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This is a duplicate of this question, but for a different gaming system; Pathfinder v1 vs DnD 5e. Also relevant, but slightly different is this question about intersection.

So the gist is, a wizard is sitting in the middle of an empty 50'x50' room. They have cast Antimagic Field on themselves (10' radius so well within the confines of the room with no obstructions).

Another character opens the door and doesn't trust the wizard with the wry smile and casts Detect Magic (30' radius, so big enough to reach the wizard 25' away but well outside the Antimagic Field).

Things within the Antimagic Field appear mundane, as per the spell description. But does the other character sense the abjuration spell itself?

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  • \$\begingroup\$ I have just opened a question about detect magic detecting itself, because I theory the anti magic zone may make a hole in the aura, so you can detect it by noticing the aura stopping in a suspiciously circular manner \$\endgroup\$ – SeriousBri Dec 5 '19 at 21:07
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You would not feel the presence of the field

This answer stated the opposite before, but upon further reading of the Anti-magic field spell I realized it was wrong.

As per Detect Magic's description:

For the duration you sense the presence of magic within 30 ft of you. If you sense magic in this way, [...], and you learn its school of magic if any.

Anti-magic field effect on spells:

Spells. Any active spell or other magical effect on a creature or an object in the sphere is suppressed while the creature or object is in it.

Areas of Magic. The area of another spell or magical effect, such as fireball, can't extend into the sphere. If the sphere overlaps an area of magic, the part of the area that is covered by the sphere is suppressed. For example, the flames created by a wall of fire are suppressed within the sphere, creating a gap in the wall if the overlap is large enough.

According to the description in Areas of Magic, Detect Magic's area of effect would be cut short the moment it reached the field, looking as though theres no magic there.

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Executive Summary

Like Dispel Magic, Detect Magic should "have no effect on the [Antimagic Field] sphere" meaning that you will not detect a faint aura from the sphere itself.

Relevant Quotes

Detect Magic allows the caster to:

[S]ee a faint aura around any visible creature or object in the area that bears magic...

Antimagic Field says:

Spells and other magical effects... are suppressed in the sphere and can’t protrude into it... Spells and magical effects such as Dispel Magic have no effect on the sphere.

Special Situations

If there is a "visible creature or object" that generates the antimagic field from outside the Field, or a magical part of it protrudes from the Field, then Detect Magic might let you perceive an aura around the generator, if cast and viewed from an appropriate direction (not through the field). I'm imagining a magical staff, the end of which creates an anti-magic cone. The cone itself is not detectable by Detect Magic, but the rest of the staff, or the staff when deactivated, is detectable.

Other Ways of Discovering an Antimagic Field

The presence of a rune or creature that generates an antimagic field can be discovered via non-magical means, such as a Perception, or targeted Investigation skill check. A subsequent Arcana check may determine some aspects what the rune or creature does. These checks are not guaranteed to work. Nor do they necessarily provide the school of magic, or other relevant details.

If you suspect an antimagic field, you might also detect it indirectly by casting Light (or some other spell with an obvious effect) onto an object and propelling that object into the field. You can observe the light being snuffed by the field without having to make a skill check. The player and character could then deduce that an anti-magic field was the likely culprit.

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    \$\begingroup\$ Can you support your thoughts? \$\endgroup\$ – NautArch Dec 5 '19 at 18:09
  • \$\begingroup\$ I'm not sure I agree (see my answer), but I do like your use of light to detect it. It does assume that they're testing for the antimagic field itself and not just "there's something up with this wizard", but it's a neat idea. Additionally, your use of a perception of arcana check to learn that there is unseen magic present seems potentially problematic given the specific purpose of detect magic. If you can do that without a spell, why have the spell? \$\endgroup\$ – NautArch Dec 5 '19 at 19:03
  • \$\begingroup\$ Cast detect magic when fleeing a treasure room in a hurry, to just grab whatever is near and glowing. Notice enemies with magic items in total darkness. Pick out the enemy wielding a magic weapon. Find every last magical item in a large horde. Guaranteed results (given line of sight). Perception or Investigation could alert a character to something interesting (enemy or a trap). Arcana could tell you that a particular monster or rune creates an anti-magic field. But these take time, they can fail, and making multiple checks still doesn't tell you the same information as the spell. \$\endgroup\$ – GlenPeterson Dec 5 '19 at 19:15
  • \$\begingroup\$ FYI, antimagic field is cast on the caster. THey can get around that using a glyph of warding in terms of the casting event, but at the end of the day, it's a spell with a target of self. The field is centered on the caster. \$\endgroup\$ – NautArch Dec 5 '19 at 19:27
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No, there is a null result

Detect Magic (PHB, 231) works as follows:

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

So what we have is a character casting detect magic withing range of the wizard. Magic in range should be sensed, after which the character could use an action to see a faint aura about the wizard and learn the school of magic.

But then the rules around the antimagic field (PHB, 213) come into play.

Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere and can't protrude into it.

Areas of Magic. The area of another spell or magical effect, such as fireball, can't extend into the sphere. If the sphere overlaps an area of magic, the part of the area that is covered by the sphere is suppressed. For example, the flames created by a wall of fire are suppressed within the sphere, creating a gap in the wall if the overlap is large enough

Spells. Any active spell or other magical effect on a creature or an object in the sphere is suppressed while the creature or object is in it.

Dispel Magic. Spells and magical effects such as dispel magic have no effect on the sphere. Likewise, the spheres created by different antimagic field spells don't nullify each other.

Okay, so we've got a few things in here.

  1. Detect Magic can inherently be stopped by certain barriers. An anti-magic field is a barrier of sorts. While it isn't explicitly mentioned in detect magic, there is an existing rule that the detect magic can be stopped. We can now look at the language of antimagic field to see if it would qualify.

  2. Antimagic field clearly suppresses any magic (except those created by an artifact or a deity.) Detect magic is not created by an artifact or deity so should be suppressed as a whole. It can't protrude into it, and therefore wouldn't be detected. There is nothing to detect. The additional stipulations in areas of magic and **spells* also are clear in that no magic passes the barrier.

  3. The rules in antimagic field about dispel magic also imply that interactions between the field and other spells have no effect. Just like how dispel magic can't interact with the field, neither can detect magic.

Narratively, the character casts detect magic, but they get a null result from the field. If the character knows that they have magic items on them and they don't get a response, then they'll know something is up. But otherwise, the reading from detect magic comes up with no magic (at least from the antimagic field.)

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