what if physics questions

How to Avoid Breaking Physics With Your “What If” Question

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We get a lot of “what if” questions here on Physics Forums. This article will explore three different types and then some suggestions for students who feel their question may fall into one of those types.

  • “what if” questions that contradict physics as we know it
  • “what if” questions that are themselves self-contradictory
  • “what if” questions that can be answered but with some difficulty
  • suggestions for the student

“What If” Questions That Contradict Physics as We Know It


Often here on the Physics Forum, people ask questions that, unrealized by them, amount to being exactly “if the laws of physics don’t apply, what do the laws of physics say about <insert contradiction of your choice>?”.

Sometimes this is just because of a poorly formulated question. For example, if someone asks “what would happen to the Earth if the sun were to suddenly disappear”, it could well be that what they are really want to know is “does the gravity of the sun affect the Earth instantaneously”. The first question is problematic because it posits magic but the second question is a quite reasonable scientific question.

Much more often, however, the problem is that the questioner does not realize the interconnectedness of things in science. Sometimes it appears that the questioner does have some idea that their question is problematic and so will specify it as “hypothetical” or “just a thought experiment”. Unfortunately, that still doesn’t remove the problem.

An example: a recent question was “What would be the effect if there were no back EMF in a DC motor”. The context of the rest of the questioner’s statement made it clear that the question was not “is there some way to prevent back EMF?” but truly “what if there just was no such thing as back EMF?”.

The problem is that that requires Maxwell’s equations are considered invalid and the problem with that is that if Maxwell’s equations are invalid then you have to invent a whole new set of rules for how electricity works. This is typical of such questions, where someone wants to invalidate one part of the ramifications of known physics without, somehow, affecting the rest. Physics just doesn’t work like that. There just is no answer to the question “If Maxwell’s equations are invalid, what do Maxwell’s equations say about <insert contradiction of your choice>?”

Scientists have spent centuries doing experiments, formulating theories to explain the results of the experiments, refining the theories, and on and on and what we have ended up with is, as just one example, Maxwell’s equations. If you throw out Maxwell’s equations, you have to come up with some other explanation for how to explain the results of all those experiments. Since those equations have been validated by innumerable experiments, well, … good luck with that.

That example has beaten on a particular question and Maxwell’s equations but the problem is much broader than that which is why this section started with, and I repeat here: There is no answer to the question “If the laws of physics do not apply, what do the laws of physics say about <insert contradiction of your choice>?”

You cannot arbitrarily discard the interconnectedness of things in science and what we try to do here on PF is help people understand how that might apply in a particular situation they have in mind. This can lead to their improved understanding of the science relating to their question and possibly to a rethinking of, and a reformulation of, the original question so as to make it one that can be answered.

Just as a sampling, here are a few of the many other questions that have shown up on PF that fall into the category of “if the laws of physics do not apply, what do the laws of physics say about …”

What if:

  • oxygen, nitrogen, and ozone did not absorb UV radiation?
  • you had an FTL drive that works only with respect to the CMB rest frame?
  • gravity was not geometry?
  • the atom really was the smallest thing in existence?
  • entanglement could be used to exchange information?

Every one of these questions posits the tossing out of a significant amount of interconnected physics and defies all experimental results of the theories describing those areas of physics.

“What If” Questions That Are Themselves Self-contradictory


There is another form where the question is self-contradictory without the questioner realizing it. The most common of such questions are of various forms such as

What if

  • we could ride with a photon; what would we see?
  • a photon itself had a point of view; what would it see?
  • the passage of time applied to a photon; how would that work?

The particular problem here is that light is the same speed in all inertial frames of reference but “speed” is something that has to be measured in an inertial rest frame. Since light travels at the same speed in all inertial frames, it is contradictory to say that light has a rest frame. That is, you would be saying that there is a frame in which light is at rest, and in that frame, light travels at c. Obviously “at rest” and “at c” are not the same thing and so cannot be equated in this way, which immediately means that all such questions are self-contradictory and thus not meaningful.

“What If” Questions That Can Be Answered but with Some Difficulty


Some “what if” questions may vastly exceed our present or anticipated future technological limitations, but although we don’t know how they could be accomplished we can still apply the laws of physics to determine what would happen. For example: “what would happen to the earth’s orbit if the sun were to suddenly explode into nothing but massless photons?” Although we don’t know how to do that on the scale of the sun, we do have lots of experience converting small amounts of matter into photons, and we do have theories that describe the gravitational effect of light, so this question exceeds our technological capability to implement, but not our theoretical capability to analyze. Usually, forum members will not hesitate to answer such questions as posed.

Sometimes it is exceptionally difficult for students to recognize the distinction between an answerable (sun turns into photons) and a non-answerable (sun disappears) “what if” question. What we often try to do with such questions is to propose a modification that is answerable, usually along with an explanation about why the original question was not answerable.

Unfortunately, sometimes the questioner misclassifies their “what if” question in their own mind. They think that it is of the answerable “technological limitations” type and do not realize that it is of the unanswerable “violates physical laws” type. Frequent responses that indicate this are along the lines of “it is just hypothetical” or “this is only a thought experiment”. Unfortunately, this response indicates a failure to grasp the physical law which is being violated and understand the nature of the “what if” question. If this is your situation, then we encourage you to ask about the details of the physical law that is being violated and how it applies to your scenario. Often, this is where the interconnectedness of scientific laws becomes apparent and can lead to a much better understanding.

Suggestions For the Student


First off, keep in mind that we are here to help you answer your questions and we want you to ask questions, so help us avoid having a thread get bogged down by having responders rush to correct incorrect statements, others comment on the corrections and then the original question is forgotten. Also, if you see people trying to guess what you asked about, that is a clue that your question was unclear.

First and foremost try to determine whether or not your question violates a known law of physics. If you have posted your question as “hypothetical” or “just a thought experiment” that should be a red flag to you that you might have done so. Also, be specific. “Tell me about light speed.” is too vague. “How do relative velocities work with a constant speed of light?” is more specific and would be a great question. Just don’t follow a good question with your own theory about the answer because that also leads to problems rather than solutions and can quickly derail a question away from what you really want to know. We’d rather help you than correct you.

One thing to do is to think about the physics you might be violating and see if you can formulate your question so that it is a question about that specific area of physics rather than one that violates it. First, it is a good idea to do at least some basic research on your own. For example, if you start off wanting to know if the atom is the smallest thing in existence, just Google that question directly and you’ll have an answer that will help you formulate a valid physics question such as “what experimental evidence do we have for the existence of quarks?”

As another possibility is to just think about just posing your question more from the point of view of “I know this is probably wrong, but I don’t understand why”. For example, rather than asking “is the atom really was the smallest thing in existence?” ask “I know that the atom is not the smallest thing in existence but could physics be made to work if quarks did not exist?”. That could lead to a useful discussion about quark charge and how it determines the charges of protons and neutrons and the fact that if quarks did not exist then you would have to come up with some other explanation for the charges. This kind of discussion would help you see the interconnectedness of things as regards your particular question.

Keep in mind that the Physics Forum is not really a Q and A type forum but rather one where we try to help you learn how to figure out answers on your own. Consequently, we appreciate it if you start with at least some basic research on your own so as to best formulate your question, and consider the suggestions just discussed so as to help us help you.

** This article was written with input from Anorlunda, Dale, and ZapperZ and my thanks to them

Comment Thread

4 replies
  1. phinds says:

    eltodesukane said

    I know, but it would have made more sense to define it as the ratio of a circle’s radius to its diameter.

    I think you may want to rethink what you just said.

  2. phinds says:

    eltodesukane said

    Just like 2*pi should be the real pi, but it is way too late to heal that.

    No, it should not. Pi is the ratio of a circle’s circumference to its diameter. That is Pi, not 2*Pi

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