Can there be any acceleration without mass?

[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?

 

[Dale]

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.

 

[DrStupid]

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.

 

[sophiecentaur]

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

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]

It does.

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

 

[sophiecentaur]

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

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

 

[DrStupid]

But for 'rectilinear propagation'?

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

 

[sophiecentaur]

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

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

 

[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.

 

[DrStupid]

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

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

 

[sophiecentaur]

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

So where does this take the thread?

 

[Dr.D]

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

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

 

[DrStupid]

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

Which point are you talking about?

 

[sophiecentaur]

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]

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.

 

[DrStupid]

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.

 

[Dr.D]

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.

 

[DrStupid]

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.

 

[Dr.D]

Therefore "massless geometrical point" is a tautology.

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

 

[sophiecentaur]

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

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.

 

[rcgldr]

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

 

[sophiecentaur]

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

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.

 

[David Lewis]

What mass do you think Euclid ascribed to a point?

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

 

[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.

 

[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.

 

[DrStupid]

Example. In a region of space $\varepsilon$ and $\mu$ vary from one point to another

That requires an optical dense medium. Such a medium has mass and the photon crossing it can't be clearly distinguished from this medium. It is a matter of definition where the photon ends and the medium begins. But no matter how you define it - if it has energy and moves with less than c, than it has mass.

Light does not have $m_o$, we know that. [...]

Light actually can have mass, but that's off-topic. We are talking about acceleration without mass.

[...] in Newtonian physics [...]

Newtonian physics is not valid for objects with m=0.

Is it also consistent with quantum theory?

It's difficult to talk about acceleration if you don't even have a defined position.

 

[Nugatory]

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.

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).

 

[David Lewis]

...in what way do you think that points, lines, planes, etc. are imaginary objects?

They're perfect. The only place perfect things exist is in our imagination.

 

[Dr.D]

They're perfect. The only place perfect things exist is in our imagination.

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]

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?

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]

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'?

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. That does not imply that root(2) is not real, only that the geometric ideas of displacement, velocity, and acceleration don't seem to apply there in the same way.

 

[Vanadium 50]

Whenever the word "real" appears in a Physics Forums post

Can you add "actually" to that list?

 

[sophiecentaur]

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.

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]

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.

Hard to say whether this is relevant to the OP or not, but it is certainly relevant to the discussion others have entered into. There are many other similar situations. For example, consider two radial lines, each rotating and extending out from a different point. The intersection of those two lines defines a moving and accelerating point, but again, there is no mass involved.

 

[DrStupid]

Hard to say whether this is relevant to the OP or not

Most probably not. The first post refers to dynamics and not to kinematics.

 

[Dr.D]

Most probably not. The first post refers to dynamics and not to kinematics.

Maybe, but maybe not . Here is a repeat of the first post:

92 / 3

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?

It certainly does not reference the word "dynamics," per se, but it does speak of acceleration. Acceleration is specifically a kinematic concept that merely happens to have relevance in dynamics. The OP says, "A force is needed to cause an acceleration," which is not true at all unless there is mass involved. Accelerations of massless points happen in many circumstances with no force involved.

 

[PeroK]

Hard to say whether this is relevant to the OP or not, but it is certainly relevant to the discussion others have entered into. There are many other similar situations. For example, consider two radial lines, each rotating and extending out from a different point. The intersection of those two lines defines a moving and accelerating point, but again, there is no mass involved.

One problem with this is that you are ascribing motion to an illusion of motion. For example, if you have an array of lights going on in a sequence, then that creates the illusion of something moving with a speed and an acceleration.If you accept that, then what happens if two lights go on at once and stay on? Do you have a single object in two places at once or two objects?

The fact is that a sequence of illuminated lights does not represent the motion of any thing. It's purely arbitrary that you have chosen to associate the different lights with each other in that way. You can simply drop that association and consider it a set of lights, each of which us either on or off at a given time.

Technically, therefore, your point of intersection is not moving or accelerating. It's an arbitrary association of points in space over time.

Informally, of course, you can say it moves, but technically it isn't motion.

 

[DrStupid]

The OP says, "A force is needed to cause an acceleration," which is not true at all unless there is mass involved.

The quotation from the OP is Newton's first law of motion. That's the reference to dynamics I was talking about. This law is true if physics describes reality correctly. You are right that it is not true for m=0 in classical mechanics, but in this case classical mechanics fails at all. In relativity it always holds.

The fact is that a sequence of illuminated lights does not represent the motion of any thing.

That depends on the definition of "thing".

 

[Outhouse]

It does. If the 'entity' only exists at velocity c then when would it be accelerating?

In a lab when light passes through say fiber optics is not the speed reduced by 31% ?

 

[Outhouse]

If you search "slow light" you will find out light can be stopped, started and slowed down.

 

[jbriggs444]

In a lab when light passes through say fiber optics is not the speed reduced by 31% ?

The letter "c" denotes the speed of light in vacuum -- the invariant speed limit in relativity. Light in fiber does not move at c.