(John Baez) wrote

So, if you hit a hydrogen atom, it
acts like it has a kinetic energy roughly equal to 100 billion
times its usual mass-energy.

I'm not saying this makes it *impossible*, but you'd have to really
want to get there fast to take the trouble to shield against such
energetic hydrogen atoms - much less the slight chance of running into
something bigger.



Assuming cruising speed, shouldn't hitting dangerously energetic
hydrogen atoms be as rare an event in the traveller's inertial frame
as it is in ours?

I do not think that the velocity disttribution of intergalactic
hydrogen atoms varies from one inertial frame (say the space
traveller's) to the other (ours). Does it?

Regards

Italo Vecchi

In message u, Leonard
writes
In a discussion of getting to Andromeda in 10 minutes, no one (Julian
Moore) wrote:
...And assuming you accelerate for 5 minutes (with the intention of
decelerating at the half-way point) you might well be fried in the
Unruh radiation, but which gets you first on the cosmic Cannonball run
- that or the blue-shifted CMBR I'm not sure.

I'm not sure the Unruh temperature would be all that high if one
accelerates to near the
speed of light in 5 minutes.

It seems that by this rough calculation the unruh temperature would be
16 orders of magnitude less than room temperature.

(please correct me if I am mistaken) the Unruh temperature associated
with the acceleration
does not seem to be a problem.

[Moderator's note: This is quite right. The Unruh temperature
associated with an acceleration in which an object becomes
relativistic in five minutes is about 10^(-13) K. -TB]

Physics an interest not a speciality (apart from basic BSc), so I'm not
going to be correcting anyone here in the near future but now I know
something I didn't know before and didn't have a quick reference to
(Unruh calculation). Thanks

Julian

"Italo Vecchi" wrote in message

So, if you hit a hydrogen atom, it
acts like it has a kinetic energy roughly equal to 100 billion
times its usual mass-energy.

I'm not saying this makes it *impossible*, but you'd have to
really want to get there fast to take the trouble to shield
against such energetic hydrogen atoms - much less the slight
chance of running into something bigger.



Assuming cruising speed, shouldn't hitting dangerously energetic
hydrogen atoms be as rare an event in the traveller's inertial frame
as it is in ours?

I do not think that the velocity disttribution of intergalactic
hydrogen atoms varies from one inertial frame (say the space
traveller's) to the other (ours). Does it?

Yes. In intergalactic space the co-moving frame (in which the
CMB is homogenous and isotropic) is the preferred
frame.

Cheers,
Michael C Price
----------------------------------------
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http://www.hedweb.com/manworld.htm