Travelling close to the speed of light

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Discussion Overview

The discussion centers on the implications of traveling at relativistic speeds, specifically 0.99c, and the resulting time dilation effects experienced by a spaceship compared to Earth. Participants explore the relationship between the elapsed time on the ship and the corresponding time on Earth, questioning the distance traveled and the nature of speed at relativistic velocities.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the validity of ignoring acceleration and deceleration stages when considering the spaceship's journey at 0.99c.
  • Another participant calculates that at 0.99c, one day on the ship corresponds to one week on Earth, leading to a conclusion that the ship would be one lightweek away from Earth when it stops.
  • There is a discussion about the implications of time dilation, with one participant suggesting that if the ship traveled for a day, it would not mean the crew experienced traveling faster than light, as they still measure their speed at 0.99c.
  • One participant introduces the concept of Lorentz contraction, comparing it to the behavior of cosmic ray muons, suggesting that the ship's frame of reference alters the perception of distances and times.
  • Another participant mentions that to achieve a significant time dilation ratio, such as 2 million years passing on Earth while traveling, speeds greater than 0.9999c would be necessary.

Areas of Agreement / Disagreement

Participants express varying interpretations of time dilation and distance traveled, with no consensus reached on the implications of these relativistic effects. Some calculations and assumptions are challenged, indicating ongoing debate.

Contextual Notes

Participants acknowledge that the calculations depend on specific relativistic formulas and assumptions about the nature of time and distance in different frames of reference. There is uncertainty regarding the exact speeds required for certain time dilation effects.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in special relativity, time dilation, and the effects of traveling at relativistic speeds, particularly in theoretical physics and astrophysics contexts.

kbaumen
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I'm sure this has been asked a million times but a quick search didn't give me quite definitive answers so here it goes:

Imagine a spaceship traveling away from the Earth at 0.99c. For argument's sake, let's ignore the acceleration and deceleration stages of the ship (is that valid?). Simply the ship starts traveling at 0.99c. Now the ship stops and according to the ship's clock, they have traveled for a day which on Earth is a year or so (or whatever the difference works out to be). How far is the ship from the Earth? Surely if it has been a year, then it has traveled almost a lightyear within a day (according to ship's clock) which can't be true. So where and why is it?

P.S. This is not homework, merely a question that came to mind when watching this.
 
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kbaumen said:
I'm sure this has been asked a million times but a quick search didn't give me quite definitive answers so here it goes:

Imagine a spaceship traveling away from the Earth at 0.99c. For argument's sake, let's ignore the acceleration and deceleration stages of the ship (is that valid?). Simply the ship starts traveling at 0.99c. Now the ship stops and according to the ship's clock, they have traveled for a day which on Earth is a year or so (or whatever the difference works out to be). How far is the ship from the Earth? Surely if it has been a year, then it has traveled almost a lightyear within a day (according to ship's clock) which can't be true. So where and why is it?

P.S. This is not homework, merely a question that came to mind when watching this.
At 0.99c, one day on the ship is one week on Earth so the spaceship will be one lightweek away from Earth when it stops. For it to be one year, the ship would have to be traveling at greater than 0.999996c. Aside from that minor detail, everything else you described is correct.
 
ghwellsjr said:
At 0.99c, one day on the ship is one week on Earth so the spaceship will be one lightweek away from Earth when it stops. For it to be one year, the ship would have to be traveling at greater than 0.999996c. Aside from that minor detail, everything else you described is correct.

Ok, yes, I admit, that year was a shot in the dark with the main thing being that different times have elapsed.

So you're saying that the ship is then a lightweek away from the Earth when it stops. But the ship's clock says that is has traveled for only a day. Does that mean that the people on the ship experienced traveling faster than the speed of light?
 
kbaumen said:
Ok, yes, I admit, that year was a shot in the dark with the main thing being that different times have elapsed.
Actually, after I posted, I looked at the video and it says that at 99% of the speed of light, a single day onboard is a full year of Earth time (4 minutes into the video). This is wrong which makes me wonder how many other their other numbers are wrong.
kbaumen said:
So you're saying that the ship is then a lightweek away from the Earth when it stops. But the ship's clock says that is has traveled for only a day. Does that mean that the people on the ship experienced traveling faster than the speed of light?
No, they still measure their speed at 0.99c, however, in the Frame of Reference in which they are at rest during the trip, everything around them is length contracted and traveling past them and the Earth only has to travel behind them by about a lightday while their destination was only a lightday away at the beginning of the trip.
 
kbaumen said:
So you're saying that the ship is then a lightweek away from the Earth when it stops. But the ship's clock says that is has traveled for only a day. Does that mean that the people on the ship experienced traveling faster than the speed of light?

Space Lorentz contracts towards you if you are in the spaceship. It's the same way with the cosmic ray muon that can only live for a very short while, not long enough to travel from the upper atmosphere to the surface of the Earth (even at the insane speeds they travel), yet we still measure them a large number of muons at sea level. The muon essentially sees Earth "Lorentz contract" up towards itself so that it is able to make the journey from upper atmosphere to sea level within its very short lifetime.

You can travel across the galaxy if you want in your lifetime (200,000 light years across about). Assuming you travel at like 0.9999c and are comfortable with the rest of Earth getting like 2,000,000 years older while you complete the journey..
 
dydxforsn said:
You can travel across the galaxy if you want in your lifetime (200,000 light years across about). Assuming you travel at like 0.9999c and are comfortable with the rest of Earth getting like 2,000,000 years older while you complete the journey..
At 0.9999c, the ratio of Earth time to ship time is only 70. You have to go way faster than that to get 2 million years to pass on Earth. As I said earlier, even 0.999996c is less than a ratio of one Earth year to one ship day.
 
Ok, cheers guys, that explains it.
 

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