Traveling at Near-Light Speed: Time Dilation and Your Perspective Explained

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Discussion Overview

The discussion revolves around the effects of traveling at near-light speed, particularly focusing on time dilation and how time is perceived differently by travelers versus stationary observers. Participants explore theoretical scenarios involving long-distance space travel and the implications of relativistic speeds on time experienced by individuals in a spaceship compared to those on Earth.

Discussion Character

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

Main Points Raised

  • Some participants propose that traveling just short of the speed of light would result in a significantly shorter travel time from the perspective of the traveler compared to an observer on Earth.
  • Others argue that while it may take about a year to reach a destination from Earth's perspective, the traveler could experience only a fraction of that time.
  • A participant questions the feasibility of traveling to distant locations, such as the center of the galaxy, and suggests that relativistic speeds could allow for significant distances to be covered in a short subjective time.
  • Some contributions discuss the implications of constant acceleration at 1 g, suggesting that it could enable travel across vast distances within a single lifetime from the traveler's perspective.
  • A later reply introduces the concept of 'rapidity' as a factor in calculating travel times and distances in relativistic travel scenarios.
  • Another participant mentions the time-difference between the traveler's experience and the messages received from Earth, highlighting the complexities of time dilation over long journeys.
  • There is a correction regarding the distance to the center of the galaxy, clarifying it is approximately 50,000 light-years rather than 50 million, but the discussion acknowledges the long travel times involved regardless.
  • Some participants reference fictional scenarios, such as those from Ursula LeGuin's works, to illustrate theoretical concepts of relativistic travel.

Areas of Agreement / Disagreement

Participants generally agree on the concept of time dilation and its effects on travel at relativistic speeds, but multiple competing views remain regarding the implications of these effects on long-distance travel and the calculations involved. The discussion remains unresolved on some specific points, particularly regarding the exact nature of distances and subjective experiences of time.

Contextual Notes

Limitations include assumptions about constant acceleration and the definitions of terms like 'rapidity' and 'time-difference,' which may not be universally understood or agreed upon. The discussion also reflects varying interpretations of relativistic effects and their implications for space travel.

csnsc14320
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I'm just wondering:

Say you obtain a spaceship that can travel just short of the speed of light.

If you travel to ao object "one light year" away, would it take you one year to get there from YOUR perspective, or from some stationary person on Earth's perspective?

Because I know that the closer you get to the speed of light, the slower time passes for you relative to people at rest

So, basically, I'm wondering if, from your perspective, it takes one year to get there or some shorter time
 
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If you travel just short of the speed of light - from the perspective of someone on Earth - then to them it takes you about a year to get there. From your perspective, it could be a very short period of time.
 
russ_watters said:
If you travel just short of the speed of light - from the perspective of someone on Earth - then to them it takes you about a year to get there. From your perspective, it could be a very short period of time.

really? because i have always wondered that, if it is 50 million light years to the center of our galaxy, then there is really no hope of ever getting too far since that's an excruciatingly long amount of time

so if you were to travel just short of light speed, you could theoretically travel huge distances in what seems like a short amount of time for you? (although your home would be 50 million years older)
 
csnsc14320 said:
really? because i have always wondered that, if it is 50 million light years to the center of our galaxy, then there is really no hope of ever getting too far since that's an excruciatingly long amount of time

so if you were to travel just short of light speed, you could theoretically travel huge distances in what seems like a short amount of time for you? (although your home would be 50 million years older)
Yep, that's right :smile:
 
diazona said:
Yep, that's right :smile:

crazy stuff :p

thanks
 
Even crazier things happen if you don't return. Say your friends back home are sending you messages. How many years would you see go by in their messages?

Depends on your means of getting close to lightspeed, but say you're accelerating at 1 gee. Then once you've traveled more than a few light-years, the time-difference between when you left and the messages from home you receive on coming to a halt at the Galactic Core, or wherever, is just 1.9 years. That's the difference between light's travel time and yours, regardless of how far you go, if you accelerate constantly the whole way.
 
think about russ' response above for a moment - from the perspective of someone on Earth watching you, it would take you just over a year to get there - BUT from your own perspective, you will have traveled that "one light year" in far less that a year - which brings up the question of "just how far away is that star?" from your perspective, you have either traveled faster than the speed of light, or the star is not "really" 1 LY away.

from Earth's perspective, again, it is 1 LY, but from your ship's perspective, it is perhaps only a few million miles (depending on how close to C you are traveling). if you could actually travel at C, people on Earth would say it took you exactly one year to arrive, but from your perspective, you would be there instantaneously, since time does not exist at C. so, how far away is that star, really?
 
csnsc14320 said:
really? because i have always wondered that, if it is 50 million light years to the center of our galaxy, then there is really no hope of ever getting too far since that's an excruciatingly long amount of time

so if you were to travel just short of light speed, you could theoretically travel huge distances in what seems like a short amount of time for you? (although your home would be 50 million years older)
FYI, it is "only" about 50,000 light-years to the center of our galaxy, not 50 million. But even that is an excruciatingly long distance / travel time.
 
PhysicsDilettante said:
FYI, it is "only" about 50,000 light-years to the center of our galaxy, not 50 million. But even that is an excruciatingly long distance / travel time.

50,000 ly to the Core and back. The current best estimate puts the Core about ~26,000 ly away. But if one was flying to the very heart of the Virgo Super-Cluster then it'd be about 50 million ly. Would be quite a challenging trip.

Ursula LeGuin's Nearly-as-Fast-as-Light (NAFAL) starships, from her Hainish stories, can do 256 ly in 10.65 hours of subjective time. If acceleration was (near) instantaneous then the time-distortion factor (TDF) would be ~210,000. Constant acceleration would need to be at 25,000 g and a peak TDF of ~2,700,000. For constant acceleration the ratio of cosmic time to subjective time is ln(2*TDF)/(TDF) for high TDF (above about 10.)

BTW what I'm calling TDF is normally referred to as γ <gamma>, and is equal to ~1/√[1 - (v/c)²] so the speed is incredibly close to lightspeed, a gnat's whisker per century difference in speed for that trip to Virgo...
 
  • #10
they say that if you could continually accelerate at one g (from your perspective I guess) then you could go virtually anywhere in the universe in one lifetime. I've always found that to be a very surprising result. I think you have to use 'rapidity' to calculate it.
 
Last edited:
  • #11
granpa said:
they say that if you could continually accelerate at one g (from your perspective I guess) then you could go virtually anywhere in the universe in one lifetime. I've always found that to be a very surprising result. I think you have to use 'rapidity' to calculate it.

See

https://www.physicsforums.com/showthread.php?p=1193451#post1193451.
 
  • #12
around 2.4 months from your perspective. 1 year to the rest of us.
 

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