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Is Faster-Than-Light Travel or Communication Possible? (1997)

math.ucr.edu

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Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#21

Post by placeybordeaux »

xg15 wrote: post #16
pippy wrote: post #2

One way would be to fire an entangled photon down both ends of fibre optic cable in the middle. On the sending end you could observe/ ignore it, and on reviving end you'd observe it. We'd have to develop cables capable of preserving the entangled state of light particles, and a beam splitter that's more reliable. You could improve the bandwidth considerably by measuring the polarisation of the particle, even more if…

You couldn't use this for communication as there is no way to "set" the spin of an entangled photon. You'd effectively have two receivers for the same random signal but no way to actually influence the signal. I think there was a HN link a while ago about ideas to use this as a perfectly secure key exchange mechanism for crypto. I don't know how serious this idea is, though.

I believe OP was thinking that there was a way to tell if the photon had already collapsed it's state once before you collapse it by observing it. If that was the case you could convey information by collapsing the state or not. However as far as I know there is no information about if the photo is in a superposition just prior to observing or not.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#22

Post by simias »

sgt101 wrote: post #7
pippy wrote: post #2

One way would be to fire an entangled photon down both ends of fibre optic cable in the middle. On the sending end you could observe/ ignore it, and on reviving end you'd observe it. We'd have to develop cables capable of preserving the entangled state of light particles, and a beam splitter that's more reliable. You could improve the bandwidth considerably by measuring the polarisation of the particle, even more if…

Pretty harsh to downvote this person for writing out an idea - ok, if you know the physics you might dismiss this, but dismiss with words not votes!

TFA addresses this point directly so one can assume the parent didn't read the article before posting.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#23

Post by scobar »

A thought I've had about this is that perhaps the speed limit within a universe (299,792,458 m/s in ours) is relative to a constant at its Big Bang moment such as total energy contained or maximum rate of expansion of space-time. If the smallest particles of our universe are smaller universes and our universe is a tiny particle within a larger universe, then the speed limit within each could be different.

This theory would allow FTL travel in a larger universe relative to the speed limit of our universe, but not FTL communication. FTL travel within one universe may be the escape velocity required to enter the next larger universe outside of the origin universe's boundary. But a transmission, originally traveling FTL, sent into a smaller universe may be slowed by its speed limit (similar to light propagating through a non-vacuum medium). In a universe expanding like ours, the transmission would never reach the destination.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#24

Post by spacehome »

> But multiverses are entirely out of keeping with the Ockham's Razor approach to doing science, and constitute more of a popular interpretation of quantum mechanics than a serious physical theory.

The multiverse is not a theory - it's a logical consequence of a simple theory. This claim is logically akin to claiming that the universe is identical to the observable universe, because to propose that matter exists outside of direct observation is a violation of Ockam's Razor.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#25

Post by spacehome »

Retra wrote: post #18
DennisP wrote: post #15

Earlier quoted context omitted.

Our galaxy is only 100K lightyears across. A civilization that attained only 10% lightspeed could populate the entire galaxy in a million years.

That's still ~500x faster than any man-made object has ever moved.

Well, as long as we're fantasizing, make the speed 0.00001c and a billion years.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#26

Post by archgoon »

rwmj wrote: post #4

Is even near (but slower than) light travel possible? Doesn't your spaceship get destroyed the first time you hit some interstellar dust?

> Is even near (but slower than) light travel possible?

Sure; send yourself as data. ;) Just don't use comcast.

On a less flippant note, in addition to the interstellar dust issue, another fun engineering problem is providing the energy necessary. If you want to send a 50 kilogram person to .99c, you'll have to provide about 4 months worth of the United States current energy production (7 quadrillion btu per month or about 7 exajoules)[1].

Now, if you are wiling to increase your flight time by a factor of ten (reducing yourself to .1 c), you can cut this down to merely 15 days worth of California's total consumption[2]. Mind you, that's just the poor 50 kilogram astronaut. The space shuttle weighed in at 75,000 kilograms (empty). So plan on using about 9 months of the United States energy production capacity for your dinky .1 c.

Given that you'll probably want to STOP once you get where you're going, antimatter engines are probably a prerequisite for relativistic space travel.

[1] http://www.eia.gov/totalenergy/

[2] http://www.ecdms.energy.ca.gov/elecbycounty.aspx

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#27

Post by DennisP »

Retra wrote: post #18
DennisP wrote: post #15

Earlier quoted context omitted.

Our galaxy is only 100K lightyears across. A civilization that attained only 10% lightspeed could populate the entire galaxy in a million years.

That's still ~500x faster than any man-made object has ever moved.

I don't think there's any reason to think that we've reached any kind of serious limit at the moment, or that a technical civilization, say, 10,000 years older wouldn't be much more capable. There's certainly no fundamental physics in the way of that sort of velocity.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#28

Post by jljljl »

Retra wrote: post #18
DennisP wrote: post #15

Earlier quoted context omitted.

Our galaxy is only 100K lightyears across. A civilization that attained only 10% lightspeed could populate the entire galaxy in a million years.

That's still ~500x faster than any man-made object has ever moved.

While true, Isn't this more an "engineering problem" barrier (like the sound barrier) instead of a "theoretically impossible" barrier?

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#29

Post by lmm »

Retra wrote: post #18
DennisP wrote: post #15

Earlier quoted context omitted.

Our galaxy is only 100K lightyears across. A civilization that attained only 10% lightspeed could populate the entire galaxy in a million years.

That's still ~500x faster than any man-made object has ever moved.

It's in the same order of magnitude as the Project Orion speed estimates, based on present-day technology - it ought to be possible if the political will was there.
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