<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Why is it not a valid inertial frame of reference ? <br /> Posted by UncertainH</DIV></p><p><em>Special principle of relativity</em>: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the <em>same</em> laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K.</p><p> – <cite>Albert Einstein: <em>The foundation of the general theory of relativity.</em></cite></p><p>If I may rephrase that, K represents an inertial frame of reference <strong>if</strong> it is chosen so that the physical laws hold good in relation to it. That being the case, then the same laws hold good in relation to other inertial frames of reference.</p><p>But if light is K, the physical laws do <strong>not</strong> hold good in relation to it, as no time would pass for a photon. So, you answered your own question as to why it is not a valid inertial frame of reference. As you put it, "If the photon experiences no time then from its perspective it has no energy or motion and in fact without time it wouldn't have any laws of physics whatsoever." In valid frames of reference, a second always lasts a second.</p><p>In the real world, nothing will ever be able look at things from the point of view of a photon. It, or anything moving at c, <em>has no point of view</em>. It is a null viewpoint</p><p>Or put another way...</p><p>Consider the point of view of a photon travelling at c. No time passes. Nothing ever happens. The universe doesn't exist. There's your frame of reference, but what use is it if the universe doesn't exist in that frame of reference?<br /> </p><p>Consider the point of view of a spaceship that manages to impossibly accelerate to c. The universe ceases to exist. Nothing more can ever happen from the point of view of that spaceship, whilst it is at c. Why? Because it will, from a valid (if you allow the impossible) frame of reference, continue to travel at c forever, or until some <em>(impossible?)</em> outside force causes it to decelerate. As no time is passing on the spaceship, it cannot change its own circumstances in any way.</p><p>So what if it meets something that impossibly slows it down? From its own (now impossibly valid again!) less than c frame, the universe springs into existence again (unless the ship hits something hard and is vapourised)! No time has passed since the ship achieved c. Whilst it was at c, it had no point of view, it had no frame of reference. For the people on that impossible spaceship, the universe they ceased to experience would be termed a <em>discontinuity</em>. From their valid (less than c) frames of reference, you might think the discontinuity represents instantaneous travel, but that is only if you invoke impossible conditions, which render the frame of reference invalid. </p><p> </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>