# Light Speed & Acceleration: Is it Instantaneous?

• smslca

#### smslca

I know the speed of light is 3*10^8m/sec. And its acceleration is Zero.

1. I think we can take photon as light. Its mass=0, acceleration=0, charge=0.
By eqn F=ma=0*0=0 i.e any particle with or without mass cannot impel force on a photon.
From Newtons third law vice versa is also true.
I think force between particles (attraction or repulsion) represents their interaction with
each other.
Then what is this absorption of photon i.e interaction by jumping electrons(containing mass) actually mean.

2. When a photon is released,
is its speed is instantaneous with a speed of light?? If it is instantaneous could you tell
me, how does it attain that speed from the start i.e at t=0.

If its not instantaneous, Is photon accelerating for its first second to attain the speed of
light, since speed is 3*10^8 m for each second , and then it continued to travel with no
acceleration after t=1.

I don't know the physics too deep. I just know the basics. I am trying to learn into its depths. So please notify me if there are any wrong in my questions.

Welcome to PF!

Hi smslca ! Welcome to PF! I know the speed of light is 3*10^8m/sec. And its acceleration is Zero.

No, the speed (a number) is constant, but the velocity (a vector) isn't …

the velocity can be c in any direction, and if the direction changes, then there is an acceleration (perpendicular to the velocity). 1. I think we can take photon as light. Its mass=0, acceleration=0, charge=0.
By eqn F=ma=0*0=0 i.e any particle with or without mass cannot impel force on a photon.
From Newtons third law vice versa is also true.
I think force between particles (attraction or repulsion) represents their interaction with
each other.
Then what is this absorption of photon i.e interaction by jumping electrons(containing mass) actually mean.

I'm not sure what you're asking here, but a photon can "collide" with an electron (see eg the Compton experiment) …

they exchange momentum (and energy). 2. When a photon is released,
is its speed is instantaneous with a speed of light??

Yes, it is created at the speed of light. 1. I think we can take photon as light. Its mass=0, acceleration=0, charge=0.
By eqn F=ma=0*0=0 i.e any particle with or without mass cannot impel force on a photon.
From Newtons third law vice versa is also true.
Welcome to PF!

Note, that Newton's laws are just an approximation for v<<c and need to be adapted when dealing with relativistic speeds. Specifically, Newton's second law for relativistic objects should be written $F=dp/d\tau$ where p is the energy-momentum four-vector and $\tau$ is the proper time. In this case, although a photon has no mass it does have momentum and therefore can exert a force on a massive particle.

Yes, it is created at the speed of light. How do you know that?

Because otherwise momentum would not be conserved.

Hi smslca ! Welcome to PF! No, the speed (a number) is constant, but the velocity (a vector) isn't …

the velocity can be c in any direction, and if the direction changes, then there is an acceleration (perpendicular to the velocity). I'm not sure what you're asking here, but a photon can "collide" with an electron (see eg the Compton experiment) …

they exchange momentum (and energy). Yes, it is created at the speed of light. Hi ,tiny-tim,
thanks for your answer ,even i was stuck with the same type of question

I learned a new point about velocity. I had studied about velocity and vectors in school and in college. How can they miss such a minute points. They always said that If there is change in velocity it gives the acceleration, because that's what written in the textbooks. Cant they just add the small and important missing points to the lecture from their memory. I think even teachers have no knowledge about it. Then where is the quality in education. In India they just want money, to earn that they don't care about knowledge.

Please ignore above statements if u want to. Its my frustration about the educational system in India. I am not judging just with this one example.

Thanks to Tiny tim for adding the sentence
if the direction changes, then there is an acceleration (perpendicular to the velocity).

Dalespam, Are u saying that momentum of the particles with no mass and traveling with speed of light doesn't need mass to gain momentum. In http://en.wikipedia.org/wiki/Photon it is said that momentum of photon depends only on its frequency.
Is there any particles like photon??. I like to learn about them. Physics is so interesting.

Does the particle with no mass must travel with speed of light??

Because otherwise momentum would not be conserved.

Not sure I understand that. The effective mass of a photon should be zero so the m x v equation is going to be zero. I also can't see why you couldn't have a change of momentum when it is first produced which would be canceled when it finally impingies on something, hence momentum being conserved.

Hi smslca! Is there any particles like photon??. I like to learn about them. Physics is so interesting.

You mean, are there any other particles that have zero mass, and so travel at the speed of light?

We used to think that the neutrino with (spin-half) did, but now experiment suggests that it does have a very small mass.

The graviton (with spin two), which has not yet been discovered, is believed to have zero mass.
Does the particle with no mass must travel with speed of light??

(In vacuum,) yes. No, the speed (a number) is constant, but the velocity (a vector) isn't …

the velocity can be c in any direction, and if the direction changes, then there is an acceleration (perpendicular to the velocity).

This change in direction, If The light traveling in a straight line turned left or right or back or at an angle, does mean the light accelerated.

If the above assumption is true, Does reflection and refraction produces acceleration in light. At that time does reflected or refracted light travels with speed of light.

This change in direction, If The light traveling in a straight line turned left or right or back or at an angle, does mean the light accelerated.

If the above assumption is true, Does reflection and refraction produces acceleration in light. At that time does reflected or refracted light travels with speed of light.

Yes!... Light always travels at the speed of light, but not always at the vacuum value of 'c'.
In fact, my very favourite type of radiation applies here... Čerenkov Radiation! It's a fine example of how light keeps on trucking, and even creates something very like a sonic 'n wave' when it "brakes" into its local speed.

EDIT: Remember, light is EM radiation, all EM radiation behaves as light re "speed"

Dalespam, Are u saying that momentum of the particles with no mass and traveling with speed of light doesn't need mass to gain momentum. In http://en.wikipedia.org/wiki/Photon it is said that momentum of photon depends only on its frequency.
Yes, that is correct. However, the more general principle is that the momentum of any particle is proportional to its wavenumber (1/wavelength). For photons, since they are massless, the frequency is proportional to the magnitude of the wavenumber so it works out that what you said above is correct.

This change in direction, If The light traveling in a straight line turned left or right or back or at an angle, does mean the light accelerated.

If light (or anything) changes direction, it accelerates.

If light (or anything) changes direction but stays the same speed, it accelerates perpendicular to its velocity.
If the above assumption is true, Does reflection and refraction produces acceleration in light. At that time does reflected or refracted light travels with speed of light.

That rather depends on whether you regard it as the same photon.

When light is reflected or refracted, quantum theory tends to convert the photon into a wave while it interacts with the glass (or other medium), and back into a photon when it leaves.

But if you regard it as the same photon, then yes it must have accelerated.

Not sure I understand that. The effective mass of a photon should be zero so the m x v equation is going to be zero. I also can't see why you couldn't have a change of momentum when it is first produced which would be canceled when it finally impingies on something, hence momentum being conserved.

Remember that only the rest mass of the photon is said to be zero. Since a photon is never (as far as we can tell) at rest, the photon's momentum is determined according to its frequency.

An alternative way to look at it: Dr. Haus has shown that the condition for radiation is that the Fourier Transform of the particle's current contains components synchronous with the velocity of light. In other words, some part of the current is already moving at the speed of light.