Let $P_0$, $P_1$, $P_2$ be three points on the circumference of a circle with radius $1$, where $P_1P_2 = t < 2$. For each $i \ge 3$, define $P_i$ to be the centre of the circumcircle of $\triangle P_{i-1} P_{i-2} P_{i-3}$.

(1)  Prove that the points $P_1, P_5, P_9, P_{13},\cdots$ are collinear.

(2)  Let $x$ be the distance from $P_1$ to $P_{1001}$, and let $y$ be the distance from $P_{1001}$ to $P_{2001}$. Determine all values of $t$ for which $\sqrt[500]{ \frac xy}$ is an integer.
### Part (1)

1. **Define the initial points and circumcenter:**
   - Let \( P_0, P_1, P_2 \) be points on the circumference of a circle with radius 1.
   - Given \( P_1P_2 = t < 2 \).
   - Define \( P_3 \) as the circumcenter of \( \triangle P_0P_1P_2 \).

2. **Properties of \( P_3 \):**
   - Since \( P_3 \) is the circumcenter, \( P_3 \) is equidistant from \( P_0, P_1, \) and \( P_2 \).
   - The triangle \( \triangle P_1P_2P_3 \) is isosceles with \( P_1P_3 = P_2P_3 \).

3. **Angles and perpendicular bisectors:**
   - Let \( \phi = \angle P_1P_2P_3 = \angle P_2P_1P_3 \).
   - \( P_3 \) and \( P_4 \) lie on the perpendicular bisector of \( P_1P_2 \), so \( P_1P_2 \perp P_3P_4 \).

4. **Isosceles triangles and angles:**
   - The triangle \( \triangle P_2P_3P_4 \) is isosceles with \( P_2P_4 = P_3P_4 \).
   - The angle \( \angle P_2P_4P_3 = 2 \angle P_2P_1P_3 = 2\phi \).
   - The base angles \( \angle P_2P_3P_4 = \angle P_3P_2P_4 = 90^\circ - \phi \).

5. **Similarity and rotation:**
   - The triangle \( \triangle P_3P_4P_5 \) is isosceles with base angles \( \angle P_3P_4P_5 = \angle P_4P_3P_5 = \phi \).
   - The triangles \( \triangle P_1P_2P_3 \sim \triangle P_3P_4P_5 \) are similar, and the base \( P_3P_4 \) is rotated \( 90^\circ \) with respect to the base \( P_1P_2 \).

6. **Homothety and concurrency:**
   - The triangles \( \triangle P_1P_2P_3 \sim \triangle P_5P_6P_7 \) are similar, and the base \( P_5P_6 \) is rotated \( 90^\circ \) with respect to the base \( P_3P_4 \).
   - The lines \( P_1P_5, P_2P_6, P_3P_7 \) connecting corresponding vertices concur at their homothety center \( H \).

7. **General concurrency:**
   - The lines \( P_{n}P_{n+4} \) concur at \( H \) for \( n > 0 \).
   - This implies that the points \( H, P_1, P_5, P_9, P_{13}, \ldots \) are collinear.

\(\blacksquare\)

### Part (2)

1. **Define distances \( x \) and \( y \):**
   - Let \( x \) be the distance from \( P_1 \) to \( P_{1001} \).
   - Let \( y \) be the distance from \( P_{1001} \) to \( P_{2001} \).

2. **Distance calculation:**
   - Since the points \( P_1, P_5, P_9, \ldots \) are collinear, the distance between consecutive points in this sequence is constant.
   - The distance between \( P_1 \) and \( P_{1001} \) is \( 250 \) times the distance between \( P_1 \) and \( P_5 \).
   - Similarly, the distance between \( P_{1001} \) and \( P_{2001} \) is \( 250 \) times the distance between \( P_1 \) and \( P_5 \).

3. **Ratio of distances:**
   - Therefore, \( x = 250 \times d \) and \( y = 250 \times d \), where \( d \) is the distance between \( P_1 \) and \( P_5 \).
   - The ratio \( \frac{x}{y} = \frac{250d}{250d} = 1 \).

4. **Integer condition:**
   - We need \( \sqrt[500]{\frac{x}{y}} \) to be an integer.
   - Since \( \frac{x}{y} = 1 \), \( \sqrt[500]{1} = 1 \), which is an integer.

5. **Conclusion:**
   - The value of \( t \) does not affect the ratio \( \frac{x}{y} \) being 1.
   - Therefore, all values of \( t \) satisfy the condition.

\(\blacksquare\)

The final answer is all values of \( \boxed{ t } \).