# Would close to light speed travel be possible if all atoms in the system were instantly given “forward motion”?

I have heard people say that based on our current understanding and ideas for possible/current technology no person would ever be able to even travel at a solid (70-80%) the speed of light because getting up to that speed fast would kill the person inside (pizza) because first, the spaceship will accelerate into the person, then the atoms on that persons' back will be accelerated towards the ones in front and so on... In the end, the person is flat and dead.

So I was wondering if we were able to give each and every atom within the system that is the spaceship the same forward motion as is being applied by the rocket's thrust (that includes all atoms making up the oxygen molecules ect.), would that then solve this problem of very fast acceleration necessary for light speed?

• Flattening is not the problem with traveling at speeds near $c$. If you accelerate comfortably at 1g for a year you’ll be going close to the speed of light. The problem is the energy it would take. – G. Smith Jun 6 at 16:22
• If you are “ignoring the energy problem” then you are doing fantasy physics, which is off-topic here. – G. Smith Jun 6 at 17:07
• @G.Smith, I am asking a specific question related to one part of the problem – Outsider Jun 6 at 17:15
• @G.Smith, That would depend on whether "ignoring the energy problem" means ignoring the practical limits of our technology, or ignoring physical laws. If nobody's allowed to talk about experiments that can not be realized with existing technology, then you've pretty much outlawed any discussion of "barn and pole", or "clocks on a moving train", or any other discussion about relativistic motion of anything much bigger than an alpha particle. – Solomon Slow Jun 6 at 17:21
• I think the OP asked a very concrete question, I don't see any need for calling it out as off-topic and for the close votes. The OP is asking whether, if it were possible to accelerate all particles in the body at once, would one be able to avoid the problem the the OP is referring to. They answer is yes, and such an acceleration is possible, it is called gravity. – Peter Kravchuk Jul 1 at 0:34

I have heard people say that based on our current understanding and ideas for possible/current technology no person would ever be able to even travel at a solid (70-80%) the speed of light because getting up to that speed fast would kill the person inside (pizza) because first, the spaceship will accelerate into the person, then the atoms on that persons' back will be accelerated towards the ones in front and so on... In the end, the person is flat and dead.

This is not really a problem with traveling at a high speed; it's only a problem with travelling at high acceleration. It's true that if you were to accelerate a spaceship too fast, the occupants would be pressed towards the "rear" of the spaceship with a tremendous force (a so-called inertial force); but nothing prevents the spaceship from accelerating at a slower rate over a long period of time, and in that case the occupants of the spaceship will be perfectly fine. If you were somehow able to continuously accelerate the ship at $$g = 9.8 \text{ m/s}^2$$, the occupants of the ship would feel the same weight they do on Earth, and they'd reach 90% the speed of light in less than three years (according to their own measurements.)

Think of it this way: when you hit the gas in a car, you feel a force pressing you into your seat. But you don't feel that force pressing you back when the car is moving at constant speed, regardless of whether you're driving slowly down your driveway or speeding at 120 km/h down the highway. Our current understanding of physics is that this is true no matter what speed you travel at: you only feel an inertial force when you change speed, not when you're moving at constant speed.

• Ahh ok so this is less of a problem than I thought although it still is a problem (say we wanted to reach 90% light speed instantly)... so only energy is the main issue. – Outsider Jun 6 at 17:04

The concept of "instantly" is problematic in physics, but we can ask whether our bodies could tolerate an arbitrarily high acceleration (say a trillion times g) if all atoms were accelerated at exactly the same pace: you would indeed tolerate it perfectly well, in fact you would feel weightlessness! The problem is that achieving equal acceleration for all atoms in your body, e.g. with some sort of motor, is probably impossible. But there is one thing that does precisely that: gravity!

Imagine you were falling from outer space straight into a really massive planet without any atmosphere. You could be accelerating at arbitrarily high acceleration with respect to the planet (by making the planet arbitrarily massive) without feeling absolutely nothing, just weightlessness like the guys at the International Space Station! This is related to something called "equivalence principle".

Notice however that as you get closer to the planet, you will start experiencing tidal forces: there starts to be an appreciable difference in the acceleration experienced by your head vs the acceleration experience by your feet. This would become very uncomfortable and ultimately rip you apart. But as long as you stay far enough from the planet, you can always make the acceleration arbitrarily large, and the tidal forces arbitrarily small.