What is the difference between the phrases "hypothetically speaking" and "theoretically speaking"? If one wants to make a point using an example that would likely never happen, which phrase would be more appropriate?
A hypothesis and a theory are different – the exact differences depend on what field you are in. But “hypothetically speaking” vs “theoretically speaking” have a different set of meaning.
“hypothetically speaking” would be "if X were true what would it mean for Y”.
e.g. “hypothetically speaking, if the sun were a binary star what would it mean for Earth”
While “theoretically speaking” means it is possible in theory but not necessarily in practice.
e.g. “theoretically speaking, you can solve all of chemistry by just knowing the wave equation for the entire system” (although in practice it's impossible for all but the simplest case)
[TFD-Collins] 4. existing only as an idea or concept: (a time machine is a hypothetical device);
[TFD-WordNet3.0] based primarily on surmise rather than adequate evidence;
theoretical: 1. Of, relating to, or based on theory.
In 'hypothetically speaking', the implication is 'This is not going to happen, though. It's only conjectural, suppose....
On the other hand, in 'theoretically speaking', the meaning is just that, you are backed by theory; you are speaking with reference to theoretical concepts; also, 'We are only dealing with the theory here, don't ask if it has any practical use, that's not in scope.'
In the given usage context of -ly speaking, there's neither an overlap not ambiguity in meaning between the words.
It's a different matter, though, that the words per se, are broad, overlapping concepts used in various senses.
"Theoretically speaking," and its more scientifically correct cousin "hypothetically speaking," are used to introduce an informal idea or question, often one that might seem silly or out of place, in order to open debate on the matter or get people thinking, or to answer with a best guess based on some domain knowledge: “Theoretically speaking, what if we used frog DNA to fill in dinosaurs’ missing genes?” “If we can get a gene splicer, then theoretically speaking, it should work.”
Sometimes it’s a serious question, sometimes it’s just to introduce a random off-the-wall stray thought (the bread and butter of a ditz in sitcoms). “Hypothetically speaking, if we switch to cubic time, does that mean Time Cube is all true?” You can sometimes leave off the “speaking” without changing the meaning, for simplicity.
Because in popular use, theory means “hypothesis,” both forms mean pretty much the exact same thing — unless you’re talking to a scientist, in which case “theoretically speaking” might actually mean looking for a solid theoretical basis, rather than a best guess. I can think of times when I personally would choose one or the other, but I don't think there's any real difference in meaning to most people, just in personal preference.
A common subversion is to use the phrase to obliquely ask about the consequences of a disaster that's already happened: "Theoretically, how bad would it be if I filled the tank with gasoline instead of diesel?” “Really bad, why?” “Errr….” After a beat, cue white faces and a rush to salvage the disaster. Extremely common in sitcoms, but played with in all kinds of media and real life.
I'm not sure why so many answers are discussing the difference between theory and hypothesis, when that has nothing to do with the usage of the phrase.
An hypothesis is a testable unproven statement used to build an experiment. The theory enplanes what happened in the experiment. This is the core of the scientific method. Theoretical is used to discuss what we think we know. Hypothetical is used to discuss what we want to know. Theoretically is for when we build on what we know. Hypothetically is used for what we guess or won't admit to knowing. Speaking only adds uncertainty.
If I were a gambling man, theoretically speaking I am 99.8% certain I will win hand like a royal flush. And hypothetically speaking have only 50/50 chance win with no straight the flush. Albeit, 'probably speaking' unlikely to win any chance in hell any more than 0% when caught cheating. So to get out from the two options false dilemma, add another option the 'probaly speaking' key and your out of the cage/trap. It's customary to hear 'probably' to mean it's not going to happen anyway, probably speaking.
In general usage, the two words are very similar, and can both mean existing only as an idea or concept, without actual physical existence. They are often used interchangeably. To look deeper, you can take the meanings of theoretical to mean relating to or based on a theory, and hypothetical to mean relating to, or based on a hypothesis. Here is where there is subtle, but important difference, especially in the world of the scientific method.
A hypothesis is a tentative explanation to account for an observed phenomenon. The important characteristic about a hypothesis is that it has to be able to be tested. Therefore a hypothetical situation must be possible, even it is extremely unlikely.
A theory is unifying set of ideas that can be used to account and predict phenomena. They are generally a widely applicable to account for a set of proven hypotheses. The critical characteristic about a theory is that it does not have to be able to tested. This means that a theoretical situation is one that does not ever have to exist.
Therefore, I believe that the expression that best suits your needs is theoretically speaking.