To date, there is absolutely no scientific evidence to support that orbs are ghosts or anything else paranormal. However, there is much objective data to support that orbs in photographs and video are explainable photo/video artifacts that can be reliably replicated. The orb craze gained traction after the popularity of early digital cameras. While it is possible to capture orbs with 35 mm cameras, it is far less common. The point and shoot digital cameras had a greater depth of field compared to 35mm cameras, because of a smaller focal distance, and with the flash source closer to the lens, it made capturing airborne particles much easier.
Here are just a few articles explaining orbs or "light anomalies" caught by cameras:
This is ASSAP's (Association for Scientific Study of Anomalous Phenomena) site map linking to various articles about explanations of orbs, including cause of their color, transparency, shapes, etc.:
Here is an photographic experiment conducted by Midnite Walkers Paranormal Research Society that has been widely circulated on the internet for reference of the various colors and shapes of orbs caught on film:
Camera and film manufacturers have also provided an explanation for these artifacts. This one comes from Sony: (Thank you to River Cities Paranormal Society for sharing.)
Orbs or Dusts: A Practical Guide to False Positives by Kenneth Biddle is an excellent book which not only explains orbs and other photographic artifacts, but also demonstrates detailed experiments for the reader to recreate for them themselves:
"Yeah, but dust can't suddenly change direction!" Well, yes it can:
Air is dynamic, not static, even inside buildings. Air currents are not caused only by movement or wind, but also by changes in temperature and humidity as this article explains:
Even if we exclude air currents as a cause of movement, there is Brownian motion to consider:
If a larger particle (such as a dust particle) collides with a large set of smaller particles (molecules of a gas) which move with different velocities in different random directions, it can change direction. Air is a mixture of gases - 78% nitrogen and 21% oxygen - with traces of water vapor, carbon dioxide, argon, and various other components:
"But I saw an orb with my own eyes!" Unfortunately though, our eyes aren't very trustworthy:
Only a portion of the visual process takes place in the eye - the majority up to how the brain interprets the information. Misperception is caused by various physiological, psychological, and environmental factors. That is not to say the observer is crazy, lying or stupid. It simply means that sometimes our brain, by design, misinterprets information. Add suggestibility or expectation, and the chance of misperception increases. But for the sake of argument, let's rule out misperception for a moment. Even if one sees an orb, it is a huge leap in logic to conclude it is a ghost or something paranormal. There are many other factors to carefully consider and rule out first, including: electrical discharges, static electricity, ball lighting, bioluminescence, piezoelecticity, and ignis fatui.
Unfortunately there are some people who will still cling to the notion that orbs are ghosts even after they are presented with facts to the contrary. Cognitive bias is very hard to overcome, especially if a belief somehow serves an agenda. Some ghost hunters want "proof" of the paranormal more than they want valid answers: