# Time Dilation Effects on a Probe

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1. Feb 9, 2015

### bodykey

I was having a discussion with a colleague earlier today regarding exoplanet exploration. Being that the biggest challenge for mankind to travel to distant planets is that time dilation would make such an extreme effect that by the time we reached our destination thousands of years may have already passed on Earth, my thought was that the Voyager is traveling roughly 5% the speed of light currently and although the time dilation effects would be minimal at this speed, the radio communications are unaffected by the time dilation itself.

My friend argued alternatively, saying that time dilation would affect the radio communication equally and would be greatly affected if a probe were moving close to the speed of light.

So I suppose my question is this:

Would it be entirely unreasonable to send probes to potentially habitable planets right now at close to the speed of light so that we don't have to waste a human life with such an incredible journey? My thought is that even though the probe is traveling this fast, it can still communicate with light waves (lasers, etc.) which are already moving at the speed of light, in which case time dilation wouldn't affect them...am I right here?

2. Feb 9, 2015

### phyzguy

(1) The Voyager probes are currently traveling at about 16 km/sec relative to the sun. This is about 0.005% of the speed of light, not 5%.
(2) We are not even close to having the technology to accelerate something as massive as a space probe up to even 1% of the speed of light.
(3) If we could accelerate a space probe up to a significant fraction of the speed of light, you are correct in that it could still communicate with us with light signals (or radio waves, or any electromagnetic signal). These signals would be Doppler shifted because of the probe's speed, but this could be easily compensated for and we do this routinely when communicating with space probes.

3. Feb 9, 2015

### Ibix

We could (in principle) send robot probes to other star systems. I think the biggest challenge would be a propulsion system capable of pushing anything worthwhile up to a significant fraction of the speed of light. The equations describing relativistic rockets make depressing reading for SF fans.

Time dilation isn't really a challenge. The time, as measured on Earth, that it will take to get to an exoplanet is simple to calculate. It's the familiar distance-over-time formula, using the distance and the speed as measured from Earth - so if you go at 1/2 light speed to Alpha Centauri (about 4 light years away), it'll take 8 years. If you go to a star a hundred light years away, it'll take 200 years. However, the elapsed time measured by the probe will be less, by a factor of $\sqrt{1-v^2/c^2}$. That comes out at about 6.9 years in the Alpha Centauri case - obviously you'd have to go a lot faster if you, personally, want to arrive at a destination 100 light years away.

Note that there is no way to get the probe to the star quickly according to Earth. If the star is 100 light years away, the probe will need a tiny bit longer than 100 years to get to that star - the guys who sent the probe will be dead by the time it gets there whatever they do.

This isn't directly relevant to communication. The probe can always communicate by laser, as long as the signal is strong enough to pick out at however many light years' distance. The recipients on Earth will notice that the laser will be red-shifted by the probe's velocity. They will also observe that, once they have corrected for the probe's velocity, everything on the probe will appear to be slowed down. None of this presents any particular problem, so far as I an aware. The fact that the communication is by laser isn't special in a theoretical sense, either. The probe could perfectly well communicate by firing USB sticks back towards Earth. That isn't practical for a number of reasons (limited supplies of USB sticks, difficulty of accelerating them), but it would be possible.

4. Feb 9, 2015

### bodykey

phyzguy, hmmmm seems you're right about the speed. When I was doing my research for some reason I got a number much greater than that one. You are correct, Voyager 1 is only traveling 0.005% the speed of light.

I somewhat disagree with the notion that we aren't even close to being able to cause something to move close to the speed of light. With time, any force of acceleration can and will achieve this velocity relative to us as the observer. That simply depends on fuel and thrust, which has significantly been advanced upon since the Voyager 1 launch. I believe with a little push something like this could be most certainly likely. <-- this, however, is neither here nor there.

I think a plausible approach to such an experiment would be to shoot probes out to star systems only within a 50ly sphere of influence. However, the other portion of this equation I think we're missing is that a probe will also have to slow down in order to not pass through the system once it reaches its destination, so I imagine this will add significant changes in how long it would take a probe to reach its destination. I guess the good/bad thing is that the slower the craft is going, the less time it will take to slow down.

I'm not sure how USB sticks will really help anyone there. They would be unretrivable from space at those distances and if they ever did reach earth, they would burn up in our atmosphere before ever reaching the ground -- and that would have to be quite precise. I think laser comms would be the quickest and most reliable form of communication between probes and/or earth.

Some obvious issues with such an experiment would be as such:

1. Thrust, as you mentioned above, having such thrust would require a very large probe, which requires more fuel and weight to push.
2. Computing power. In order for a probe to successfully reach the destination it's going for and to be able to make a proper analysis of the system it enters, it's going to have to have a LOT of computer power.
3. Cosmic rays. I hear they are extremely destructive for electronics in space.
4. Battery power. Whether getting it from the nearby star, our star, or from reserves, it's gonna have to have enough to not just last the mission but year and years to come. It's unlikely anyone in the near future would want to go there physically, or want to wait another 50 years to send another probe, so it's best to send the first probe with all the equipment (and extra probes?) to do whatever necessary in that system, such as mapping the entire system, sending scout probes to the planets in the system to do graphical mapping, etc.
5. I would imagine it would have to have its own navigation system on board, things can change fast in an unpredictable environment, for example, we may detect three planets, but it could actually be five planets, we just don't know, and the probe would have to be able to detect that and adjust its course appropriately on its own without human intervention all at the same time staying in communication with Earth.

All of the above is pretty much crackpottery though, since it's never even been attempted outside of Science Fiction anyways. However, I don't see our ability as being so far off from being able to do such a thing. And I think, at least my own person opinion, that the idea "well it would take all this stuff...and it might not work" or "it's going to take this long to get there, so we might as well not even try" doesn't make any sense to me. If there's a time to do it it's right now, since before we see any returns from it it would be when we're all dead anyways.

5. Feb 9, 2015

### Ibix

There are hundreds of practical objections to the USB stick idea. You pretty much need Star Trek magi-tech to make it work. None of the objections relate to time dilation, though, which was the point I was making. Time dilation doesn't really cause problems for communication.

6. Feb 10, 2015

### phyzguy

Don't get me wrong. I'm not saying that it is not theoretically possible - it certainly is. I'm saying that our current technology is not up to the task. Concepts exist that could get a probe up to a sizable fraction of the speed of light, but right now they are just concepts. As you say, it is simply a matter of fuel and thrust. Try doing some calculations. Say you have a 1000 kg probe that you want to accelerate to 10% of the speed of light. Look up the thrust of current rocket engines and calculate the mass at take-off you would need to achieve this. Let us know what you find.

7. Feb 10, 2015

### bodykey

Hmm....ok, now I understand what you're saying. I suppose, what I'm trying to get to is this:

Let's say a probe was hypothetically able to achieve .99% the speed of light, and as it's accelerating and moving through space, it's constantly shooting back a laser towards Earth for communication purposes, telemetry, vital signs, etc. etc. From what I've read above, the probe itself would physically experience time dilation effects, and at .99%, those would be huge. However, given that the laser is not affected by the time dilation with the exception of red shifting, it would appear as though the probe didn't reach its destination yet to us as the observer even though it would've already reached the destination as the traveler, however, we would receive laser telemetry regardless of the time dilation effects, in 'real time' (of course as fast as light can travel, so if it's physically 20 light years away, it'd still take 20 years for us to receive that data).

I think that's a more precise statement of what I'm trying to figure out here. Can communication function properly even though with respect to time dilation our probe hasn't even reached its destination yet?

8. Feb 10, 2015

### Staff: Mentor

Um, what? When you get a laser image from the probe, it will show the status of the probe when the image was transmitted. If the probe had reached the destination when the image was emitted, that's what the image will show when it is received. Time dilation doesn't affect this at all. Time dilation only affects what clocks on the probe will read when the probe arrives at its destination (relative to what a clock at the destination that was synchronized with clocks on Earth would read).

What does "real time" mean here? We receive the laser telemetry, as you say, in the time it takes for light to travel from the probe to us. But this time is frame-dependent; if it takes 20 years for the telemetry to get to Earth according to Earth clocks, it will take a lot less time according to clocks on the probe if the probe is moving at 99% of the speed of light relative to Earth.

Sure, why not? (Allowing for redshift, of course.)

9. Feb 10, 2015

### Ibix

If the probe travels at .99c, it'll take about 20.2 years to reach a star 20 ly away. It's "made it!" message would take 20 years to get home, so we'd receive a close-up of the star 40.2 years after the probe set off.

All this is true in a Newtonian universe as well as an Einsteinian one. No relativity has been invoked so far.

Where time dilation comes into it is that the clocks on the probe run slow, as measured by people on the Earth. That means that the "made it" message would be timestamped 2.8 years after departure. That's all. There's no "made it but hasn't made it yet". That would make no sense.

If the probe sent continuous signals home, you'd see them slowed down by a factor of about 7 (even if you accounted for the increasing transmission delay). If the probe is programmed to send out a daily (by its onboard clock) status report, it'll send it out weekly according to Earth's clocks.

That's why time dilation isn't a problem. It doesn't make clocks go crazy; it just means that they tick at different rates.

10. Feb 10, 2015

### Staff: Mentor

To expand on this a bit: what you would actually observe in the signals would be a slowdown by a factor of about 14 (the relativistic Doppler factor). When you accounted for the fact that signals emitted later have further to travel, so take longer to get to you, you would calculate that the clocks on the probe were slowed down by a factor of about 7 (the time dilation factor).

11. Feb 12, 2015

### bodykey

Thank you guys, this pretty much answered my question. This is pretty cool, I thought I had a good grasp of relativity, but this makes it so much clearer for me now. Thanks a lot! :)