- #1

- 176

- 6

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter Sundown444
- Start date

- #1

- 176

- 6

- #2

- 31,974

- 8,887

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

- 2,167

- 499

Anything without mass must move at c at all times. It cannot accelerate.

I don't think that this is a valid argumentation. Constant speed doesn't mean that there is no acceleration.

- #4

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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.

- #5

- 2,167

- 499

It does.

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

- #6

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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

- 2,167

- 499

But for 'rectilinear propagation'?

Of course linear acceleration is not possible with constant speed. However, there is Shapiro delay.

- #8

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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.

- #9

- 2,409

- 709

- #10

- 2,167

- 499

But the speed, measured at any point would still be c (??).

Yes, the locally measured speed of massless objects is always c.

- #11

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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

- #12

- 2,409

- 709

Yes, the locally measured speed of massless objects is always c.

This seems like a nonsense statement. Suppose the point is at rest?

- #13

- 2,167

- 499

This seems like a nonsense statement. Suppose the point is at rest?

Which point are you talking about?

- #14

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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?

- #15

- 2,409

- 709

Which point are you talking about?

If there is no mass, then all that exists there is a geometrical point. That is point of which I am speaking.

- #16

- 2,167

- 499

If there is no mass, then all that exists there is a geometrical point.

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.

- #17

- 2,409

- 709

The adjective "massless" doesn't make much sense with a geometrical point.

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.

- #18

- 2,167

- 499

The adjective "massless" applied to a geometrical point makes perfect sense.

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.

- #19

- 2,409

- 709

Therefore "massless geometrical point" is a tautology.

By all means, have it your way. This thread seems pretty pointless anyway.

- #20

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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.

- #21

rcgldr

Homework Helper

- 8,770

- 569

- #22

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 26,568

- 5,588

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.

- #23

- 827

- 197

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

- #24

- 2,409

- 709

- #25

- 93

- 16

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.

Share:

- Replies
- 2

- Views
- 297