# A time dilation question as a biologist

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1. Feb 18, 2016

### merwyn

Iam a Microbiologist, not an expert in math and relativity. I have a simple question.

Assumption
Mr 'X' travells to the star 'S'
In earth reference the star is 5 light years ie 9,500,000,000,000 x 5 Kms from earth
Mr X travels @ 80% of lights speed (80% speed from leaving and reaching back; Immediate acceleration)
In earths time Mr X returns after 12.5 (6.25x2) years travelling 9.5^13 Kms
In Mr ' X' clock, Assume, he has returned to earth in 9 years due to time dilation.
As per Mr 'X' clock, he has traveled only 8.55^13 (9 Years X 9,500,000,000,000 kms)

Question is
1. How can it be?Whether Distance is also relative as time?
2. If Distance is relative, then how can be the speed of light is a constant?

I guess, i have made you to understand my point
Please No mathematics and calculations, Biologists are fools to understand numbers, so use simple terms like "Time runs slow @ higher speed" to explain.

With Regards
Merwyn

2. Feb 18, 2016

### Orodruin

Staff Emeritus
No, in his frame he has not travelled at all - it is the star which has moved. That being said, the star has moved a shorter distance yes. This is called length contraction. Both time and space are relative concepts.

Because time is also relative and velocity is distance per time.

3. Feb 18, 2016

### phinds

@merwyn, the math needed to understand Special Relativity, which is the topic that covers what you are asking, uses only high school algebra and very little of that and there are many beginner's tutorials on the Internet.

4. Feb 18, 2016

### Orodruin

Staff Emeritus
Well, I would say this depends on what you mean by "understand". It is certainly enough to get the very basics though.

5. Feb 18, 2016

### Staff: Mentor

For the specific question though, anyone who passed middle school pre-algebra should be able to handle it:

d=s*t

So if s is constant and t changes, d must...?

6. Feb 18, 2016

### jartsa

Mr X's traveling method was something like this:
1: Contract the distance to star S
2: Travel the contracted distance
3: Expand the distance
4: Contract the distance to Earth
5: Travel the contracted distance
6: Expand the distance
7: Declare "I traveled 10 light years in 9 years"

Everybody agrees that the clock that measured the time 9 years was shaken, therefore it was not a valid clock, except for the purpose of measuring proper time.

7. Feb 18, 2016

### Orodruin

Staff Emeritus
If you want to put it like that, you should do it properly:
2: Let S and Earth travel the contracted distance.
5: Let S and Earth travel the contracted distance.
7: Declare "I went to that star S 5 light years away in the rest frame of the Earth and the star and I aged 9 years during my trip. Isn't time dilation wonderful?"