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I'm a philosophy undergraduate student who has an interest in, and respect for (as most people do) what physicists know about the world.

In philosophy, the traditional criterion of physical existence has been extension in some dimension of space (not necessarily the three most familiar dimensions). For example, we might say that no instance of my experience of the colour red (not the wavelengths of light that correspond to red) physically exists since no instance of my experience of red has extension in space.

However, I wouldn't be surprised to learn that there are some things that do not have extension in some dimension of space, and if that's the case, then the traditional criterion fails.

I could have asked this question on the philosophy stack, but I'm more curious about the properties that a physicist might say physical things (the objects of study in physics) have that non-physical things (ideas, experiences, etc) do not have.

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  • $\begingroup$ Interesting. I suppose I'd argue that anything measurable would have physical existence, which would seem to coincide with your argument for extension, but I'm not sure that it's an all-inclusive and/or rigorous argument. $\endgroup$ – Kyle Kanos Feb 13 '14 at 21:33
  • $\begingroup$ What is the definition of extension in some dimension of space. @KyleKanos The wavefuction is not measureable but I think it could exist. $\endgroup$ – jinawee Feb 13 '14 at 21:41
  • $\begingroup$ @jinawee tough call, I can imagine a few ways one might define it, but I'm not confident about any of them. I imagine it's been defined well by someone. The little I know about the consequences of relativity seem to require that we be able to say that something is in one place and not in another. $\endgroup$ – Hal Feb 14 '14 at 1:19
  • $\begingroup$ I would say that electric charge, mass, etc. exist. But I can't see how they have extension in some dimesion of space. $\endgroup$ – jinawee Feb 14 '14 at 18:05
  • $\begingroup$ my criterion would be energy-momentum, but I wouldn't want to formalize that argument while avoiding circularity; also, compound system (like, say, the universe as a whole) can have zero total energy-momentum... $\endgroup$ – Christoph Feb 14 '14 at 20:59
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Giving a precise answer to this is I suspect impossible, as the very notion of physical existence is quite subjective. Please therefore treat this answer as subjective - I would not expect all physicists to agree with it. But here goes…

As a first stab, I’d be inclined to reason a bit like this: Your suggested philosophical definition of ‘having extension in space’ does correspond to human instinct about what existence means, so let’s see if we can refine it to match physics:

  1. You need to say, ‘spacetime’ not space, since time is – so far as we can tell - not separate from space.
  2. It may be better to say something like forms part of spacetime rather than ‘in’ spacetime. That’s because most physicists would view spacetime itself as having physical existence.

Now you have a definition something like ‘X physically exists if X forms a part of spacetime’. Which is quite good, because all the physical objects around us (like electrons, and photons, and tables, and snazzy new hi-fis) can be regarded as perturbations of spacetime, and therefore being part of space-time rather than merely existing in it. So this definition does match intuition quite well.

There is something more though. Physics is ultimately only interested in whether something causes measurable effects – because that is after all the only thing we are capable of discovering about the Universe we live in. Suppose something existed in the Universe – say 2.5m x 1.5 m x 0.5m in size - that had no ability to affect us or any of the objects we can measure in any manner whatsoever (the ‘invisible pink unicorn’ ;-) ). Such an object would be completely undetectable to us and therefore of no interest to physics. Indeed, a scientist would probably argue that my words in the previous sentence ‘suppose something existed...’ are nonsensical, since, if there’s no way for us to detect it, then as far as we are concerned, it is completely imaginary, and therefore cannot be seen as existing in any real sense. I suspect that is where physics would differ from philosophy: Physics would not consider such an object to have any physical existence, whereas philosophy would consider the theoretical possibility of that object existing (after all, I’ve just given its dimension, so it does have extension in space!)

Because of that reasoning, I’d be inclined to add an additional criteria to a physicists definition of physical existence: X physically exists if X is part of spacetime AND is able (either directly or indirectly) to influence other objects that are a part of spacetime and are detectable to our senses.

I still don’t think that definition is quite complete, although it may well be as close as you can reasonably get. The main issue with it is it doesn’t account for things like ‘logic’ or ‘ideas’ or ‘beauty’ – clearly, all those things do influence us, and therefore are detectable (albeit subjective), but most people would say they do not exist in any physical sense. I’m not sure off the top of my head what precisely makes those things ‘not physical’, and therefore what else might need to be added to my definition.

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  • $\begingroup$ great answer. I'll try and provide an equal useful reply. 1) It's worth remarking that philosophy as a whole doesn't consider anything as being any one way. The discipline, for the last 100 years, has been interested in logical possibilities given certain assumptions. Most philosophical work is about looking for logical consequences and inconsistencies of claims 2) your statement about the the undetectable thing having existence to philosophers: philosophy agrees that we couldn't say anything of consequence about such a thing, and says (cont) $\endgroup$ – Hal Feb 14 '14 at 1:28
  • $\begingroup$ 'yes maybe in some manner of thinking there could be such a thing, but whether or not that is true is necessarily inconsequential - and there is nothing else we can say about it other than that, so let's ignore it.' 3) To clarify the idea of red not being physical - consider that you can only find correlates of red, but not red itself. So consider that you and I both claimed that we are only concerned with possibly detectable things, and suppose someone is currently beholding a cup of water, I cannot find that cup of water as they experience it anywhere (cont) $\endgroup$ – Hal Feb 14 '14 at 1:35
  • $\begingroup$ I can only observe the neuronal impulses that correlate with their experience of it, and the photons that excited those impulses etc. And so we require things to be detectable to say they exist, for things to be detectable they must somehow perturb space-time, a person's experiences are all that is detectable to her, but her experiences do not perturb space-time. So we have something of a special case when talk about mental content. $\endgroup$ – Hal Feb 14 '14 at 1:40
  • $\begingroup$ What if something was able to directly or indirectly influence other object that are detectable to our senses, but was not part of space-time, what would you call that? (My answer: something that exists, of course. The whole first part of your definition is superfluous, and in any case is based on a specific physical theory that could on principle be falsified, leaving it on rather shaky ground, philosophically speaking.) $\endgroup$ – Nathaniel Feb 14 '14 at 8:37
  • $\begingroup$ Hal, a stack exchange tip: once you accept an answer, people tend not to post any more. For questions like this it's best not to accept too soon. $\endgroup$ – Nathaniel Feb 14 '14 at 8:39
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What is gravitation's extension in space to an observer in vacuum free fall versus one not? Does time exist in space?

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  • $\begingroup$ Ha. Yeah, good points. Time might be a dimension, which I'll stipulate as having existence for the sake of this question. $\endgroup$ – Hal Feb 14 '14 at 1:15
  • $\begingroup$ en.wikipedia.org/wiki/Time_dilation_of_moving_particles (x, y, z, -ict), Complicated, but observable and calculable. What would reality be like if it crunched numbers? Slower? $\endgroup$ – Uncle Al Feb 14 '14 at 1:44

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