# Faster than Speed of light

#### kogaht

I know Einstein said it is impossible to travel faster than speed of light as the mass would get infinitely large. But if black holes exist and even a beam of light would be pulled back by gravity. Then Newton's 2nd law of motion, F=ma suggest faster than speed of light is possible.

can somebody explain what im thinking wrong.

thanks.

Related Special and General Relativity News on Phys.org

#### Doc Al

Mentor
You didn't give a full statement of your reasoning. In any case, 'F = ma' must be modified for application at high speeds.

#### skeptic2

On the other hand if a spaceship were to travel to Proxima Centauri from earth at over 70% of the speed of light (from the earth's perspective), he would reach Proxima Centauri in less than 4.3 years by his own clock, obviously faster than the speed of light.

(just kidding)

#### kogaht

You didn't give a full statement of your reasoning. In any case, 'F = ma' must be modified for application at high speeds.
Say light can be sucked by black holes then there must be a force to do so. Since there is a force acting to the light, then the light must be accelerating. So the speed of light plus the extra velocity gained by accelerating = faster than speed of light.

no?

#### Fredrik

Staff Emeritus
Gold Member
In GR, there's no force acting on an object in free fall. F=ma still makes sense, but the a is a measure of the deviation from geodesic (free-fall) motion.

#### JesseM

I know Einstein said it is impossible to travel faster than speed of light as the mass would get infinitely large. But if black holes exist and even a beam of light would be pulled back by gravity.
Are you suggesting the idea that since "mass increases" as you approach the speed of light, then objects will turn into black holes before they can reach the speed of light? If so this is a misconception, see this entry from the Usenet Physics FAQ along with pervect's post #9 on this thread.

#### stevmg

I know Einstein said it is impossible to travel faster than speed of light as the mass would get infinitely large. But if black holes exist and even a beam of light would be pulled back by gravity. Then Newton's 2nd law of motion, F=ma suggest faster than speed of light is possible.
can somebody explain what im thinking wrong.

thanks.
The second law is a microcosm of SR. As v $\Rightarrow$ c, a gets asymptotically smaller to zero and is at "just before" zero "just before" v = c so an object is never "bumped up" to the speed of light.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving