# Massless objects and speed

1. Aug 22, 2015

The main obstacle to FTL is that mass increases with velocity and in the book of my imagination there is a blank spot on how to nullify the mass of a star ship. I cant help but think the answer is hidden somewhere in Albert's equation E=MC2.

2. Aug 22, 2015

### Staff: Mentor

It does not. At least not in the way "mass" has been used the last 50+ years.
The main obstacle to FTL is the fact that every finite energy leads to a speed slower than the speed of light.
Even massless objects are not faster than the speed of light. They move exactly at the speed of light.

3. Aug 22, 2015

### Orodruin

Staff Emeritus
4. Aug 22, 2015

Yes, but does not the infinite energy issue directly connect to the increasing mass? In nature massless objects are limited in speed by the inertia at the time of their creation ?

5. Aug 22, 2015

### Orodruin

Staff Emeritus
6. Aug 24, 2015

### Kostik

Ohanian (Classical Electrodynamics) has very neat argument that if any signal can travel FTL, then in another inertial frame it travels at infinite speed. Consequently the signal in that frame is received by the sender before he sent it : unacceptable causality violation.

7. Aug 24, 2015

### Markus Hanke

Mass does not increase at all. What you are thinking of is probably the old concept of "relativistic mass", which is generally speaking not much used any more these days because it tends to lead new students to all sorts of misconceptions. The consensus in most texts on relativity these days is that "mass" usually refers to rest mass, unless specifically stated otherwise - and rest mass is an invariant quantity. Relativistic mass is best understood as a measure of total energy a system possesses, which is an observer-dependent quantity.

8. Aug 24, 2015

### bcrowell

Staff Emeritus
SR has a defense in depth against FTL. I have a discussion of this in section 4.7.1 of my SR book http://www.lightandmatter.com/sr/ , listing four logically independent reasons.