Problem on loading a new template file using jQuery. Call to undefined function

Hello, I am trying to reload the fresh posts using jQuery. As far as I know, I can’t reload the contents of a div inside the page so I reload a file into that div. (Is that correct?)

The problem is that my loaded file gives me a **Fatal error: Call to undefined function wp_query()**

How can I implement functions to a newly created file inside the theme directory to work properly?

***This is my jQuery***

<script language="JavaScript">
    $(function() {
        var SANAjax = function(){
            $('#reservationdetails').empty().addClass('loading')
              .load('wp-content/themes/theme/reloadhomeposts.php', function() {
                 $(this).removeClass('loading')
              });
        }
        setInterval(SANAjax, 15000 );
    });
    </script>
    <div id="reservationdetails"></div>

***And this is what i have in reloadhomeposts.php (I have deleted the content though)***

<?php $recent = new WP_Query("cat=3,4,5&showposts=10"); while($recent->have_posts()) : $recent->the_post();?>
    .
    .
    .
    <?php endwhile; ?>