# Decelerating from light speed

1. Aug 26, 2009

### That was mine

I heard that an infinite amount of time would pass as an object decelerates from the speed of light, is that true?

2. Aug 26, 2009

### nicksauce

Only massless objects travel at the speed of light, and massless objects always travel at the speed of light, so this question doesn't really make sense.

3. Aug 26, 2009

### That was mine

Assuming that the object was already traveling at light speed, what would happen if it decelerated?
The question was asked to me earlier today, and I couldn't think of an answer.

4. Aug 26, 2009

### That was mine

Assuming that the object was already traveling at light speed, what would happen if it decelerated?
The question was asked to me earlier today, and I couldn't think of an answer.

5. Aug 26, 2009

### That was mine

Here is the exact quote:
"now , you have a person go goes into a spacecraft and begins to travel the speed of light (exactly) in circles around his wife. (yes i know thats not possible , but this is a thought experiment) His wife is standing in the yard , and he is going light speed around her , over and over. Now . Stop the craft 3 months later. What will he see and why? And what about her?"

6. Aug 26, 2009

### I_am_learning

you can always skip the answer nicksauce posted "Mass can't travel at speed of light, so your question doesn't make sense" by asking "What will happed if we decelerate from the speed of 0.9999999999999999999999c (take the limiting case)" If the declaration time tends to infinite as V tends to C, you got your result.

7. Aug 26, 2009

### That was mine

So what happens?

8. Aug 26, 2009

### Staff: Mentor

Your lack of an answer was the correct answer. A massive object has a timelike four-momentum and an object travelling at c has a lightlike four momentum. So the question is fundamentally flawed in the same way as any question that began "What if 1 equaled 0"

9. Aug 27, 2009

Staff Emeritus
No it's not. A thought experiment describes something that is not, by construction, impossible. Dale is exactly right, this is like starting "supposing 1 = 0".

10. Aug 27, 2009

### A.T.

If by "object" you mean something with rest mass: There's no answer because it never moves at c.

If by "object" you mean some energy moving at c as light: It becomes matter + antimatter if you "decelerate" it below c:
http://en.wikipedia.org/wiki/Pair_production

11. Aug 27, 2009

Staff Emeritus
That's really, really misleading. Yes, one can pair produce particles with photons, but calling this a consequence of deceleration is really not right. If I apply a force to a photon, I can change it's energy or direction without any particle production.

12. Aug 27, 2009

### That was mine

I should note that he is, allegedly, a construction worker with a ninth-grade education. Not saying that he couldn't have dabbled in SR, but maybe his knowledge is a little off.
This is the answer that he gave:

13. Aug 27, 2009

### DaveC426913

I am not sure what question the above post answers. (It is all nonsense anyway. Travelling at near light speed does not allow you to pass your hand through invisible objects!)

If the question is still "what would happen if you decelerated from the speed of light", then the answer is still "this is an impossible scenario".

If you want a sensical answer, you'll have to reform the question.

14. Aug 27, 2009

### That was mine

I know that the question makes no sense.
I quoted the question above and the answer they gave.

I would like to know what seasoned physicists would say about doing this.

15. Aug 27, 2009

### That was mine

I still need help!

16. Aug 27, 2009

### DaveC426913

What help do you need? The question is nonsense, as is the answer.

17. Aug 27, 2009

### DaveC426913

This question can be answered (though I don't understand the point of the three month delay):

"now , you have a person go goes into a spacecraft and begins to travel NEARLY the speed of light in circles around his wife. His wife is standing in the yard , and he is going ALMOST light speed around her , over and over. Now . Stop the craft 3 months later. What will he see and why? And what about her?"

If she looked in on the spacecraft as it was travelling at .999999999c, she would see her husband virtually frozen, unmoving. As the craft decelerated, she would see him "come to life" slowly, until after three months, he would be moving normally.

This whole nonsense about light being "three months old" is nonsense. The light is travelling from him to her and her to him, which is - what? - the width of their backyard - a few dozen metres? Light can do that standing on its head.

18. Aug 28, 2009

### tickle_monste

Let's say that the man swallowed an anti-matter pill, subsequently transforming himself into pure energy, and his wife was large enough that her gravitational pull caused him to now orbit her, for 3 months, after which he, in his pure-energy form, swallows his matter-anti-matter annihilation reversal pill which is also composed entirely of pure-energy and so is allowed to travel with him to be swallowed (does this fit into the bounds of a thought experiment?), and all of his pure-energy undergoes a highly ordered particle-anti-particle pair production process that leaves him feeling 20 years younger. I know that when he stops, everything will be back to normal and that the 3 month old light doesn't make sense anyway, but I'm curious as to what he would see during his orbit. Nothing at all? Certainly light can't catch up to him, but he's constantly intersecting light along his path, how does he 'see' this?

19. Aug 28, 2009

### sym666

Here is an actual journey I made from Proxima Centauri to our solar system heading Sol first, which required a correction of the route as you can see, and then to Earth. I put on ship's screen the planets' orbits in order to better visualize relativistic effects. I hope you enjoy it

http://www.dailymotion.com/video/xaaxdu_proximacsolearth

Thanks to Dr. Mitchell McKaine who realized this amazing software. I used an unregistered version of it.

20. Aug 28, 2009

### DaveC426913

As energy moving a light speed, he is composed of massless photons. Photons do not have a frame of reference and do not experience time.