Alpha particles aren't highly trackable. They can be shielded by a piece of paper, or by a short distance of air.

It's gamma rays that have a lot of penetrating power, isn't it? Though I don't see how those would leave a residue you could track, they'd either be absorbed by nearby matter or expanding at lightspeed from the source point.

Tom and Matt are right. Geiger counters need to be, like, a centimeter away from the source to see alpha particles. Beta particles (which are basically electrons or positrons) can penetrate further, with gamma rays being the most able to penetrate (since they are high energy light waves).

For betas and gammas there would be a trail of ionized molecules as they were absorbed by nearby matter, and gammas you could detect with something that sees light at that frequency. Speed of light shouldn't matter because the source would be constant, like it doesn't matter when you see a flashlight in a dark room.

They should've just gone with tachyons.

Interesting factoid I teach my students. PET (positron emission tomography) scanners work by feeding a patient a radioactive isotope that produces positron beta particles as it decays. Positrons are antimatter electrons, so as soon as runs into a normal electron (like instantly) they annihilate themselves and the electrons producing energy in the form of gamma rays inside the body of the patient that a detector outside the body can track to provide diagnostic info. There is no word on whether the side effects of PET scans include anger management issues and a greenish tinge to the skin.

Thanks, I didn't realize gamma rays left ionized atoms in their wake.

DH and I both gasped when Waller got it. Did NOT see that coming. Thought there were some lovely scenes with Oliver and Felicity. Stephen Amell gives good Love-eyes.

I love it when y'all get all sciencey.

I'm finally catching up on the past few seasons of Doctor Who. I miss Amy and Rory. I don't like Clara. I despite the whole Clara leads a double life and actively lies to Danny.

Hated Clara was in love with the Doctor and freaked when he got old but there was Madame Vastra which was bonus points.

Not sure if I like the current Doctor. I mean I think I like Peter what his name's acting but the weird manic energy mixed with anger and attempts at detachment don't work for me. Or maybe because I'm sick.

I want...a companion that is not from current times and in her 20s and from Earth. Alien would be good. And more stuff going on in space. These episodes based around the school are way less interesting than the UNIT episodes.

I just watched Vincent and the Doctor. That was really lovely. If I weren't in tech I could probably finish S5 before it disappears from Hulu and Netflix, but I don't know that I can make that happen.

I fast forwarded through parts but watched most. Missy wasn't as annoying as I thought and still fucked up and playing the Doctor and him still wanting Gallifrey to be there. Poor Danny.

Although I didn't watch the Christmas episode.

I'm finally getting the hang if this regeneration. I still don't like Clara and I can't give a good reason other than she annoys me

I'm finally catching up on the past few seasons of Doctor Who. I miss Amy and Rory. I don't like Clara. I despite the whole Clara leads a double life and actively lies to Danny.

Sadness! Season 8 is my favourite season of the new era - possibly my favourite ever - and Clara is the main reason for that. She went from being one of the most generic characters ever, in Season 7, to perhaps the best-drawn. (Not that I'm wanting to change your own mind, but I do like talking about it.) Also probably the first time ever that they've had an arc actually driven by character flaws (contrast Amy's and Rory's flaws being used more or less exclusively for laughs). I've heard it said that under Russell T. Davies, the focus was the Doctor's effect on the world, while under Steven Moffat, it was the world's effect on the Doctor. With the Twelfth Doctor, it's become the Doctor's effect on his companion, which I think works much better.

Her best scenes, for me:

• Deep Breath: going one-on-one with the robot, after the Doctor's just abandoned her. She does this often - takes the lead against the villain and/or takes the Doctor's role. (The only other companion with that ability was Romana.) She does it in Robot of Sherwood, Flatline, Death in Heaven (with similar occasions in Listen and Dark Water). Also Cold War and Nightmare in Silver, last season. But in this one she does it while clearly terrified.

• Kill the Moon: tearing strips off the Doctor at the end. She makes the emotion in that scene more complex than just anger. Also an important corrective to the Doctor's behaviour up to now. I feel it was necessary for the show to acknowledge that this regeneration, to date, wasn't right.

• Mummy on the Orient Express: again, right at the end, when she decides to keep travelling with him. Given the set-up, I was expecting the Doctor to engage in some partial redemption, and Clara accepts that despite his manner, his heart(s) are still in the right place. But then she lies to Danny (and the Doctor), and her decision to stay becomes so much more complex, more to do with her than with the Doctor, and something profoundly unhealthy.

• Dark Water: The scene by the volcano, of course. (Or I should say, the scene that starts by the volcano.) I can't think of any scene between Doctor and companion as gripping as this. Jenna Coleman and Peter Capaldi act the hell out of it.

And special mention to her line from Deep Breath: "Nothing is more important than my egomania!"

Though giving Clara a stronger personality, especially a seriously flawed one in some ways, means people can form stronger opinions of her. (I did find her quite unlikable in Forest of the Night. getting addicted to the thrill of adventure is one thing, neglecting the kids she's responsible for is quite another. Not to mention that when she does focus on them again, it's to decide they'd be better off dead than surviving as orphans. The hell?)

Hated Clara was in love with the Doctor and freaked when he got old but there was Madame Vastra which was bonus points.

Oh, that points to another thing I very much like about the Twelfth Doctor, namely the decision to get away from the romantic tension in the Tardis. (I don't actually mind him having a love life, but it never serves the companions well to be caught up in it.) But I'm not sure that she really was in love with Eleven. That was Vastra's accusation, but Clara offered a forceful rebuttal (of which Vastra apparently approved), and then there's this exchange towards the end:

The Doctor: "I’m the Doctor. I’ve lived for over two thousand years. And not all of them were good. I’ve made many mistakes, and it’s about time that I did something about that. ...Clara, I’m not your boyfriend."
Clara: "I never thought you were."
The Doctor: "I never said it was your mistake."

I think Clara having such trouble accepting his regeneration was kind of ham-fisted. It put her out of character so they could try and steer the audience's reaction to accept Capaldi as the Doctor.