# Massless particles

1. Aug 23, 2008

### mite

Can we find massless particles at rest? and why?

2. Aug 23, 2008

### HallsofIvy

Staff Emeritus
No. All massless particles (photons are the only ones we are absolutely certain exist) must always move at the speed of light.

3. Aug 23, 2008

### Hootenanny

Staff Emeritus

4. Aug 23, 2008

### Staff: Mentor

Note: by "below," Hoot means "all the way at the bottom of the page."

5. Aug 24, 2008

### mite

I understand massless particles travel at light speed & we cannot have massless particles at rest.
but if we just consider we have a massless particle at rest & suppose we apply a small force on it.
As massless particle wouldn't oppose being accelerated so its velocity will keep on increasing to infinite speed but ofcourse special relativity doesn't allow that.
So these massless particles reaches the ultimate speed possible the speed of light.
Is this right or wrong?

6. Aug 24, 2008

### Hootenanny

Staff Emeritus
This is wrong. Firstly, you agree that SR requires that a massless particle travel at c. In other words, physics currently predicts that all massless particles must travel at c. So can you now see the problem with asking physics to predict what would happen if we had a massless particle that didn't travel at c?

7. Aug 24, 2008

### mite

Hi Hootenanny
Sorry for bothering

I saw the proof that massless particles should travel at the speed light in another thread.
I was just trying to prove it myself thinking wt would happen if we have a massless particle .
I think it is wrong.

8. Aug 25, 2008

### Nickelodeon

Are you sure that massless particles are real? It would seem to me that they are more likely to be a particle/anti-particle oscillating combination (ie mass/antimass) so would not necessarily be travelling at the speed of light.

9. Aug 25, 2008

### matheinste

Hello all

Just a thought. Are we perhaps a bit hung up about mass. We have particles that lack other properties such as charge, spin etc. so why can't we have particles with no mass. We tend to think of mass as something that everything real has, but why? Is it just another property that something may or may not have?

Matheinste.

10. Aug 25, 2008

### paw

I think the problem with this is that as mass approaches zero the force required to achieve a given acceleration also approaches zero. In fact in the limit as mass --> 0 no force at all is required to achieve any acceleration, including infinite acceleration.

As soon as we accept infinite acceleration we have to accept the baggage that goes with it. For example, we'd have to accept infinite velocity because, under infinite acceleration a particle would attain infinite velocity in an infinitely short time. Since we don't observe massless particles moving infinitely fast I think we just have to accept that massless particles can only travel at c.

11. Aug 25, 2008

### matheinste

Hello paw.

I have no problem with photons having zero mass or zero mass particles having to travel at c. I was just getting a bit off topic. I understand your point.

Matheinste.

12. Aug 25, 2008

### yuiop

Have a look at the Minkowski space time diagram of particles undergoing "Born rigid acceleration" on this mathpages link http://www.mathpages.com/home/kmath422/kmath422.htm

The curved lines are worldlines of the accelerating particles as seen by an inertial observer. The acceleration in the diagram is proportional to 1/x. When x=0 the acceleration is infinite and the worldline of such a particle is a null worldline or that of a photon (The dotted diagonal straight lines in the diagram). It is easy to see in this diagram that infinite acceleration corresponds to a constant velocity of c from the point of view of an inertial observer. The curved worldlines also represent lines of constant spatial location from the point of view of an accelerated observers comoving with the particles. From this point of view a photon is not moving from the point of view of an observer at x=0 (infinite acceleration) but remains on the dotted null line that represents the event horizon from the point of view of the inertial observers. (Of course the infinitely accelerated observer at x=0 would have to be massless too, as it would require infinite energy to infinitely accelerate an observer with rest mass so we are talking about a purely hypothetical observer in this case.)

13. Aug 25, 2008

### paw

Sure, I know you had no problem with it. I thought it was a more philosophical question, hence my reply.

14. Aug 25, 2008

### paw

Interesting. I haven't seen this before. Thanks.