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In a somewhat famous Star Trek: The Next Generation quote Data corrects Riker on the 'correct term' for a scenario where a hatch on a space capsule breaks open and all the crew are killed.

Riker: "You were right. Somebody blew out the hatch. They were all sucked out into space."

Data: "Correction, sir, that's blown out."

Riker: "Thank you, Data."

Data: "A common mistake, sir."

Is there a technical or scientific and objective correct answer? Was Data, from a technical perspective, correct in his assertion or are both statements equally valid?

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    $\begingroup$ Regardless of the suck/blow distinction, the air is definitely expanding. It just has a lot of volume to expand into. $\endgroup$
    – Erin Anne
    Apr 19, 2018 at 5:22
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    $\begingroup$ This question is on the edge of being "opinion-based". There are two possible non-opinion-based questions you might be asking: (a) is one of these terms in wider use in <some defined community> or (b) is one of these terms preferred in some official policy. Could you clarify the questin to ask one or other of these? $\endgroup$
    – Steve Linton
    Apr 19, 2018 at 10:09
  • $\begingroup$ A lot of activity on this situation suddenly. I wonder if it was on rerun last night. $\endgroup$
    – user6587
    Apr 19, 2018 at 16:05
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    $\begingroup$ Maybe it is related to the Southwest accident: nytimes.com/2018/04/18/business/… $\endgroup$
    – Rory Alsop ♦
    Apr 19, 2018 at 18:17
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    $\begingroup$ Thinking about this some more, this is not opinion-based. The answer is a result of physics: gases exert pressure (due to molecules bumping into each other). This is what causes a gas to expand to occupy all available volume. The only force at work is repellent, so the answer must be "blown out". $\endgroup$
    – Hobbes
    Apr 20, 2018 at 11:00

9 Answers 9

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Both are correct, it just depends on who/what is doing the verb (sucking/blowing). In english we would call this the subject of the sentence.

Consider the following scenario. You have two large tanks, sitting in your garage (presumably on planet earth). One has a vacuum in it at 0.01 ATM, the other has some pressurized air, lets say at 10 ATM. The two tanks are connected by a sealed pipe with a valve in the middle.

Now lets say you open this valve so air now starts rushing from the pressurized tank into the vacuum tank. Is the air being sucked or blown? Well the answer is, both, just depends how you construct the sentence. You could say "The low pressure tank sucked the air out of the high pressure tank". Alternatively, you could also say "The high pressure tank blew the air into the low pressure tank". Both statements are true, the verb just changes depending on what is doing the action (the subject).

So back to your original Star Trek quote. Both are right. The people were sucked out by the vacuum of space. But it is also true that the people were blown out into space by the space station.

Technical Physics Explanation

The technical definition of suck, or rather suction, is according to wikipedia:

Suction is the colloquial term to describe the air pressure differential between areas... When the pressure in one part of a system is reduced relative to another. [1]

Similarly the dictionary defines it as such:

The force that, by a pressure differential, attracts a substance or object to the region of lower pressure. [2]

In other words, suck, or suction, does not suggest that the force comes from the low pressure side, only that the force exerted on the object results in an attractive force towards the side with the lower pressure.

In the case of people, or a space door hatch, it satisfies these definitions, there is a pressure differential across them and from the perspective of the low pressure side, since objects are attracted to it by the produced force, there is suction.

Most of the answers which suggest the "motive force" is coming from the high pressure side is true, but this ignores the definition of the word suction, which does not in anyway rely on where the force comes from but only its direction.

In fact, for that reason, the word "suck" is often used in scientific literature and is quite acceptable, despite Mr. Data's objections.

Examples in Scientific Literature

An Elementary Treatment of the Reverse Sprinkler

In an attempt to back up my claim that both suck and blow are equally valid I tried to find some scholarly source on the matter. Ultimately this is an English question, and I could not find a scholarly English source. I was however able to find a peer-reviewed physics paper published on arXiv, a well respected journal, that used similar language.

Here are some quotes from the paper:

the box will turn in the direction of the long arrow when blown into and in the direction of the short arrow when sucked on.

and

Steam bubbles cause water to be alternately blown out of and sucked into the tank.

and

If, therefore, an elastic ball, which has one escape-tube, be attached to the reaction-wheel, in the manner represented in [Fig. 8(a)], and be alternately squeezed so that the same quantity of air is by turns blown out and sucked in, the wheel will continue to revolve rapidly in the same direction as it did in the case in which we blew into it

All of the above quotes can be found in the following physics paper:

https://arxiv.org/abs/physics/0312087v1

Study of Combined Rice Husk Gasifier Thermoelectric Generator

Similarly here is another separate paper, also peer-reviewed, using suck and blow as equally valid terms in the paper.

A blower was used to suck the ambient air to cool the heat sink and blow the air from the heat sink to the reactor of the gasifier.

Which can be found in this paper:

https://www.sciencedirect.com/science/article/pii/S187661021400928X

As we can see in the above again suck and blow are both used in a scholarly scientific context with peer review. So clearly both terms are considered acceptable and correct within the scientific community.

Conclusion

I think its a safe bet that if multiple research scientists and physicists can publish papers and have it pass peer-review using blow and suck, then either word, when in agreement with the subject (as described above), is acceptable to use. Clearly the scientific community finds both words equally valid and I think it's common sense that these are valid English usages as well.

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    $\begingroup$ This answer is better than the highest voted one. $\endgroup$
    – Magic Octopus Urn
    Apr 19, 2018 at 14:53
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    $\begingroup$ "You could say "The low pressure tank sucked the air out of the high pressure tank". .. you could say that, but you would be wrong. The vacuum does nothing at all. The motive force only comes from the air molecules themselves. The air blows itself out into the vacuum. This site should not upvote answers because they sound clever, but because they are correct. $\endgroup$
    – Innovine
    Jun 28, 2021 at 20:46
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    $\begingroup$ @Innovine the word "sucked" does not care where the motive force comes from. There is no english rule to suggest that motive force is the dictating factor in how one must construct their sentences. "sucked" is absolutely correct and proper. The answer was upvoted literally because it is linguistically correct, not because it sounds clever. $\endgroup$
    – Jeffrey Phillips Freeman
    Jun 29, 2021 at 21:34
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    $\begingroup$ @Innovine In response to your suggestion that this was wrong I added a section where I actually cite a scientific source demonstrating the use of either word as acceptable to the physics community (which seems to be the domain your arguing from). $\endgroup$
    – Jeffrey Phillips Freeman
    Jun 30, 2021 at 1:21
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    $\begingroup$ @JeffreyPhillipsFreeman It only pops up in certain conditions. Not to worry--mods always get notified when the thread gets too long. $\endgroup$
    – called2voyage ♦
    Jul 6, 2021 at 12:30
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Edit:

Thinking about this some more, the answer is a result of physics: gases exert pressure (due to molecules bumping into each other). This is what causes a gas to expand to occupy all available volume. The only force at work is a repellent one, so the answer must be "blown out".

In everyday language, there are some exceptions. We say a vacuum cleaner sucks. Physically, it pumps air from the vacuum chamber to the outside, lowering the pressure in the vacuum chamber and enabling air to rush in through the nozzle.

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  • $\begingroup$ Actually, it sounds like the distinction you are making is that a vacuum moves in toward a central location and a compressor moves out from the central location. So things are only ever "sucked in" or "blown out." "Sucked out" makes no sense. Is this correct? If so, that is the most sense I've ever heard of this. $\endgroup$
    – user6587
    Nov 15, 2014 at 21:29
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    $\begingroup$ No. If you have a vacuum test chamber, the air is sucked out to create the vacuum. $\endgroup$
    – Hobbes
    Nov 16, 2014 at 9:49
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    $\begingroup$ But that is relevant to unnatural pressure differences from an air pump, which is actively moving air against the natural pressure differences (ie using energy). In a decompression scenario, the air passively moves due to the natural pressure differences that were maintained because of the impassable barrier (ie release of potential energy). $\endgroup$
    – user6587
    Nov 16, 2014 at 17:49
  • $\begingroup$ @fredsbend: While the whole matter has a lot of blurred edge cases where the answer would be unclear, generally history of creation of the vacuum or overpressure matters: You have two identical chambers connected by a valve, chamber 1 is connected to compressor, chamber 2 to a vacuum pump. You create 1 bar pressure delta between them, then disconnect the compressor and vacuum pump. And while the flow caused by opening the valve is the same whether the delta was caused by pump or compressor, the nomenclature is not. It's silly like that. $\endgroup$
    – SF.
    Apr 19, 2018 at 15:48
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    $\begingroup$ Essentially, the side that has "atypical pressure" due to mechanical action (directly or indirectly, e.g. pressurized from a tank that was earlier pressurized by a compressor), this side "takes responsibility" - is blowing/sucking. Since void is "naturally empty" and ship is "artificially inflated", it's blowing out. (but take the prior example, pump air from tank 1 into tank 2, then open the valve between them... there's no correct answer because one equally sucks as the other blows.) $\endgroup$
    – SF.
    Apr 19, 2018 at 15:51
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One simple rule: You follow the energy. Where the energy comes from defines the process.

So while an ice-cube on your hand does result in a cooling effect, it is your hand transferring energy to the ice-cube, melting it, not the ice-cube melting to draw heat from your hand.

We see the same with pressure gradients. A common turn of phrase in the winter might be:

"close the door, you are letting the cold in"

This is a poor description of the process that is occurring. In this case, the energetic warm air is providing the energy that leads to the convection and thus the transfer of air. What we should say is that we are

"letting the heat out"

In a venting spacecraft, the energy comes from the atmosphere inside. It's akin to holding your hand over a plug-hole - it may feel like it is being "sucked" onto the plug-hole, but it's really the force (in this case weight) of the water pushing your hand onto it.

This means that

"Blown out" is the correct terminology.

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    $\begingroup$ Can you provide a reference, semantically or scientifically, that "where the energy comes from defines the process"? It seems very strange that under this definition, a refrigerator is a device that warms everything in your house except the food. Similarly, under this definition, there is no distinction between a vacuum cleaner and a leaf blower, since they both "blow" debris using relatively high pressure air. But in reality, they're very different - one generates negative pressure, the other positive, so one sucks and one blows. $\endgroup$
    – Nuclear Hoagie
    Apr 21, 2020 at 14:22
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If we think of the atoms and molecules of the gas in the airlock, the only thing that happens are collisons between two gas atoms or collisons of gas atoms with the wall.

There is no force pulling or sucking the gas atoms out into the vacuum. Atoms hitting the wall are reflected, atoms hitting another gas atom will bounce back. The possibility of interactions between gas atoms out in the vacuum is very low due to the large distance from atom to atom. Therefore these atoms keep moving in the same direction.

If a gas atom is moving by chance into the direction of the open hatch, it will leave the airlock. Only another collison with a gas atom before leaving the hatch may change the direction of the atom.

An astronaut in the airlock will be hit by many atoms pushing him into the direction of the hatch, but very, very few atoms pushing him away from the hatch. Unfortunately the resulting force on the astronaut is moving him out of the airlock.

So the gas is not sucked out, it flows out into space driven by the thermal movement of the gas atoms and molecules.

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    $\begingroup$ This is the correct answer -- too bad it was submitted late. When the hatch is present, it reflects the gas molecules. When the hatch is absent, objects in motion remain in that motion, and the gas escapes. $\endgroup$
    – DrSheldon
    May 24, 2019 at 13:17
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Could it be the difference about on whom the vacuum acts?

When the hatch is opened, the vacuum's zero pressure causes all the air to rush out. However, the people, the hatch or anything else are blown out by the air which is rushing out to fill the vacuum.

Because without the air inside, there wouldn't be a "blow out"... (like when astronauts go out for space walk, they open the hatch after decompressing the chamber)

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Air is actually neither sucked out nor blown out. Instead, there is just a system approaching thermodynamic equilibrium in the context of gas pressure gradients. That's from the objective, physical point of view.

In a more detail description of this hypothetical setting, an isolated system (consisting of just-broken capsule with the air still inside, and the whole rest of the universe) spontaneously undergoes a change in entropy; the entropy of isolated systems left to spontaneous evolution cannot decrease with time and those systems always ultimately arrive at a state of thermodynamic equilibrium where entropy is the highest. Hence, thermodynamically irreversible process of gas expansion spontaneously happens.

This question's spirit is in some sense a red herring, because the question is actually a matter of language semantics rather than physics, and language semantics in this case depend on subjective context. Even though the observed "suction force", seemingly presenting itself as the reverse of pressure force, does not physically exist, it is would be perfectly reasonable term to use in such a situation. Air was blown out, because that is exactly what a hypothetical vacuum-dwelling observer would experience as he/she would feel the sudden breeze of fresh air escaping the just-broken capsule. But air was also simultaneously sucked out, because that is exactly what apparently would happen from the subjective point of view of the crew. However, at the same time none of that happened as well because, from the air's frame of reference, it was just let loose and got spread out from a confined space into the void. Gases tend to expand in their attempt to occupy the whole volume of a container, so that's exactly what cabin's air does without any need for blowing nor sucking force. And since the container in this context happens to be the whole universe, let's not distract the cabin's air from the journey for this futile quest's fulfillment by pigeonholing its actions with unnecessarily strict terms, and especially considering that any discussion attempting to strictly define that event using either of those two words, while potentially entertaining mental exercise, is of linguistic rather than scientific nature.

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Actually, Data is also wrong - possibly!

A pressure due to incident wind (blowing) and a pressure difference across an object due to a sudden pressure gradient are two separate things!

The air pressure pushes on the hatch, the hatch exerts a pressure back on the gas, and the hatch-latch in turn pushes on the hatch.

When the hatch-latch stops holding, the hatch pressure drops to zero, and the air pressure accelerates the hatch.

Now there is a pressure gradient in the air. The air closest to the opening can not push back on the air farther inside. The air deeper in the ship pushes and accelerates the air closer to the opening.

"Blowing" usually refers to being pushed by a wind or velocity. You might have used pressure to create the wind, but "wind pressure" is not the same as a pressure gradient. For example, wind pressure depends on the mass density of the fluid and the velocity of the fluid. A pressure gradient is just about pressure.

The air "Pushes" itself out, it doesn't really get "Blown" out.

Same for the people. While there will be a force associated with the moving air blowing on them, there is a much much BIGGER force associated with the pressure gradient.

So I think that Data is also wrong. They were "Pushed" out by the sudden, transient pressure gradient.

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    $\begingroup$ Yes, they are pushed out by the airflow, but something being pushed by a flow of air is usually referred to as being "blown". $\endgroup$
    – Baldrickk
    Apr 19, 2018 at 13:21
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    $\begingroup$ "A pressure due to incident wind (blowing) and a pressure difference across an object due to a sudden pressure gradient are two separate things!" I don't follow your reasoning on this. For there to be a pressure force on an object, there must be a differential pressure. $\endgroup$
    – Organic Marble
    Jan 8, 2020 at 14:11
  • $\begingroup$ @OrganicMarble Wait! Where have I said that they are the same thing? Where have I said that there was no differential pressure? Think about hanging from a skyhook in the jet stream, or from a rope in a fast river. There is a strong wind or flow that continues for hundreds of kilometers, but the pressure difference between one point and another point one foot downstream is nearly zero. But when you insert a person, a pressure difference across them will be formed by the blockage. That pressure differential wasn't there before, it was formed by the person blocking the flow. $\endgroup$
    – uhoh
    Jan 8, 2020 at 14:27
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    $\begingroup$ I wouldn't touch this one. $\endgroup$
    – Organic Marble
    Jan 8, 2020 at 14:30
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    $\begingroup$ @OrganicMarble I'm gonna pursue this now, it's driving me crazy! I'll post a question or two in Physics SE soon and ping you with a link. $\endgroup$
    – uhoh
    Jan 9, 2020 at 2:47
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Air is blown out not sucked out. The force of air being blown out is a result of collisions of the molecules inside the vessel. The hotter the air, the more active are the air molecules, and the more molecules that are forced into the set volume then the higher the air pressure

Ultimately it is the air pressure that is pushing out the gas.

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    $\begingroup$ Welcome to Stack Exchange! We disagree but I think this problem has some wiggle room. I conclude that ...Data is also wrong. They were "Pushed" out by the sudden, transient pressure gradient. but given the binary choice between blown and sucked, the former is definitely preferable. $\endgroup$
    – uhoh
    Jun 29, 2021 at 14:33
  • $\begingroup$ Welcome, great to have you here. The problem with this explanation is that what is exerting the force has no relevance to the definition of "suck" or "suction". Suction by definition is a force that arises from a pressure differential that is attractive to the low pressure side. So even if that force arises due to the high pressure side it is still correct to say the low pressure side produces suction. $\endgroup$
    – Jeffrey Phillips Freeman
    Jul 2, 2021 at 0:13
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As the first answer points out, suction is a perfectly legitimate concept even though it is an emergent effect caused by forces pressing on the receding side of the object experiencing the suction.

It's important to learn that, when you suck a drink up a straw, it is really being pushed up by the pressure of the ambient air. That's why a pedant might say "he wasn't really sucked out", which makes it totally in character for Data. But that phenomenon is exactly what suction is, which is why both are right.

It is like talking about the sun setting. It's perfectly correct to use that expression even though we all know it's the earth that's moving.

Finally, the water in trees really is pulled up to the top by the mutual attraction of water molecules, rather than being pushed by air pressure, and I have never heard that characterized as suction.

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  • $\begingroup$ Thanks, yea this was mostly my point. Suction does not imply it is the source of the force. People are basically arguing that a verb can only describe a thing which produces a force. Yet we have countless counterexamples to show that is not the case and a verb does not rely on where the force is coming from. $\endgroup$
    – Jeffrey Phillips Freeman
    Jul 2, 2021 at 17:52

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