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Dale

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I would say yes. Anything without mass must move at c at all times. It cannot accelerate.is mass required for accelerations to happen? Why or why not?

- #3

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I don't think that this is a valid argumentation. Constant speed doesn't mean that there is no acceleration.Anything without mass must move at c at all times. It cannot accelerate.

- #4

sophiecentaur

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It does. If the 'entity' only exists at velocity c then when would it be accelerating? It would emerge from whatever reaction/ interaction generated it at c. Slower than c and it would not be in existence.I don't think that this is a valid argumentation. Constant speed doesn't mean that there is no acceleration.

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Just a little hint: speed is constant for v·a=0.It does.

- #6

sophiecentaur

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But for 'rectilinear propagation'?Just a little hint: speed is constant for v·a=0.

Though I must say I had ignored motion in a circle.

- #7

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Of course linear acceleration is not possible with constant speed. However, there is Shapiro delay.But for 'rectilinear propagation'?

- #8

sophiecentaur

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But the speed, measured at any point would still be c (??). Isn't that the basis of GR?Of course linear acceleration is not possible with constant speed. However, there is Shapiro delay.

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Yes, the locally measured speed of massless objects is always c.But the speed, measured at any point would still be c (??).

- #11

sophiecentaur

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So where does this take the thread?Yes, the locally measured speed of massless objects is always c.

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This seems like a nonsense statement. Suppose the point is at rest?Yes, the locally measured speed of massless objects is always c.

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Which point are you talking about?This seems like a nonsense statement. Suppose the point is at rest?

- #14

sophiecentaur

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Under what circumstances could the object be at rest? What would be a 'stationary' photon be like?This seems like a nonsense statement. Suppose the point is at rest?

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If there is no mass, then all that exists there is a geometrical point. That is point of which I am speaking.Which point are you talking about?

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The adjective "massless" doesn't make much sense with a geometrical point. I am talking about objects that are subject to E²/c² = m²c² + p². Such objects can only be at rest with m>0 and they always move with c (locally measured) with m=0.If there is no mass, then all that exists there is a geometrical point.

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The adjective "massless" applied to a geometrical point makes perfect sense. What mass do you think Euclid ascribed to a point? No, the thing that is a stretch is the idea of a mass point. The latter is a useful fiction, but it really does not make rigorous sense.The adjective "massless" doesn't make much sense with a geometrical point.

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No, it doesn't because geometrical points never have mass. In theory you can have a point size object with mass located at a geometrical point but not a geometrical point with mass. Therefore "massless geometrical point" is a tautology.The adjective "massless" applied to a geometrical point makes perfect sense.

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By all means, have it your way. This thread seems pretty pointless anyway.Therefore "massless geometrical point" is a tautology.

- #20

sophiecentaur

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Also, a photon is not a point particle. It has no defined extent so it is pretty meaningless to assume you could use a stopwatch and push the button when it goes past. Using a very mechanical model is just not appropriate.The adjective "massless" doesn't make much sense with a geometrical point.

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rcgldr

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- #22

sophiecentaur

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No speed limit there! You are talking Virtual. You can let your eye travel at many times c if you scan from one galaxy to the next on a dark night.

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Euclid was a mathematician. His discussions dealt solely with imaginary objects.What mass do you think Euclid ascribed to a point?

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1. Material mass ##m_o## is not necessary to observe acceleration. Example. In a region of space ##\varepsilon## and ##\mu## vary from one point to another, so that there is a path where the speed of light varies. In kinematic terms you can express the acceleration of light when it crosses the region. It's acceleration without ##m_o##.

2. Light does not have ##m_o##, we know that. But do it have another type of mass? In case of having it, in the previous example there is acceleration and mass.

3. If you are interested in the fundamentals of physics and not in practical situations, in Newtonian physics and in Einstein's postulates validity is given to the conclusion obtained by Galileo, that is, in a vacuum the gravitational acceleration is independent of mass. So, an infinitesimal mass experiences the same acceleration as the finite masses. An infinitesimal mass is the limit of a mass that tends to zero. If that may correspond to your idea of "without mass", then Newton and Einstein, from the foundations of physics, are answering affirmatively to your question. The acceleration without mass is conceivable and is consistent with both theories, Newtonian and Einsteinian.

4. Is it also consistent with quantum theory? Maybe someone in the specialty can help us understand a little.

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