B Can there be any acceleration without mass?

1. Mar 11, 2018

Sundown444

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?

2. Mar 11, 2018

Staff: Mentor

I would say yes. Anything without mass must move at c at all times. It cannot accelerate.

3. Mar 11, 2018

DrStupid

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

4. Mar 11, 2018

sophiecentaur

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.

5. Mar 11, 2018

DrStupid

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

6. Mar 11, 2018

sophiecentaur

But for 'rectilinear propagation'?
Though I must say I had ignored motion in a circle.

7. Mar 11, 2018

DrStupid

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

8. Mar 11, 2018

sophiecentaur

But the speed, measured at any point would still be c (??). Isn't that the basis of GR?

9. Mar 11, 2018

Dr.D

We can discuss the motion of a point under various conditions and constraints without reference to any mass at all. It makes perfect sense without reference to either force or mass.

10. Mar 12, 2018

DrStupid

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

11. Mar 12, 2018

sophiecentaur

So where does this take the thread?

12. Mar 12, 2018

Dr.D

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

13. Mar 12, 2018

DrStupid

Which point are you talking about?

14. Mar 12, 2018

sophiecentaur

Under what circumstances could the object be at rest? What would be a 'stationary' photon be like?

15. Mar 12, 2018

Dr.D

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

16. Mar 12, 2018

DrStupid

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. Mar 12, 2018

Dr.D

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. Mar 12, 2018

DrStupid

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. Mar 12, 2018

Dr.D

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

20. Mar 12, 2018

sophiecentaur

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.

21. Mar 12, 2018

rcgldr

What about something without a direct connection to mass, such as a shadow sweeping / accelerating across an observer's view?

22. Mar 13, 2018

sophiecentaur

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. Mar 15, 2018

David Lewis

Euclid was a mathematician. His discussions dealt solely with imaginary objects.

24. Mar 15, 2018

Dr.D

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.

25. Mar 15, 2018

slow

Let's try to advance in stages.

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.