My players are currently in a futuristic setting where they are being shot at by lasers and such. One of my players tried to justify that an antimagic field should be able to disable future tech because it's "basically magic". My understanding is that all magic comes from The Weave, and anything outside of that is just considered technology (like an electric light bulb). Am I wrong in this assumption?
Technology is not considered to be a magical effect unless it is powered by magic
A player's perception of what appears to be magical or not has no bearing on the rules for whether it actually is considered to be magical by the rules. The only thing that matters is the magical nature of the object.
Something must be connected to the weave to be considered magical regardless of the technological level of your campaign/item. That is what defines being magic. There are many things in the D&D world which look magic and are not, as well as things that don't look magical at all but are.
Sage Advice gives us the rules for determining if something is considered magical:
Is it a magic item?
Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
Is it a spell attack?
Is it fueled by the use of spell slots?
Does its description say it’s magical?
By the book - lasers/future tech are non-magical
By default, D&D does not provide details on how technology works, but it does give us examples of specific items in the DMG on p. 268 (including laser rifles and other such items). As listed in the book, these definitely are not magical because the weapons do not pass any of the tests listed above for magical items.
You have indicated that this is the way you also are treating technology in your campaign. In this case, anti-magic field would certainly not affect any tech in the game because of its lack of magical nature.
If you are making your own technology - it is up to you
However, a DM looking to add detail could certainly add to or change what the book says. Also, not all technology has to be treated in the same way as long as it makes sense in the world and is clear to the players.
If you are making your own custom setting/tech then you get to decide the answers to these questions with regards to how technology works in your world.
If you decide that technology is powered by spells or magic then they would be magical.
If you decided that your technology is based off of in-world science (and completely devoid of any connection to the weave or any kind of spells) then it would not be considered magical.
Only magic and magical effects are affected by an anti-magic field.
Either way, the perception that something is magical has nothing to do with what the rules consider to be magical.
Sidenote: by default, there are always tools to distinguish between magical and non-magical things
Your (original) title said that you are referring to something technological that is "indistinguishable" from magic (presumably referring to Clark's Third Law). That technically isn't possible given how the game concretely defines magic and even gives you tools to determine if something is magical or not (identify, detect magic, antimagic field). Since there is a concrete definition of what is magical there are in-universe tests to determine if that is the case. In other words there is theoretically always a way to distinguish between technology and magic if you try hard enough and have the tools. This doesn't really change anything about the nature of this answer or your question, but I thought it was worth noting.
D&D 5e hasn’t released any supplements detailing how futuristic technology works, so there really isn’t any kind of official or authoritative answer to your question. Ultimately, since you have introduced these concepts to the game at your table, it is up to you (and/or your table) to define them.
You could easily go either way with these. The flip-side of Clarke’s Third Law is that “Any sufficiently analyzed magic is indistinguishable from science,” as Agatha Heterodyne put it. If we take a magical world à la D&D and advance it into the future, magic is going to be just as serious a discipline of scientific study as physics or chemistry. Actually, it is very likely to be more so, considering the power and convenience of magic. Just think how many early “scientists”1 in our own history were primarily bent on studying the various magics they believed in, despite the fact that no magic is known to exist here! When magic is provably a real thing, it would only garner that much more attention.
So scientists would study magic. Their findings and solid conclusions would be streamlined and taught to architects and engineers. The world would be built upon a foundation that relied on magic as much as it relied on gears, electricity, chemical reactions, and the like.
Thus any particular thing may well have been implemented with magic.
On the other hand, maybe it wasn’t: maybe dispelling or suppressing magic was a severe enough risk to be worth the effort of doing things without magic. Or maybe, once non-magical means were discovered, there was little reason to use magic—maybe magic isn’t amenable to mass production and industrialization and so it became cheaper to implement things non-magically.
Either way is entirely plausible. You could easily decide on a case-by-case basis, too. And while there may be things that your players could object to on the grounds that they aren’t possible without magic, part of the point of playing a futuristic fantasy is to have them know how to do things we currently consider to be impossible. We, of course, have no way of knowing what things we are wrong to consider impossible, so that leaves all of them potentially up for being hand-waved away.
Ultimately, the real criterion you should use is “what will make the game better?” Players who bitterly feel as though you’re cheaply obviating their characters’ skills are not good for the game. Giving in to the players’ every demand, however, tends not to make a good game either. This is a great opportunity for a table discussion about how you all want to handle this. Have a discussion, divorced from any specific example, bring up some of the points I’ve raised here, and get feedback from your players. Look for what they want, consider what you want, and try to find a compromise.
- Quotations because the “scientific method” hadn’t been invented yet and there are many, many ways in which these early researchers are rather distinct from what we would consider a scientist today.
No. Clarke's third law is not a 5e rule.
Magic also does not have any sort of paradigmatic basis in 5e (like in some games). It doesn't care who the caster is or what they believe about the world, it just does what it says it does in the description. If it did, then Antimagic Field would protect a Hill Giant from a Crossbow bolt.
As an alternative view, yes: Sufficiently advanced technology may be considered to have a magical effect, provided the technology is not explicitly marked as non-magical.
Now I come from 2nd edition and haven't updated my books too much, but as I've DM'd magic is a fundamental of physical phenomenon (in the science sense, but may be intangible).
While the counterargument presented by @MarkTO that "[i]f it did, then Antimagic Field would protect a Hill Giant from a Crossbow bolt", is an excellent point, depending on the DM's interpretation, it may simply be that quality of magic requires some level of INT / WIS. Thus an Antimagic Field that protects against a crossbow bolt fundamentally requires understanding of what you are protecting against and becomes mutually exclusive.
Further on this point, think of it this way: ask how many "whys" can be asked and explained.
Gem stone example: If the source is a glowing green emerald, and the reason is "because", it is likely still magic to you.
Crossbow bolt: If it is a physical object that moves fast, makes you go ouch 'cause of the pointy bit, and gets it speed from "a rubber band thingy", you might have out INT yourself from magical defense against it.
It could be based on the party, individual or society level of understanding, pick it and stick with it.