So this seems like an open and closed question. What is an Ultra Beam? Many of you no doubt want to answer "A Beam an Ultra shoots". Congrats, you've just won the idiot award, expect you award in the mail, never. Passive-aggressive (was that even 'passive'?) humor aside, what makes up an Ultra Beam? A 'beam' is not always as simple as a laser, you have plasma beams, particles beams, and with the latter, you have so many more beams falling under that definition. What is an Ultra Beam, can we figure it out...well I wrote this blog, so I may as well get on with the theory...
So it's time for Sol to once again make up some quasi-scientific idea and make it sound sensible (it's a theory based on a franchise that's best describe as Science 'Fantasy', If I wasn't this self aware by now things would be so sad...still I like doing this so, meh)
Note: this is not about HOW Ultras fire their beams...but the 'physical' composition of said beams.
What do we know for sure?[]
A few things we know for sure about Ultra Beams. They are all based on substances based off Specium, as the Specium Redoublizer (see the end of Mebius) more than implies. I say 'more than implies', because if it wasn't present in both attacks....the machine would not have had the effect it had on Mebius and Zoffy's beams. Also in one of the Retsuden, Zero stated that the Specium Ray is the most basic Ultra Beam. This means every other beam is a variation of this basic technique.
The second thing we know, again from Mebius (the episode with Femigon), is that Ultra Beams have a "molecular deconstructive effect", which is a fancy way of saying, they disrupt/break molecular bonds. Here's the thing...if you heat up something enough, then yes, molecular bonds will weaken, that's how we get solid to liquid and then gas. However, that's not how it's being demonstrated, described or implied. Instead, the idea is, what ever is hit by an Ultra Beam, immediately the bonds are being disrupted, hence the explosion. It's not heat that's causing the explosion (though heat does seem to hold a connection with how strong a beam is), but the physical properties of the beam.
A third thing we know is that, the Ultra Beam, holds a connection to the inner energy/light of an Ultra. This is why the signature beams of different Ultras are such, even in cases where different Ultras are all using the Specium Ray, they are different colors (the Ultraman Cosmos Musical Special, where the Ultras fight Sky Demon) though granted I'm not sure that should be considered canon... In Nexus, using the energy signature of the titular Ultra, the Night Raiders were able to use the Ultimate Vanisher, a beam weapon that was just a mechanically recreated Over Ray Schtrom, Nexus' signature ray in Junis Mode. It was even shown that the photon-electrons used, could be 'constructed in a different way' to be a current of pure energy to recharge an Ultra. Which would explain why in Ultraseven the Salome aliens needed a 'formula' for the Ultra Beams, and couldn't just figure it out.
Speculation Time[]
So from all this...what specifically is a Ultra Beam. Well...I think it's a hybrid of beams. A plasma beam, particle beam, and the classic Laser, or perhaps...a Phaser...
For those who don't know what a Phaser is...don't feel bad, a lot of people watch Star Trek and don't know what a phaser is either. Long story short, it's a laser, a short story long, it's a laser that is modulating between two or more frequencies while it's being fired. This modulation is why it's so destructive, and yet it can have a 'stun' setting. Now the reason is speculate this may be what may be going on (please note, phasers are not real...nor do I know of any scientific research or experiments that give credence to the idea) is because, like Ultra Beams, Phasers are otherwise simple lasers that make things explode, or basically cause large explosions. However such a destructive effect on matter is not from simple burning, and those things could literally vaporize a person...not unlike Nexus/Next...
Now considering that specium is the prime ingrediant for this stuff, we can't forget about that, the question being...how does it connect to the laser part...? Well allow me to rant.
Let's go back to the idea that Specium turns into plasma/initiates nuclear fusion at room temperature, so being fired in a laser, turns it into such a state. Then we have the Photon Electrons, basically an electric current connecting the particles/plasma with the photons (does that even work...meh Photon 'Particles' and Science 'Fantasy'). So we have a beam of photons, a typical laser, electrically connected to particles, of most likely plasma, (again, this isn't exactly science...) in a single beam. Yet this is not why Ultra Beams are so destructive. Simply being a laser, or plasma beam is not enough to explain why, what they hit explodes. For this, lets go back to the phaser idea.
Here we go.
Remember that instance I spoke about, when several Ultras seemed to be using the Specium Ray, various Ultras, from Ultraman to Ultraman Cosmos, and Zearth. They used it, but they all looked different, they all looked like their signature beams in some cases. Heck, it's the picture in the infobox on the Ultra Beam page.
Now the logical explanation for these differences is the unique energy signature of each Ultras 'Inner Light'/Energy Field, was resulted in these differences. These are the signatures that each beam's photons are following, explaining their appearance. Now...lets got back to the idea of a phaser, each ray's energy signature is more than just a frequency of light, it's a complicated affair, and if the photons are adhering to that matrix of frequencies (if what Nexus showed is anything for a standard) then assuming the laser part of an Ultra Beam, acts like a phaser...makes sense. This phase/frequency change, may also what changes Specium into the other elements, shown in other Ultra Beams.
Now you're probably wondering why then would the 'Specium Ray' be the most basic ray attack. Well, in that case, it's less about the signature and more about the application. Every Ultra Beam, is based on specium, but perhaps with some others, it's less about the laser side of the beam, and more about the plasma beam side of the attack...or the other stuff is still present, but the beam is still mostly Specium.
As for why some monsters are able to survive Ultra Beams...I don't know... By all logic, nothing should survive these things, except perhaps Ultras. My best guess is reflective armor, really dense energy fields like Antlar, or other similar monsters. Maybe they have skin not unlike Ultra Armor, but instead of absorbing for nuclear fusion, they convert energy for nuclear fission or something. I don't know. Still it seems to be the type of beam, such as Ultraman using the Ultra Attack Beam on Antlar when the Specium Ray failed, or with really powerful monsters, in which case a large amount of Ultra Beam energy, usually supplied by several more Ultras, does the job just as well. So maybe some monsters have evolved natural ways to counter/reflect the phaser effect of Ultra Beams.
This would also explain combination beams, as the two attacks are connecting based on their shared element. Anything I missed but you want answered/theorized, ask them in the comment section.