B Not understanding relativity

1. Mar 27, 2016

Alltimegreat1

Let's assume we have the technology to travel in space at exactly the speed of light and embark on a journey to a star located 50 light years away. When the ship arrives at its destination, people on Earth will have aged 50 years. How much will the astronauts on board have aged?

2. Mar 27, 2016

Borg

Nothing except light can travel at the speed of light.

3. Mar 27, 2016

mathman

0. To be practical (travel at the speed of light is impossible) assume traveling near the speed of light. The Lorentz transformation will shorten the distance to the star, so travel will take whatever time needed to cover the foreshortened distance.

4. Mar 27, 2016

jfizzix

Since accelerating a ship exactly the speed of light is impossible in the current formulations of physics, we can ask a very similar question; What would happen if we could travel just barely below the speed of light?

The people on the ship wouldn't age much at all, and this would make sense, since in their frame of reference, the distance to that star would be Lorentz-contracted to a much smaller value. In the ship's frame of reference, the journey wouldn't take much time at all because the distance would literally be shorter.

But this journey is not without its perils.

Traveling so fast, micro-meteroids would be devastating to the ship, which in the ship's reference frame would be like bullets moving at relativistic speed.
If we say the ship has good shields, there's still the radiation to deal with.

In all directions, there is a microwave glow in space called the cosmic microwave background. If you were travelling that quickly, that microwave background would be blue-shifted in front of you into the infra-red, visible, ultra-violet, and beyond depending on how fast you were going. Also, for the same reason that more rain hits your windshield when your car is moving, this background radiation would get blindingly bright, likely cooking your ship.

5. Mar 27, 2016

Staff Emeritus
Relativity forbids massive objects from traveling "exactly at the speed of light". The best you can do is take the limit - if they travel almost at the speed of light, they experience almost no aging.

6. Mar 27, 2016

micromass

Staff Emeritus
Are you sure of that?

7. Mar 27, 2016

rootone

So if we disregard the need to accelerate to 'almost light speed'. and inconvenient dust and radiation, the ship could in principle go to the star 50 ly away, turn around and return to Earth 100 (+ a bit) years later, but for the ship occupants only one or two seconds will have elapsed.
?

Last edited: Mar 27, 2016
8. Mar 27, 2016

phinds

Yes, and since in practical terms all this is just silly anyway, you can skip the acceleration problem to show the effects of SR by positing a scenario where one ship is passing by the Earth at .999999999999c and heading to a star 50 LY away from Earth and just as it passes that star another ship heading the other way passes both it and the star at the same time and heads on to Earth. The occupants of the first ship will age a few seconds on the way to the star and the occupants of the second will age a few seconds as they travel to Earth.

9. Mar 27, 2016

rootone

Passengers in ship A could wave at the passengers in ship B while they passed by, but they would be need to be waving in gamma ray wavelengths to be noticed.

Last edited: Mar 27, 2016
10. Mar 27, 2016

phinds

At those speeds I'm not sure something that slow would work

11. Mar 28, 2016

Borg

I was using light to loosely represent all electromagnetic radiation. I guess that I should also include gravity waves. Am I missing something?

12. Mar 28, 2016

ProfuselyQuarky

This is outside of my realm, but someone was telling me a while back that quantum entanglement and negative matter move faster than light . . . I could be wrong.

13. Mar 28, 2016

rootone

I think you may be referring to hypothetical particles named 'tachyons'.
As far as I know these are mathematical constructs which there is no reason to believe can physically exist.
In the same sense that the concept of negative quantity is not hard to understand mathematically,
You may owe somebody three apples and can agree to obtain some apples next week to settle the debt.
Meanwhile though there are no negative apples in existence as physical reality.

14. Mar 28, 2016

phinds

Quantum entanglement does not "move" at any rate. The effects are instantaneous but nothing moves. Negative matter is believe to be an nonphysical result of math and not something that occurs in the real world.

15. Mar 28, 2016

ProfuselyQuarky

Thanks, phinds! The person was referring to this, I believe: click here if you dare

Could you help clarify quantum entanglement further?

16. Mar 28, 2016

micromass

Staff Emeritus
Take everything that Kaku says with a grain of salt. He's a very intelligent physicist, but also willing to distort the true nature of physics to make it seem more exciting.

17. Mar 28, 2016

ProfuselyQuarky

Thank you for letting me know. I don't really read who the authors of articles are; I just jump into the text. Perhaps a bad habit?

18. Mar 28, 2016

phinds

I REALLY dislike Kaku. As micromass pointed out, you have to take him with a grain of salt. For me, I think it's more like you need to douse him w/ the entire salt shaker.

Entanglement works like this, basically: a pair of particles get entangled and their spin, for example, becomes a characteristic of the pair not of either one of them exactly. When you measure one of them you are measuring one half of a pair exactly as you would be if you had a pair of gloves in your pocket and pulled one of them out and looked at it. When you see that the glove you are looking at is left handed, you know that the other on is right handed. When you measure the spin of half of an entangled pair, if it measure up, you know that the other is down. No information is communicated and nothing moves.

There are LOTS of threads on entanglement here on PF

19. Mar 28, 2016

ProfuselyQuarky

Thank you, again, phinds. Beautiful explanation. I'll search the other threads, as well.
I don't have salt at the moment, but I do have a lovely bottle of cayenne. I'll use that, instead, and tell the guy who shared with me the article to do the same

20. Mar 28, 2016

micromass

Staff Emeritus
That is not a good explanation since the entire weirdness with entanglement is precisely because it does not behave like a pair of gloves! https://www.physicsforums.com/threads/whats-so-unusual-about-entanglement.841824/