# Gravity Accelerating Objects To The Speed Of Light

1. Jul 30, 2009

### ScienceNerd36

If you were to drop a penny down an elevator shaft that was infinite in the downward direction, what would begin to happen when the penny is accelerated by gravity to the speed of light or at least very close?

Last edited: Jul 30, 2009
2. Jul 30, 2009

### DavidSnider

How can the elevator shaft have an infinite downward direction and still have substantial gravitational pull in that direction?

I'm guessing the spirit of your question is what keeps the velocity of a constantly accelerating object below the speed of light?

The simple answer that I usually hear is that it requires energy to accelerate an object and the faster you go the more energy is needed to continue to accelerate because your mass increases as you speed up.

The special-relativity answer I don't understand but has something to do with it violating the geometry of space-time.

Last edited: Jul 30, 2009
3. Jul 30, 2009

### rcgldr

How about an object when it crosses the event horizon into a black hole before it impacts with the core?

4. Jul 30, 2009

### Staff: Mentor

What is, "escape velocity at the event horizon of a black hole", Alex...?

5. Jul 30, 2009

### DavidSnider

Elevator shaft and "downward direction" aren't exactly metaphors that would make me think he was implying the sort of scenario that involves black holes.

6. Jul 30, 2009

### Staff: Mentor

Sorry, that was a Jepoardy reference. Jeff was proposing the only scenario where the question has any connection to reality - an object falling into a black hole approaches C as it approaches the event horizon.

7. Jul 31, 2009

### vin300

To accelerate to infinite speed, the field must be so strong.. which happens only in case of black holes and you won't get your penny back

8. Jul 31, 2009

### sermatt

Wouldn't you want to take air resistance into account? Correct me if I'm wrong, but if there was air, the object would stop accelerating after a while.

9. Jul 31, 2009

### Staff: Mentor

I'm sorry, but neither of those two posts are relevant/useful here: You can't accelerate something to infinite speed and air resistance isn't relevant if you accelerate something in space.

10. Jul 31, 2009

### vin300

Yes it can, if it crosses the event horizon.The escape velocity of a body becomes c at the mass-equivalent radius which implies infinite proper velocity

Last edited: Jul 31, 2009
11. Jul 31, 2009

### ScienceNerd36

OK, let me rephrase the question.

(1) Imagine that space-time is warped as it would be in the presence of a black hole. And I'm speaking only of the geometry.
(2) Now let's imagine dropping a penny down this infinite bit of space-time curvature.
(3) As it falls and falls it accelerates until it is at a speed of 99.999999999% the speed of light. Will the penny then stop accelerating?

And if there are any mistakes in this form of the question please point them out, but don't start a conversation over what I might have meant.

12. Jul 31, 2009

### vin300

Yes, once it is inside the singularity
You could say c, because mass inside the singularity is said to have infinite density and energy

13. Jul 31, 2009

### fleem

Current theory is....

Classically speaking, the penny's time rate approaches zero as it approaches the event horizon, and we never see it cross the event horizon. Quantum mechanically speaking, the particles of the penny tunnel across the event horizon to the singularity.

But then "current theory" might change in a few decades.