How to Ask ‘What If’ Physics Questions: Avoiding Pitfalls

Overview of “What If” Questions

We receive many “what if” questions here on Physics Forums. This article explores three common types and offers suggestions for students whose questions 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 students who ask “what if” questions.

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

Often on Physics Forums, people ask questions that unintentionally amount to, “If the laws of physics don’t apply, what do the laws of physics say about <insert contradiction>?”

Sometimes this is just due to a poorly formulated question. For example, someone might ask, “What would happen to the Earth if the Sun were to suddenly disappear?” What they often mean is, “Does the gravity of the Sun affect the Earth instantaneously?” The first question posits magic; the second is a reasonable scientific question.

More often, the issue is that the questioner does not realize the interconnectedness of scientific principles. They may label the scenario “hypothetical” or “just a thought experiment,” but that label alone does not remove the underlying contradiction.

Example: Back EMF in a DC Motor

A recent question asked, “What would be the effect if there were no back EMF in a DC motor?” The context made it clear the asker was not asking, “Is there some way to prevent back EMF?” but truly, “What if back EMF simply did not exist?”

That hypothetical requires discarding Maxwell’s equations. If Maxwell’s equations are invalid, you must invent a new set of rules to explain the results of the many experiments that support those equations. Because Maxwell’s equations have been validated by innumerable experiments, the scenario “no back EMF” is not answerable within our current physical theories.

Reformulating Unanswerable Questions

There is no meaningful answer to the question, “If the laws of physics do not apply, what do the laws of physics say about <insert contradiction>?” You cannot arbitrarily discard the interconnectedness of scientific laws. What we try to do on Physics Forums is help people understand how those connections apply to a particular situation, which often leads to reformulating the original question into one that can be answered.

Other Unanswerable “What If” Examples

Here are a few examples of questions that effectively discard large bodies of validated physics:

• What if oxygen, nitrogen, and ozone did not absorb UV radiation?
• What if you had an FTL drive that worked only relative to the CMB rest frame?
• What if gravity were not geometry?
• What if the atom were the smallest thing in existence?
• What if entanglement could be used to exchange information?

Each of these posits tossing out interconnected, experimentally verified parts of physics and therefore cannot be answered without inventing an entirely new consistent framework.

“What If” Questions That Are Themselves Self-Contradictory

Another common form is questions that are logically self-contradictory without the asker realizing it. Many of these involve improper application of inertial frames to light, for example:

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

The problem is that light travels at the same speed in all inertial frames of reference, and “speed” is defined relative to an inertial rest frame. Claiming that light has a rest frame is contradictory: you would be asserting there is a frame in which light is at rest and yet travels at c. “At rest” and “at c” are not equivalent, so these questions are not meaningful within relativity.

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

Borderline but Analyzable Examples

Some “what if” scenarios exceed current technology but are still analyzable using established physics. For instance: “What would happen to Earth’s orbit if the Sun exploded into nothing but massless photons?” We cannot do that experimentally at solar scales, but we have experience converting small amounts of mass into photons and theories describing the gravitational effect of radiation. That makes the question difficult but answerable in principle.

Forum members will often answer such questions as posed. When a question is borderline — technologically implausible but theoretically analyzable — it’s usually helpful if responders explain the assumptions and limitations in their analysis.

Frequently, students misclassify their “what if” question. They think it is of the answerable, “technology-limited” type, when it actually violates physical laws. Phrases like “it is just hypothetical” or “this is only a thought experiment” can indicate a misunderstanding of which physical law is being violated. If you’re unsure, ask specifically about the law that seems to be violated; that often clarifies the issue and shows how interconnected laws constrain the scenario.

Suggestions for the Student

We want you to ask questions, and we’re here to help. To keep threads productive and avoid having responders spend time correcting misconceptions while the original question gets lost, follow these suggestions:

• Check whether your question violates a known law of physics. Labeling a scenario “hypothetical” can be a red flag that you may be discarding an important constraint.
• Be specific. Instead of “Tell me about light speed,” ask “How do relative velocities work with a constant speed of light?” Specific questions invite specific answers.
• Do some basic research first. For example, if you wonder whether the atom is the smallest thing, a quick search will show evidence for subatomic particles and guide you to ask a sharper question like “What experimental evidence do we have for quarks?”
• Try framing the question as “I know this is probably wrong, but I don’t understand why.” For example, rather than asking “Is the atom the smallest thing?” ask “I know the atom is not the smallest thing, but could physics be made to work if quarks did not exist?” That invites a discussion of charges, particle structure, and experimental constraints.
• Remember: Physics Forums aims to help you learn how to figure things out, not just give short answers. Starting with some research and a focused question helps us help you better.

Acknowledgements

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

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