Cannot get alt text working

  <div class="page-header">
    <h1><?php wp_title(''); ?></h1>
  </div>

  <?php 

    $args = array(
      'post_type'     => 'post',
      'category_name' => 'featured' 
    );

    $the_query = new WP_Query( $args );

  ?>

  <div id="carousel-example-generic" class="carousel slide" data-ride="carousel">
    <!-- Indicators -->
    <ol class="carousel-indicators">
      <?php if ( have_posts() ) : while ( $the_query->have_posts() ) : $the_query->the_post(); ?>

      <li data-target="#carousel-example-generic" data-slide-to="<?php echo $the_query->current_post; ?>" class="<?php if( $the_query->current_post == 0 ):; ?>active<?php endif; ?>"></li>

      <?php endwhile; endif; ?>
    </ol>

    <?php rewind_posts(); ?>

    <!-- Wrapper for slides -->
    <div class="carousel-inner" role="listbox">

      <?php if ( have_posts() ) : while ( $the_query->have_posts() ) : $the_query->the_post(); ?>


      <div class="item <?php if( $the_query->current_post == 0 ):; ?>active<?php endif; ?>">

        <?php 
          $thumbnail_id = get_post_thumbnail_id(); 
          $thumbnail_url = wp_get_attachment_image_src( $thumbnail_id, 'thumbnail-size', true );
          $thumbnail_meta = get_post_meta( $thumbnail_id, '_wp_attachment_image_alt', true );
        ?>

        <a href="<?php the_permalink(); ?>"><img src="<?php echo $thumbnail_url[0]; ?>" alt="<?php echo $thumbnail_meta; ?> featured image"></a>
        <div class="carousel-caption"><?php the_title(); ?></div>
      </div>

      <?php endwhile; endif; ?>

    </div>

    <!-- Controls -->
    <a class="left carousel-control" href="#carousel-example-generic" role="button" data-slide="prev">
      <span class="glyphicon glyphicon-chevron-left" aria-hidden="true"></span>
      <span class="sr-only">Previous</span>
    </a>
    <a class="right carousel-control" href="#carousel-example-generic" role="button" data-slide="next">
      <span class="glyphicon glyphicon-chevron-right" aria-hidden="true"></span>
      <span class="sr-only">Next</span>
    </a>
  </div>

  <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

  <article class="post">
    <h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
    <p><em>
      By <?php the_author(); ?> 
      on <?php echo the_time('l, F jS, Y'); ?> 
      in <?php the_category( ', ' ); ?>.
      <a href="<?php comments_link(); ?>"><?php comments_number(); ?></a>
    </em></p>

    <?php the_excerpt(); ?>

    <hr>

  </article>

<?php endwhile; else: ?>

  <div class="page-header">
    <h1>Oh no!</h1>
  </div>

  <p>No content is appearing for this page!</p>

<?php endif; ?>

</div>

<?php get_sidebar( 'blog' ); ?>