part of his first comment is this:

I see no reason to believe that the cause must occur before the effect. For example, if I throw a brick through a window, the brick does not pass through the window before the window breaks. At the precise moment the window breaks the brick is acting upon the window. Cause and effect are simultaneous.

He’s responding to my argument that time (and thus the material universe of space-time) must exist before cause and effect is possible, because cause and effect exist in a chronological order, and the cause has to happen before the effect. I know what I want to say in response, but perhaps some of you who know subatomic physics could comment on whether or not I’m on the right track.

I’ve done some thinking about this and I’ve wavered back and forth on the point of whether or not a cause can indeed be simultaneous with its effect. Currently, I’m still inclined to say that it can’t, at least in terms of what I’d call real cause and effect (as opposed to mere apparent cause and effect). In the specific example of the brick through the window, I’d say that the motion of the brick and the breaking of the glass appear to be simultaneous, but that’s merely an artifact of our perceptual limitations. If we could pick just the instant where the brick first makes contact with the glass, and could zero in to a near-atomic scale physically, with a corresponding slowdown in the passage of time, we would observe that the electron shells in the atoms of the brick do not actually touch the electron shells of the atoms in the glass, but rather the fields interact with each other, such that the kinetic energy in the brick’s atoms is transferred to the atoms in the glass. At a certain level, this transfer of energy is not instantaneous, but is subject to the limitations of the speed of light (i.e. the influence of the cause cannot propagate to the target object at a rate that exceeds the speed of light). Since there is a non-zero interval of time between the generation of the cause and the propagation of its influence to the effect, the cause still comes first.

Is that legit? Is it sound physics to say that the influence of the cause cannot propagate to the effect at a rate faster than the speed of light? And on an even more esoteric note, is it possible or even meaningful to speak of two events as occurring simultaneously? Technical answers welcome, technically-correct answers in layman’s terms even more welcome.



  1. Randomfactor says

    The “cause” of the broken window is the momentum of the brick. That momentum was imparted well before the brick reached the window and requires time existing before the impact.

  2. Ms. Crazy Pants says

    My answer isn’t exactly scientific, but I think it might work.

    It doesn’t take a brick passing through glass to break it. If you had a brick wall behind the sheet of glass, throwing a brick at it would still break the glass. So, the effect happens at the point of impact.

    Even up close it may look like two actions happening at once, but on a cellular level, it takes a bump of a an atom to make a non-moving atom move. The bump happens first to the atom, then it moves. In larger structures, you have thousands of atoms all being bumped and moved, but not at exactly the same nanosecond.

  3. Ben says

    Randomfactor has it right, it’s much simpler than physics, etc… the cause of the brick breaking the window is the person propelling it. Bricks don’t just appear in windows and shatter them of no accord. It’s worrying that this backwards fellow has graduated from throwing ‘first stones’ to bricks… hope he doesn’t have anything heavier around.

  4. cnjnrs says

    Simultaneity is relative. If two events A and B are simultaneous, which is to say they happen at different locations but at the same time, in one reference frame, then typically there exist frames where A precedes B and vice versa. Normally, we assume that a cause must precede its effect in all frames, so the effect must be in the future “light cone” of the cause. The light cone includes all events that can be reached from the cause event, without going faster than the speed of light.

    In the case of the brick hitting the glass, though, one problem is that we don’t have a well-defined pair of events. I think the cause is supposed to be the brick hitting the glass, and the event the glass breaking; but when, exactly, is the glass defined to be broken? If the condition is that there is a visible crack, this certainly does not happen until after the hitting event. In fact, I think it would be hard to provide a sensible definition that would make the glass-breaks event simultaneous with the brick-hits event.

    However, I also suspect causality to be a fundamentally subjective thing. The laws of physics are normally invariant under a time reversal (although in the quantum world, this basically requires that particles and antiparticles be swapped; it is called the CPT theorem). Unfortunately I don’t know enough about this to make any strong (or useful) statements, though.

  5. quantheory says

    Well, in order for the brick to hit the window, it needs to not be in contact with the window, and then later to be moving through the window. In a sense, whether or not the “cause” of the brick hitting the window and the “effect” of the window breaking are considered simultaneous, the particular cause we are talking about implies the existence of time, because otherwise the event of the brick first making contact with the window really makes no sense. Similarly, for the window to break, there needs to be an unbroken state proceeding a broken state.

    That said, it’s also true that brittle materials tend to shatter no faster than the speed of sound in that material. Within a suitably defined “reasonable” energetic regime, that’s the fastest that vibrations can be transmitted. And the speed of light, of course, remains unbreakable.

    One reason for this is that “events” or positions in space time are only partially ordered. If event A is in the past light cone of another event B, then A proceeds B. But if neither event is in the other’s light-cone (a “space-like” separation), then neither is necessarily before the other. Indeed, different observers may perceive them as being simultaneous, or as either being before the other.

    However, events that we experience in our daily lives take a long time to happen over a short distance (compared to the speed of light), so events are rarely space-like separated, so we are able to put different events in a strict chronological order. Generally speaking, the brick will hit the glass before the glass sheers enough to be considered “broken”.

    All of that said, I’m actually not positive on the relationship between causality and time. So far as we know, the universe obeys something called “CPT” symmetry, which means that, with a few sign changes, the laws of physics work the same forwards and backwards in time. This presents a conundrum, in that there’s no clear physical reason to label antecedent events, rather than later events, as “causes”.

    However, the “psychological” arrow of time seems to be related to changes in entropy over the course of the universe’s evolution. We remember the past, and not the future, because as time progresses information about an event “spreads out” from that event, in a way that can at least sometimes be physically quantified in terms of increasing entropy. The change in entropy also explains why the reverse doesn’t happen, why influences from across the universe don’t routinely gather together to cause some particular, “fated” event to happen.

    That’s also why we only know about events ahead of time when we have expended work to gather information about the causes of those events. So I’d expect that causality has more to do with the conditions under which conscious minds can work (only in the direction of increasing entropy), than with an inherent causality property of the universe. The details here are far from settled though, so this is largely my own (albeit informed) speculation.

    But if we’re talking about the intentions of God, then it’s not clear how causality works anymore. For that matter, it’s not clear what it even means to have a timeless, unchanging, omniscient mind. Does it have memories? What of? Does it think? How can you define “think” in a way that doesn’t involve producing new knowledge in a process, or define such a process in a way that doesn’t involve some dimension of time along which such a process of thought can be carried out? Does it perceive? If it is already aware of everything that could possibly exist, what does that perception consist of? Can something be willed or desired, without the conception of a state without something being antecedent to a state with that thing?

    The thing being described seems to be more like a static array of cognitive elements or concepts more than something “conscious” with a will. Only if God is “simple”, it is not even as much as the former, because it can’t have multiple component parts. It’s hard to believe that any definition of the word “person”, assumed to encompass consciousness on the human label, could also accurately describe such an entity.

    I think that this is the deep-down problem with WLC’s conception of God. He seems to think that “person” is one of the basic categories of thing in the universe. But there’s no coherent description of what a “person” really is that doesn’t rely on physical or quasi-physical ideas such as time and interaction of component parts (or faculties, such as memory, perception, language, planning, intention, imagination, emotion, and so on).

    • quantheory says

      I apologize a bit for my terrible sentence construction; today is not a clear day for me. Some better sentences:

      “We remember the past, and not the future, because information about an event “spreads out” over time. We sometimes even quantify this process in terms of increasing entropy.”

      “Doesn’t thought necessarily involve producing new ideas from old ideas in a creative process? Can you have such a process without some dimension of time along which the process can be carried out?”

      “Can something be willed or desired without conceiving of a progression, from a state of absence to a state of presence?”

  6. mikespeir says

    My concern is more linguistic. Why, the very definition of “cause” is virtually, “that which produces an effect.” If we’re going to make definitions that, well, indefinite, then the sounds we make with our mouths and the marks we put on paper don’t qualify as language anymore, because language is only language if it conveys meaning. To confuse “cause” and “effect” is to rob the words of their meaning and, therefore, of their, uh…wordiness?

  7. Patrick says

    Causes cannot precede effects, nor can they be simultaneous with effects. Arguments to the contrary tend to either be scientifically illiterate, or else dishonest. The usual trick is this: take a macro-event that is technically loads of smaller things happening, and use that to muddy the waters. That’s what seems to be going on in the brick example.

    The example I’ve seen most often is to claim that a truck pulling a trailer is an example of a cause that’s simultaneous to an effect- the truck is the cause of the trailer moving, etc. But to anyone with the slightest scientific literacy, a truck is a collection of molecules, as is a trailer hitch, and as is a trailer. And this collection of molecules is linked together by forces on the atomic scale in a vibrating matrix of molecular attachments and repulsions. Any movement from the truck that causes any movement in the trailer is in fact a series of small interactions traveling through the chain of molecules, with every single interaction involving a cause preceding an effect.

    Its not a difficult subject once you stop letting them smear mud in your eyes. Presume mendacity, and you’ll go far.

  8. ah58 says

    I would think all you need to do is show a slowed-down film of your brick hitting the glass. It would be obvious that the brick makes contact, transfers energy, the stress on the glass builds to a point that its structure fails, and the glass then breaks.

    Just because this happens too fast to be seen at normal speed, doesn’t mean it doesn’t happen in that order.

  9. Robert B. says

    You’re basically right. “Simultaneous” is a concept that depends on your frame of reference, and has no important physical meaning. The closest equivalent that actually exists is a space-like interval – a distance across space and time that has a lot of space and not much time, so that to cross it you’d have to go faster than the universal speed limit. If two events are separated by a space-like interval, then different observers at different speeds can see them happening in either order, or at the same time. But cause-and-effect (in physics-speak, transmission of information) can only occur across a time-like interval, which is the exact opposite of space-like: a distance that has a lot of time and not much space, which makes it possible for something to go from one end to the other without breaking the speed limit.

    In other words, the glass can only “know” to break when it gets the news that a brick has arrived, and that information can’t be sent faster than light. So, cause and effect will never appear simultaneous or reversed in any frame of reference.

    Your interlocutor was being sneaky, using the fact that the glass is broken before the brick is all the way through. But the cause of the glass breaking is the front corner of the brick; the rest of the brick that passes through after the glass breaks, was not the part that hit it and caused it to break.

    An interesting point: the electric force between electrons in the brick and glass, which you correctly cite as the fundamental interaction going on here, is mediated by virtual photons, so in fact it occurs at exactly the speed of light. (Propagation of the shattering effect through the glass is slower, though.)

  10. F says

    Field interaction and simultaneity are already beyond the error. The statement is begging the question, and a false assumption once the question is examined.

    The glass is breaking as the brick is passing through it. The brick does not pause, wait for the glass to get off its duff, and obligingly break so that the brick may continue on its merry way.

    If he wants to play word games, we can say the brick is already through the window before even contacting the glass excepting in cases where there is no singular window opening, but a glass curtain wall (which you can just as easily call a wall and make a case that it is not, in fact, even a window at all) on a building for which the framing members are somehow exactly flush or recessed.

    Of course, this says nothing of glass which is not in windows, or of windows which have no glass.

  11. kft says

    From a physical standpoint, the brick breaking the window is pretty obviously causal and properly oriented in time. Macroscopic objects like the brick and the window are going to interact via the electromagnetic interaction (the outer shells of electrons in their atoms repel each other – this is why solid objects don’t pass through each other). This force is carried by the photon, the particle of light, which, as you may have heard, travels at the speed of light. It thus takes a finite amount of time for the brick to transmit its presence to the glass, so the process is pretty obviously properly ordered in time.

    Those photons that carry the electromagnetic force between the brick and window are transporting the momentum that others have mentioned. But the point is that they travel at a finite speed, meaning the two events, while causally related, CANNOT happen truly simultaneously.

  12. Steve says

    Whether the window breaks “because” of force interaction between electron shells is an illusory red herring. Cause and effect is a human invention to help us isolate and model regularities in the universe, the universe just happens. Its state changes deterministically, or stochastically if you like the Copenhagen Interpretation, but always and only in accordance with the mathematically precise rules constraining it.

    Judea Pearl, the greatest living expert on cause and effect, has a book about it. He also has a slideshow explaining it:

  13. John Morales says


    I’ve done some thinking about this and I’ve wavered back and forth on the point of whether or not a cause can indeed be simultaneous with its effect.

    Causation is a time-bound concept, so: no.

  14. Chrisj says

    If you’re watching on a suitable time-scale, you’ll find that not only does the brick come into contact with the glass before the glass breaks, the glass bends before it breaks. The proximal cause of the break is bending and micro-cracking of the glass; the brick merely causes that bending and cracking.

    So your religious interlocutor is, quite simply, flat-out wrong.

    The question of cause and effect more generally is a rather more complex one, and less directly in my field, but essentially it’s impossible for (a) to cause (b) unless (a) happens before (b) in every possible frame of reference. Anything else is likely to require a hasty rethink of relativity.

  15. Randomfactor says

    These guys are always trying to apply “common sense”* to a unique event. Every event in the universe has a “before” which can be referenced, except one–the Big Bang.

    *(And coming up empty.)

  16. josh says

    It looks like other commenters have covered this, but in modern physics there is no absolute notion of simultaneous since time and distance vary depending on your frame of reference. Nonetheless, there is a concept of ‘causality’ encoded in the equations which means that a measurement at one point in spacetime cannot affect a measurement at another point outside its light cone. So, for a given frame, something that happens at A can’t affect the outcome at B if B is farther away in that frame than the speed of light can travel in the amount of time between the events at A and B in that frame. It’s also true that if B appears to happen at a later time than A in one frame, then in no frame can it appear to happen at an earlier time, so there is some notion of order.

    On the other hand, the perception of the direction of time seems to be a trait of the human mind related to entropy and not a quality of the fundamental physics. If you can predict the event at B given the information at A, then, with enough information at B you can post-dict A. The order may just be an anthropic effect and a traditional notion of cause and effect goes out the window. It may be like saying that having three sides causes the summed internal angles of a polygon to be 180 degrees, when you could say with equal validity that the sum of the angles causes the number of sides.

    • quantheory says

      Er, I can only read the first page from home, but that paper looks like an example, I think, of philosophers running into trouble by applying laymen’s language to mathematical concepts.

      At the moment two things collide, no reasonable physicist would say that that represents a moment when the two entities would either “overlap” or “occupy adjacent points”. We would rather say that the union of the set of points that compose the surface of one object, and the set of points that compose the surface of the other object, is a non-empty set. That’s what our intuitive feeling for the word “touch” really refers to; the point in time when two surfaces share a point in space. The “surface” of each object can either be considered not part of the object (but defined as the “edge” of the object using limits), or we can say that the surface is part of the object, and that two colliding objects “overlap” in the sense that they share points, but that they do so only in a finite area, and thus the overlap has a volume of zero, which doesn’t seem like much of a problem.

      This is easy enough to describe in one dimension. If you have an object occupying the interval [0,1) on the number line, and another occupying the interval [1,2), then they would seem to be touching. There are no “empty” points between them, and yet they don’t overlap at all. Depending on precise definitions, you might also consider (0,1) and (1,2) to be touching, since there is one point, or zero distance, between them, or [0,1] and [1,2] to be touching, since they overlap at only a single point, and thus the distance over which they overlap is zero.

      Of course, from a physicists’ perspective the precise definition of “contact” or “touching” doesn’t much matter, firstly because it’s a semantic distinction with no physical consequences, and secondly because the world turns out to be composed of fields deep down (or if you prefer, the wave-like “particles” that are excitations of those fields), so the “mechanistic” (billiard ball) model of matter is empirically wrong anyway.

      And of course, I get the feeling that the paper is a bit tongue-in-cheek anyway.

      • Andrew Woods says

        It looks like a reformulation of Achilles and the Tortoise; notice that statement (2) defines touching in terms of “adjacent points in space”, even though their existence is denied by statement (4), so this amounts to creating an incorrect definition of collision and then pointing out that it is incorrect.

        If you introduce time, you can show that at one moment the bodies do not overlap, and at a subsequent moment the bodies will overlap unless at least one velocity changes; as overlap is impossible, a collision necessarily occurs between those two moments. (Clearly this requires the intervention of God, so we have here yet another proof of His existence for the delectation of sophisticated thinkers like Dr Craig.)

      • says

        but that paper looks like an example, I think, of philosophers running into trouble by applying laymen’s language to mathematical concepts.

        At the moment two things collide, no reasonable physicist would say that that represents a moment when the two entities would either “overlap” or “occupy adjacent points”.

        Precisely. (Notice any parallels in the brick hitting the window?)

        Likewise, no reasonable physicist would say that a brick hitting a glass window and the window shattering occurs simultaneously. A window undergoing a transformation from unbroken to broken does not happen in zero time, since atoms have to move for that change, and they do so at a finite velocity. Therefore, a brick hitting an unbroken window cannot be simultaneous with a brick hitting the broken window.

        And of course, I get the feeling that the paper is a bit tongue-in-cheek anyway.

        So do I. Which is why arguing with a non-physicist abusing physics make not drive home the point like exactly such a philosophical argument.

  17. F says

    Likewise, no reasonable physicist would say that a brick hitting a glass window and the window shattering occurs simultaneously. A window undergoing a transformation from unbroken to broken does not happen in zero time, since atoms have to move for that change, and they do so at a finite velocity. Therefore, a brick hitting an unbroken window cannot be simultaneous with a brick hitting the broken window.

    The really amazing thing is that something as simple as cameras can tell us this.

    What a fantastically interesting thread. I’ve lost count of how may ways this His First Comment is wrong.

  18. jakc says

    Ladies & gentlemen! Stop with all of the theoretical bibble-babble! What we need is solid experimentation. I propose we go to the original commentator’s home, throw bricks through windows and then ask him/her if thrown bricks cause broken Windows.

    • Deacon Duncan says

      Somehow I really doubt this would work as a rebuttal of the OP’s claim that thrown bricks cause broken windows.

      • jakc says

        Jaysus, Mary & Joseph, Duncan, I am not aware of all internet conventions. OP? Is this “original post”, or is it some reference to the “bricks-windows simultaneous” poster?

        In the first case, my experiment is meant to confirm, not rebut, the idea that thrown bricks cause broken windows.

        If it is the second case, well as Peter Griffin says when he’s trying to get the couch off of the Death Star and unto the Millenium Falcon, “we’re making this harder than it really is.”

        You don’t need quantum mechanics to understand that the brick breaks the window; the claim that the “brick hitting the window/window breaking” can be simultaneous is a bit like the old joke: “2+2=5, for sufficiently large values of 2.” The point of Einstein is that even if you can find a reference frame where an observer would see an two events as simultaneous, in every different reference frame, those events would not be simultaneous. The poster presents this problem as a way around “cause-and-effect” but we only have a problem if we accept the flawed premise. I mean, it’s very easy to explain errors in the Bible if you reject a premise like “it’s the inerrant word of God”, and a lot harder job if you’re stuck with an inaccurate premise like inerrancy.

        In the bricks and windows example, simple observation shows us that the events are not simultaneous, and yes, sometimes you can trust eyes. And, even if you find a reference frame where the actions appear simultaneous, it doesn’t really help our bricks and window guy. After all, he’s trying to show that God isn’t bound by the arrow of time or by the rules of cause and effect – cause first, then effect. Imagine that God has a reference frame where God is simultaneous with the creation of the universe. Even if true, you can’t prove it to be true – from our non-God reference frame, the events no longer appear simultaneous. There is no absolute simultaneity, any more than there is absolute time. Either God or the universe would be there first; it’s much simpler to imagine the universe there without God. If cause doesn’t need to be before effect, than you don’t need cause there at all.

        In the case of the thrown brick and broken window, the correct answer is that the thrown brick and broken window are NOT simultaneous, and we need no further explanation or discussion of what’s really happening at a quantum level when the brick hits the window. And I’d love to see a reference frame where the window breaks before the brick is thrown (The bartender says “We don’t serve neutrinos here. A neutrino walks into a bar.”)

        Einstein, or perhaps Woody Allen, once said “Time is the universe’s way of keeping everything from happening at once.”

        We know that time has an arrow and are uncertain as to whether time can be quaticized, but your original instinct that the brick hitting and the window breaking are not simultaneous is not only right, but demanded by relativity.

        Gallileo dropped balls to prove a point. I’nm just saying you could prove your point with thrown bricks.

  19. chaos-engineer says

    After reading this about 50 times, I think I see the point he’s trying to make. “The brick’s impact caused the window to break” is a bad analogy; it would be clearer if he’d said “The laws of physics caused the brick to break the window”. Obviously the laws of physics exist simultaneously with the breaking of the window.

    So the argument is, “Even if time didn’t exist before the Big Bang, ‘existence’ must have some kind of structure that allowed the Big Bang to occur. This structure might be ‘outside of time-as-we-know-it’. Therefore, God exists.”

    That’s a pretty good argument up until the last step. The last step needs to be, “If this structure is self-aware and purposeful, we would call it ‘God’. If it’s self-aware but not purposeful, we would call it ‘Demiurge’. If it’s not self-aware, we would call it ‘Natural Law’. All three of these possibilities are consistent with observed reality and you’re free to pick one based on personal preference. Obviously you’re not allowed to use this as an excuse to believe in a specific sort of God/Demiurge/Natural Law that’s not consistent with observed reality.”

    • Deacon Duncan says

      Well, no, I think the point he’s trying to make is simply the one he says he’s making: that the brick passing through the window is simultaneous with the window breaking. The fact that this is technically not true doesn’t mean that’s not what he’s claiming.

      And while I’m quibbling, I don’t think you can quite say that “the laws of physics caused the brick to break the window.” There are plenty of bricks, and plenty of unbroken windows, so if the laws of physics were causing the bricks to jump up and break the windows, we ought to see it happening right now. The laws of physics are the context within which physical causes produce physical effects, but that’s something a bit different than being the actual cause itself.

      As for the “structure that permits the Big Bang,” see today’s post. 🙂

  20. dephlogisticated says

    The commenter’s brick/glass analogy is not valid as has been noted by yourself and other commentators. The brick hits the glass, though it never atomically touches the atoms of the glass. The glass will bend until it reaches its modulus of elasticity. Once the normal stress limit is reached, the glass will shatter. These are all forces, where, as an example; F=ma (a is acceleration, which is time-based). So, it cannot be simultaneous. Plus, something had to give the brick motion.

    But, that is also relative. To the glass, it is motionless, while the brick is the object in motion. But, to the brick, it is the one that is motionless, with the glass being the one in motion.

    I believe what your bible scholar commenter is trying say, is trying to equate this to the creation of the universe. However, this is not valid. As has been proven by particle physicists; there need not be a cause to have an effect. It has been proven that neutrons can appear, and then disappear, without cause.

    Discovery Channel’s Curiosity had a show: “Did God Create the Universe?” with Stephen Hawking. Which explains that there need not be a cause for the creation of the universe.

    If you add up all matter and energy in the universe, the net result is 0: nothing. So, something is not created out of nothing.

    Time is actually a man-made concept. (Why do we have 60 seconds in a minute; 60 minutes in an hour, and 24 hours in a day? It’s because those values come to us, via the ancient Sumerians. The numbering system we use today is Base10 (0-9). The Sumerians used Base60.) We use it to measure change. It has no physical basis. If there is no change (energy, mass, velocity, acceleration, etc.), there is no time.

    If nothing existed, there can be no change. If there is no change, there can be no time.

    Since homo sapiens is a terraformer, if you will, we build things for a reason. Therefore, people such as your commenter transposes this reason, or purpose, onto everything else that humankind did not create. They then therefore declare that there must be a creator. (This, of course, is a logical fallacy) They equate cause to reason/purpose (which, again, is a logical fallacy).

    They simply do not understand that there need not be a cause/reason/purpose, for why things happen.

  21. says

    On this simultaneous cause/effect thing, I’ve heard apologists talk about a pillow with a bowling ball. The bowling ball caused the depression in the pillow, but it’s not like billiard ball 1 caused the motion of billiard ball 2.

    I don’t find this pillow example persuasive because the “effect” is insubstantial. Somehow, “a depression in a pillow” seems to be of a different kind than a broken window.

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