Nature mostly prefers simultaneous events: Acceleration is produced without any delay on applying force. Angular acceleration is produced without any delay on applying torque. A bulb glows simultaneously as we close the circuit. Heat is transferred from one place to another as we allow it without any delay.

I don't think there are any such events except transmission of waves which takes time in physics. Why is this so?


I really appreciate the answer given by Aaron Stevens but there is something still unclear to me.Calculus as well as Newtonian mechanics start with approximations but those approximations are really beneficial to us as they simply the complex mathematical equations involved in several problems.

As Aaron Stevens wrote in his answer that on microscopic scale there is a delay between force and acceleration.I think that in our standard model we take no delay of time which is still an approximation.We give answers to the problems based on our standard models which are based on numerous no. of approximations and assumptions.Similarly in the next example he said that there is nothing rigid on microscopic scale.I again say that we assume that every thing is rigid.It is my gentle request that please give answers based on the assumptions we made while building our standard model.

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    $\begingroup$ "A bulb glows simultaneously as we close the circuit. Heat is transferred from one place to another as we allow it without any delay." Neither of these are simultaneous. The glow of the bulb is a growth item as the temp increases. Heat via radiation travels at c, and conduction is much slower than that. Your other two examples are the subjects of philosophical debate. $\endgroup$
    – Bill N
    Aug 28, 2019 at 17:17
  • $\begingroup$ If you're referring to incandescent bulbs, they do not glow instantaneously when the circuit is closed. For example, the incandescent bulbs used in automobile brake lights take around 170-200 milliseconds to reach full brightness - one reason they're being replaced with LEDs, which reach full brightness much faster. $\endgroup$
    – jamesqf
    Aug 29, 2019 at 3:02
  • $\begingroup$ I am not sure what your edit is asking for. Of course if we assume that things happen instantaneously then things happen instantaneously in our models. I never said these approximations aren't useful. I am just saying that we need to recognize that these are approximations and not reality (as we currently understand reality to be). $\endgroup$ Aug 29, 2019 at 17:33
  • $\begingroup$ "Nature mostly prefers simultaneous event" No it doesn't, we generally prefer to look at quantities rather than their time-integrals, since that tends to give the simplest explanation. "As Aaron Stevens wrote in his answer that on microscopic scale there is a delay between force and acceleration." Only because there's no such thing as a perfectly inelastic body. $\endgroup$
    – smci
    Aug 29, 2019 at 20:31
  • 1
    $\begingroup$ This question has zero net votes and yet its top answer has 56 upvotes. I don't get that. $\endgroup$ Sep 1, 2019 at 19:38

4 Answers 4


None of the processes you describe are instantaneous.

Acceleration is produced without any delay on applying force. Angular acceleration is produced without any delay on applying torque.

If you are looking at the microscopic scale, it takes time for fields to change in order for forces to be produces. For example, E&M changes propagate at the speed of light.

If you are looking at macroscopic bodies, there is no such thing as a rigid body. "Information" about the presence of a force propagates through the body at the speed of sound in the body. This propagation also takes time. A cool example of this is shown in this video

bulb glows simultaneously as we close the circuit

No, it actually takes time for the current to build up in a circuit. This happens very quickly to us, but it is not instantaneous

Heat is transferred from one place to another as we allow it without any delay.

Heat transfer is probably the slowest process you have listed here. Think about cooking on a stove top, or preheating your oven for baking. It takes time to transfer heat that depends on the thermal diffusivity of the objects in question.

If you are arguing that the heat transfer starts instantaneously, then that still is not correct. You would have to define some sort of energy threshold that determines when you say the heat transfer has officially started, and this threshold will always be obtained in some finite amount of time.

I don't think there are any such events except transmission of waves which takes time in physics.

Even neglecting the above cases, there are plenty of processes in physics that take a finite amount of time. An object hitting the ground after falling from a table. Two galaxies colliding. The charging of a capacitor in an RC circuit. The list goes on and on.

Any process can be considered to be "instantaneous" if you are looking on slow enough time scales, and when operating on these time scales it is perfectly reasonable to say that certain processes are instantaneous. However, don't confuse approximation with reality. If you look fast enough, you will always find delays.

  • $\begingroup$ You will always find delays, except when you make a measurement on a pair of entangled particles. They "know" which state the other particle is in instantaneously. Of course, the rest of the universe only learns about the measurements at the speed of light, so you cannot use it to communicate faster than light. Arguably that means that even in that case there are delays. $\endgroup$
    – Graipher
    Aug 29, 2019 at 9:42
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    $\begingroup$ @Graipher: Measurement of one of an entangled pair is not a physical event. No physical change takes place in the other by virtue of measurement of one. $\endgroup$ Aug 29, 2019 at 12:24
  • $\begingroup$ @R.. eh, it's a bit trickier than that. Nonlocal correlations violating local hidden variable bounds imply that, in some sense, the entangled parties do affect one another, as the produced correlations cannot in such cases be described as due to additional "hidden" shared information. What is true is that these correlations are such that it is nonetheless impossible to harness them to transmit information from one particle to the other FTL. $\endgroup$
    – glS
    Sep 15, 2019 at 16:42
  • $\begingroup$ @glS: The word "affect" is generally understood as some sort of causation, which absolutely is not present here. That QM admits models where there is clearly no causal relationship is sufficient to conclude that QM does not imply causation here. If you want to use "affect" to simply mean there's a correlation, then yes of course, but I think that's misleading. $\endgroup$ Sep 16, 2019 at 19:34

This depends on our own role as observers. Our own feeling of time is of such a nature that these things look instantaneous for us. When we close a circuit it takes at least some nanoseconds for the bulb even to be able to "know" about this and some microseconds to heat up and some nanoseconds for this to reach our eye. However, human time resolution is at some 10 milliseconds.

If we look at all these events from a smaller time unit, it would take ages.

So your observation says a lot about us as observers (and not so much about nature).


What would be the alternative? If there was a delay between e.g. force and acceleration, cause and effect, then there would have to be a moment where there is no force and no acceleration. And then the acceleration starts, so the information that a force was applied has to be stored somehow somewhere.

This leaves two possibilities: Either you can measure this stored information somehow, or you can not. In the latter case, that would mean that there are hidden variables. In the first case, where you can measure that information, you're back to simultaneous events again, because there would be no delay between force and infomation, and then information and acceleration.

"Why"-questions about fundamental laws of our univers can usually only be answered with "Because it is like it is". Unless you ask the creator, if you believe in such a thing.

A less spiritual reasoning could be that the universe prefers simplicity and hidden variables are not simple.

  • $\begingroup$ thermodynamics has simple laws, and it is part of the description of nature, but it emerges from the complexity of statistical mechanics, which are the hidden variables of thermodynamics. Classical electrodynamics emerges from the "hidden variables" of quantum electrodynamics. At the moment main stream physics believes that the quantum mechanical framework has not hidden variables, though there are well known physicists who are working on such theories that will describe nature. ( for example see physics.stackexchange.com/users/11205/g-t-hooft ) $\endgroup$
    – anna v
    Aug 29, 2019 at 10:48
  • $\begingroup$ @annav I'm not a physicist, but AFAIK the current model of the world does not contain hidden variables, not even quantum mechanics. What kind of hidden variables are you referring to? $\endgroup$
    – Sentry
    Aug 29, 2019 at 12:27
  • $\begingroup$ I just gave examples of theories with hidden variables, and a link for a famous physicist looking at theories of quantum mechanics with hidden variables. Nature is complex, as far as our theories go, which try to simplify it at various levels . $\endgroup$
    – anna v
    Aug 29, 2019 at 13:51
  • $\begingroup$ this is wrong on many levels. There is indeed always a delay between force and acceleration. Some cases are discussed e.g. here. More generally, you can either describe the acceleration as due to the state of the system in the past, or use fields (electromagnetic, gravitational, etc.) to "store the information". All of this has nothing to do with hidden variables in quantum mechanics, which is a wholly different problem. $\endgroup$
    – glS
    Sep 15, 2019 at 16:37
  • $\begingroup$ @glS Are you referring to my answer or a comment? $\endgroup$
    – Sentry
    Sep 16, 2019 at 9:37

The principle, or, if you want, "paradigm" of instantaneous reactions to forces can be said to reflect Newton's worldview. There was no reason not to assume that a body reacted instantaneously to an applied force. The theory is sufficiently exact to describe phenomena on space and time scales which are neither too big nor to small, and at velocities small relative to the speed of light, $c$.

The big paradigm change came, of course, with the theory of relativity which developed around 1900 and was formulated by Einstein in 1905. The theory establishes $c$ as an upper speed limit not only for bodies but, perhaps less obviously, for anything. In particular, no information passes between two points in space time faster than with $c$. No radiation, no gravity wave, nothing.

Things which happen at a remote spot are not only unknown here until the information arrives; they are non-existing a far as we are concerned. If the event is outside our light cone it will never have happened in our universe. If you lift up a rod which is 3 m long the other end will not know about it before at least $3m/c = 3m/3*10^8m/s = 10^{-8}s = 10ns$ have passed. This is a short time, so short that we usually do not notice; but it is measurable.

If you look at the quantum effects underlying our reality it makes sense to quantize space time as well, if for no other reason than computability. This leads to models like lattice gauge theory which basically subdivides reality in nodes on a space-time lattice which carry specific properties and interact only with their neighbors (entanglement aside). Any disturbance propagates from node to node, one node per "step", which results in a communication speed of $c$. The whole thing is basically a cellular automaton, much like Conway's Game of Life. I'm not saying that reality indeed functions this way, but the model surely describes many of its aspects well. The core paradigm is that all interaction is local, neighbor to neighbor. There is no global force or plan; there is no coordination or remote communication of any kind.


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