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"Speed of Light"
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<blockquote data-quote="Umbran" data-source="post: 6256362" data-attributes="member: 177"><p>Yeah, both of tomBitonti's descriptions are reasonable ways to look at the situation.</p><p></p><p>For #1, we can try this.</p><p></p><p>Einstein told us E=mc^2. Energy and mass are interchangeable.</p><p></p><p>Most things we call physical matter have what's called a "rest mass". This is the mass of objects in your everyday experience - the mass you'd measure them having when they look to you like they are standing still (you are in their "frame of rest" or "rest frame"). By Einstein, then, an object at rest still has some energy just from being - we call that the object's "rest energy".</p><p></p><p>But Einstein's E is not particular. It applies to <em>any and all</em> energy the object has. If it has kinetic energy from moving, say, we add that to the bucket of total energy, and recalculate the mass. So, the faster the thing moves, the more effective mass it has. </p><p></p><p>Now, photons are screwy. When we say they don't have mass, what we really mean is that they don't have a rest mass. However you may be moving, the photon looks like it is moving at <em>c</em> relative to you. You *cannot* be in the rest frame of the photon, because you cannot move at its speed. But, it still has well-defined energy, and E=mc^2 still applies, so we can still think of it as interacting with gravity using that mass-from-energy.</p></blockquote><p></p>
[QUOTE="Umbran, post: 6256362, member: 177"] Yeah, both of tomBitonti's descriptions are reasonable ways to look at the situation. For #1, we can try this. Einstein told us E=mc^2. Energy and mass are interchangeable. Most things we call physical matter have what's called a "rest mass". This is the mass of objects in your everyday experience - the mass you'd measure them having when they look to you like they are standing still (you are in their "frame of rest" or "rest frame"). By Einstein, then, an object at rest still has some energy just from being - we call that the object's "rest energy". But Einstein's E is not particular. It applies to [I]any and all[/I] energy the object has. If it has kinetic energy from moving, say, we add that to the bucket of total energy, and recalculate the mass. So, the faster the thing moves, the more effective mass it has. Now, photons are screwy. When we say they don't have mass, what we really mean is that they don't have a rest mass. However you may be moving, the photon looks like it is moving at [I]c[/I] relative to you. You *cannot* be in the rest frame of the photon, because you cannot move at its speed. But, it still has well-defined energy, and E=mc^2 still applies, so we can still think of it as interacting with gravity using that mass-from-energy. [/QUOTE]
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