I am working with engineering equations in a vacuum system and want to emphasize that a certain set of parameters will not work. Usually, this is due to real world effects (friction, pump efficiency factors, wear and tear of bearings, etc), but I want to emphasize that a particular setup wouldn't work even under perfect textbook conditions.

I would like to say it is "theoretically impossible", but that has a ring of just the opposite of what I would like to say! It sounds like:

a safety rating better than 100% is theoretically impossible, but the Tesla Model S did it!

What is a word or phrase to say something is absolutely forbidden by fundamental, physical laws, even under ideal conditions?

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    In physics, a bare 'forbidden' is used. Commented May 26, 2015 at 23:03
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    @EdwinAshworth I thought it would be difficult to understand "forbidden". Shall I change it back?
    – Centaurus
    Commented May 26, 2015 at 23:07
  • @Centaurus, thank you for editing the body, but changing the title from "impossible" to "infeasible" really changes the heart of the question! Commented May 26, 2015 at 23:07
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    Obviously, it is inconceivable.
    – jxh
    Commented May 27, 2015 at 2:04
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    @jxh, I do not think it means what you think it means. Commented May 27, 2015 at 2:09

10 Answers 10


I'd suggest fundamentally impossible when writing for an audience which is not familiar with the field, while obviously impossible would work for an audience which is familiar with it.

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    +1 for fundamentally to avoid the theory/practice connotations of theoretically. I prefer to avoid obviously / clearly because it's usually unnecessary and feels a little rude.
    – BoppreH
    Commented May 27, 2015 at 6:05
  • Well, it would be more rude to suggest to a knowledgeable audience that you think they might not realize that the subject is impossible even in theory. It's a matter of who you care about being rude to: the person who suggested the setup or the audience. Commented May 27, 2015 at 12:15
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    @WhatRoughBeast I don't think that's true in general. In my experience, at least in scientific writing, it's quite common that things which are obvious to the authors are far from obvious to the readers, even to experts. And I think you overestimate the degree to which most experts would be offended by not stating that something is obvious.
    – David Z
    Commented May 27, 2015 at 12:31
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    I ended up using fundamentally for the reasons @BoppreH laid out. Obvious would be inappropriate because the conclusion that the propsed system violates physical laws took a bit of research. Commented May 27, 2015 at 12:34
  • Well, then - it's not obvious. ;) Commented May 27, 2015 at 12:48

Since you're talking about physics, why not "physically impossible"?

  • I like this one.
    – KPM
    Commented May 27, 2015 at 7:18
  • Me too. This is something I could see myself using in a paper. Physicist-approved :-P
    – David Z
    Commented May 27, 2015 at 12:31
  • I think physically impossible implies theoretical as well as practicible limitations: "It is physically impossible for someone 1000 miles away to hear you scream" as opposed to "It is fundamentally impossible to travel faster than light". Commented May 27, 2015 at 12:42
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    I don't think it is "physically impossible" for someone 1000 miles away to hear you scream. Whale song travels that sort of order of distance. It may not be likely that your scream will travel that far, but whilst a set of circumstances exist that make it possible, it's not impossible.
    – Sobrique
    Commented May 27, 2015 at 14:26
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    Depending on why it's impossible, you might prefer mathematically impossible (e.g. "it is mathematically impossible for a candidate to receive more than 100% of the vote").
    – Kevin
    Commented May 27, 2015 at 18:19

I would suggest intrinsically unworkable where the important part is intrinsic, expressing the fact that the problems with the experiment are internal, not related at all to any changeable conditions.

I realize this recasts your question a bit, but I think it better conveys the underlying concept that a given proposal can't ever work. Intrinsically flawed could also do the job.

  • This is great. I can see lots of similar situations where intrinsically incapable or intrinisically limited/flawed or whatever will be helpful. Commented May 27, 2015 at 12:36

It "violates the laws of physics". You could be more specific and state which laws it violates (for example, conservation of energy).

  • I don't think this answers the question very well since writing "the proposed pump parameters violate Boyle's Law and the continuity equation, as well as exceed the manufacturer's maximum allowable compression for this pump..." is not a phrase or word to recap the situation. However, upvoted because it otherwise satisfies all the other requirements Commented May 27, 2015 at 12:48
  • @user1717828 I'm sorry but I don't understand your objection. What requirement stated in the question am I violating? Commented May 27, 2015 at 14:28
  • I'm new to English.SE, but I thought by tagging single word request and phrase request I constrained answers to a few words. Your latter suggestion (which is what my previous comment was referring to; sorry for that omission) of citing the broken laws/theorems does not fit that. Commented May 27, 2015 at 14:55
  • @user1717828 OK, I see what you mean. I interpreted that as meaning they were looking for a word or a phrase so I supplied a phrase (I doubt there's a single word for such a complicated concept). Commented May 27, 2015 at 16:40

Just from reading the question's title, I would've suggested taboo... but, obviously, that doesn't fit in the physics world.

So instead, I'd recommend something like infeasible, i.e. the antonym of feasible, defined as "capable of being done, effected, or accomplished". (Note that you could also use unfeasible.)

Alternatively, borrowing from a more biological background, another option would be nonviable: "not practicable or workable".

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    I think these examples carry the connotation that something is merely too hard to do. Like building a man-made mountain the size of Mount Everest.
    – user66219
    Commented May 27, 2015 at 12:03
  • @Hurkyl: True - and intentionally so. The OP's example specifically overcomes these huge odds, so it can't really have been fundamentally impossible, can it? ;-) Commented May 27, 2015 at 13:06
  • @AmosM.Carpenter, I have miscommunicated. The example with the Tesla was supposed to illustrate what I didn't want. Commented May 27, 2015 at 14:56
  • @user1717828: Ah, ok... my bad, not yours. Re-reading your question, my initial interpretation is clearly wrong. Commented May 27, 2015 at 15:03

Physics, and all of science, is based on the assumption that "Laws", as deduced from thousands (and in some cases millions or billions) of observations will always yield the same result. [Of course, with our understanding of quantum mechanics this means that an experiment will faithfully reproduce the probability that we can calculate for its outcome.] Anything that appears to violate such Laws is simply "impossible".

Unfortunately, sometimes Laws need to be updated (tweaked?) as we learn more with increased experimentation, typically with improved precision. So, for "explicitly forbidden by the most fundamental laws" we can use "impossible", but keep in mind that science is open to revision based on valid new experimental data. Einstein's relativity is the classic example of revision to Newtonian physics - but we still teach the latter to undergraduates and most engineering has little need for relativity.


The correct word, if not choosing impossible, would be insuperable: rendered insuperable by all known physical laws of the Universe.

  • +1 for a great word, but not one the audience is likely to know.
    – Taejang
    Commented May 27, 2015 at 17:33
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    That's not what the word means! Insuperable is something that cannot be surmounted, it is not something that is impossible. It could be used to describe the problem that makes something else impossible but it cannot be used to describe something that cannot be achieved.
    – terdon
    Commented May 28, 2015 at 15:48

In this case the problem is not "theory" of physical "law", but rather the linguistic (and mathematical) meanings of "100% safety". It's more a collision between our agreed upon notions of the range of probability which are "by definition" bounded between 0% and 100%. I would have worded this:

a safety rating better than 100% is impossible _by_definition_, but the Tesla Model S did it!"


We just say "impossible" or "literally impossible" to clarify that we aren't exaggerating.

Also applicable is "self-contradictory" if the claim is internally inconsistent such as "I did the impossible!".


"Inherently unrealizeable" works for me.

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    Oh,my... A drive-by downvote? No rhyme, no reason, no attempt to communicate; just vitriol. Must be a pretty good answer, methinks. ;)
    – EM Fields
    Commented May 29, 2015 at 19:06

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