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Q & A: How can you tell if you are in free-fall near a massive binary system

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Most recent answer: 10/31/2015
You are in a box free-falling towards a "rapidly" rotating, massive binary star system. (Assume both stars have equal mass.) You are in the plane of their mutual orbit and outside the radius of their orbit. You have all the modern scientific instruments aboard, but no windows to see outside the box. Can you detect that this is a binary system rather than a single star? If so, do you need to know more than Newtonian physics?
- Jim (age 67)

Hi Jim,

Nice question.  If you are in an enclosed box and being sucked into a massive star system you won't experience any net force since your acceleration is the same as the gravitational force. However you can feel, at least measure, a tidal force.   This is due to the gradient of the 1/r2 force and  is responsible  for the tides of the earth, both solar and lunar.    Poor stars that are captured by a black hole get ripped apart by these tidal forces.  

If the system is a pair of massive orbiting bodies it is possible in principle to determine that fact. As they rotate  the gradient would add a small oscillating component at twice their orbital frequency.

How to mesure tidal forces in your space ship?   Well, I could imagine some sort of apparatus consisting of springs with small masses attached to each end.  The separation of the masses would be proportional to the tidal force.


See   for some more information.


(published on 10/31/2015)

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