In a vacuum, how would a single atom such as hydrogen react when speed close to the speed of light?

The fact that the atom is in a vacuum makes no difference. In fact, all atoms are in local vacuums; air is made of molecules as well, remember, and in the space between molecules there is nothing (sort of). The concept of vacuum versus atmosphere only makes sense on a macroscopic scale. So, the atom will become infinitely massive as it is accelerated and so cannot be accelerated to the speed of light, although it can come arbitrarily close, because to do so would require that it be imbued with an infinite amount of energy.

It will behave in the same way as a system of particles like a spaceship. A spaceship reacts to relativistic speeds in the way it does because each particle comprising the spaceship reacts in that manner. It will get heavier, appear to shorten along the plane of motion, and it will appear to have lengthened time. Because of the time dilation it will experience, the atom will also appear to be much more stable since the probabilities that dictate its behavior are dependent on time. (For an example, see Atmospheric Muons)

Actually space is full of high-speed particles. One cause of this are supernovae.

I really should have studied more in science class.

@mattbrowne, yes, hence the (sort of). Actually, I suppose that in a vacuum without Higgs bosons, the atom would have no mass no matter its speed, and there would be nothing preventing it from reaching the speed of light. But I really know far too little about the subject to say for sure.

It would not “react”—it would not “sense” that it was not at rest, because velocity means something only relative to some other object. If (contravening “vacuum”) there were another object relative to which velocity could be measured, this conscious hydrogen atom would judge that it was the other object that was undergoing the relativistic effects (increased mass, foreshortening, and so on).

It would not “react”—it would not “sense” that it was not at rest, because velocity means something only relative to some other object. If (contravening “vacuum”) there were another object relative to which velocity could be measured, this conscious hydrogen atom would judge that it was at rest and that the other object was the one undergoing the relativistic effects (increased mass, foreshortening, and so on).

@mattbrowne : that was an ingenious speculation about its being effectively massless, and therefore capable of reaching “c”, because of the absence of Higgs bosons! Pretty sly—i’ll have to think about that.

@LexWordsmith; that was me :)

Let’s hope we’ll ‘see’ the Higgs at LHC. Repairs should almost be complete.

@Jayne : oops, sorry! i grovel, really—no intention to disrespect.

Please say that you forgive me—i’ll do my best to stay more alert.

I will grant you my forgiveness, this one time- in return for chocolate, of course ;)

rats—currently i’m fresh out of real chocolate, but i could give you some virtual chocolate for your virtual forgiveness.<grin>

good luck with your studies. chocolate contains caffeine, so it’s an excellent pick-me-up for late-night studying.<double grin>