# Aging during space travel

1. Oct 6, 2009

### Radek01

Hello, I have been lately very interested in time dilation. I have found out many information about it and it's very interesting for me. But I have few problems I dont quite understand, so I will be glad for your help.

Let's start:
There's a man traveling in a spaceship from point A to point B.
distance A to B: d=1 light year
speed of spaceship: v=0,99999999999999
The man is 30 years old

So here are the options:
A) on the day of his 30 birthday he start his journey from A to B, with the spacewhip. As soon as he get to the point B, will his age be approx. 31? (mostly I heard this to be the truth. That from his point of view in the spaceship, the time elapses the "normal" speed for him)

B) He will arrive to the point B almost instantly - from his point of view (I dont trust this option much, it's making a mess in what I know about it. And I find this option unlikely to be truth. But also I heard the photons moving v=1c, basicly don't know what time is. Or in other words the time for photons is t=0. And from this the photons should "feel" to be everywhere instantly)

So where's truth? I am not complete "noob", but also not an expert.
I am 99,9% sure that the man will be 31 years old when arriving to B.
But also it seems to me logic that photons moving v=1c don't have time - t=0, and v=1.

Last edited: Oct 6, 2009
2. Oct 6, 2009

### JesseM

The answer is B. Keep in mind that in his own frame, the distance from A to B is shrunk by a factor of 7073895.4 due to length contraction, so the distance is far less than 1 light year. If B is traveling towards him at v=0.99999999999999c from his perspective, and when he is passing A his distance to B is only 1/7073895.4 = 0.00000014 light years, naturally it will only take about 0.00000014 years (about 4.5 seconds) for B to reach him. So time does elapse at normal speed in his frame, despite this short time. On the other hand, in the frame of A and B, the distance is 1 light year, but his clock is slowed down by the same factor of 7073895.4 as he travels.

Incidentally, in relativity only slower than light objects have their own inertial rest frames, so it isn't really meaningful to ask what things look like from the perspective of photons themselves (see here, here and here for some previous discussions).

Last edited: Oct 6, 2009
3. Oct 7, 2009

### Radek01

Ok, I think I understand.

1)
From his point of view, the B point is closing to him. Since there's a length contraction, the "his" lenght between ship and B is small. But how long will he travel then from his frame?
Will he think he spent 1 year in ship, or less?

2)Andd as soon as he reach the point B, he stops. Therefor change his frame from inertial to not-inertial - so I assume he will be aged 31?

4. Oct 7, 2009

### Simon76

Unless I've misunderstood his answer,

1) 4.5 seconds as stated
2)No

(Please correct if I've misunderstood this).

In either case, I would assume that unless the method of travel has changed and is non-linear, it won't make much difference. Having been catapulted at the speed of light, what's left of him is unlikely to be identified as any particular age.

5. Oct 7, 2009

### HallsofIvy

Staff Emeritus
No, he will have spent about 45 seconds.

No, you do not immediately age when you come to a halt. People on B will, if they were watching, have seen him coming for over a year, and will have seen him (really, really good telescope!) aging very slowly. When he stops he will begin aging at the same rate as people on B but he won't suddenly "make up" for that year.

6. Oct 7, 2009

### Simon76

I may have misread this..

Surely if he is travelling beyond the speed of light, his "travelling image" will arrive after he has in fact arrived himself? Therefore, the spectators won't see anything until he ceases to move (having impacted the ground somewhere beyond lightspeed and distributed himself generously over the surrounding area).

(Is this under the same reasoning that quantum entanglement can be used to reflect state changes regardless of distance in theory beyond the speed of light? Not something I've read much about, but am curious if anyone knows more and wants to add.)

7. Oct 7, 2009

### JesseM

No, I already told you at the beginning of my post that the answer is the option you labeled B) (he will arrive almost instantly from his perspective), not the one you labeled A) (he will be aged 31 when he arrives). Like I said, with a velocity of v=0.99999999999999c and a distance of only 0.00000014 light years, naturally since in any frame velocity = distance / time, that means time = distance/velocity, in this case 0.00000014 light years / 0.99999999999999 light years per year = about 0.00000014 years, which is equal to about 4.5 seconds. That's how long it'll take for him, so his age when passing B will only be 30 years + 4.5 seconds.

8. Oct 7, 2009

### JesseM

He's not, it's impossible to accelerate past light speed (would require infinite energy). He's only traveling at 0.99999999999999c, which is slightly less than 1c (the speed of light).

Even if an object could travel faster than light (perhaps a hypothetical object made of tachyons which was always traveling FTL rather than accelerating past light speed), if it emitted photons as it went then travelers would see it at the same time it past right next to them, since when it was next to them the distance the light would have to travel to reach their eyes would be nearly zero. The odd thing from their perspective would that they wouldn't see images of earlier points on its journey until later times, because the light from earlier points took longer to get to them than the FTL object itself...so, first they would see it passing them at the same time it actually did pass them, then they would see an image of it traveling backwards, like a movie played in reverse.
It's not known if any FTL is going on "behind the scenes" in QM, different "interpretations" of QM would say different things about this (many proponents of the many-worlds interpretation argue that it explains entanglement without the need for FTL effects). Either way, it is known that from an observable perspective it is impossible for us to use entanglement to transmit messages FTL, and all the interpretations of QM are observationally indistinguishable so no tests can determine which, if any, is true.

9. Oct 8, 2009

### Radek01

So if there's A to B points = distance 1 light year.
A= earth
B= destination_planet
Ship travels v=0,999c, from A to B.

From ship's view of point:
The cosmonaut will not spend approx 1 year in space.
He will fly on distance of 1ly divided by the y (d=1ly/22=0,045 ly)
So the time will be t=d/v, t=0,045/0,999=0,045 years
16 days

So the cosmonaut will be older only by 16 days.
And people on earth (point A - departure point) will be older by 16*22= 352 days.
And people on destination_planet (pont B - target planet) will be also older by 352 days?

--------

So he will spend 16 days flying from A to B.
But how long it will be for his clocks on the ship?

The ship's clock will show fly time to be 16 days? or 1 year?

Thank you in advance for the answer on these 2 questions.

10. Oct 8, 2009

### Simon76

Aside from a few other concerns, why is it stated that it's impossible for mass to move beyond the speed of light?

Please explain what happens in the case of a black hole. Not specifically the singularity, but the surrounding area. On reflection, probably the wrong question to ask, but what does happen to the "light" in these regions? (You may hit the word gravity here?)

For that matter, light isn't constant is it? If light is indeed affected by gravity, temperature and other factors... There have been numerous experiments aimed to slow light etc.

Last edited: Oct 8, 2009
11. Oct 8, 2009

### HallsofIvy

Staff Emeritus
No one said anything about traveling "beyond the speed of light". In this case, his speed was postulated to be slightly less than the speed of light.

But you are right that I should not have said "People on B will, if they were watching, have seen him coming for over a year". The light from his takeoff will reach B only slightly before he arrives himself.

Last edited: Oct 8, 2009
12. Oct 8, 2009

### DaveC426913

The entire reference frame is affected.

You must understand, this is not an aberration applied to the spaceship, resulting in some special physical properties. It is a completely valid frame of reference, just as valid as all the people on Earth.

We here on Earth are also moving relavitive to Pluto or Alpha Centauri or the centre of the galaxy, and we are experiecning dilation relative to those reference points. We do not go around wondering if our clocks are accurate.

Please start a new thread to ask this question.

13. Oct 8, 2009

### Simon76

http://www.universetoday.com/2009/06/30/device-makes-radio-waves-travel-faster-than-light/

Depends on your interpretation of the problem. I wouldn't imagine in that scenario that mass would be sent in a linear fashion or via "catapult".

Any updates in regards to tachyon theory / quantum tunnelling / superluminal electromagnetic fields are welcome..

Since we're in the "humour the imaginary scenario" mood, what would be the result if "beyond-light-speed radio waves" were used to transmit information from point A to point B (over 1 light year), and that information was used to "reconstruct" said mass? hehe

14. Oct 8, 2009

### Radek01

I know its valid frame of reference, not different from our's on earth.

My point is that there is ship and A to B route
Ship is one frame of reference and
A to B is also one frame of reference (they are not moving to each other)

My point is that if a ship is moving, so people on earth are 352 days older in comparation to the ship, but how old will be people on B? Also 352 years older?

And also I want to know what cosmonaut in ship is experiencing.
As I calculated he would spend 16 days in the ship.
But the ship's clock would measured it as 16 days, or 1 year? (AtoB trip is 1 light year, and his v= 0.999 of c)

15. Oct 8, 2009

### DaveC426913

Faeries and ghosts. Transmission of signals faster than the speed of light is impossible, despite what the overly-simplified article is trying to imply.

16. Oct 8, 2009

### DaveC426913

No, you don't.

If you did, you would not ask:
Here on Earth, when we experience 16 days passing, what do our clocks show? They show 16 days passing.

Earth is a spaceship traveling through the universe at .999c with respect to the spaceship. It is an exactly equal frame of reference. The Earth is not special.

The 16 days aboard the spaceship is not some "biological illusion" such that clocks aren't affected. 16 days is how much trime has elapsed in the spaceship. Period.

17. Oct 8, 2009

### JesseM

Technically relativity doesn't rule out the possibility of FTL signals, but what it does say is that if the laws of physics applying to FTL worked the same way in every inertial frame (which they'd have to or else relativity would be incorrect), then since any FTL signal will be seen to have arrived at an earlier time than it was sent in some frames (due to the way the relativity of simultaneity works), a combination of FTL signals sent by devices at rest in different frames could be used to transmit information backwards in time, violating our ordinary ideas about causality. For this reason physicists think it's unlikely the fundamental laws of physics will allow tachyons, but as I said it can't be absolutely ruled out by relativity alone.

18. Oct 8, 2009

### JesseM

It's impossible for matter to move faster than light as measured in an inertial frame of reference. If you use a non-inertial coordinate system in SR, like Rindler coordinates, then matter (and photons!) may move at speeds other than c in these coordinates. In general relativity, it's impossible for any coordinate system that covers a large region of curved spacetime to qualify as "inertial", so you can't expect light to necessarily move at c in a coordinate system covering a black hole spacetime, like Schwarzschild coordinates (in fact in Schwarzschild coordinates, the speed of light gets smaller and smaller as you approach the horizon, until exactly at the horizon the speed of light is zero). However, in GR there is the "equivalence principle" (see the introduction http://www.aei.mpg.de/einsteinOnline/en/spotlights/equivalence_principle/index.html [Broken]) which says that if you zoom in on a very small local region of spacetime, the curvature in that region becomes negligible, and the observations of an observer in freefall passing through that reason will be just like the observations of an inertial observer in SR. So in this sense we can still say that the speed of light as measured by these "local inertial observers" is always c in GR, and that they will never observe any massive object moving at c or faster in their local inertial coordinate system.

Last edited by a moderator: May 4, 2017
19. Oct 8, 2009

### Radek01

Dave, thank you a lot. It's much clearer for me now. thx

20. Oct 8, 2009

### DaveC426913

Anytime. Ask away. We live for questions.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook