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## The Question

(Submitted May 16, 1997)

A Space traveler is under constant acceleration, ignore for a moment the propulsion unit or fuel requirements. As the traveler approaches the Speed Of light, "The way I understand it " the hull of the ship would be undergoing tremendous stresses, High Energy Photons would be bombarding the ship at an ever increasing energy level. While behind the ship, light would be shifting into the infrared.

Light from the approaching Red Giant would be shifted into the Gamma Ray region.

Time would be coming to a standstill. The Ship would be attaining great mass.

That the Speed of Light the occupant would be seared by "All Radiation " "All Energy " "All Mass " and be at "All Time ". Is this basically the correct idea?

If it is, the other question is: Speed is linked to Time and Mass/Gravity, so it leads me to think there is some sort of "agent " such as "air molecules in a planetary atmosphere " that is getting compressed or otherwise affected. As though a fabric is being stretched.

I understand that we live on the edge/surface of an expanding "Soap Bubble ", the Past being at the nonexistent center while the future being what we are expanding into. Have I got it right so far?

## The Answer

I asked my colleague Demos Kazanas to address your question. His answer follows:

The observer which keeps constant acceleration will not feel any stresses other than the normal when he approaches the speed of light. stresses are caused by acceleration, not velocity and since the acceleration is constant there will be no more stresses when the velocity is large or small.

It is true there will be a blueshift of all forward coming photons and if one wants to compute that, yes there will be stresses from their interaction with the space ship. Other than that, in as much as the observer does not interact with the outside he should feel as comfortable as when he sits in his office.

The power requirements of course become humongous because of the amount of fuel necessary to approach the speed of light arbitrarily close. One can compute at which point the mass of the fuel required to keep the ship accelerating becomes so high that its self gravity would cause it to collapse as a black hole. Of course the observer (assumed human) would have been crushed long before due to the intense gravitational field of his own ship.

Yes, your picture of the expanding universe is (we believe) basically correct if we assume that the universe is indeed closed. if it is open then the soap bubble analogy does not hold and one would have to consider a different kind of hypersurface which extends to infinity.

I hope that this does answer some of your questions.

Jim Lochner
for Ask an Astrophysicist

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