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Q & A: Can invisible objects have shadows?

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Most recent answer: 02/19/2014
Q:
light caste shadow of those things only through which it cannot pass and those things also have mass and are also visible to us.so is it possible thing which is not at all visible also have shadow?
- pooja (age 18)
lucknow,uttar pradesh,india
A:

Hi Pooja,

Objects become visible when they interact with light, for example by scattering it. If an object didn't interact with light, you couldn't see it, and light would travel straight through it. So, as you suspect, no shadow would be formed.

Such an object is basically "completely see-through". This is almost the case for many objects at certain wavelength ranges. For example, radio waves can (obviously) pass through walls, so they don't interact strongly, and have a correspondingly weaker "shadow".

But this "shadowlessness" is most pronounced for dark matter, the mysterious substance scattered throughout space that physicists are still working to understand. Dark matter is a substance which doesn't interact with electromagnetic waves at all, which is why they don't scatter light, and we can't see them. As such, light would travel straight through them, and they wouldn't cast shadows at all.

Another interesting thing to note is that, since dark matter doesn't interact with electric or magnetic forces, it probably can't form typical molecular bonds, so it's not clear how you could actually put it together into small objects (e.g. to make an invisible house or something). On larger scales, of course, dark matter will cling together by gravity.

Cheers,

David Schmid


(published on 02/19/2014)

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