# Noob with a question about lightspeed travel.

1. Feb 27, 2008

### JikeSpingleto

I would like to preface this idea by stating that I am a total noob and in no way have any right to be here haha.

I was just watching a show on the science channel "Carl Sagan's Cosmos". I am an avid viewer of basically anything on TV about the universe or physics in general. I find everything about it completely fascinating.

Anyways, In this show Carl sagan talked about the Time Dilation phenomenon and its impact on travel at the speed of light. It got me thinking about if a human were to travel at that speed, lets say to a star 50 lightyears away. He stated that it would take that individual 50 years to reach his destination but due to time dilation, those on earth would experience some thousand years of time because of this. I am wondering how communication between earth and the traveler would be possible with someone traveling at speeds approaching lightspeed. Obviously verbal communication would be impossible due to the vast differences in speed between sound and light. However, i may be incorrect, but i believe that thoughts themselves occur at a speed much faster than sound. So my thinking is this: If a human's brain who is travelling at speed approaching the speed of light were to be connected to a device which translated thought and converted it into laser flickers or something of the sort and it was pointed at the earth. Then the travellers thoughts could be used as a communication medium, but even moreso the communication process would occur in direct opposition to the time dilation phenomenon because it would not take earth the same thousands of years to receive the communication from the traveler. Am i making sense? is this possible? If an object travelling from one point to another at the speed of light were itself to broadcast light back to its origin would time dilation apply?

Confusing, but interesting... am i completely assinine in making these statements, or proposing these questions? Can someone explain to me what they believe the outcome would be? I'm very interested to hear what a professional's opinion on this is.

2. Feb 27, 2008

### RandallB

NO and NO

I did not see the show but I have no doubt that Carl Sagan did not comment on “ travel at the speed of light”, but maybe “travel NEAR the speed of light”.
And the only “time dilation” to apply would be on the traveler aging much slower than time on earth.

Nothing indicates ANY kind of communication (including ESP) that could confirm an arrival at the destination back to earth any faster that the information could be transmitted by light. That would be 50 years after the landing (not thosands). If the earth observer saw a speed that indicated a travel time of 55 years they could deduce and assume an arrival at 55 years, but could not know it be true until confirmed in 50 more years at best.

The time dilation only address the ageing of the traveler as being much much smaller than the travel time as measured in the Earth / Destination reference frame.

3. Feb 27, 2008

### JikeSpingleto

That is what I meant, NEAR (approaching) i misspoke in the first couple of sentences.

You are right, Carl Sagan did not speak about communication between the traveler and the observer. His statements on the effects of near lightspeed travel and time dilation got me thinking about how communication could possibly occur THROUGHOUT the entire journey. What im asking is:

If a device capable of quantum computing (lets just leave people out of this.) was launched at a star 50 Lightyears away traveling at 99.9% the speed of light, but was simultaneously transmitting information back to its exact point of origin via some sort of light based communication medium. Would the information sent back be received at the point of origin within a timeframe relative to the distance from the traveling device?

ie: if the device is 2 lightdays away it would send data back to earth which would be received approx. 2 days from the exact time of departure.

Or would the data be received at 4 days from the exact departure time due to the device's distance from the point of origin, and the data having to travel the same distance back?

4. Feb 27, 2008

### DLuckyE

Don't know all that much about relativity and all either, but unless i'm mistaken if you'd travel to a star 50 lightyears away at near light speed, you'd be there in no time (just seconds if you're really close to light speed) but it would seem like 50 years for the rest of the universe.

So basically at lightspeed you'd be everywhere instantly. Would be rather hard to stop where'd you like to be since there'd be no distance either...

Makes me wonder how light knows where it's going

5. Feb 27, 2008

### Cinimod

I'm no expert, but from what I can tell, the apparent difference in time would be made up during the acceleration up to and the deceleration from 0.999c. Otherwise you would end up with paradoxes. One I can think of off the top of my head if the time difference was not compensated for is a father being younger than his son/daughter.

Hopefully someone who knows more on the subject will be able to verify that statement.

6. Feb 27, 2008

### RandallB

the data will be received at ABOUT 4 days from the departure time. Due to the device's distance from the point of origin you must allow time for the data to travel back.

The "you'd be everywhere instantly"
and "time would be made up during the acceleration"

You are close to answering your own questions -
Take some time to read up on "Twins Paradox" & Think about it for awhile - give yourself a few days to actually think about what you read - See how well you can do,

Start with search function on these threads and in Google. You will find plenty.

7. Feb 27, 2008

### chroot

Staff Emeritus
Welcome to PF, JikeSpingleto.

Unfortunately, the ultimate limit imposed by relativity is that information cannot travel faster than light. It doesn't really matter how you package you transmissions, or whether your travelers use neurally-linked computers or plain ol' keyboards. Ultimately, all of the information you want to transmit is encoded into electromagnetic radiation -- light -- and light obviously travels at the speed of light.

- Warren

8. Feb 27, 2008

### JesseM

Why quantum computing? Everything on board the ship will be slowed down by exactly the same factor in the Earth's frame--a normal computer, a quantum computer, a human brain, an ordinary clock, etc.
Yes, if the object was 2 light-days away in the Earth's frame, then it would take 2 days in the Earth's frame for the signal to reach Earth. But suppose the ship was sending signals at regular intervals according to its own clocks--say, every time a clock on board the ship struck 12 pm, a signal would be sent back to Earth (once every day in the ship's frame). With a speed of 0.999c in the Earth's frame, the clock on board the ship is slowed down by $$\frac{1}{\sqrt{1 - 0.999^2}}$$ in the Earth's frame, or about 22.366, so in the Earth's frame the signals are only sent out once every 22.366 days. The Earth does not actually receive the signals this often though--you also have to take into account the Doppler effect caused by the fact that the ship is moving so successive signals are sent from different distances. In 22.366 days the ship will have moved 0.999*22.366 = 22.344 light-days further away, so each successive signal has 22.344 more light-days to travel to reach Earth than the previous one, so the Earth actually receives signals once every 22.366 + 22.344 = 44.7 days. On the other hand, if the ship was moving at 0.999c towards the Earth each signal would have 22.344 fewer light-days to travel to reach Earth, so the time between signals on Earth would be only 22.366 - 22.344 = 0.022 days. You could also get these numbers directly using the equation for the relativistic doppler effect which incorporates both time dilation and the different distances successive signals must travel, which tells you that in the first case the signals are received at $$\sqrt{\frac{1 - 0.999}{1 + 0.999}}$$ = 0.022 the rate they are emitted in the emitter's frame (i.e. once every 1/0.022 = 44.7 days), and in the second case the signals are received at $$\sqrt{\frac{1 + 0.999}{1 - 0.999}}$$ = 44.7 the rate they are emitted in the emitter's frame (i.e. once every 1/44.7 = 0.022 days).

Last edited: Feb 27, 2008