Does this thought experiment violate de Broglie hypothesis? A friend of mine, Nima Fathali, asked me a question for which I could not find a clear answer. De Broglie's hypothesis asserts that when the (de Broglie) wavelength is small in comparison to the size of the slits in, say, two-slit experiment, the wave behavior of the object is not detectable, and thus the configuration of the interference pattern is meaningless.
However, since the de Broglie's wavelength depends on the object's momentum, we can arbitrarily increase the de Broglie's wavelength of a macroscopic object by lowering its velocity sufficiently. Now, assume that we set too many similar marbles in motion, each having a mass of $0.1\space Kg$ and a diameter of $0.01\space m$, towards the slits of the size of $0.02\space m$. 
If the marbles move slowly enough at, say, $v=6.63×10^{-31}\space m/s$, the de Broglie wavelength implies:
$$\lambda=\frac{h}{mv}=\frac{6.63×10^{-34}}{0.1×6.63×10^{-31}}=0.01\space m\space.$$
This wavelength is of the order of the marbles' diameter as well as the size of the slits. Therefore, is it correct to anticipate an interference pattern on the film for the marbles as they pass through the slits?! If so, do you think it is rational?!  
 A: If the marble is moving at $v \approx 10^{-31}\text{ m/s}$, it would take $10^{31}\text{ s} \approx 10^{23}\text{ years}$ for the marble to move a single meter. This is orders of magnitudes more than the current age of the universe. The number is as absurd as the result we're talking about. 
In principle, quantum effects not only take place at the very small and the very low energies, but also for the very long time spans. The time spans are so mind-numbingly long that they cannot be used in any practical observation.
There is also another problem: the marble needs to be kept completely isolated during the entire duration, or else its so-called "wavefunction" would be "collapsed" before the experiment is finished. I'm not completely sure, but I think blackbody radiation from the marble interacting with the environment would cause decoherence early on. This would effectively nullify any quantum effects that could happen at long time scales, rendering the thought experiment impossible. 
A: Interference has been observed for molecules as heavy as nearly 1300 proton masses. There is no fundamental reason to assume an upper limit. See https://scitechdaily.com/quantum-interference-shown-experimentally-in-larger-molecules/ 
A: Yes there will be an interference pattern as stated above (eventually). However, you are essentially thinking like Bohr, i.e. whether a quantum system appears as a particle or as a wave depends on the type of experiment you subject it to.
Now Wheeler thought that the quantum system was neither a particle or wave (this is the modern QFT thought, i.e.quantum systems are quantum fields), it just takes on either 'coat' after observation. This of course kicked off the whole 'something exists only when observed' idea that still is around. 
What is more probable (Adami) is the quantum system stays neither and appear the way you choose you want to see them, when you interrogate a quantum state with classical devices.  
A: Apart from the enormous time taken (as pointed out by other answers)and also the (probable)lack of current experimental apparatus to achieve such slow speeds,the necessary condition for interference being coherence,preparing a human body with all its particle in a coherent state and to maintain it for such a long time may almost, would be impossible.
Also,One wont be able to move the macroscopic object at such low speed without ripping it apart because it is a system of particles,and even the slowest movement will contribute greatly to the motion of the individual components of the body,and increase the velocity of the internal particles comprising the body move fast and destroying the wave nature.The body will be moving at low speed,but the particles inside wont be.The only possible option left is to separate out the particles and making them move with such low speeds,which is clearly destructive...
