Scivillage.com Casual Discussion Science Forum - Misconception: The real reason we can't travel FTL + Einstein against black holes

Mass misconception: The real reason we can’t outpace light speed
https://bigthink.com/hard-science/light-...istic-mass

KEY POINTS: Einstein’s theory of relativity sets a cosmic speed limit: nothing can travel faster than the speed of light, posing challenges for space exploration. A widespread but incorrect explanation suggests that objects gain mass as they approach light speed, making further acceleration impossible. In reality, an object’s mass remains constant, while its inertia changes with speed, ultimately preventing travel at or beyond the speed of light... (MORE - details)

Einstein against black holes
https://iai.tv/articles/what-einstein-go..._auid=2020

INTRO: The event horizon of a black hole marks the point of no return. Once someone has passed it, even though spacetime in its vicinity is quite regular, they can no longer escape. Einstein and a roster of his leading contemporaries in mathematics and physics, however, in direct contradiction with our modern understanding, regarded spacetime there as breaking down, as ‘singular’, where terms in the equations disappear to zero or blow up to infinity. As University of Pittsburgh historian and philosopher of science John Norton explains, Einstein's concern about these terms now appears to us as a puzzling novice error. But a closer look at Einstein and the mathematical methods used reveals a different story... (MORE - details)

(Aug 31, 2025 05:14 PM)C C Wrote: [ -> ]Mass misconception: The real reason we can’t outpace light speed
https://bigthink.com/hard-science/light-...istic-mass

KEY POINTS: Einstein’s theory of relativity sets a cosmic speed limit: nothing can travel faster than the speed of light, posing challenges for space exploration. A widespread but incorrect explanation suggests that objects gain mass as they approach light speed, making further acceleration impossible. In reality, an object’s mass remains constant, while its inertia changes with speed, ultimately preventing travel at or beyond the speed of light... (MORE - details)

Where does this leave the equivalence principle?
If the active mass, that determines an object's gravity, and inertial mass, that determines an object's resistance to acceleration, are not equivalent, how does this not violate the equivalence principle?

(Aug 31, 2025 10:11 PM)Syne Wrote: [ -> ]
(Aug 31, 2025 05:14 PM)C C Wrote: [ -> ]Mass misconception: The real reason we can’t outpace light speed
https://bigthink.com/hard-science/light-...istic-mass

KEY POINTS: Einstein’s theory of relativity sets a cosmic speed limit: nothing can travel faster than the speed of light, posing challenges for space exploration. A widespread but incorrect explanation suggests that objects gain mass as they approach light speed, making further acceleration impossible. In reality, an object’s mass remains constant, while its inertia changes with speed, ultimately preventing travel at or beyond the speed of light... (MORE - details)

Where does this leave the equivalence principle?
If the active mass, that determines an object's gravity, and inertial mass, that determines an object's resistance to acceleration, are not equivalent, how does this not violate the equivalence principle?

("Inertial mass is a measure of an object's resistance to acceleration when a force is applied.")

So one can view it as the observed effects in both scenarios that have to be equivalent. Just as in free fall, it is not really lack of a gravitational field that is causing objects to "float", so likewise it is not the presence of a massive gravitational field that causes objects to be crushed at _X_ fantastic rate of acceleration. (Spotlight on equivalence principle)

Clocks slow down when close to super-massive bodies, and accordingly a rocket has to accelerate at whatever _X_ rate for a clock inside it to achieve the same diminished pace. Just as a human passenger ages slower (and a dropped wine glass drops slower) in a rocket that attains relativistic speeds, compared to similar events back on Earth.

But the latter effects in the rocket are not due to gravity (mass growing), but instead the result of macroscopic objects (like the rocket) catching up to how fast photons and other boson particles mediate the matter interactions that make change possible. Whereas near a massive body, it is the curved space creating longer paths for photons/bosons to travel that slows down time or events/changes.

So the mass of the rocket does not have to increase to simulate the effects of being near a neutron star, black hole, etc. The rocket's mass remains constant, but its inertia (resistance to changes in motion) increases as its speed increases, eventually reaching a maximum that prevents further increases in rate of movement.

So equivalent effect, between felt gravity near a body and felt gravity within an accelerating frame, but no change in active mass observed from any relative frame.