[Kryptid (OP)]

Something we often hear is that gamma rays can easily penetrate even thick blocks of matter. I assume this is related to their high energy, but also know that absorption of photons is not quite that straightforward (some materials can absorb higher energy ultraviolet light while allowing weaker visible light to get through more easily: some glass, for example).

Now let's consider a black hole in its dying moments as it releases its last burst of Hawking radiation. At this point, the hole has a diameter almost as small as the Planck length and the resulting gamma rays have energies near the Planck energy. Is it safe to assume, because those photons are so many, many orders of magnitude stronger than the gamma rays we normally experience, that they would pass unhindered through any kind of material shielding we could put in place? Would any amount of significant energy be transferred to the material causing it to heat up? The very small size of those photons also makes it seem like the vast majority of them would zip between the electrons and nuclei without hitting anything.

I've also read that gamma rays with an energy higher than the rest mass of two electrons can also transform into an electron-positron pair when interacting with charged particles. Since a Planck energy photon would have an energy much, much higher than any known particle mass, that seems like it should be able to change into any kind of particle. Would that increase the odds of interacting with the blocking material?

[Bored chemist]

Is it safe to assume, because those photons are so many, many orders of magnitude stronger than the gamma rays we normally experience, that they would pass unhindered through any kind of material shielding we could put in place?

I don't think that assumption is safe.
For a start, I think it would undergo pair creation even if there was nothing in its way 
and I also think that an energy of about a gigajoule is a lot for a photon it's only about a tonne of TNT equivalent.

A well designed concrete barrier could stop it (once).

[Kryptid (OP)]

For a start, I think it would undergo pair creation even if there was nothing in its way 

I'm not sure about that, since, in some reference frames, the kinetic energy of the photon would be below that needed for pair creation. I think interaction with another particle would be necessary in that frame since the other particle would be the one carrying immense kinetic energy instead.

[Bored chemist]

I'm not sure about that, since, in some reference frames, the kinetic energy of the photon would be below that needed for pair creation.

I exist.
In my reference frame the photon has plenty of energy.
So, how do you resolve the idea that, in my perspective it must form pairs but in other frames, it doesn't?

[Kryptid (OP)]

So, how do you resolve the idea that, in my perspective it must form pairs but in other frames, it doesn't?

By saying that it doesn't form pairs unless it collides with other particles. In that case, you get pairs in all frames. At least, interaction with other particles is stated as a requirement in this article: https://en.wikipedia.org/wiki/Pair_production

"The photon must be near a nucleus in order to satisfy conservation of momentum, as an electron–positron pair produced in free space cannot satisfy conservation of both energy and momentum."

[Bored chemist]

"The photon must be near...

The key word there is "near".
You don't need a collision.
You don't need a direct hit, just an EM field.

And I'm fairly sure that a virtual particle is enough to do that (The transition probability is tiny but I think it increases with photon energy)..
Also "near" isn't well defined.
I'm "near" it on some scales.
Again, we are talking about probability rather than yes/ no.

So a very high energy particle travelling through space will eventually undergo pair production.
It won't even pass "unhindered" through nothing.

And that's before we consider the problem of hitting the CMBR.
It looks like harmless microwaves to us, but if we were travelling at nearly C those photons would hurt.