Time and space travel at light speed

In summary, the conversation discusses the concept of traveling at or near the speed of light and the effects it has on time. It is revealed that when one person is traveling at this speed, time passes differently for them compared to others who are not moving as fast. There is no "one true" time base and it is relative to the individual's experience. This is demonstrated through the example of Bill and Ted, where they both experience different amounts of time passing even though they are traveling together. Additionally, the conversation touches on the issue of measuring speed when traveling at these high speeds and the potential implications it can have on one's perception of reality.
  • #1
mike777
12
0
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 traveling 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 traveling 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?
 
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  • #2
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
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 let's say. But if you mean on Teds watch it took nine years then much more time elapsed for Bill maybe 100 years, let's 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
micky_gta said:
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 let's say. But if you mean on Teds watch it took nine years then much more time elapsed for Bill maybe 100 years, let's say?

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

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.

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
Chestermiller said:
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.

Good job, Chestermiller. Good to see you jump in there.
 
  • #6
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 traveling 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
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 traveling at thousands of times the speed of light??
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
mike777 said:
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 traveling 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...
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
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
mike777 said:
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...
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
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.
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
Many thanks for all your replies. Little Ted and I will study special relativity and hopefully post more concise questions in future!
 
  • #13
And please answer the questions I give to you.
 
  • #14
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 traveling '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 traveling 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
micky_gta said:
we can NEVER figure this out.
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
Thanks. Yes I suppose that's what I was getting at with Little Ted thinking he was traveling 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 traveling 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
mfb said:
We move with ~400km/s relative to the cosmic microwave background.

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
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
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 traveling at light speed from their point of view.
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.

What are the implications if we ourselves are already pretty near the speed of light
None. We are for some reference frames, and we are not for others.

for starters would it mean the universe is very significantly older than we think when considered from outside our reference point?
If you move quick relative to the cosmic microwave background (CMB), the universe looks like it has a different age in different directions.
 
  • #20
From the perspective of light itself would it be traveling at almost infinity speed if we removed any other variables?
 
  • #21
Algr said:
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?

Yes and no.

Direct observations are not going to show an immediate and dramatic shift.

But a calculation of how far away the [moving] far-away star is "right now" is going to change dramatically during the acceleration because the standard of synchronization is changing. The notion of "right now" is not fixed.

The speed of the star is sub-light in any inertial frame of reference. The fact that its "speed" is greater than light speed in a particular accelerating frame does not violate any physical law. One way of thinking about it is that this speed is more a matter of moving coordinates past the star than moving the star past coordinates.
 
  • #22
mike777 said:
From the perspective of light itself
That is not possible in relativity.
 
  • #23
jbriggs444 said:
But a calculation of how far away the [moving] far-away star is "right now" is going to change dramatically during the acceleration because the standard of synchronization is changing. The notion of "right now" is not fixed.

The speed of the star is sub-light in any inertial frame of reference. The fact that its "speed" is greater than light speed in a particular accelerating frame does not violate any physical law. One way of thinking about it is that this speed is more a matter of moving coordinates past the star than moving the star past coordinates.

I'm not sure, but it sounds like you are saying "Yes".

(I'm presuming that the start and end locations are not moving relative to each other.) Any measure of speed must involve two points in time. If an object goes from 9 light years away to .9 light years away in anything less then 8.1 years, then by definition it has moved faster then light relative to the observer. You seem to be saying that this IS possible, provided that the observer & clock are accelerating or in a gravity well?

========
Another post mentions "Seeing galaxies receding at faster then light." Due to the expansion of the universe. Is this really possible? I thought that all light from such galaxies would be red shifted to zero, and thus would never reach Earth. Such galaxies have ceased to exist from Earth's perspective because no information from them will ever reach us. (Unless the universe turns around and starts contracting!)
 
  • #24
Algr said:
I'm not sure, but it sounds like you are saying "Yes".

Yes, it is possible for objects to move faster than light -- if you are not careful about what you mean by "move".

For instance, take a rotating coordinate system in which the Earth is motionless. In this coordinate system, Alpha Centauri revolves around the Earth once every 23 hours 56 minutes.

That's a lot faster than light. But it's not what you or I would think about as "really" being faster than light. Like I said before, that's more like moving coordinates past the object and not so much moving the object past coordinates.

An accelerating frame in Special Relativity is the equivalent of a rotating coordinate system. It's just that you are rotating in four dimensions and it's a hyperbolic rotation instead of a circular rotation.
 
  • #25
Big Ted should teach Little Ted the definition of velocity. It is dx/dt and not dx'/dt. I.e. Both x and t need to be measured in the same inertial reference frame or it simply isn't velocity.
 
  • #26
micky_gta said:
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.

Everything in the univere IS moving at close to the speed of light, including you right now as you read this. You just have to pick a frame of reference in which that statement is true for the object you are considering.

In other words, your statement "everything in the universe can be moving close to the speed of light" is meaningless becasue you have not said RELATIVE TO WHAT? Speed is always relative but you have mentioned it as though it is absolute.
 

1. What is the speed of light and why is it important in time and space travel?

The speed of light is approximately 299,792,458 meters per second in a vacuum. It is considered the fastest possible speed in the universe and is important in time and space travel because it is a fundamental limit that cannot be exceeded.

2. How does traveling at light speed affect time?

According to Einstein's theory of relativity, time is relative and is affected by the speed an object is traveling. As an object approaches the speed of light, time slows down for that object relative to an outside observer. This means that time would pass slower for someone traveling at light speed compared to someone on Earth.

3. Is it possible to travel faster than the speed of light?

According to our current understanding of physics, it is not possible to travel faster than the speed of light. As an object approaches the speed of light, its mass increases and it would require an infinite amount of energy to accelerate it to that speed.

4. Can time travel be achieved by traveling at light speed?

While traveling at light speed can cause time dilation, where time passes slower for the traveler, it is not considered time travel in the traditional sense. Time travel to the past is currently not possible according to our current understanding of physics.

5. What are the potential consequences of traveling at light speed?

Traveling at light speed can have significant consequences for the traveler, such as time dilation and increased mass. It can also have potential effects on the surrounding space and time, such as causing disturbances in the fabric of space-time known as gravitational waves.

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