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 its "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.

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

If you are going by the future tech as outlined in the DMG on p. 268 (including laser rifles and other such items) then these definitely are not magical because the weapons do not pass any of the tests listed above for magical items.

If you are making your own technology - it is up to you

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.

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-waived 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.

1. _{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.