# Interstellar astronauts would face years-long communication delays due to time dilation

#### VVet1968

The very first science fiction book that I read in the 1950s was Heinlein's "Time for the Stars" which focused on this same problem. Nearly seventy years later I still remember being blown away by the concepts as it was my first introduction to relativity. Sure, it's not an "adult" scifi, but if you've never read it, pick up a copy.

#### Helio

Interstellar travel is perhaps the best case for using particle entanglement for instant communication since one set of them must be carried to the distant locations.

Of course, there has been no means found for a working model, but scientists are very clever, so I bet they’ll get ‘er done…someday,

#### billslugg

Entanglement cannot be used for communication for two basic reasons. The sender can only send a random message. The timing at the receiving end is either fixed by a predetermined schedule or is indeterminate. Neither a message nor a time can be communicated.

#### Helio

Entanglement cannot be used for communication for two basic reasons. The sender can only send a random message. The timing at the receiving end is either fixed by a predetermined schedule or is indeterminate. Neither a message nor a time can be communicated.
In a pair of entangled particles, my limited understanding is that whenever one particle collapses, say "heads", the other will instantly collapse to say "tails", regardless of distance. This would allow for excellent and instant communication. BUT, of course, the problem is getting the "first" one to say "heads" in a way that doesn't break their entanglement. My hope is that someone will find a way to do it.

billslugg

#### Helio

"But signals sent from the spacecraft to the destination would not arrive until shortly before the craft itself got there, at which time all of the sent messages would pile up on each other, announcing the arrival of the craft."

This is interesting because the travelers who, say reach alpha Centauri in one year, will argue their travel transmissions were never more than 1 year from Earth. But those on Earth would, I think, disagree and say that, for instance, when the travelers were 6 months out (half way) then Earth would see the transmission from 2.15 lyrs distance, so it would take 2.15 years to get the transmission, which is 1.15 years after the ship arrived.

So, WAIM ( What Am I Missing?).

These realities mean that communication with near-light-speed spacecraft would be very challenging. All interstellar vehicles must operate independently, because after a certain amount of time, they will be cut off from Earth. If a problem arises, they will be able to tell people on Earth about it, but they won't be able to hear a response.
Under SR, I don't see how this is true, eventually any signal sent will take one year for every lightyear distance the ship is from Earth when the transmission was sent. WAIM?

Also, distant colonies wouldn't know about the launch of a spacecraft in their direction until shortly before the craft arrived there.
This is puzzling as well. Assuming SR effects reduce their travel time (or distance) then any Earth broadcast will come after their arrival, not before.

If the spacecraft sends a transmission to the destination after it has slowed enough, then this would make sense, admittedly.

#### unclefishbits

I have a silly saying: "No internet is fine. Having internet is great" Bad internet is a frustrating nightmare".
Now put this at an existential level of loneliness and hope, and you've ramped up that situation to horrific levels.
I'd just right off that comms won't work, and make sure you plan ahead to never exist in context of the earth or human race, ever again. Seeding the galaxy is going to be quite lonely. And we thought Tom Hanks in Castaway seemed crazed with lack of connections.

Time to rewatch Aniara.

#### dgmesser

In a pair of entangled particles, my limited understanding is that whenever one particle collapses, say "heads", the other will instantly collapse to say "tails", regardless of distance. This would allow for excellent and instant communication. BUT, of course, the problem is getting the "first" one to say "heads" in a way that doesn't break their entanglement. My hope is that someone will find a way to do it.
This would violate Einstein's principle that information cannot travel faster in an inertial frame than the speed of light. If that is violated, it would be a very big deal.

billslugg

#### dgmesser

"But signals sent from the spacecraft to the destination would not arrive until shortly before the craft itself got there, at which time all of the sent messages would pile up on each other, announcing the arrival of the craft."

This is interesting because the travelers who, say reach alpha Centauri in one year, will argue their travel transmissions were never more than 1 year from Earth. But those on Earth would, I think, disagree and say that, for instance, when the travelers were 6 months out (half way) then Earth would see the transmission from 2.15 lyrs distance, so it would take 2.15 years to get the transmission, which is 1.15 years after the ship arrived.

So, WAIM ( What Am I Missing?).

Under SR, I don't see how this is true, eventually any signal sent will take one year for every lightyear distance the ship is from Earth when the transmission was sent. WAIM?

This is puzzling as well. Assuming SR effects reduce their travel time (or distance) then any Earth broadcast will come after their arrival, not before.

If the spacecraft sends a transmission to the destination after it has slowed enough, then this would make sense, admittedly.
The strongest conclusions of the article dont really apply at Alpha Centauri distances, so that doesnt make a suitable thought experiment. Since AC is "only" 4 ly away, even a spaceship traveling at 10% of the speed of light can arrive within a human lifetime (as measured by clocks at both the origin and on the ship).

Take as a better example a star at distance 1000 ly. Then travel at near the speed of light becomes necessary, and in that case the elapsed time on the ship for a 1-g ship is only about 12 years. Doable, from an astronaut perspective. However, the results of the paper indicate that the event horizon is about a year. Any signals leaving earth after a year never catch up with the ship as long as its acceleration persists. This is because the ship is approaching the trajectory of a photon leaving the origin about 1 year later than the ship.

From a destination perspective, the paper shows that signals are received from the ship during an approx two year period prior to arrival, even though the ship has taken more than 1000 years on its journey. This is because the ship is almost keeping up with any photons it emits in its direction of travel.

#### Helio

This would violate Einstein's principle that information cannot travel faster in an inertial frame than the speed of light. If that is violated, it would be a very big deal.
"Spooky action at a distance", as Einstein called it. "Instantaneous" action has been demonstrated to be correct, thanks greatly to John Bell.

#### dgmesser

"Spooky action at a distance", as Einstein called it. "Instantaneous" action has been demonstrated to be correct, thanks greatly to John Bell.
"Action" yes." Information transfer", no. That is because the action is completely random (maximum entropy) and thus conveys no information.

billslugg

#### Crashlander

Due to the mind-blowing distances and speeds required, interstellar travel remains a major spaceflight challenge. But new research highlights yet another hurdle: Communication blackouts.

Interstellar astronauts would face years-long communication delays due to time dilation : Read more
Hey Paul, You might mention that while for "...the passengers...only weeks or months might pass.", back here on earth years, decades or centuries might pass. In that sense, it's a one way trip. An effect explored in great (and often sad) detail in 20th century science fiction. Many stories (including I vaguely recall the recent movie 'Interstellar'), describe cases where earth bound technology surpasess that of those in transit and beats them to their destination. The reality of the universe in general and space-time in particular is mind boggling.

My money is on finding a way to 'fold' or (dare I say it?) 'warp' space-time, and base communications on an 'entangled particle' system. Problems solved. And your efforts to educate the general public, greatly appreciated.

However, I would recommend Brian Greene's 'The Fabric of the Cosmos' to anyone (and I do mean anyone) wanting a much better and comprehensive discussion and explanation of all this (and the history preceding it). Greene's genius is his ability to clearly, succinctly and understandably describe all this and more to the lay person, while also interesting those with an appetite for the theory and math involved through extensive chapter notes.

Bon appetite!

#### billslugg

Energy requirements pretty much rule out interstellar travel within a human lifetime. Even fusion technology is insufficient.

Entanglement cannot be used to convey information, only random noise.

"Folded space" or "wormhole" is not ruled out but you must go to the far end first to set up the other end. Only individual subatomic particles could make the trip.

#### Marc2001

We will figure out how to travel & communicate faster than light when we learn more about physics. We will not be satisfied being restricted to the solar system forever!

#### George²

One ticket to TC², please! I'll be sleeping on the flight, so the presence or absence of superluminal chatter with my remaining Earth relatives is of no concern, nor is explaining myself to the local Taucetirians before I've even arrived. When I land and we sit down at the table for a drink, we'll talk as much as we want.

#### mamouda

We need to look after quantum entanglement to solve this Pb

#### jamestmallow

Interstellar travel is perhaps the best case for using particle entanglement for instant communication since one set of them must be carried to the distant locations.

Of course, there has been no means found for a working model, but scientists are very clever, so I bet they’ll get ‘er done…someday,
Interesting video on the topic. Kipping thinks it can't work.

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billslugg

Well a couple of things, the major one being using 1 g acceleration to near the speed of light.
The amount of fuel, even using a nuclear powered vehicle, with an ISP far greater than we have today, would take an enormous amount of fuel. The mass of which then makes accelerating (and having the nearly same amount of fuel to decelerate as you near your destination) such a mass essentially impossible.
Note, nobody is going to fly to another star system except to land there. There would be no possibility of return.
Which means not only do you need the mass of the landers, but sufficient supplies/equipment to give the people landing on this new world, a chance to survive. The number of people sufficient to start a new colony would also be a major factor. Hard to believe you could do it with less than 100 people.
Considering how long the trip would be, and the mass of people, food, water and power generation, along with the descent stage and materials, the mass of said spacecraft would be huge. Which drives the mass of the propellent required.
Second problem is space is not empty, and at anywhere near the speed of light, no material would withstand the energy imparted from collisions even at space densities.
C/5 might be possible, but even then, going 4 LY would take more than several decades, and you wouldn't be going fast enough for time dilation to reduce the length of the trip.
As to communication, its not just the time it takes for a signal to reach its destination, its the amount of power one would need to send a signal that distance, which for the spacecraft at least would be highly problematical, to have both that much available power and an antenna large enough for the task.

#### jamestmallow

Energy requirements pretty much rule out interstellar travel within a human lifetime. Even fusion technology is insufficient.

Entanglement cannot be used to convey information, only random noise.

"Folded space" or "wormhole" is not ruled out but you must go to the far end first to set up the other end. Only individual subatomic particles could make the trip.
Not to mention the dangers of running into a grain of sand at 90% light speed even if was possible. We are effectively alone and isolated. Breakthrough Starshot is an interesting concept though.

billslugg

#### jamestmallow

We will figure out how to travel & communicate faster than light when we learn more about physics. We will not be satisfied being restricted to the solar system forever!

billslugg

#### Helio

Interesting video on the topic. Kipping thinks it can't work.
Yes, nice one! There's been a great deal of effort to find ways to use entanglement since entanglement is the only thing that gives instantaneous action at a distance. If there is a law, and it seems there might be one, that specifically disallows this for communication then even that would advance physics, though unhelpful for communication. I'm optimistic, but the pessimists currently have the best odds.

#### Classical Motion

We are not going anywhere, the best to hope for is an arrival. And for that it would take inconceivable technology, and we could only interest them for life is scarce. Probably a covert study. With the tech to transverse space, I wouldn't think custom mass and resources would be a problem.

Or the cosmos is full of insects. And time/distance is our only protection.

So, it will never happen, or, if it did, this world and the life on it will be forever changed. It will start a new history. Everything in ones life will change. All attitudes about everything will change.

It will be the largest disturbance. Probably the largest violence. Even if the arrival was peaceful and welcome.

We will demonstrate our true nature to them at once.

We might be the insects. Supposition is fun.

#### billslugg

Kipping says there is proof that entanglement cannot transmit information in Hawking's Chronology Protection Conjecture.

Also, actual measurement shows the speed of the "spooky action" is at least 10,000 c.

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Helio

#### Classical Motion

There might be a way to detect entanglement change with detection instead of measurement. Some of these quantum sensors and some of these condensates might allow this.

The detection of change is all that's needed, not the beginning states. That change is instant, so that change can be modulated with infinite bandwidth. In theory.

The only limit is processing speeds. So it could take milliseconds for a dozen light years distance. Not too shabby.

If entanglement as proposed is true. But if the entanglement only works with measurement, then it's useless. In other words, the flip only occurs with measurement. And NOT at same time other is flipped. Or you MUST measure to detect.

If a system where measurement(stimulus) is only needed to transmit, and could detect(without stimulus) the other end, then we might do it.

The dynamic would require zillions of entanglements and the transportation of such.

Or some method of long distance entanglement. The best solution.

#### billslugg

Quantum mechanics says you cannot detect the state of something in superposition. That's what superposition means. "Above position", that is, there is no position, until it chooses one state over another. You must collapse it in order to measure it. You can't even tell whether it was collapsed or not before you measured it. When you open the box all you see is a randomly selected 1 or 0. You don't know if he did it or if you did it.

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