Perceived time and distance at Speed of light

In summary: If they were moving faster than the speed of light, time for them would appear to pass more slowly for the observer on Earth, and so their twin would appear to have aged more than 5 years.
  • #1
Dave Apsey
11
0
If a spacecraft were traveling at the speed of light (OK, forget the difficulties for now) relative to an observer on Earth then would it appear to an occupant on that ship that they had traveled instantly from one point to another?
 
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  • #2
A spaceship cannot travel at the speed of light. See the FAQ: [thread]511170[/thread].
 
  • #3
Um not saying it is possible but what I am trying to get my head around is the old conundrum of time "passing" slower for an astronaut traveling close to light speed relative to an observer so that when they returned after say ten years their twin brother on Earth would have aged 500 years or whatever. O,K they would have been dead hundreds of years but that is another tangent.

If the speed of light is fixed for both observers then to the observer on Earth it would appear that the ship took say 5 years to reach it's destination. Would it also seem to the traveller that it took 5 years or would it seem to them that the distance had shrunk to zero and so it took no time?
 
  • #4
Dave Apsey said:
If a spacecraft were traveling at the speed of light (OK, forget the difficulties for now) relative to an observer on Earth then would it appear to an occupant on that ship that they had traveled instantly from one point to another?
Although a spacecraft cannot quite achieve the speed of light, it can, forgetting the difficulties, as you say, get as close to it as you want. So here is a formula that you can use to determine what speed you have to travel at to traverse any distance, d, in any time, t:

v = 1 / √[1 + (t/d)²]

Note that the velocity is a fraction of the speed of light and that d and t are in compatible units like light-years and years. Also, be aware that d is the distance for the observer on Earth and t is the time for the spacecraft and it must be greater than zero. No matter how large d is and how small t is (as long as it is not zero), the velocity will always be less than 1.

So although the trip cannot be done instantly, it can be as short as you would like, say a nanosecond. You need to have a calculator with a lot of precision if you want to see what velocity that will take.
 
  • #5
If you let go of 'at the speed of light', which seems mostly irrelevant to your question, and instead ask 'exceedingly close to the speed of light', then (in principle):

1) You could travel to a star while aging only a minute.
2) While going that fast, the distance to the star would appear to be a little less than one light minute, so you still see the star moving less than c (of course, you consider yourself stationary relative to yourself).

This scenario is verified zillions of times a day: muons produced by cosmic rays high in the atmosphere should decay within some hundreds of meters. Virtually none should reach the ground. In fact, almost all reach the ground.
 
  • #6
Thank you for the replies. One further question please.

Does this mean that when our traveller disembarked at their destination, time for them would pass at the same rate as the observer on Earth? Let's not worry about stars moving apart or the universe expanding for now. Their twin would be 5 years older but they would now both age at the same rate.
 
  • #7
Dave Apsey said:
Thank you for the replies. One further question please.

Does this mean that when our traveller disembarked at their destination, time for them would pass at the same rate as the observer on Earth? Let's not worry about stars moving apart or the universe expanding for now. Their twin would be 5 years older but they would now both age at the same rate.

Once they are co-located and not moving fast relative to each other, they age at the same rate. If you ignore gravity (general relativity), you don't even need to worry about co-location, only relative speed. So yes, after disembarking, the traveler would age at the same rate as their Earth twin - remaining 5 years younger (for example).
 
  • #8
Thank you all. So now all we need is faster than light communications and we have a viable colony. Anyone invented a communication device using Quantum Entanglement yet?
 
  • #9
Dave Apsey said:
Thank you for the replies. One further question please.

Does this mean that when our traveller disembarked at their destination, time for them would pass at the same rate as the observer on Earth? Let's not worry about stars moving apart or the universe expanding for now. Their twin would be 5 years older but they would now both age at the same rate.
Yes, they would now both age at the same rate and all frames will agree, but it is only in their common rest frame that we can say the traveling twin would be 5 years younger than the Earth twin. Other frames would not agree. It is only when the traveling twin makes a quick return trip while the Earth twin ages another 5 years that all frames will agree on their age difference.
 
  • #10
Dave Apsey said:
Thank you all. So now all we need is faster than light communications and we have a viable colony. Anyone invented a communication device using Quantum Entanglement yet?

It mathematically proven that entanglement cannot be used to send messages.
 
  • #11
Dave Apsey said:
Thank you all. So now all we need is faster than light communications and we have a viable colony. Anyone invented a communication device using Quantum Entanglement yet?

Also, though relativity per se sets no limit on rocket speed, there are very strong reasons to doubt it will ever be achieved. For example, if you are traveling at a mere 90% of lightspeed (which is not enough for dramatic time dilation), hitting 1 gram particle will release energy greater than the Nagasaki atom bomb. It is really hart to see how effective shielding could exist for near light speed (where a speck of dust could be more dangerous than all the H-bombs in the world).
 
  • #12
Well the speed of light is also a mathematically proven fact but the OPERA experiment casts doubts on that.

Plus if the experiments prove that actions on one of a tangled pair affects the other then they must have observed the effect on the other. In other words they have passed on information. It's just a matter of encoding it surely?
 
  • #13
Dave Apsey said:
Well the speed of light is also a mathematically proven fact but the OPERA experiment casts doubts on that.

Plus if the experiments prove that actions on one of a tangled pair affects the other then they must have observed the effect on the other. In other words they have passed on information. It's just a matter of encoding it surely?

Speed of light limit and its consequences are experimentally established (to a high degree), not mathematical facts. OPERA result, if confirmed, would be new evidence requiring theory modification. Your question was asked in light of current theory and knowledge.

The way quantum entanglement works is that no prior agreement plus local observations can distinguish an entangled particle from an ordinary particle - the results will be random. Only on receiving (by some other means - thus light speed limited) the results of the other party's measurements do you detect correlation that can only be explained by entanglement.

If you want to play the game of 'what if the world is different' this is not the proper forum for that.
 
  • #14
PAllen said:
if you are traveling at a mere 90% of lightspeed (which is not enough for dramatic time dilation), hitting 1 gram particle will release energy greater than the Nagasaki atom bomb.
Could you show the calculations or a reference?
 
  • #15
Passionflower said:
Could you show the calculations or a reference?

The calculation is simple. Using the relativistic KE formula, at around .86c the KE = mc^2 (that is, KE = rest energy). Then compare m c^2 in ergs (for example) to the Nagasaki bomb in ergs (available, for example, from wikipedia). I did it once when I was kid, and once in the last year to ensure I hadn't made a mistake. I remember this fact as as useful order of magnitude to keep in mind.
 
  • #16
PAllen said:
The calculation is simple.
I am sure it is, so why not work out this example, I am sure people will appreciate it. :)
 
  • #17
Passionflower said:
I am sure it is, so why not work out this example, I am sure people will appreciate it. :)

This is really, really silly. I thoroughly outlined the computation. However, to humor a silly request:


1) For v=.9c, gamma = 1/sqrt(1 - v^2/c^2) = 2.294

2) This implies KE = 1.294 m c^2. For energy in ergs, use grams, and cm/sec. Thus for 1 gram, KE will then be: 1.165 * 10^21 ergs.

3) The Nagasaki bomb was about 21 kilotons or about 88 terajoules (http://en.wikipedia.org/wiki/Nagasaki_bomb#Nagasaki). A joule is 10^7 erg. Thus 8.8 * 10^20 erg.

4) 1 gm at .9c has KE of 1.32 Nagasaki bombs.
 
  • #18
Another comment on the realizability of extreme time dilation by hypothetical rockets: the same factor (gamma) specifies both time dilation and kinetic energy of debris. Thus, based on the result that for gamma of 2, 1 gm debris is approx 1 Nagasaki bomb, you can immediately conclude that to travel 100 light years in 1 month ship time, you need time dilation factor = gamma of 1200. Then each gram of debris has KE of 1199 Nagasaki bombs.
 

What is the speed of light and how does it affect our perception of time and distance?

The speed of light is a fundamental constant in physics and is approximately 299,792,458 meters per second in a vacuum. According to Einstein's theory of relativity, the speed of light is also the maximum speed at which anything in the universe can travel. This means that as an object approaches the speed of light, time and distance are distorted for an outside observer.

How does time dilation occur at the speed of light?

Time dilation is a phenomenon that occurs when an object moves at speeds close to the speed of light. This means that time appears to pass slower for objects that are moving at high speeds. This is due to the fact that as an object's speed increases, its mass also increases, making it harder for time to pass for that object.

How does the perception of distance change at the speed of light?

As an object approaches the speed of light, its length in the direction of motion appears to decrease for an outside observer. This phenomenon is known as length contraction. This means that an object traveling at the speed of light would appear to have no length at all to an outside observer.

What is the theory of relativity and how does it relate to perceived time and distance at the speed of light?

The theory of relativity is a fundamental concept in physics that explains how the laws of physics are the same for all observers, regardless of their relative motion. This theory also explains how time and distance are relative concepts and can be affected by an object's speed, particularly at the speed of light.

Is it possible to travel at the speed of light and experience these distortions in time and distance?

Currently, it is not possible for any object with mass to travel at the speed of light. As an object approaches the speed of light, its mass becomes infinite, requiring an infinite amount of energy to continue accelerating. However, scientists continue to explore ways to travel at high speeds and potentially experience these distortions in time and distance.

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