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Sundown444
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So, we know that force equals mass times acceleration. A force is needed to cause an acceleration. I am wondering though, is mass required for accelerations to happen? Why or why not?
I would say yes. Anything without mass must move at c at all times. It cannot accelerate.Sundown444 said:is mass required for accelerations to happen? Why or why not?
Dale said:Anything without mass must move at c at all times. It cannot accelerate.
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.DrStupid said:I don't think that this is a valid argumentation. Constant speed doesn't mean that there is no acceleration.
sophiecentaur said:It does.
But for 'rectilinear propagation'?DrStupid said:Just a little hint: speed is constant for v·a=0.
sophiecentaur said:But for 'rectilinear propagation'?
But the speed, measured at any point would still be c (??). Isn't that the basis of GR?DrStupid said:Of course linear acceleration is not possible with constant speed. However, there is Shapiro delay.
sophiecentaur said:But the speed, measured at any point would still be c (??).
So where does this take the thread?DrStupid said:Yes, the locally measured speed of massless objects is always c.
DrStupid said:Yes, the locally measured speed of massless objects is always c.
Dr.D said:This seems like a nonsense statement. Suppose the point is at rest?
Under what circumstances could the object be at rest? What would be a 'stationary' photon be like?Dr.D said:This seems like a nonsense statement. Suppose the point is at rest?
DrStupid said:Which point are you talking about?
Dr.D said:If there is no mass, then all that exists there is a geometrical point.
DrStupid said:The adjective "massless" doesn't make much sense with a geometrical point.
Dr.D said:The adjective "massless" applied to a geometrical point makes perfect sense.
DrStupid said:Therefore "massless geometrical point" is a tautology.
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.DrStupid said:The adjective "massless" doesn't make much sense with a geometrical point.
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.rcgldr said:What about something without a direct connection to mass, such as a shadow sweeping / accelerating across an observer's view?
Euclid was a mathematician. His discussions dealt solely with imaginary objects.Dr.D said:What mass do you think Euclid ascribed to a point?
slow said:Example. In a region of space ##\varepsilon## and ##\mu## vary from one point to another
slow said:Light does not have ##m_o##, we know that. [...]
slow said:[...] in Newtonian physics [...]
slow said:Is it also consistent with quantum theory?
Whenever the word "real" appears in a Physics Forums post, an alarm sounds in Mentor Central (which, if you're wondering, is located in a secure facility in the basement of Avengers's Mansion).Dr.D said:David, in what way do you think that points, lines, planes, etc. are imaginary objects? As I see it, they are very much real, just non-physical.
They're perfect. The only place perfect things exist is in our imagination.Dr.D said:...in what way do you think that points, lines, planes, etc. are imaginary objects?
David Lewis said:They're perfect. The only place perfect things exist is in our imagination.
I could say the same about the number 7.563 . It's a totally virtual idea and is not 'real' (except in the convention used for complex numbers) What about a nice irrational number (√2) or even a transcendental number (π)? Could one of them 'accelerate'?Dr.D said:No, I don't think so. If you talk about a line, a point, a plane, etc. and I understand exactly what you mean, then are they (1) in your imagination, (2) my imagination, (3) or are they real?
sophiecentaur said:I could say the same about the number 7.563 . It's a totally virtual idea and is not 'real' (except in the convention used for complex numbers) What about a nice irrational number (√2) or even a transcendental number (π)? Could one of them 'accelerate'?
Nugatory said:Whenever the word "real" appears in a Physics Forums post
Not the same class, agreed. I have just reached a small problem for my initial idea. If I take a geometrical circle (not made of anything so no mass). Then I rotate the circle, a point on the circle is now accelerating towards to centre. No force was involved and the point has no mass but there is still acceleration.Dr.D said:I really have no idea how root(2) could accelerate, but I don't think it is in the same class of objects as a point.
sophiecentaur said:Not the same class, agreed. I have just reached a small problem for my initial idea. If I take a geometrical circle (not made of anything so no mass). Then I rotate the circle, a point on the circle is now accelerating towards to centre. No force was involved and the point has no mass but there is still acceleration.
But whether this is relevant to the spirit of the OP,I am not sure.
Dr.D said:Hard to say whether this is relevant to the OP or not