Can one observe an absence of something?

Question

It seems to me that because there is an absence of something, it should be impossible to observe it. Therefore, if I wanted to say

This is supported by the experimentally observed absence of...

what word would I use instead of observed? I need to keep the implication that no experiments have observed this particular phenomenon, but the word observed itself seems oxymoronic.

Answer 1

Observed is perfectly fine here. You are not observing the phenomenon, you are observing that it's absent.

The Free Dictionary gives this:

ob·serve
1. To be or become aware of, especially through careful and directed attention; notice.

You became aware, noticed, that the phenomenon was absent. I can readily observe that the Chicago Bulls play no female players...

Answer 2

There seem to be two relevant definitions of "observe":

observe — notice something: to see or notice something, especially while watching carefully

observe — watch something attentively: to watch somebody or something attentively, especially for scientific purposes

You can certainly apply the former to an absence:

I noticed something was absent.

I observed something was absent.

The second usage is a more interesting debate:

1. I watched the experiment for an absence of X.
2. I observed the experiment for an absence of X.
3. I saw/noticed an absence of X.
4. I observed an absence of X.

There isn't anything wrong with these sentences or, by extension, your example. They aren't any different than saying something like, "I saw that the person wasn't there."

Answer 3

Of the two phrases

• experimentally observing the absence
• experimentally verifying the absence

the first phrase is weaker than the second. It implies the use of a passive means of observation and is not necessarily conclusive. The second implies that an active effort was taken to arrive at the conclusion.

Here are two examples:

My recon team observed the absence of any opposition in the nearby village. However, my combat team verified that a pocket of resistance still exists, after taking fire from the church steeple.

My recon team observed the absence of any opposition in the nearby village. So I sent my combat team in to verify the absence by doing a building-to-building search.

Answer 4

I have certainly heard the expression that something or someone was 'noticeable by their absence'. It contains a note of irony and also can imply sarcasm e.g. 'The member who had made such a fool of himself at the last meeting was very noticeable by his absence'.

But on a more straightforward level this is a philosophical matter, comprehensively addressed by Plato in his story of the prisoners in the cave. Did anything which they could not see, exist? And did the things which they did see, exist in the way that they saw them, which involved considerable distortions from a wider reality?

Before radio was invented radio waves existed even though no one could see them, and no one had even heard of them. So it does rather confirm the point that the 'observing' of the absence of something is a less certain use of the term 'observe' than it is when you positively see something before your eyes.

Notwithstanding all that, what does it mean to 'observe'. Plato's prisoners saw what they saw. It wasn't reality as it would have appeared to someone with a better vantage point, but nonetheless they 'observed'. If I am colour blind and a red light appears green to me, it is perfectly correct for me to say 'I observe a green light'.

So the answer is 'yes'. I think you can 'observe' the absence of something. 'Observe' simply refers to vision, and what someone sees, or doesn't see, irrespective of any wider reality.