# B Clocking Cosmic Rays. . .

1. Dec 20, 2016

### Dr Wu

I understand that cosmic rays can (and do) propagate through space extremely rapidly. But just how rapid, I wonder? (For reasons I know not, and despite having googled questions like "Speed of cosmic rays" and "How fast do cosmic rays travel?", I keep drawing a blank. I gather that subatomic energies and their measurements - like 'Gev' etc - are fundamental to the nature of cosmic rays; but I'd still like to know how fast the little blighters go. . . that is in km/s, for e.g.) Even an educated guess would be helpful. Thanks in advance.

2. Dec 20, 2016

### Orodruin

Staff Emeritus
Relative to what? Most cosmic rays consist of massive particles and therefore do not have a definite velocity, it depends on what you want to know the velocity relative to.

If you want to know the velocity in the Earth's rest frame, it is essentially at light speed. As a rule of thumb, if the energy of a particle is much larger than its mass, a particle will travel at almost the speed of light. For such particles, the speed is approximately
$v = \left(1 - \frac{m^2c^4}{2E^2}\right) c.$

3. Dec 21, 2016

### Dr Wu

Relative, say. . . to the distance between Alpha C and Sol. . . yes? no?

4. Dec 21, 2016

### Orodruin

Staff Emeritus
That is not an object with a velocity.

5. Dec 21, 2016

### Dr Wu

I feel as if we are still talking past one another. I'll need to rephase the original question - but first, a clarification: I understand that nothing material in the universe has a "definite velocity", as you put it. This includes small objects like human beings and racing cars to large objects like galaxies. It also must include cosmic rays, which despite their name are particles, nonetheless: not massless electromagnetic waves. We are in agreement here, I would hazard. Therefore I was perplexed by your citing that because they (cosmic rays) are massive particles, that "therefore" their velocities are indeterminate. A blunt response to that statement would be to say: well, so what? You then point out that highly energetic particles propagate through space at almost light-speed. . . which is gratifying to know. Unfortunately - and I'm sorry if I seem to be sympathising with your tagline on Groucho Marx here - but I'm afraid I don't have the mathematical nous to solve the formula you have posited (in mitigation I did peg this post at the 'basic' level). It's okay, though: a friend of mine will be able to unravel that formula for me.

The reason why I posed the question in the first place was in response to reading about the so-called 'Oh-My-God' particle. The stats about that left me wondering what the characteristics might be for the more common-or-garden variety of cosmic particles, not high-flyers like OMGP. These are far more numerous and since a number of them sleet through my being every few seconds, I feel duty bound to find out more about them. . . which includes their velocity (approximations notwithstanding).

Thanks for getting back to me.

6. Dec 21, 2016

### Drakkith

Staff Emeritus
Hmmm. By my calculations, an average cosmic ray particles travels at around 250,000 km/s, while the OMG particle was traveling at more like 299,792.457999 km/s. Probably with a few more 9's tacked on to the end of that last number (had to stop there, the online calculator kept telling me I was inputting the speed of light).

7. Dec 21, 2016

### phyzguy

I think you are still not understanding what orodruin is saying. We can only talk about the speed of some object relative to some other object. Relative to us here on Earth, the OMG particle was traveling at nearly the speed of light. However, relative to an observer traveling along with the particle, the OMG particle was motionless and it was the Earth that was moving.

8. Dec 21, 2016

### Chronos

From http://www.srl.caltech.edu/personnel/dick/cos_encyc.html: "Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light ... Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. Cosmic rays also include high energy electrons, positrons, and other subatomic particles." Hopefully this source is sufficiently authoritative and clarifies the scientific definition of cosmic rays as well as answers the 'how fast' question. The speed of light, given its invariance under the principles of GR and SR, is an irrelevancy - at least within the context of this discussion..

9. Dec 21, 2016

### Dr Wu

Thanks again. Yes, I'm extremely grateful at having all this explained to me. I did realise - too late at it turned out - that I should have inserted a frame of reference when discussing the notional movements, in this instance, of cosmic rays. I had hoped in retrospect that it would be implicit in my general argument. But if nothing else it serves to remind me not to take anything for granted. . .

Regards