Usually, yes. Silence specifically prevents sound, not all vibration. (Remember that heat is a vibration as well - a complete lack of vibration would indicate absolute zero, and probably herald the end of times.) It prevents only those vibrations that occur in ranges that can be heard. This stops most vibration-based direct attack spells (sound lance) and some vibration-based area-effect attack spells (soundburst), specifically those with the [sonic] keyword. At the DM/GM's discretion, it could stop or weaken some attacks besides spells that are a form of vibrative force if the judge rules that they're primarily or entirely composed of sound waves, but this should probably be done only if the supporting description of that attack describes it as being sound or sonic.
Magic is oddly arbitrary at times, but in any world where feather fall exists, we should be prepared for magic to interact strangely with physics.
It should have no effect on spells or abilities that use any other vibration frequency, although it likely has incidental effects on how the characters experience those spells - for example, a silent earthquake or meteor swarm is likely a very surreal experience. It can still be felt, and objects (and structures!) can be seen falling and smashing, but none of it can be heard - this would be strange enough for us in the modern day, just watching earthquake footage on YouTube with the sound muted; having it actually occurring around you with all the volume of a sensory deprivation chamber has got to be mind-bending.
To directly answer your examples:
Could they still be damaged by an Earthquake (PHB) or similar spell like Wrack Earth (PHB2)?
Definitely. Spells specifically let you know if they use sound in their effect by way of the [sonic] keyword.
How about the Shockwave spell? (Sharn: City of Towers)
I don't have this one, but assuming the spell has no [sonic] keyword, definitely. (If it does, clearly, it should be prevented.)
What about waves dropping heavy objects (for instance a boulder into a pond)?
Only if the sound itself is enough to cause damage. The water displaced by the boulder retains all its momentum as it enters the silence spell - that will hit at full effect. (Generally, if something hit something else hard enough to produce damaging sound waves, the actual effect of the hit is probably enough to make the sound damage negligible. For something crashing to earth, if it hits hard enough that the sound itself could damage a human standing 100 feet, the explosive impact and clouds of debris are probably going to kill that human.
What about weapons that cause concussive force types of damage?
Besides ones like the sound lance spell or a thundering greatsword or a sonic pistol, these should be unaffected by a silence spell - except the part where the opponent can't hear the weapons being used, so it's less likely to send up alarms, especially if the silenced combatant is in melee so the shouts and cries of his enemies can't be heard.
Interesting! On a similar note, would the same types of rules/ ideas apply to a deafened character? (or does a character need to be able to hear in order to be damaged by sound/ sonic attacks)
This might require a bit more case-by-case evaluation, but generally, a deafened character is only protected against [sonic] effects if they are also [mind-affecting] and/or [language-dependent]. A sound lance or sonic rifle will rip a deaf person's body apart and he won't even know what's happening to him, since he can't hear the cacophonous roar of a discharged physical sonic weapon. But spells like command and suggestion will completely fail to affect him, because they require him to hear and understand the caster's spoken words. A deafened character is very difficult to sway through normal Diplomacy or Bluff as well as many forms of magical persuasion short of outright domination. Non-combat uses of magical persuasion might be able to be rendered as a written message, depending on the spell; likewise, I've personally ruled that a deaf character was affected by command because they'd spent a Linguistics choice to gain the ability to read lips, and the conditions were met for them to automatically do so without making a Perception check. (Had they been further away or the caster not directly facing the deaf character speaking his one-word command slowly, I'd have permitted the deaf character to choose to fail their Perception check - in fact, I'd done so twice already that fight before the bad guy figured out why his spells weren't taking hold.)
This can work
And congratulations on thinking up of a creative use of a buff spell. I like this unconventional application of Haste, as I love that spell myself. Of course, you would probably get more use out of this if you buffed your allies instead. :)
Does the casting of haste fail because the target is not willing anymore? Does the target stop being a willing target and the haste spell will end? Does the spell just work as usual?
- RAI: Yes, haste does end. If you take a look at this Sage Advice, Jeremy Crawford says that a spell which requires a willing target takes the creature's will to help fuel the spell. This implies that their ongoing consent is a requirement to keep the magic alive.
Can the caster stop concentrating on the haste spell during the target's turn to make it waste 2 turns?
Yes, as you can stop concentrating at any time without expending any resources. This includes at the start of their turn.
If the target becomes an invalid target, the spell ends on them. Haste says: "When the spell ends, the target can’t move or take actions until after its next turn, as a wave of lethargy sweeps over it." So they cannot take an action until the end of their next turn.
Normally, you only lose 1 turn when Haste ends. This is because it typically ends at the end of your turn (after which it has fulfilled the full 1 minute duration), or you lose concentration of it during some other creature's turn (therefore there is no current turn to lose). But let's say you cast it on yourself, and you lose concentration on it at the start of your turn, you actually become lethargic immediately, and therefore lose that current turn as well as your own next turn.
This means, if the target becomes unwilling at the end of their turn, taking into account the answer from #1, they only lose one turn due to the spell ending on them.
Making a Target Willing Against Their Will
The most unambiguous and cheapest means to do this is Charm Person. It does not require concentration and lasts for an hour. A creature charmed by this spell regards you as a friendly acquaintance.
- Depending on the DM, the fact they are friendly to you may not be enough to call them willing targets of the haste spell. In which case, you may have to make a Persuasion check to convince them to let their speed be bolstered by your magic, as you are friendly acquaintances, after all.
A more ambiguous -- but still reasonably valid -- method of making a creature willing is the Command spell. If you give it a command to become willing, then it becomes willing for 1 round without concentration. Commands that might accomplish this would be "submit", "surrender", "yield", or "succumb".
A high level spell which can definitely do the trick is Mass Suggestion, which is Suggestion but without concentration. So you can suggest to them to "become willing to allow me to enhance your speed with my magic".
Another spell which can technically do this, but is not really useful in combat, is Geas. If you command them to "always be willing to let me enhance your speed with my magic", then they must be willing targets of haste or take 5d10 psychic damage. If they become unwilling of their own accord, they take the damage and lose a turn.
It doesn't work for a target under Suggestion, Dominate X, Friends, etc
If you made them willing by using a concentration spell such as Suggestion/Dominate X/Friends, then when you cast haste on them, your first spell will end and there is no time of overlap. This means it is questionable if they are willing in the first place, and hence they may be invalid targets of haste even before haste has been cast on them.
It is completely up to your DM to decide if they still become willing after the two spells (charm and haste) switch up.
If both spells overlap for a short period of time, they lose only 1 turn
Let's say your DM ruled that they remained to be willing targets of haste even after you lost concentration of your Suggestion/Dominate X/Friends spell. So they come under the effects of haste. However, if they immediately become unwilling targets after haste has been cast, haste ends immediately. They lose only their next turn, because it is still your turn now while you are switching the charm and haste spells.
Best Answer
Definition
Willing means:
Definition 1 is effectively motiveless, in this sense "willing" means that you are prepared to do something; your motivation for doing so is irrelevant.
Definitions 2 & 3 expand on definition 1 to incorporate a motive, you are "willing" if you are "acting gladly", "eagerly", "voluntary" or "ungrudgingly".
My personal preference for a "willing creature" is to adopt definition 1 - if they consent to the spell then their motives are irrelevant. The other definitions are problematic as the motive must always be a matter of degree.
For example, if you were to agree in a negotiation to do something then you are clearly willing by definition 1 but you may or may not be willing by definition 2 and 3. When you ride a roller coaster in spite of your fears, are you willing? Or are you unwilling because you harbour doubts or are submitting to peer pressure?
Are you a "willing worker" because you derive deep satisfaction from the job you do or because you have a mortgage to pay? If the latter, are you in fact willing for definition 2?
What is willing
Prima facie, for all definitions, if you ask, and they say "yes" then when you cast the spell, it will work.
Further, the target of a "willing" spell must knowingly consent to be a target (PHB p.201):
If you were to simply cast the spell on a creature it would not know that it was the target of a spell. In general, it would be fair to assume that a creature would be unwilling as a default state as being willing would expose them to anything any spell caster wanted to do to them.
Making the unwilling willing
So, if they are unwilling, what can make them willing?
Charm makes it easier to do any or all of the above but would not of itself be sufficient:
Suggestion seems perfectly feasible (for all definitions), if you can word the suggestion to make subjecting themselves to the spell seem reasonable, it says:
Dominate again problematical (under definition 2 & 3). The command "be willing for this spell" may be sufficient to overcome this but the DM would need to decide.
Personally, I think "which it does its best to obey" makes it willing for definition 1 and I think that is the threshold but others may disagree.
I would think that using this part of the Dominate spells would make the creature willing:
Conditions What are the effects of various conditions on a creatures willingness?
I believe the following would make no difference: Blinded, Deafened, Frightened, Grappled, Incapacitated, Invisible (though it may make them an invalid target; Hidden would be similar although not a condition), Paralysed, Exhausted, Poisoned, Prone, Restrained and Stunned.
Charmed has been dealt with above.
So that leaves Petrified, Unconscious and Dead (technically not a condition but close enough). For the first two (and presumably dead) the creature "is unaware of its surroundings" so, can it be willing or unwilling or even treated as an object rather than a creature? Clearly a DMs call but I think that it should depend on the particular spell. I will only give one example but there are some imponderables here.
Consider Teleporting with an unconscious creature; if the creature were an ally of the caster then they would (but for their unconsciousness) be willing to be teleported (e.g. away from the ravening dragon), conversely, an enemy would be unwilling (e.g. to prison). To my mind it makes more sense to treat them as an object i.e. their willingness or otherwise is irrelevant and they count against the weight limit rather than the creature limit. However, it is equally valid to say all unconscious creatures are willing or unwilling or as they would be but for their unconsciousness. Your call.