I went to a concert the other day and noticed that the bows (violins, cellos, violas) were all coordinated as the orchestra played. It looks neat, but is there an audible difference if they're not in sync bow-wise? If there's a difference, is it more or less obvious depending on the player/instrument?
It's easiest for the player to put downward pressure on the instrument string when the bow is making contact with the string near the end that the player holds (the 'frog' end). This means that on a downstroke (extending the arm), it's easier to start the bowing action with firmer pressure.
Players tend to take advantage of this by using downstrokes to play strong beats, and upstrokes to play weak beats. Obviously it may not be possible to maintain this rule strictly, depending on the rhythm of the passage being played; if not, players will have to compensate by applying more pressure on the upstrokes.
Another consideration is that if a section indicates repeated strokes in the same direction, the bow will have to be lifted between each stroke, leading to shorter strokes even if rests aren't indicated between the notes.
In theory it's also possible that the direction of the horsehair makes a difference - but although some think so, Others don't (1) (2), so it's possible that bows aren't always re-haired in the same direction anyway. If there is any effect, it's likely to be very small as the scales on the hair are very tiny (p28).
To your question, then : Can one tell by ear ? The answer is probably that it depends how skilled at compensating for pressure differences between down and up strokes the player is, and of course whether or not they are actually trying to compensate. In most situations though, a player would use the natural difference between an up and downstroke to their advantage, rather than try to fight against it.
Yes, it is obvious. A note played "up-bow" sounds discernably different than a note played "down-bow". A string player can also play a series of notes continuously using the same long bowing motion, or the player can "saw" back-and-forth, reversing direction between each note. There are several other kinds of bowing articulation techniques as well. They all create different sounds and effects.
Bowing direction, up or down, is often written into the sheet music by the composer, as a symbol above each note.
String sections in orchestras can spend a lot of time in rehearsal on planning out and coordinating bowing motions. Earlier music, such as from the Baroque era, generally didn't include up-and-down bowing symbols written into the sheet music. In that case, in the present day, the members of the string section, usually led by the concertmaster, will devise and agree upon precise bowing directions for certain notes in rehearsal, pencil in bowing markings on their sheet music, and practice the agreed-upon articulations for each note. This shapes the phrasing and the rhythm of the parts being played.
Someone above made the comment that down-bows are pulling the bow and up-bows are pushing the bow. This may be true, but it's not my understanding. Clearly, down-bows are pulling the bow over the string. And I think the Russian style is for the hand to be somewhat rigid on the bow, so up-bows are indeed pushing on the string. But the in the Franco-Belgian style, both directions are really pulling on the bow. On a down bow you keep your hand loose, and change the fingers so that the hand leads the fingers. This is pretty obvious. To do this on an up-bow, you make your hand loose, so that the hand, again, leads the fingers. If you ever have the chance to see a violinst play long, whole-bow notes very legato, you will see that their fingers change from knuckles-up, tips down on the up-bows and knuckles-down, fingers up on the down-bows. This is the reason. Also, I think a loose hand helps mediate the weight of the hand and bow on the string, and helps find the right bow speed and weight combination.
So, why do I think up bows sound different from down bows? I think it's because the up-bows start with the tip and the down bows start with the handle end, which we call the frog. The tip is lighter than the frog, so notes on the tip are not as loud as those played on the frog. Notes played on the frog may have so much weight that they squash the string and make it scratch.
Fiddling is not just fiddling around, you know.
Of course, I could be completely wrong about this.