"Tom Roberts" wrote in message
.. .
| wrote:
| A common question is posed when trying to find solutions to the
problem
| of near light speed travel (assuming we had knowledge to produce
such
| speeds): "How does your ship avoid getting pulverized by tiny grains
in
| space that would impact your ship with enormous energy?"
|
| There are two basic approaches:
| A. only move at such speeds in regions where the chance of hitting
| such "grains" is negligible.
| B. construct shields on the ship that can withstand these impacts;
| it's not clear to me that this can be done.
|
| Of course the current, more pratical approach is:
| C. Don't move at speeds anywhere close to c (relative to such
grains).

Egads! Roberts knows Galilean relativity!

Pity he believes and teaches
[quote]
we establish by definition that the "time" required by light to travel
from A to B equals the "time" it requires to travel from B to A.
[end quote]
Ref: http://www.fourmilab.ch/etexts/einstein/specrel/www/


|
|
| From what I understand as your ship approaches the speed of idlight,
its
| mass would also increase to near infinite. These tiny objects would
| not have enormous amounts of energy; the ship would, so the ship
would
| be the one doing the pulverizing.
|
| You must be careful to remember in which frame you are discussing
| things. In the earth frame the grains move with speed c and the ship
| (by your hypothesis) is moving with speed approaching c. Yes, the
| collision of a small grain and a ship could well be catastrophic to
both.
|
|
| There are everyday examples of this: Tornados can accelerate common
hay
| or dried grass to speeds of 300 mph, increasing their mass,
|
| The relativistic "mass increase" is negligible here -- on the order of
1
| part per million. And this isn't really an increase in mass, it is
| rather an increase in energy of ~1 ppm more than Newton's 0.5*m*v^2.

I'd edufart Roberts, but he's a stubborn fool.
|
|
| and drive
| them right through trees and telephone poles.
|
| Sure. But relativity is not needed to understand this.


Of course it is.
http://webexhibits.org/calendars/year-text-Galileo.html

| Straws can be
| quite strong along their length -- in high school we could routinely
| drive a paper drinking straw through a history book. But only ~1 in
100
| attempts actually succeeded (lunchtime was boring (:-)). In a tornado,
| it's clear that only an incredibly small fraction of the hay or grass
| caught up by the storm is actually driven through trees or poles. The
| impact must be EXACTLY along the straw's length....
|
|
| If you accelerated a
| tree or telephone pole to 300 mph, and shot it through a field of
| hay, it would mow down the hay, not split into a thousand pieces.
|
| Sure. But to accelerate a tree to such a speed requires ENORMOUSLY
more
| energy thatn a piece of straw, or even a thousand pieces (which in
| aggregate still weigh considerably less than the tree).
|
| This is more a function of the total energy involved than any details
of
| hay, grass, straw, or tree. Note that in practice hurling a tree at a
| field of hay would not mow any hay at all (it would bend, and some
| ending up under the tree would be crushed, but little or none would be
| mown [i.e. cut]).

What if you burn the tree, use the heat to raise steam and harvest the
crop with a steam tractor?
I think the Victorians thought of that.


| So why do I continuously see this question posed as a problem of
near
| light speed travel? I understand larger objects that have mass more
| equal to the near-light speed ship (comet, planet, etc.) would be a
| danger, but not tiny particles.
|
| If your spaceship actually achieved ~0.9c relative to the "grains",
then
| each grain would impace the front of the ship with enormous energy. I
| doubt any material could withstand this for more than a handful of
such
| impacts, so you must select a region with very low density of such
| grains. At some higher speed, nuclear reactions between grain and ship
| would become likely....
|
| Tom Roberts

Yes, the grains would impace the front of the ship with enormous
energy.
E = 1/2mv^2 much. And m is? (I'm nt to surw about "imapace!)
Androcles