Time and space travel at light speed

1. Nov 11, 2012

mike777

Apologies if this is a really stupid question but I've never seen it satisfactorily explained although that is perhaps because it is a really stupid question!

Bill stays on earth, Ted flies off into space travelling randomly at or very near the speed of light. He's back after a few days and discovers hundreds of years have passed on earth.

That's fine but what if Ted has a more definite journey - ie. to the nearest star and back. Bill and Ted both know it will take 9 years and Bill has the reception party waiting in 9 years time.

But what does Ted actually experience relative to himself - does he find that he gets there in seconds, ie. his instruments indicate he is travelling at many times the speed of light?
Or if he still exeriences everything as normal and believes 9 years have passed but actually gets back in just a few seconds having aged 9 years, then surely that is the wrong way round, ie more time has passed for him than on earth?

2. Nov 11, 2012

Staff: Mentor

By Ted's reckoning, the distance is much shorter (length contraction). So he gets there and back in less time (on his clock), never exceeding the speed limit (c). By Bill's reckoning, the distance is ~9 light years, and the time it takes Ted is ~9 years (on Bill's clock). What an Excellent Adventure!!!

3. Nov 11, 2012

micky_gta

ok "Bill and Ted both know it will take 9 years " this is before he takes off. So if you mean on Bills watch it took 9 years, then for Ted it was a lot shorter, maybe a few days lets say. But if you mean on Teds watch it took nine years then much more time elapsed for Bill maybe 100 years, lets say? So basically they can never synchronize their watches together if one of them is moving really fast. There is no 'one true' time base, its all relative to one person, what his consciousness experiences.

4. Nov 11, 2012

Staff: Mentor

I mean on Bill's watch it took 9 years, but on Ted's watch it was a lot shorter.

Yes. That is exactly the situation. There is no 'one true time base'. But it is not only his consciousness that is experiencing this. It is his biological clock as well. When Ted comes back, he will be about the same age as when he left, but he will encounter a Bill who is 9 years older.

5. Nov 11, 2012

bobc2

Good job, Chestermiller. Good to see you jump in there.

6. Nov 12, 2012

mike777

That's great, thanks for all your replies.

I'm very happy with that explanation, although Ted has emailed to say he's still confused about something... his speedometer calculates his current speed by looking at the time and the various known markers that it passes by on the journey and therefore it tells Ted he is currently travelling at thousands of times the speed of light??

Ted understands that it's perhaps just a faulty speedo but he's brought his young son with him and he worries that once he's passed away his son, who plans never to return to earth, will assume that in his reality faster than light travel is very normal?

...apologies...

7. Nov 12, 2012

Staff: Mentor

That is not the usual way speed is measured (and defined). With that definition, Ted will measure a "speed" (as "light years in earth system per years in ship system") larger than c.

8. Nov 12, 2012

ghwellsjr

Ted sent you an email? Have you already got it? How long ago did he leave? How fast was he going? When according to his clock did he send it?

9. Nov 12, 2012

mike777

Ok fine, i deserve that.
Ted's son though, Little Ted, still thinks in a simplistic manner that he travels faster than the speed of light because he calculates a planet is 5 light years away but it then only takes him 5 minutes to get there.
He will come to earth and swear until he's blue in the face that faster than light travel is possible because that's his own experience.

So, does light travel at 186000 miles per second in our perception only whilst in other perceptions it actually travels at almost infinity miles per zero seconds, which is the ultimate reason why nothing can travel faster?

Or worse yet, if distance contracts does that mean the known universe is infinitely small??

Spare a thought for Little Teds sanity...

10. Nov 12, 2012

ghwellsjr

We're not going to get anywhere if you don't answer my questions. I have no idea what you are trying to communicate by simply saying, "Ok fine, i deserve that." And then you simply ask more questions before we get a chance to resolve earlier issues.

You also have put out so many different conflicting scenarios. You said earlier that Ted is going go make a round trip visit to a star that will take 9 years of earth time and now you're claiming that Little Ted calculates a planet that is 5 light years away. Is this a different planet orbiting a different star than the first one (which cannot be more than 4.5 light years away)?

11. Nov 12, 2012

Staff: Mentor

He misunderstood special relativity then. For the spaceship, the star is not 5 light years away - it is about 5 light minutes away in this scenario. The ship needs 5 minutes to reach it (or the star needs 5 minutes to reach the ship), and the calculated speed is close to the speed of light, but slightly lower.

12. Nov 12, 2012

mike777

Many thanks for all your replies. Little Ted and I will study special relativity and hopefully post more concise questions in future!

13. Nov 12, 2012

ghwellsjr

14. Nov 12, 2012

micky_gta

Let me clarify my last post

bobc2 "It is his biological clock as well. When Ted comes back, he will be about the same age as when he left, but he will encounter a Bill who is 9 years older."

Actually any thing travelling 'with' you at what ever speed ages the same. So yes Ted's conscious experience will seem normal to him, he will also age normally (in his experience) and his spaceship also, etc...

And we must not forget that (in his experience) his mass does not increase, he does not shrink and time seems to tick normally. Yet someone watching/measuring him from earth will see increase in mass, size shrinkage and his time slowing.

Now what's really going to break your noodle is, how fast are we already travelling through space? the earth orbits the sun, the sun orbits the galaxy, and the galaxy is also moving (relative to other galaxies) - so how fast are we really moving?? we can NEVER figure this out.

Nature designed the universe so that where ever you are and no matter how fast you are travelling, for YOUR EXPERIENCE everything will seem/work just fine!

15. Nov 12, 2012

Staff: Mentor

That is not just an experimental issue, "our speed" is not defined without some arbitrary reference frame to measure it in.

We move with ~30km/s relative to the sun.
We move with ~200km/s relative to our galactic center.
We move with ~400km/s relative to the cosmic microwave background.
We move with 0.9c relative to some reference frame which moves with 0.9c relative to us.

16. Nov 12, 2012

mike777

Thanks. Yes I suppose thats what I was getting at with Little Ted thinking he was travelling faster than light - can we actually be sure about anything ourselves with any more certainty than Little Ted if we can only depend on own frame of reference?

Distant galaxies appear to be receding at near or in some cases even faster than the speed of light due to the universe expansion or whatever is going on - or in other words we appear to be travelling at light speed from their point of view.

What are the implications if we ourselves are already pretty near the speed of light, I can't get my head around that one - for starters would it mean the universe is very significantly older than we think when considered from outside our reference point?

17. Nov 12, 2012

micky_gta

And how fast is the cosmic microwave moving??
For all we know everything in the universe can be moving close to the speed of light. We can NEVER know.

18. Nov 12, 2012

Algr

Little Ted might have an old map that _says_ that the planet is 5 light years away, but everything he might do on the ship will tell him that the planet was never that far away. The only way to calculate a speed faster then light would be to use the ship's clock, but ignore the ship's telescopes in favor of the old map. I'm sure little Ted would find that procedure flawed.

But now my own question:

When the ship is at rest relative to the destination, the star will appear 9 light years away. Then the ship accelerates and the distances contract. At traveling speed, the star appears to be 0.9 light years away. Wouldn't observations during the acceleration phase appear to show the destination star approaching faster then light?

19. Nov 12, 2012

Staff: Mentor

That is an effect of general relativity, and the speed depends on your choice of a coordinate system. You don't have the same issue in special relativity.

None. We are for some reference frames, and we are not for others.

If you move quick relative to the cosmic microwave background (CMB), the universe looks like it has a different age in different directions.

20. Nov 13, 2012

mike777

From the perspective of light itself would it be travelling at almost infinity speed if we removed any other variables?